https://openjdk.java.net/ 
/*
 * Copyright (c) 2000, 2016, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
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 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
 * by Oracle in the LICENSE file that accompanied this code.
 *
 * This code is distributed in the hope that it will be useful, but WITHOUT
 * ANY WARRANTY; without even the implied warranty of MERCHANTABILITY or
 * FITNESS FOR A PARTICULAR PURPOSE.  See the GNU General Public License
 * version 2 for more details (a copy is included in the LICENSE file that
 * accompanied this code).
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 * You should have received a copy of the GNU General Public License version
 * 2 along with this work; if not, write to the Free Software Foundation,
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 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
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 */

package java.util.prefs;

import java.util.*;
import java.io.*;
import java.security.AccessController;
import java.security.PrivilegedAction;
// These imports needed only as a workaround for a JavaDoc bug
import java.lang.Integer;
import java.lang.Long;
import java.lang.Float;
import java.lang.Double;


This class provides a skeletal implementation of the Preferences class, greatly easing the task of implementing it. 
This class is for Preferences implementers only. Normal users of the Preferences facility should have no need to consult this documentation. The Preferences documentation should suffice.

Implementors must override the nine abstract service-provider interface (SPI) methods: getSpi(String), putSpi(String, String), removeSpi(String), childSpi(String), removeNodeSpi(), keysSpi(), childrenNamesSpi(), syncSpi() and flushSpi(). All of the concrete methods specify precisely how they are implemented atop these SPI methods. The implementor may, at his discretion, override one or more of the concrete methods if the default implementation is unsatisfactory for any reason, such as performance. 
The SPI methods fall into three groups concerning exception behavior. The getSpi method should never throw exceptions, but it doesn't really matter, as any exception thrown by this method will be intercepted by get(String, String), which will return the specified default value to the caller. The removeNodeSpi, keysSpi,
childrenNamesSpi, syncSpi and flushSpi methods are specified to throw BackingStoreException, and the implementation is required to throw this checked exception if it is unable to perform the operation. The exception propagates outward, causing the corresponding API method to fail. 
The remaining SPI methods putSpi(String, String), removeSpi(String) and childSpi(String) have more complicated exception behavior. They are not specified to throw BackingStoreException, as they can generally obey their contracts even if the backing store is unavailable. This is true because they return no information and their effects are not required to become permanent until a subsequent call to Preferences.flush() or Preferences.sync(). Generally speaking, these SPI methods should not throw exceptions. In some implementations, there may be circumstances under which these calls cannot even enqueue the requested operation for later processing. Even under these circumstances it is generally better to simply ignore the invocation and return, rather than throwing an exception. Under these circumstances, however, subsequently invoking flush() or sync would not imply that all previous operations had successfully been made permanent. 
There is one circumstance under which putSpi, removeSpi and
childSpi should throw an exception: if the caller lacks sufficient privileges on the underlying operating system to perform the requested operation. This will, for instance, occur on most systems if a non-privileged user attempts to modify system preferences. (The required privileges will vary from implementation to implementation. On some implementations, they are the right to modify the contents of some directory in the file system; on others they are the right to modify contents of some key in a registry.) Under any of these circumstances, it would generally be undesirable to let the program continue executing as if these operations would become permanent at a later time. While implementations are not required to throw an exception under these circumstances, they are encouraged to do so. A SecurityException would be appropriate. 
Most of the SPI methods require the implementation to read or write
information at a preferences node.  The implementor should beware of the
fact that another VM may have concurrently deleted this node from the
backing store.  It is the implementation's responsibility to recreate the
node if it has been deleted.

Implementation note: In Sun's default Preferences implementations, the user's identity is inherited from the underlying operating system and does not change for the lifetime of the virtual machine. It is recognized that server-side Preferences implementations may have the user identity change from request to request, implicitly passed to Preferences methods via the use of a static ThreadLocal instance. Authors of such implementations are strongly encouraged to determine the user at the time preferences are accessed (for example by the get(String, String) or put(String, String) method) rather than permanently associating a user with each Preferences instance. The latter behavior conflicts with normal Preferences usage and would lead to great confusion. 
Author: Josh Bloch
See Also:
Since:  1.4
/**
 * This class provides a skeletal implementation of the {@link Preferences}
 * class, greatly easing the task of implementing it.
 *
 * <p><strong>This class is for {@code Preferences} implementers only.
 * Normal users of the {@code Preferences} facility should have no need to
 * consult this documentation.  The {@link Preferences} documentation
 * should suffice.</strong>
 *
 * <p>Implementors must override the nine abstract service-provider interface
 * (SPI) methods: {@link #getSpi(String)}, {@link #putSpi(String,String)},
 * {@link #removeSpi(String)}, {@link #childSpi(String)}, {@link
 * #removeNodeSpi()}, {@link #keysSpi()}, {@link #childrenNamesSpi()}, {@link
 * #syncSpi()} and {@link #flushSpi()}.  All of the concrete methods specify
 * precisely how they are implemented atop these SPI methods.  The implementor
 * may, at his discretion, override one or more of the concrete methods if the
 * default implementation is unsatisfactory for any reason, such as
 * performance.
 *
 * <p>The SPI methods fall into three groups concerning exception
 * behavior. The {@code getSpi} method should never throw exceptions, but it
 * doesn't really matter, as any exception thrown by this method will be
 * intercepted by {@link #get(String,String)}, which will return the specified
 * default value to the caller.  The {@code removeNodeSpi, keysSpi,
 * childrenNamesSpi, syncSpi} and {@code flushSpi} methods are specified
 * to throw {@link BackingStoreException}, and the implementation is required
 * to throw this checked exception if it is unable to perform the operation.
 * The exception propagates outward, causing the corresponding API method
 * to fail.
 *
 * <p>The remaining SPI methods {@link #putSpi(String,String)}, {@link
 * #removeSpi(String)} and {@link #childSpi(String)} have more complicated
 * exception behavior.  They are not specified to throw
 * {@code BackingStoreException}, as they can generally obey their contracts
 * even if the backing store is unavailable.  This is true because they return
 * no information and their effects are not required to become permanent until
 * a subsequent call to {@link Preferences#flush()} or
 * {@link Preferences#sync()}. Generally speaking, these SPI methods should not
 * throw exceptions.  In some implementations, there may be circumstances
 * under which these calls cannot even enqueue the requested operation for
 * later processing.  Even under these circumstances it is generally better to
 * simply ignore the invocation and return, rather than throwing an
 * exception.  Under these circumstances, however, subsequently invoking
 * {@code flush()} or {@code sync} would not imply that all previous
 * operations had successfully been made permanent.
 *
 * <p>There is one circumstance under which {@code putSpi, removeSpi and
 * childSpi} <i>should</i> throw an exception: if the caller lacks
 * sufficient privileges on the underlying operating system to perform the
 * requested operation.  This will, for instance, occur on most systems
 * if a non-privileged user attempts to modify system preferences.
 * (The required privileges will vary from implementation to
 * implementation.  On some implementations, they are the right to modify the
 * contents of some directory in the file system; on others they are the right
 * to modify contents of some key in a registry.)  Under any of these
 * circumstances, it would generally be undesirable to let the program
 * continue executing as if these operations would become permanent at a later
 * time.  While implementations are not required to throw an exception under
 * these circumstances, they are encouraged to do so.  A {@link
 * SecurityException} would be appropriate.
 *
 * <p>Most of the SPI methods require the implementation to read or write
 * information at a preferences node.  The implementor should beware of the
 * fact that another VM may have concurrently deleted this node from the
 * backing store.  It is the implementation's responsibility to recreate the
 * node if it has been deleted.
 *
 * <p>Implementation note: In Sun's default {@code Preferences}
 * implementations, the user's identity is inherited from the underlying
 * operating system and does not change for the lifetime of the virtual
 * machine.  It is recognized that server-side {@code Preferences}
 * implementations may have the user identity change from request to request,
 * implicitly passed to {@code Preferences} methods via the use of a
 * static {@link ThreadLocal} instance.  Authors of such implementations are
 * <i>strongly</i> encouraged to determine the user at the time preferences
 * are accessed (for example by the {@link #get(String,String)} or {@link
 * #put(String,String)} method) rather than permanently associating a user
 * with each {@code Preferences} instance.  The latter behavior conflicts
 * with normal {@code Preferences} usage and would lead to great confusion.
 *
 * @author  Josh Bloch
 * @see     Preferences
 * @since   1.4
 */
public abstract class AbstractPreferences extends Preferences {
    
The code point U+0000, assigned to the null control character, is the
only character encoded in Unicode and ISO/IEC 10646 that is always
invalid in any XML 1.0 and 1.1 document.

/**
     * The code point U+0000, assigned to the null control character, is the
     * only character encoded in Unicode and ISO/IEC 10646 that is always
     * invalid in any XML 1.0 and 1.1 document.
     */
    static final int CODE_POINT_U0000 = '\u0000';

    
Our name relative to parent.

/**
     * Our name relative to parent.
     */
    private final String name;

    
Our absolute path name.

/**
     * Our absolute path name.
     */
    private final String absolutePath;

    
Our parent node.

/**
     * Our parent node.
     */
    final AbstractPreferences parent;

    
Our root node.

/**
     * Our root node.
     */
    private final AbstractPreferences root; // Relative to this node

    
This field should be true if this node did not exist in the backing store prior to the creation of this object. The field is initialized to false, but may be set to true by a subclass constructor (and should not be modified thereafter). This field indicates whether a node change event should be fired when creation is complete. 
/**
     * This field should be {@code true} if this node did not exist in the
     * backing store prior to the creation of this object.  The field
     * is initialized to false, but may be set to true by a subclass
     * constructor (and should not be modified thereafter).  This field
     * indicates whether a node change event should be fired when
     * creation is complete.
     */
    protected boolean newNode = false;

    
All known unremoved children of this node.  (This "cache" is consulted
prior to calling childSpi() or getChild().

/**
     * All known unremoved children of this node.  (This "cache" is consulted
     * prior to calling childSpi() or getChild().
     */
    private Map<String, AbstractPreferences> kidCache = new HashMap<>();

    
This field is used to keep track of whether or not this node has
been removed.  Once it's set to true, it will never be reset to false.

/**
     * This field is used to keep track of whether or not this node has
     * been removed.  Once it's set to true, it will never be reset to false.
     */
    private boolean removed = false;

    
Registered preference change listeners.

/**
     * Registered preference change listeners.
     */
    private PreferenceChangeListener[] prefListeners =
        new PreferenceChangeListener[0];

    
Registered node change listeners.

/**
     * Registered node change listeners.
     */
    private NodeChangeListener[] nodeListeners = new NodeChangeListener[0];

    
An object whose monitor is used to lock this node.  This object
is used in preference to the node itself to reduce the likelihood of
intentional or unintentional denial of service due to a locked node.
To avoid deadlock, a node is never locked by a thread that
holds a lock on a descendant of that node.

/**
     * An object whose monitor is used to lock this node.  This object
     * is used in preference to the node itself to reduce the likelihood of
     * intentional or unintentional denial of service due to a locked node.
     * To avoid deadlock, a node is <i>never</i> locked by a thread that
     * holds a lock on a descendant of that node.
     */
    protected final Object lock = new Object();

    
Creates a preference node with the specified parent and the specified
name relative to its parent.

Params:
  • parent – the parent of this preference node, or null if this
              is the root.
  • name – the name of this preference node, relative to its parent, or "" if this is the root.
Throws:
/**
     * Creates a preference node with the specified parent and the specified
     * name relative to its parent.
     *
     * @param parent the parent of this preference node, or null if this
     *               is the root.
     * @param name the name of this preference node, relative to its parent,
     *             or {@code ""} if this is the root.
     * @throws IllegalArgumentException if {@code name} contains a slash
     *          ({@code '/'}),  or {@code parent} is {@code null} and
     *          name isn't {@code ""}.
     */
    protected AbstractPreferences(AbstractPreferences parent, String name) {
        if (parent==null) {
            if (!name.equals(""))
                throw new IllegalArgumentException("Root name '"+name+
                                                   "' must be \"\"");
            this.absolutePath = "/";
            root = this;
        } else {
            if (name.indexOf('/') != -1)
                throw new IllegalArgumentException("Name '" + name +
                                                 "' contains '/'");
            if (name.equals(""))
              throw new IllegalArgumentException("Illegal name: empty string");

            root = parent.root;
            absolutePath = (parent==root ? "/" + name
                                         : parent.absolutePath() + "/" + name);
        }
        this.name = name;
        this.parent = parent;
    }

    
Implements the put method as per the specification in Preferences.put(String, String). 
This implementation checks that the key and value are legal, obtains this preference node's lock, checks that the node has not been removed, invokes putSpi(String, String), and if there are any preference change listeners, enqueues a notification event for processing by the event dispatch thread. 
Params:
  • key – key with which the specified value is to be associated.
  • value – value to be associated with the specified key.
Throws:
/**
     * Implements the {@code put} method as per the specification in
     * {@link Preferences#put(String,String)}.
     *
     * <p>This implementation checks that the key and value are legal,
     * obtains this preference node's lock, checks that the node
     * has not been removed, invokes {@link #putSpi(String,String)}, and if
     * there are any preference change listeners, enqueues a notification
     * event for processing by the event dispatch thread.
     *
     * @param key key with which the specified value is to be associated.
     * @param value value to be associated with the specified key.
     * @throws NullPointerException if key or value is {@code null}.
     * @throws IllegalArgumentException if {@code key.length()} exceeds
     *       {@code MAX_KEY_LENGTH} or if {@code value.length} exceeds
     *       {@code MAX_VALUE_LENGTH}.
     * @throws IllegalArgumentException if either key or value contain
     *         the null control character, code point U+0000.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     */
    public void put(String key, String value) {
        if (key==null || value==null)
            throw new NullPointerException();
        if (key.length() > MAX_KEY_LENGTH)
            throw new IllegalArgumentException("Key too long: "+key);
        if (value.length() > MAX_VALUE_LENGTH)
            throw new IllegalArgumentException("Value too long: "+value);
        if (key.indexOf(CODE_POINT_U0000) != -1)
            throw new IllegalArgumentException("Key contains code point U+0000");
        if (value.indexOf(CODE_POINT_U0000) != -1)
            throw new IllegalArgumentException("Value contains code point U+0000");

        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node has been removed.");

            putSpi(key, value);
            enqueuePreferenceChangeEvent(key, value);
        }
    }

    
Implements the get method as per the specification in Preferences.get(String, String). 
This implementation first checks to see if key is null throwing a NullPointerException if this is the case. Then it obtains this preference node's lock, checks that the node has not been removed, invokes getSpi(String), and returns the result, unless the getSpi invocation returns null or throws an exception, in which case this invocation returns def. 
Params:
  • key – key whose associated value is to be returned.
  • def – the value to be returned in the event that this preference node has no value associated with key.
Throws:
Returns:the value associated with key, or def if no value is associated with key.
/**
     * Implements the {@code get} method as per the specification in
     * {@link Preferences#get(String,String)}.
     *
     * <p>This implementation first checks to see if {@code key} is
     * {@code null} throwing a {@code NullPointerException} if this is
     * the case.  Then it obtains this preference node's lock,
     * checks that the node has not been removed, invokes {@link
     * #getSpi(String)}, and returns the result, unless the {@code getSpi}
     * invocation returns {@code null} or throws an exception, in which case
     * this invocation returns {@code def}.
     *
     * @param key key whose associated value is to be returned.
     * @param def the value to be returned in the event that this
     *        preference node has no value associated with {@code key}.
     * @return the value associated with {@code key}, or {@code def}
     *         if no value is associated with {@code key}.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     * @throws NullPointerException if key is {@code null}.  (A
     *         {@code null} default <i>is</i> permitted.)
     * @throws IllegalArgumentException if key contains the null control
     *         character, code point U+0000.
     */
    public String get(String key, String def) {
        if (key==null)
            throw new NullPointerException("Null key");
        if (key.indexOf(CODE_POINT_U0000) != -1)
            throw new IllegalArgumentException("Key contains code point U+0000");
        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node has been removed.");

            String result = null;
            try {
                result = getSpi(key);
            } catch (Exception e) {
                // Ignoring exception causes default to be returned
            }
            return (result==null ? def : result);
        }
    }

    
Implements the remove(String) method as per the specification in Preferences.remove(String). 
This implementation obtains this preference node's lock, checks that the node has not been removed, invokes removeSpi(String) and if there are any preference change listeners, enqueues a notification event for processing by the event dispatch thread. 
Params:
  • key – key whose mapping is to be removed from the preference node.
Throws:
/**
     * Implements the {@code remove(String)} method as per the specification
     * in {@link Preferences#remove(String)}.
     *
     * <p>This implementation obtains this preference node's lock,
     * checks that the node has not been removed, invokes
     * {@link #removeSpi(String)} and if there are any preference
     * change listeners, enqueues a notification event for processing by the
     * event dispatch thread.
     *
     * @param key key whose mapping is to be removed from the preference node.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     * @throws IllegalArgumentException if key contains the null control
     *         character, code point U+0000.
     * @throws NullPointerException {@inheritDoc}.
     */
    public void remove(String key) {
        Objects.requireNonNull(key, "Specified key cannot be null");
        if (key.indexOf(CODE_POINT_U0000) != -1)
            throw new IllegalArgumentException("Key contains code point U+0000");
        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node has been removed.");

            removeSpi(key);
            enqueuePreferenceChangeEvent(key, null);
        }
    }

    
Implements the clear method as per the specification in Preferences.clear(). 
This implementation obtains this preference node's lock, invokes keys() to obtain an array of keys, and iterates over the array invoking remove(String) on each key. 
Throws:
/**
     * Implements the {@code clear} method as per the specification in
     * {@link Preferences#clear()}.
     *
     * <p>This implementation obtains this preference node's lock,
     * invokes {@link #keys()} to obtain an array of keys, and
     * iterates over the array invoking {@link #remove(String)} on each key.
     *
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     */
    public void clear() throws BackingStoreException {
        synchronized(lock) {
            for (String key : keys())
                remove(key);
        }
    }

    
Implements the putInt method as per the specification in Preferences.putInt(String, int). 
This implementation translates value to a string with Integer.toString(int) and invokes put(String, String) on the result. 
Params:
  • key – key with which the string form of value is to be associated.
  • value – value whose string form is to be associated with key.
Throws:
/**
     * Implements the {@code putInt} method as per the specification in
     * {@link Preferences#putInt(String,int)}.
     *
     * <p>This implementation translates {@code value} to a string with
     * {@link Integer#toString(int)} and invokes {@link #put(String,String)}
     * on the result.
     *
     * @param key key with which the string form of value is to be associated.
     * @param value value whose string form is to be associated with key.
     * @throws NullPointerException if key is {@code null}.
     * @throws IllegalArgumentException if {@code key.length()} exceeds
     *         {@code MAX_KEY_LENGTH}.
     * @throws IllegalArgumentException if key contains
     *         the null control character, code point U+0000.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     */
    public void putInt(String key, int value) {
        put(key, Integer.toString(value));
    }

    
Implements the getInt method as per the specification in Preferences.getInt(String, int). 
This implementation invokes get(key,
null). If the return value is non-null, the implementation attempts to translate it to an int with Integer.parseInt(String). If the attempt succeeds, the return value is returned by this method. Otherwise, def is returned. 
Params:
  • key – key whose associated value is to be returned as an int.
  • def – the value to be returned in the event that this preference node has no value associated with key or the associated value cannot be interpreted as an int.
Throws:
Returns:the int value represented by the string associated with key in this preference node, or def if the associated value does not exist or cannot be interpreted as an int.
/**
     * Implements the {@code getInt} method as per the specification in
     * {@link Preferences#getInt(String,int)}.
     *
     * <p>This implementation invokes {@link #get(String,String) get(key,
     * null)}.  If the return value is non-null, the implementation
     * attempts to translate it to an {@code int} with
     * {@link Integer#parseInt(String)}.  If the attempt succeeds, the return
     * value is returned by this method.  Otherwise, {@code def} is returned.
     *
     * @param key key whose associated value is to be returned as an int.
     * @param def the value to be returned in the event that this
     *        preference node has no value associated with {@code key}
     *        or the associated value cannot be interpreted as an int.
     * @return the int value represented by the string associated with
     *         {@code key} in this preference node, or {@code def} if the
     *         associated value does not exist or cannot be interpreted as
     *         an int.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     * @throws NullPointerException if {@code key} is {@code null}.
     * @throws IllegalArgumentException if key contains the null control
     *         character, code point U+0000.
     */
    public int getInt(String key, int def) {
        int result = def;
        try {
            String value = get(key, null);
            if (value != null)
                result = Integer.parseInt(value);
        } catch (NumberFormatException e) {
            // Ignoring exception causes specified default to be returned
        }

        return result;
    }

    
Implements the putLong method as per the specification in Preferences.putLong(String, long). 
This implementation translates value to a string with Long.toString(long) and invokes put(String, String) on the result. 
Params:
  • key – key with which the string form of value is to be associated.
  • value – value whose string form is to be associated with key.
Throws:
/**
     * Implements the {@code putLong} method as per the specification in
     * {@link Preferences#putLong(String,long)}.
     *
     * <p>This implementation translates {@code value} to a string with
     * {@link Long#toString(long)} and invokes {@link #put(String,String)}
     * on the result.
     *
     * @param key key with which the string form of value is to be associated.
     * @param value value whose string form is to be associated with key.
     * @throws NullPointerException if key is {@code null}.
     * @throws IllegalArgumentException if {@code key.length()} exceeds
     *         {@code MAX_KEY_LENGTH}.
     * @throws IllegalArgumentException if key contains
     *         the null control character, code point U+0000.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     */
    public void putLong(String key, long value) {
        put(key, Long.toString(value));
    }

    
Implements the getLong method as per the specification in Preferences.getLong(String, long). 
This implementation invokes get(key,
null). If the return value is non-null, the implementation attempts to translate it to a long with Long.parseLong(String). If the attempt succeeds, the return value is returned by this method. Otherwise, def is returned. 
Params:
  • key – key whose associated value is to be returned as a long.
  • def – the value to be returned in the event that this preference node has no value associated with key or the associated value cannot be interpreted as a long.
Throws:
Returns:the long value represented by the string associated with key in this preference node, or def if the associated value does not exist or cannot be interpreted as a long.
/**
     * Implements the {@code getLong} method as per the specification in
     * {@link Preferences#getLong(String,long)}.
     *
     * <p>This implementation invokes {@link #get(String,String) get(key,
     * null)}.  If the return value is non-null, the implementation
     * attempts to translate it to a {@code long} with
     * {@link Long#parseLong(String)}.  If the attempt succeeds, the return
     * value is returned by this method.  Otherwise, {@code def} is returned.
     *
     * @param key key whose associated value is to be returned as a long.
     * @param def the value to be returned in the event that this
     *        preference node has no value associated with {@code key}
     *        or the associated value cannot be interpreted as a long.
     * @return the long value represented by the string associated with
     *         {@code key} in this preference node, or {@code def} if the
     *         associated value does not exist or cannot be interpreted as
     *         a long.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     * @throws NullPointerException if {@code key} is {@code null}.
     * @throws IllegalArgumentException if key contains the null control
     *         character, code point U+0000.
     */
    public long getLong(String key, long def) {
        long result = def;
        try {
            String value = get(key, null);
            if (value != null)
                result = Long.parseLong(value);
        } catch (NumberFormatException e) {
            // Ignoring exception causes specified default to be returned
        }

        return result;
    }

    
Implements the putBoolean method as per the specification in Preferences.putBoolean(String, boolean). 
This implementation translates value to a string with String.valueOf(boolean) and invokes put(String, String) on the result. 
Params:
  • key – key with which the string form of value is to be associated.
  • value – value whose string form is to be associated with key.
Throws:
/**
     * Implements the {@code putBoolean} method as per the specification in
     * {@link Preferences#putBoolean(String,boolean)}.
     *
     * <p>This implementation translates {@code value} to a string with
     * {@link String#valueOf(boolean)} and invokes {@link #put(String,String)}
     * on the result.
     *
     * @param key key with which the string form of value is to be associated.
     * @param value value whose string form is to be associated with key.
     * @throws NullPointerException if key is {@code null}.
     * @throws IllegalArgumentException if {@code key.length()} exceeds
     *         {@code MAX_KEY_LENGTH}.
     * @throws IllegalArgumentException if key contains
     *         the null control character, code point U+0000.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     */
    public void putBoolean(String key, boolean value) {
        put(key, String.valueOf(value));
    }

    
Implements the getBoolean method as per the specification in Preferences.getBoolean(String, boolean). 
This implementation invokes get(key,
null). If the return value is non-null, it is compared with "true" using String.equalsIgnoreCase(String). If the comparison returns true, this invocation returns true. Otherwise, the original return value is compared with "false", again using String.equalsIgnoreCase(String). If the comparison returns true, this invocation returns false. Otherwise, this invocation returns def. 
Params:
  • key – key whose associated value is to be returned as a boolean.
  • def – the value to be returned in the event that this preference node has no value associated with key or the associated value cannot be interpreted as a boolean.
Throws:
Returns:the boolean value represented by the string associated with key in this preference node, or def if the associated value does not exist or cannot be interpreted as a boolean.
/**
     * Implements the {@code getBoolean} method as per the specification in
     * {@link Preferences#getBoolean(String,boolean)}.
     *
     * <p>This implementation invokes {@link #get(String,String) get(key,
     * null)}.  If the return value is non-null, it is compared with
     * {@code "true"} using {@link String#equalsIgnoreCase(String)}.  If the
     * comparison returns {@code true}, this invocation returns
     * {@code true}.  Otherwise, the original return value is compared with
     * {@code "false"}, again using {@link String#equalsIgnoreCase(String)}.
     * If the comparison returns {@code true}, this invocation returns
     * {@code false}.  Otherwise, this invocation returns {@code def}.
     *
     * @param key key whose associated value is to be returned as a boolean.
     * @param def the value to be returned in the event that this
     *        preference node has no value associated with {@code key}
     *        or the associated value cannot be interpreted as a boolean.
     * @return the boolean value represented by the string associated with
     *         {@code key} in this preference node, or {@code def} if the
     *         associated value does not exist or cannot be interpreted as
     *         a boolean.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     * @throws NullPointerException if {@code key} is {@code null}.
     * @throws IllegalArgumentException if key contains the null control
     *         character, code point U+0000.
     */
    public boolean getBoolean(String key, boolean def) {
        boolean result = def;
        String value = get(key, null);
        if (value != null) {
            if (value.equalsIgnoreCase("true"))
                result = true;
            else if (value.equalsIgnoreCase("false"))
                result = false;
        }

        return result;
    }

    
Implements the putFloat method as per the specification in Preferences.putFloat(String, float). 
This implementation translates value to a string with Float.toString(float) and invokes put(String, String) on the result. 
Params:
  • key – key with which the string form of value is to be associated.
  • value – value whose string form is to be associated with key.
Throws:
/**
     * Implements the {@code putFloat} method as per the specification in
     * {@link Preferences#putFloat(String,float)}.
     *
     * <p>This implementation translates {@code value} to a string with
     * {@link Float#toString(float)} and invokes {@link #put(String,String)}
     * on the result.
     *
     * @param key key with which the string form of value is to be associated.
     * @param value value whose string form is to be associated with key.
     * @throws NullPointerException if key is {@code null}.
     * @throws IllegalArgumentException if {@code key.length()} exceeds
     *         {@code MAX_KEY_LENGTH}.
     * @throws IllegalArgumentException if key contains
     *         the null control character, code point U+0000.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     */
    public void putFloat(String key, float value) {
        put(key, Float.toString(value));
    }

    
Implements the getFloat method as per the specification in Preferences.getFloat(String, float). 
This implementation invokes get(key,
null). If the return value is non-null, the implementation attempts to translate it to an float with Float.parseFloat(String). If the attempt succeeds, the return value is returned by this method. Otherwise, def is returned. 
Params:
  • key – key whose associated value is to be returned as a float.
  • def – the value to be returned in the event that this preference node has no value associated with key or the associated value cannot be interpreted as a float.
Throws:
Returns:the float value represented by the string associated with key in this preference node, or def if the associated value does not exist or cannot be interpreted as a float.
/**
     * Implements the {@code getFloat} method as per the specification in
     * {@link Preferences#getFloat(String,float)}.
     *
     * <p>This implementation invokes {@link #get(String,String) get(key,
     * null)}.  If the return value is non-null, the implementation
     * attempts to translate it to an {@code float} with
     * {@link Float#parseFloat(String)}.  If the attempt succeeds, the return
     * value is returned by this method.  Otherwise, {@code def} is returned.
     *
     * @param key key whose associated value is to be returned as a float.
     * @param def the value to be returned in the event that this
     *        preference node has no value associated with {@code key}
     *        or the associated value cannot be interpreted as a float.
     * @return the float value represented by the string associated with
     *         {@code key} in this preference node, or {@code def} if the
     *         associated value does not exist or cannot be interpreted as
     *         a float.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     * @throws NullPointerException if {@code key} is {@code null}.
     * @throws IllegalArgumentException if key contains the null control
     *         character, code point U+0000.
     */
    public float getFloat(String key, float def) {
        float result = def;
        try {
            String value = get(key, null);
            if (value != null)
                result = Float.parseFloat(value);
        } catch (NumberFormatException e) {
            // Ignoring exception causes specified default to be returned
        }

        return result;
    }

    
Implements the putDouble method as per the specification in Preferences.putDouble(String, double). 
This implementation translates value to a string with Double.toString(double) and invokes put(String, String) on the result. 
Params:
  • key – key with which the string form of value is to be associated.
  • value – value whose string form is to be associated with key.
Throws:
/**
     * Implements the {@code putDouble} method as per the specification in
     * {@link Preferences#putDouble(String,double)}.
     *
     * <p>This implementation translates {@code value} to a string with
     * {@link Double#toString(double)} and invokes {@link #put(String,String)}
     * on the result.
     *
     * @param key key with which the string form of value is to be associated.
     * @param value value whose string form is to be associated with key.
     * @throws NullPointerException if key is {@code null}.
     * @throws IllegalArgumentException if {@code key.length()} exceeds
     *         {@code MAX_KEY_LENGTH}.
     * @throws IllegalArgumentException if key contains
     *         the null control character, code point U+0000.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     */
    public void putDouble(String key, double value) {
        put(key, Double.toString(value));
    }

    
Implements the getDouble method as per the specification in Preferences.getDouble(String, double). 
This implementation invokes get(key,
null). If the return value is non-null, the implementation attempts to translate it to an double with Double.parseDouble(String). If the attempt succeeds, the return value is returned by this method. Otherwise, def is returned. 
Params:
  • key – key whose associated value is to be returned as a double.
  • def – the value to be returned in the event that this preference node has no value associated with key or the associated value cannot be interpreted as a double.
Throws:
Returns:the double value represented by the string associated with key in this preference node, or def if the associated value does not exist or cannot be interpreted as a double.
/**
     * Implements the {@code getDouble} method as per the specification in
     * {@link Preferences#getDouble(String,double)}.
     *
     * <p>This implementation invokes {@link #get(String,String) get(key,
     * null)}.  If the return value is non-null, the implementation
     * attempts to translate it to an {@code double} with
     * {@link Double#parseDouble(String)}.  If the attempt succeeds, the return
     * value is returned by this method.  Otherwise, {@code def} is returned.
     *
     * @param key key whose associated value is to be returned as a double.
     * @param def the value to be returned in the event that this
     *        preference node has no value associated with {@code key}
     *        or the associated value cannot be interpreted as a double.
     * @return the double value represented by the string associated with
     *         {@code key} in this preference node, or {@code def} if the
     *         associated value does not exist or cannot be interpreted as
     *         a double.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     * @throws NullPointerException if {@code key} is {@code null}.
     * @throws IllegalArgumentException if key contains the null control
     *         character, code point U+0000.
     */
    public double getDouble(String key, double def) {
        double result = def;
        try {
            String value = get(key, null);
            if (value != null)
                result = Double.parseDouble(value);
        } catch (NumberFormatException e) {
            // Ignoring exception causes specified default to be returned
        }

        return result;
    }

    
Implements the putByteArray method as per the specification in Preferences.putByteArray(String, byte[]). 
Params:
  • key – key with which the string form of value is to be associated.
  • value – value whose string form is to be associated with key.
Throws:
/**
     * Implements the {@code putByteArray} method as per the specification in
     * {@link Preferences#putByteArray(String,byte[])}.
     *
     * @param key key with which the string form of value is to be associated.
     * @param value value whose string form is to be associated with key.
     * @throws NullPointerException if key or value is {@code null}.
     * @throws IllegalArgumentException if key.length() exceeds MAX_KEY_LENGTH
     *         or if value.length exceeds MAX_VALUE_LENGTH*3/4.
     * @throws IllegalArgumentException if key contains
     *         the null control character, code point U+0000.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     */
    public void putByteArray(String key, byte[] value) {
        put(key, Base64.byteArrayToBase64(value));
    }

    
Implements the getByteArray method as per the specification in Preferences.getByteArray(String, byte[]). 
Params:
  • key – key whose associated value is to be returned as a byte array.
  • def – the value to be returned in the event that this preference node has no value associated with key or the associated value cannot be interpreted as a byte array.
Throws:
Returns:the byte array value represented by the string associated with key in this preference node, or def if the associated value does not exist or cannot be interpreted as a byte array.
/**
     * Implements the {@code getByteArray} method as per the specification in
     * {@link Preferences#getByteArray(String,byte[])}.
     *
     * @param key key whose associated value is to be returned as a byte array.
     * @param def the value to be returned in the event that this
     *        preference node has no value associated with {@code key}
     *        or the associated value cannot be interpreted as a byte array.
     * @return the byte array value represented by the string associated with
     *         {@code key} in this preference node, or {@code def} if the
     *         associated value does not exist or cannot be interpreted as
     *         a byte array.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     * @throws NullPointerException if {@code key} is {@code null}.  (A
     *         {@code null} value for {@code def} <i>is</i> permitted.)
     * @throws IllegalArgumentException if key contains the null control
     *         character, code point U+0000.
     */
    public byte[] getByteArray(String key, byte[] def) {
        byte[] result = def;
        String value = get(key, null);
        try {
            if (value != null)
                result = Base64.base64ToByteArray(value);
        }
        catch (RuntimeException e) {
            // Ignoring exception causes specified default to be returned
        }

        return result;
    }

    
Implements the keys method as per the specification in Preferences.keys(). 
This implementation obtains this preference node's lock, checks that the node has not been removed and invokes keysSpi(). 
Throws:
Returns:an array of the keys that have an associated value in this
        preference node.
/**
     * Implements the {@code keys} method as per the specification in
     * {@link Preferences#keys()}.
     *
     * <p>This implementation obtains this preference node's lock, checks that
     * the node has not been removed and invokes {@link #keysSpi()}.
     *
     * @return an array of the keys that have an associated value in this
     *         preference node.
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     */
    public String[] keys() throws BackingStoreException {
        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node has been removed.");

            return keysSpi();
        }
    }

    
Implements the children method as per the specification in Preferences.childrenNames(). 
This implementation obtains this preference node's lock, checks that the node has not been removed, constructs a TreeSet initialized to the names of children already cached (the children in this node's "child-cache"), invokes childrenNamesSpi(), and adds all of the returned child-names into the set. The elements of the tree set are dumped into a String array using the toArray method, and this array is returned. 
Throws:
See Also:
Returns:the names of the children of this preference node.
/**
     * Implements the {@code children} method as per the specification in
     * {@link Preferences#childrenNames()}.
     *
     * <p>This implementation obtains this preference node's lock, checks that
     * the node has not been removed, constructs a {@code TreeSet} initialized
     * to the names of children already cached (the children in this node's
     * "child-cache"), invokes {@link #childrenNamesSpi()}, and adds all of the
     * returned child-names into the set.  The elements of the tree set are
     * dumped into a {@code String} array using the {@code toArray} method,
     * and this array is returned.
     *
     * @return the names of the children of this preference node.
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     * @see #cachedChildren()
     */
    public String[] childrenNames() throws BackingStoreException {
        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node has been removed.");

            Set<String> s = new TreeSet<>(kidCache.keySet());
            for (String kid : childrenNamesSpi())
                s.add(kid);
            return s.toArray(EMPTY_STRING_ARRAY);
        }
    }

    private static final String[] EMPTY_STRING_ARRAY = new String[0];

    
Returns all known unremoved children of this node.

Returns:all known unremoved children of this node.
/**
     * Returns all known unremoved children of this node.
     *
     * @return all known unremoved children of this node.
     */
    protected final AbstractPreferences[] cachedChildren() {
        return kidCache.values().toArray(EMPTY_ABSTRACT_PREFS_ARRAY);
    }

    private static final AbstractPreferences[] EMPTY_ABSTRACT_PREFS_ARRAY
        = new AbstractPreferences[0];

    
Implements the parent method as per the specification in Preferences.parent(). 
This implementation obtains this preference node's lock, checks that
the node has not been removed and returns the parent value that was
passed to this node's constructor.

Throws:
Returns:the parent of this preference node.
/**
     * Implements the {@code parent} method as per the specification in
     * {@link Preferences#parent()}.
     *
     * <p>This implementation obtains this preference node's lock, checks that
     * the node has not been removed and returns the parent value that was
     * passed to this node's constructor.
     *
     * @return the parent of this preference node.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     */
    public Preferences parent() {
        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node has been removed.");

            return parent;
        }
    }

    
Implements the node method as per the specification in Preferences.node(String). 
This implementation obtains this preference node's lock and checks that the node has not been removed. If path is "", this node is returned; if path is "/", this node's root is returned. If the first character in path is not '/', the implementation breaks path into tokens and recursively traverses the path from this node to the named node, "consuming" a name and a slash from path at each step of the traversal. At each step, the current node is locked and the node's child-cache is checked for the named node. If it is not found, the name is checked to make sure its length does not exceed MAX_NAME_LENGTH. Then the childSpi(String) method is invoked, and the result stored in this node's child-cache. If the newly created Preferences object's newNode field is true and there are any node change listeners, a notification event is enqueued for processing by the event dispatch thread. 
When there are no more tokens, the last value found in the child-cache or returned by childSpi is returned by this method. If during the traversal, two "/" tokens occur consecutively, or the final token is "/" (rather than a name), an appropriate IllegalArgumentException is thrown. 
 If the first character of path is '/' (indicating an absolute path name) this preference node's lock is dropped prior to breaking path into tokens, and this method recursively traverses the path starting from the root (rather than starting from this node). The traversal is otherwise identical to the one described for relative path names. Dropping the lock on this node prior to commencing the traversal at the root node is essential to avoid the possibility of deadlock, as per the locking invariant. 
Params:
  • path – the path name of the preference node to return.
Throws:
  • IllegalArgumentException – if the path name is invalid (i.e.,
        it contains multiple consecutive slash characters, or ends
        with a slash character and is more than one character long).
  • IllegalStateException – if this node (or an ancestor) has been removed with the removeNode() method.
Returns:the specified preference node.
/**
     * Implements the {@code node} method as per the specification in
     * {@link Preferences#node(String)}.
     *
     * <p>This implementation obtains this preference node's lock and checks
     * that the node has not been removed.  If {@code path} is {@code ""},
     * this node is returned; if {@code path} is {@code "/"}, this node's
     * root is returned.  If the first character in {@code path} is
     * not {@code '/'}, the implementation breaks {@code path} into
     * tokens and recursively traverses the path from this node to the
     * named node, "consuming" a name and a slash from {@code path} at
     * each step of the traversal.  At each step, the current node is locked
     * and the node's child-cache is checked for the named node.  If it is
     * not found, the name is checked to make sure its length does not
     * exceed {@code MAX_NAME_LENGTH}.  Then the {@link #childSpi(String)}
     * method is invoked, and the result stored in this node's child-cache.
     * If the newly created {@code Preferences} object's {@link #newNode}
     * field is {@code true} and there are any node change listeners,
     * a notification event is enqueued for processing by the event dispatch
     * thread.
     *
     * <p>When there are no more tokens, the last value found in the
     * child-cache or returned by {@code childSpi} is returned by this
     * method.  If during the traversal, two {@code "/"} tokens occur
     * consecutively, or the final token is {@code "/"} (rather than a name),
     * an appropriate {@code IllegalArgumentException} is thrown.
     *
     * <p> If the first character of {@code path} is {@code '/'}
     * (indicating an absolute path name) this preference node's
     * lock is dropped prior to breaking {@code path} into tokens, and
     * this method recursively traverses the path starting from the root
     * (rather than starting from this node).  The traversal is otherwise
     * identical to the one described for relative path names.  Dropping
     * the lock on this node prior to commencing the traversal at the root
     * node is essential to avoid the possibility of deadlock, as per the
     * {@link #lock locking invariant}.
     *
     * @param path the path name of the preference node to return.
     * @return the specified preference node.
     * @throws IllegalArgumentException if the path name is invalid (i.e.,
     *         it contains multiple consecutive slash characters, or ends
     *         with a slash character and is more than one character long).
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     */
    public Preferences node(String path) {
        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node has been removed.");
            if (path.equals(""))
                return this;
            if (path.equals("/"))
                return root;
            if (path.charAt(0) != '/')
                return node(new StringTokenizer(path, "/", true));
        }

        // Absolute path.  Note that we've dropped our lock to avoid deadlock
        return root.node(new StringTokenizer(path.substring(1), "/", true));
    }

    
tokenizer contains  {'/' }*

/**
     * tokenizer contains <name> {'/' <name>}*
     */
    private Preferences node(StringTokenizer path) {
        String token = path.nextToken();
        if (token.equals("/"))  // Check for consecutive slashes
            throw new IllegalArgumentException("Consecutive slashes in path");
        synchronized(lock) {
            AbstractPreferences child = kidCache.get(token);
            if (child == null) {
                if (token.length() > MAX_NAME_LENGTH)
                    throw new IllegalArgumentException(
                        "Node name " + token + " too long");
                child = childSpi(token);
                if (child.newNode)
                    enqueueNodeAddedEvent(child);
                kidCache.put(token, child);
            }
            if (!path.hasMoreTokens())
                return child;
            path.nextToken();  // Consume slash
            if (!path.hasMoreTokens())
                throw new IllegalArgumentException("Path ends with slash");
            return child.node(path);
        }
    }

    
Implements the nodeExists method as per the specification in Preferences.nodeExists(String). 
This implementation is very similar to node(String), except that getChild(String) is used instead of childSpi(String). 
Params:
  • path – the path name of the node whose existence is to be checked.
Throws:
  • BackingStoreException – if this operation cannot be completed
        due to a failure in the backing store, or inability to
        communicate with it.
  • IllegalArgumentException – if the path name is invalid (i.e.,
        it contains multiple consecutive slash characters, or ends
        with a slash character and is more than one character long).
  • IllegalStateException – if this node (or an ancestor) has been removed with the removeNode() method and pathname is not the empty string ("").
Returns:true if the specified node exists.
/**
     * Implements the {@code nodeExists} method as per the specification in
     * {@link Preferences#nodeExists(String)}.
     *
     * <p>This implementation is very similar to {@link #node(String)},
     * except that {@link #getChild(String)} is used instead of {@link
     * #childSpi(String)}.
     *
     * @param path the path name of the node whose existence is to be checked.
     * @return true if the specified node exists.
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     * @throws IllegalArgumentException if the path name is invalid (i.e.,
     *         it contains multiple consecutive slash characters, or ends
     *         with a slash character and is more than one character long).
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method and
     *         {@code pathname} is not the empty string ({@code ""}).
     */
    public boolean nodeExists(String path)
        throws BackingStoreException
    {
        synchronized(lock) {
            if (path.equals(""))
                return !removed;
            if (removed)
                throw new IllegalStateException("Node has been removed.");
            if (path.equals("/"))
                return true;
            if (path.charAt(0) != '/')
                return nodeExists(new StringTokenizer(path, "/", true));
        }

        // Absolute path.  Note that we've dropped our lock to avoid deadlock
        return root.nodeExists(new StringTokenizer(path.substring(1), "/",
                                                   true));
    }

    
tokenizer contains  {'/' }*

/**
     * tokenizer contains <name> {'/' <name>}*
     */
    private boolean nodeExists(StringTokenizer path)
        throws BackingStoreException
    {
        String token = path.nextToken();
        if (token.equals("/"))  // Check for consecutive slashes
            throw new IllegalArgumentException("Consecutive slashes in path");
        synchronized(lock) {
            AbstractPreferences child = kidCache.get(token);
            if (child == null)
                child = getChild(token);
            if (child==null)
                return false;
            if (!path.hasMoreTokens())
                return true;
            path.nextToken();  // Consume slash
            if (!path.hasMoreTokens())
                throw new IllegalArgumentException("Path ends with slash");
            return child.nodeExists(path);
        }
    }

    
Implements the removeNode() method as per the specification in Preferences.removeNode(). 
This implementation checks to see that this node is the root; if so, it throws an appropriate exception. Then, it locks this node's parent, and calls a recursive helper method that traverses the subtree rooted at this node. The recursive method locks the node on which it was called, checks that it has not already been removed, and then ensures that all of its children are cached: The childrenNamesSpi() method is invoked and each returned child name is checked for containment in the child-cache. If a child is not already cached, the childSpi(String) method is invoked to create a Preferences instance for it, and this instance is put into the child-cache. Then the helper method calls itself recursively on each node contained in its child-cache. Next, it invokes removeNodeSpi(), marks itself as removed, and removes itself from its parent's child-cache. Finally, if there are any node change listeners, it enqueues a notification event for processing by the event dispatch thread. 
Note that the helper method is always invoked with all ancestors up
to the "closest non-removed ancestor" locked.

Throws:
/**

     * Implements the {@code removeNode()} method as per the specification in
     * {@link Preferences#removeNode()}.
     *
     * <p>This implementation checks to see that this node is the root; if so,
     * it throws an appropriate exception.  Then, it locks this node's parent,
     * and calls a recursive helper method that traverses the subtree rooted at
     * this node.  The recursive method locks the node on which it was called,
     * checks that it has not already been removed, and then ensures that all
     * of its children are cached: The {@link #childrenNamesSpi()} method is
     * invoked and each returned child name is checked for containment in the
     * child-cache.  If a child is not already cached, the {@link
     * #childSpi(String)} method is invoked to create a {@code Preferences}
     * instance for it, and this instance is put into the child-cache.  Then
     * the helper method calls itself recursively on each node contained in its
     * child-cache.  Next, it invokes {@link #removeNodeSpi()}, marks itself
     * as removed, and removes itself from its parent's child-cache.  Finally,
     * if there are any node change listeners, it enqueues a notification
     * event for processing by the event dispatch thread.
     *
     * <p>Note that the helper method is always invoked with all ancestors up
     * to the "closest non-removed ancestor" locked.
     *
     * @throws IllegalStateException if this node (or an ancestor) has already
     *         been removed with the {@link #removeNode()} method.
     * @throws UnsupportedOperationException if this method is invoked on
     *         the root node.
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     */
    public void removeNode() throws BackingStoreException {
        if (this==root)
            throw new UnsupportedOperationException("Can't remove the root!");
        synchronized(parent.lock) {
            removeNode2();
            parent.kidCache.remove(name);
        }
    }

    /*
     * Called with locks on all nodes on path from parent of "removal root"
     * to this (including the former but excluding the latter).
     */
    private void removeNode2() throws BackingStoreException {
        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node already removed.");

            // Ensure that all children are cached
            String[] kidNames = childrenNamesSpi();
            for (String kidName : kidNames)
                if (!kidCache.containsKey(kidName))
                    kidCache.put(kidName, childSpi(kidName));

            // Recursively remove all cached children
            for (Iterator<AbstractPreferences> i = kidCache.values().iterator();
                 i.hasNext();) {
                try {
                    i.next().removeNode2();
                    i.remove();
                } catch (BackingStoreException x) { }
            }

            // Now we have no descendants - it's time to die!
            removeNodeSpi();
            removed = true;
            parent.enqueueNodeRemovedEvent(this);
        }
    }

    
Implements the name method as per the specification in Preferences.name(). 
This implementation merely returns the name that was
passed to this node's constructor.

Returns:this preference node's name, relative to its parent.
/**
     * Implements the {@code name} method as per the specification in
     * {@link Preferences#name()}.
     *
     * <p>This implementation merely returns the name that was
     * passed to this node's constructor.
     *
     * @return this preference node's name, relative to its parent.
     */
    public String name() {
        return name;
    }

    
Implements the absolutePath method as per the specification in Preferences.absolutePath(). 
This implementation merely returns the absolute path name that
was computed at the time that this node was constructed (based on
the name that was passed to this node's constructor, and the names
that were passed to this node's ancestors' constructors).

Returns:this preference node's absolute path name.
/**
     * Implements the {@code absolutePath} method as per the specification in
     * {@link Preferences#absolutePath()}.
     *
     * <p>This implementation merely returns the absolute path name that
     * was computed at the time that this node was constructed (based on
     * the name that was passed to this node's constructor, and the names
     * that were passed to this node's ancestors' constructors).
     *
     * @return this preference node's absolute path name.
     */
    public String absolutePath() {
        return absolutePath;
    }

    
Implements the isUserNode method as per the specification in Preferences.isUserNode(). 
This implementation compares this node's root node (which is stored in a private field) with the value returned by Preferences.userRoot(). If the two object references are identical, this method returns true. 
Returns:true if this preference node is in the user preference tree, false if it's in the system preference tree.
/**
     * Implements the {@code isUserNode} method as per the specification in
     * {@link Preferences#isUserNode()}.
     *
     * <p>This implementation compares this node's root node (which is stored
     * in a private field) with the value returned by
     * {@link Preferences#userRoot()}.  If the two object references are
     * identical, this method returns true.
     *
     * @return {@code true} if this preference node is in the user
     *         preference tree, {@code false} if it's in the system
     *         preference tree.
     */
    public boolean isUserNode() {
        return AccessController.doPrivileged(
            new PrivilegedAction<Boolean>() {
                public Boolean run() {
                    return root == Preferences.userRoot();
            }
            }).booleanValue();
    }

    public void addPreferenceChangeListener(PreferenceChangeListener pcl) {
        if (pcl==null)
            throw new NullPointerException("Change listener is null.");
        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node has been removed.");

            // Copy-on-write
            PreferenceChangeListener[] old = prefListeners;
            prefListeners = new PreferenceChangeListener[old.length + 1];
            System.arraycopy(old, 0, prefListeners, 0, old.length);
            prefListeners[old.length] = pcl;
        }
        startEventDispatchThreadIfNecessary();
    }

    public void removePreferenceChangeListener(PreferenceChangeListener pcl) {
        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node has been removed.");
            if ((prefListeners == null) || (prefListeners.length == 0))
                throw new IllegalArgumentException("Listener not registered.");

            // Copy-on-write
            PreferenceChangeListener[] newPl =
                new PreferenceChangeListener[prefListeners.length - 1];
            int i = 0;
            while (i < newPl.length && prefListeners[i] != pcl)
                newPl[i] = prefListeners[i++];

            if (i == newPl.length &&  prefListeners[i] != pcl)
                throw new IllegalArgumentException("Listener not registered.");
            while (i < newPl.length)
                newPl[i] = prefListeners[++i];
            prefListeners = newPl;
        }
    }

    public void addNodeChangeListener(NodeChangeListener ncl) {
        if (ncl==null)
            throw new NullPointerException("Change listener is null.");
        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node has been removed.");

            // Copy-on-write
            if (nodeListeners == null) {
                nodeListeners = new NodeChangeListener[1];
                nodeListeners[0] = ncl;
            } else {
                NodeChangeListener[] old = nodeListeners;
                nodeListeners = new NodeChangeListener[old.length + 1];
                System.arraycopy(old, 0, nodeListeners, 0, old.length);
                nodeListeners[old.length] = ncl;
            }
        }
        startEventDispatchThreadIfNecessary();
    }

    public void removeNodeChangeListener(NodeChangeListener ncl) {
        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node has been removed.");
            if ((nodeListeners == null) || (nodeListeners.length == 0))
                throw new IllegalArgumentException("Listener not registered.");

            // Copy-on-write
            int i = 0;
            while (i < nodeListeners.length && nodeListeners[i] != ncl)
                i++;
            if (i == nodeListeners.length)
                throw new IllegalArgumentException("Listener not registered.");
            NodeChangeListener[] newNl =
                new NodeChangeListener[nodeListeners.length - 1];
            if (i != 0)
                System.arraycopy(nodeListeners, 0, newNl, 0, i);
            if (i != newNl.length)
                System.arraycopy(nodeListeners, i + 1,
                                 newNl, i, newNl.length - i);
            nodeListeners = newNl;
        }
    }

    // "SPI" METHODS

    
Put the given key-value association into this preference node. It is guaranteed that key and value are non-null and of legal length. Also, it is guaranteed that this node has not been removed. (The implementor needn't check for any of these things.) 
This method is invoked with the lock on this node held.

Params:
  • key – the key
  • value – the value
/**
     * Put the given key-value association into this preference node.  It is
     * guaranteed that {@code key} and {@code value} are non-null and of
     * legal length.  Also, it is guaranteed that this node has not been
     * removed.  (The implementor needn't check for any of these things.)
     *
     * <p>This method is invoked with the lock on this node held.
     * @param key the key
     * @param value the value
     */
    protected abstract void putSpi(String key, String value);

    
Return the value associated with the specified key at this preference node, or null if there is no association for this key, or the association cannot be determined at this time. It is guaranteed that key is non-null. Also, it is guaranteed that this node has not been removed. (The implementor needn't check for either of these things.) 
 Generally speaking, this method should not throw an exception under any circumstances. If, however, if it does throw an exception, the exception will be intercepted and treated as a null return value. 
This method is invoked with the lock on this node held.

Params:
  • key – the key
Returns:the value associated with the specified key at this preference node, or null if there is no association for this key, or the association cannot be determined at this time.
/**
     * Return the value associated with the specified key at this preference
     * node, or {@code null} if there is no association for this key, or the
     * association cannot be determined at this time.  It is guaranteed that
     * {@code key} is non-null.  Also, it is guaranteed that this node has
     * not been removed.  (The implementor needn't check for either of these
     * things.)
     *
     * <p> Generally speaking, this method should not throw an exception
     * under any circumstances.  If, however, if it does throw an exception,
     * the exception will be intercepted and treated as a {@code null}
     * return value.
     *
     * <p>This method is invoked with the lock on this node held.
     *
     * @param key the key
     * @return the value associated with the specified key at this preference
     *          node, or {@code null} if there is no association for this
     *          key, or the association cannot be determined at this time.
     */
    protected abstract String getSpi(String key);

    
Remove the association (if any) for the specified key at this preference node. It is guaranteed that key is non-null. Also, it is guaranteed that this node has not been removed. (The implementor needn't check for either of these things.) 
This method is invoked with the lock on this node held.

Params:
  • key – the key
/**
     * Remove the association (if any) for the specified key at this
     * preference node.  It is guaranteed that {@code key} is non-null.
     * Also, it is guaranteed that this node has not been removed.
     * (The implementor needn't check for either of these things.)
     *
     * <p>This method is invoked with the lock on this node held.
     * @param key the key
     */
    protected abstract void removeSpi(String key);

    
Removes this preference node, invalidating it and any preferences that it contains. The named child will have no descendants at the time this invocation is made (i.e., the Preferences.removeNode() method invokes this method repeatedly in a bottom-up fashion, removing each of a node's descendants before removing the node itself). 
This method is invoked with the lock held on this node and its parent (and all ancestors that are being removed as a result of a single invocation to Preferences.removeNode()). 
The removal of a node needn't become persistent until the flush method is invoked on this node (or an ancestor). 
If this node throws a BackingStoreException, the exception will propagate out beyond the enclosing removeNode() invocation. 
Throws:
  • BackingStoreException – if this operation cannot be completed
        due to a failure in the backing store, or inability to
        communicate with it.
/**
     * Removes this preference node, invalidating it and any preferences that
     * it contains.  The named child will have no descendants at the time this
     * invocation is made (i.e., the {@link Preferences#removeNode()} method
     * invokes this method repeatedly in a bottom-up fashion, removing each of
     * a node's descendants before removing the node itself).
     *
     * <p>This method is invoked with the lock held on this node and its
     * parent (and all ancestors that are being removed as a
     * result of a single invocation to {@link Preferences#removeNode()}).
     *
     * <p>The removal of a node needn't become persistent until the
     * {@code flush} method is invoked on this node (or an ancestor).
     *
     * <p>If this node throws a {@code BackingStoreException}, the exception
     * will propagate out beyond the enclosing {@link #removeNode()}
     * invocation.
     *
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     */
    protected abstract void removeNodeSpi() throws BackingStoreException;

    
Returns all of the keys that have an associated value in this
preference node.  (The returned array will be of size zero if
this node has no preferences.)  It is guaranteed that this node has not
been removed.

This method is invoked with the lock on this node held.

If this node throws a BackingStoreException, the exception will propagate out beyond the enclosing keys() invocation. 
Throws:
  • BackingStoreException – if this operation cannot be completed
        due to a failure in the backing store, or inability to
        communicate with it.
Returns:an array of the keys that have an associated value in this
        preference node.
/**
     * Returns all of the keys that have an associated value in this
     * preference node.  (The returned array will be of size zero if
     * this node has no preferences.)  It is guaranteed that this node has not
     * been removed.
     *
     * <p>This method is invoked with the lock on this node held.
     *
     * <p>If this node throws a {@code BackingStoreException}, the exception
     * will propagate out beyond the enclosing {@link #keys()} invocation.
     *
     * @return an array of the keys that have an associated value in this
     *         preference node.
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     */
    protected abstract String[] keysSpi() throws BackingStoreException;

    
Returns the names of the children of this preference node.  (The
returned array will be of size zero if this node has no children.)
This method need not return the names of any nodes already cached,
but may do so without harm.

This method is invoked with the lock on this node held.

If this node throws a BackingStoreException, the exception will propagate out beyond the enclosing childrenNames() invocation. 
Throws:
  • BackingStoreException – if this operation cannot be completed
        due to a failure in the backing store, or inability to
        communicate with it.
Returns:an array containing the names of the children of this
        preference node.
/**
     * Returns the names of the children of this preference node.  (The
     * returned array will be of size zero if this node has no children.)
     * This method need not return the names of any nodes already cached,
     * but may do so without harm.
     *
     * <p>This method is invoked with the lock on this node held.
     *
     * <p>If this node throws a {@code BackingStoreException}, the exception
     * will propagate out beyond the enclosing {@link #childrenNames()}
     * invocation.
     *
     * @return an array containing the names of the children of this
     *         preference node.
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     */
    protected abstract String[] childrenNamesSpi()
        throws BackingStoreException;

    
Returns the named child if it exists, or null if it does not. It is guaranteed that nodeName is non-null, non-empty, does not contain the slash character ('/'), and is no longer than Preferences.MAX_NAME_LENGTH characters. Also, it is guaranteed that this node has not been removed. (The implementor needn't check for any of these things if he chooses to override this method.) 
Finally, it is guaranteed that the named node has not been returned by a previous invocation of this method or childSpi after the last time that it was removed. In other words, a cached value will always be used in preference to invoking this method. (The implementor needn't maintain his own cache of previously returned children if he chooses to override this method.) 
This implementation obtains this preference node's lock, invokes childrenNames() to get an array of the names of this node's children, and iterates over the array comparing the name of each child with the specified node name. If a child node has the correct name, the childSpi(String) method is invoked and the resulting node is returned. If the iteration completes without finding the specified name, null is returned. 
Params:
  • nodeName – name of the child to be searched for.
Throws:
  • BackingStoreException – if this operation cannot be completed
        due to a failure in the backing store, or inability to
        communicate with it.
Returns:the named child if it exists, or null if it does not.
/**
     * Returns the named child if it exists, or {@code null} if it does not.
     * It is guaranteed that {@code nodeName} is non-null, non-empty,
     * does not contain the slash character ('/'), and is no longer than
     * {@link #MAX_NAME_LENGTH} characters.  Also, it is guaranteed
     * that this node has not been removed.  (The implementor needn't check
     * for any of these things if he chooses to override this method.)
     *
     * <p>Finally, it is guaranteed that the named node has not been returned
     * by a previous invocation of this method or {@link #childSpi} after the
     * last time that it was removed.  In other words, a cached value will
     * always be used in preference to invoking this method.  (The implementor
     * needn't maintain his own cache of previously returned children if he
     * chooses to override this method.)
     *
     * <p>This implementation obtains this preference node's lock, invokes
     * {@link #childrenNames()} to get an array of the names of this node's
     * children, and iterates over the array comparing the name of each child
     * with the specified node name.  If a child node has the correct name,
     * the {@link #childSpi(String)} method is invoked and the resulting
     * node is returned.  If the iteration completes without finding the
     * specified name, {@code null} is returned.
     *
     * @param nodeName name of the child to be searched for.
     * @return the named child if it exists, or null if it does not.
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     */
    protected AbstractPreferences getChild(String nodeName)
            throws BackingStoreException {
        synchronized(lock) {
            // assert kidCache.get(nodeName)==null;
            String[] kidNames = childrenNames();
            for (String kidName : kidNames)
                if (kidName.equals(nodeName))
                    return childSpi(kidName);
        }
        return null;
    }

    
Returns the named child of this preference node, creating it if it does not already exist. It is guaranteed that name is non-null, non-empty, does not contain the slash character ('/'), and is no longer than Preferences.MAX_NAME_LENGTH characters. Also, it is guaranteed that this node has not been removed. (The implementor needn't check for any of these things.) 
Finally, it is guaranteed that the named node has not been returned by a previous invocation of this method or getChild(String) after the last time that it was removed. In other words, a cached value will always be used in preference to invoking this method. Subclasses need not maintain their own cache of previously returned children. 
The implementer must ensure that the returned node has not been removed. If a like-named child of this node was previously removed, the implementer must return a newly constructed AbstractPreferences node; once removed, an AbstractPreferences node cannot be "resuscitated." 
If this method causes a node to be created, this node is not guaranteed to be persistent until the flush method is invoked on this node or one of its ancestors (or descendants). 
This method is invoked with the lock on this node held.

Params:
  • name – The name of the child node to return, relative to
       this preference node.
Returns:The named child node.
/**
     * Returns the named child of this preference node, creating it if it does
     * not already exist.  It is guaranteed that {@code name} is non-null,
     * non-empty, does not contain the slash character ('/'), and is no longer
     * than {@link #MAX_NAME_LENGTH} characters.  Also, it is guaranteed that
     * this node has not been removed.  (The implementor needn't check for any
     * of these things.)
     *
     * <p>Finally, it is guaranteed that the named node has not been returned
     * by a previous invocation of this method or {@link #getChild(String)}
     * after the last time that it was removed.  In other words, a cached
     * value will always be used in preference to invoking this method.
     * Subclasses need not maintain their own cache of previously returned
     * children.
     *
     * <p>The implementer must ensure that the returned node has not been
     * removed.  If a like-named child of this node was previously removed, the
     * implementer must return a newly constructed {@code AbstractPreferences}
     * node; once removed, an {@code AbstractPreferences} node
     * cannot be "resuscitated."
     *
     * <p>If this method causes a node to be created, this node is not
     * guaranteed to be persistent until the {@code flush} method is
     * invoked on this node or one of its ancestors (or descendants).
     *
     * <p>This method is invoked with the lock on this node held.
     *
     * @param name The name of the child node to return, relative to
     *        this preference node.
     * @return The named child node.
     */
    protected abstract AbstractPreferences childSpi(String name);

    
Returns the absolute path name of this preferences node.

/**
     * Returns the absolute path name of this preferences node.
     */
    public String toString() {
        return (this.isUserNode() ? "User" : "System") +
               " Preference Node: " + this.absolutePath();
    }

    
Implements the sync method as per the specification in Preferences.sync(). 
This implementation calls a recursive helper method that locks this
node, invokes syncSpi() on it, unlocks this node, and recursively
invokes this method on each "cached child."  A cached child is a child
of this node that has been created in this VM and not subsequently
removed.  In effect, this method does a depth first traversal of the
"cached subtree" rooted at this node, calling syncSpi() on each node in
the subTree while only that node is locked. Note that syncSpi() is
invoked top-down.

Throws:
See Also:
/**
     * Implements the {@code sync} method as per the specification in
     * {@link Preferences#sync()}.
     *
     * <p>This implementation calls a recursive helper method that locks this
     * node, invokes syncSpi() on it, unlocks this node, and recursively
     * invokes this method on each "cached child."  A cached child is a child
     * of this node that has been created in this VM and not subsequently
     * removed.  In effect, this method does a depth first traversal of the
     * "cached subtree" rooted at this node, calling syncSpi() on each node in
     * the subTree while only that node is locked. Note that syncSpi() is
     * invoked top-down.
     *
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     * @throws IllegalStateException if this node (or an ancestor) has been
     *         removed with the {@link #removeNode()} method.
     * @see #flush()
     */
    public void sync() throws BackingStoreException {
        sync2();
    }

    private void sync2() throws BackingStoreException {
        AbstractPreferences[] cachedKids;

        synchronized(lock) {
            if (removed)
                throw new IllegalStateException("Node has been removed");
            syncSpi();
            cachedKids = cachedChildren();
        }

        for (AbstractPreferences cachedKid : cachedKids)
            cachedKid.sync2();
    }

    
This method is invoked with this node locked.  The contract of this
method is to synchronize any cached preferences stored at this node
with any stored in the backing store.  (It is perfectly possible that
this node does not exist on the backing store, either because it has
been deleted by another VM, or because it has not yet been created.)
Note that this method should not synchronize the preferences in
any subnodes of this node.  If the backing store naturally syncs an
entire subtree at once, the implementer is encouraged to override
sync(), rather than merely overriding this method.

If this node throws a BackingStoreException, the exception will propagate out beyond the enclosing sync() invocation. 
Throws:
  • BackingStoreException – if this operation cannot be completed
        due to a failure in the backing store, or inability to
        communicate with it.
/**
     * This method is invoked with this node locked.  The contract of this
     * method is to synchronize any cached preferences stored at this node
     * with any stored in the backing store.  (It is perfectly possible that
     * this node does not exist on the backing store, either because it has
     * been deleted by another VM, or because it has not yet been created.)
     * Note that this method should <i>not</i> synchronize the preferences in
     * any subnodes of this node.  If the backing store naturally syncs an
     * entire subtree at once, the implementer is encouraged to override
     * sync(), rather than merely overriding this method.
     *
     * <p>If this node throws a {@code BackingStoreException}, the exception
     * will propagate out beyond the enclosing {@link #sync()} invocation.
     *
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     */
    protected abstract void syncSpi() throws BackingStoreException;

    
Implements the flush method as per the specification in Preferences.flush(). 
This implementation calls a recursive helper method that locks this
node, invokes flushSpi() on it, unlocks this node, and recursively
invokes this method on each "cached child."  A cached child is a child
of this node that has been created in this VM and not subsequently
removed.  In effect, this method does a depth first traversal of the
"cached subtree" rooted at this node, calling flushSpi() on each node in
the subTree while only that node is locked. Note that flushSpi() is
invoked top-down.

 If this method is invoked on a node that has been removed with the removeNode() method, flushSpi() is invoked on this node, but not on others. 
Throws:
  • BackingStoreException – if this operation cannot be completed
        due to a failure in the backing store, or inability to
        communicate with it.
See Also:
/**
     * Implements the {@code flush} method as per the specification in
     * {@link Preferences#flush()}.
     *
     * <p>This implementation calls a recursive helper method that locks this
     * node, invokes flushSpi() on it, unlocks this node, and recursively
     * invokes this method on each "cached child."  A cached child is a child
     * of this node that has been created in this VM and not subsequently
     * removed.  In effect, this method does a depth first traversal of the
     * "cached subtree" rooted at this node, calling flushSpi() on each node in
     * the subTree while only that node is locked. Note that flushSpi() is
     * invoked top-down.
     *
     * <p> If this method is invoked on a node that has been removed with
     * the {@link #removeNode()} method, flushSpi() is invoked on this node,
     * but not on others.
     *
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     * @see #flush()
     */
    public void flush() throws BackingStoreException {
        flush2();
    }

    private void flush2() throws BackingStoreException {
        AbstractPreferences[] cachedKids;

        synchronized(lock) {
            flushSpi();
            if(removed)
                return;
            cachedKids = cachedChildren();
        }

        for (AbstractPreferences cachedKid : cachedKids)
            cachedKid.flush2();
    }

    
This method is invoked with this node locked.  The contract of this
method is to force any cached changes in the contents of this
preference node to the backing store, guaranteeing their persistence.
(It is perfectly possible that this node does not exist on the backing
store, either because it has been deleted by another VM, or because it
has not yet been created.)  Note that this method should not
flush the preferences in any subnodes of this node.  If the backing
store naturally flushes an entire subtree at once, the implementer is
encouraged to override flush(), rather than merely overriding this
method.

If this node throws a BackingStoreException, the exception will propagate out beyond the enclosing flush() invocation. 
Throws:
  • BackingStoreException – if this operation cannot be completed
        due to a failure in the backing store, or inability to
        communicate with it.
/**
     * This method is invoked with this node locked.  The contract of this
     * method is to force any cached changes in the contents of this
     * preference node to the backing store, guaranteeing their persistence.
     * (It is perfectly possible that this node does not exist on the backing
     * store, either because it has been deleted by another VM, or because it
     * has not yet been created.)  Note that this method should <i>not</i>
     * flush the preferences in any subnodes of this node.  If the backing
     * store naturally flushes an entire subtree at once, the implementer is
     * encouraged to override flush(), rather than merely overriding this
     * method.
     *
     * <p>If this node throws a {@code BackingStoreException}, the exception
     * will propagate out beyond the enclosing {@link #flush()} invocation.
     *
     * @throws BackingStoreException if this operation cannot be completed
     *         due to a failure in the backing store, or inability to
     *         communicate with it.
     */
    protected abstract void flushSpi() throws BackingStoreException;

    
Returns true iff this node (or an ancestor) has been removed with the removeNode() method. This method locks this node prior to returning the contents of the private field used to track this state. 
Returns:true iff this node (or an ancestor) has been removed with the removeNode() method.
/**
     * Returns {@code true} iff this node (or an ancestor) has been
     * removed with the {@link #removeNode()} method.  This method
     * locks this node prior to returning the contents of the private
     * field used to track this state.
     *
     * @return {@code true} iff this node (or an ancestor) has been
     *       removed with the {@link #removeNode()} method.
     */
    protected boolean isRemoved() {
        synchronized(lock) {
            return removed;
        }
    }

    
Queue of pending notification events.  When a preference or node
change event for which there are one or more listeners occurs,
it is placed on this queue and the queue is notified.  A background
thread waits on this queue and delivers the events.  This decouples
event delivery from preference activity, greatly simplifying
locking and reducing opportunity for deadlock.

/**
     * Queue of pending notification events.  When a preference or node
     * change event for which there are one or more listeners occurs,
     * it is placed on this queue and the queue is notified.  A background
     * thread waits on this queue and delivers the events.  This decouples
     * event delivery from preference activity, greatly simplifying
     * locking and reducing opportunity for deadlock.
     */
    private static final List<EventObject> eventQueue = new LinkedList<>();

    
These two classes are used to distinguish NodeChangeEvents on
eventQueue so the event dispatch thread knows whether to call
childAdded or childRemoved.

/**
     * These two classes are used to distinguish NodeChangeEvents on
     * eventQueue so the event dispatch thread knows whether to call
     * childAdded or childRemoved.
     */
    private class NodeAddedEvent extends NodeChangeEvent {
        private static final long serialVersionUID = -6743557530157328528L;
        NodeAddedEvent(Preferences parent, Preferences child) {
            super(parent, child);
        }
    }
    private class NodeRemovedEvent extends NodeChangeEvent {
        private static final long serialVersionUID = 8735497392918824837L;
        NodeRemovedEvent(Preferences parent, Preferences child) {
            super(parent, child);
        }
    }

    
A single background thread ("the event notification thread") monitors
the event queue and delivers events that are placed on the queue.

/**
     * A single background thread ("the event notification thread") monitors
     * the event queue and delivers events that are placed on the queue.
     */
    private static class EventDispatchThread extends Thread {
        private EventDispatchThread() {
            super(null, null, "Event Dispatch Thread", 0, false);
        }

        public void run() {
            while(true) {
                // Wait on eventQueue till an event is present
                EventObject event = null;
                synchronized(eventQueue) {
                    try {
                        while (eventQueue.isEmpty())
                            eventQueue.wait();
                        event = eventQueue.remove(0);
                    } catch (InterruptedException e) {
                        // XXX Log "Event dispatch thread interrupted. Exiting"
                        return;
                    }
                }

                // Now we have event & hold no locks; deliver evt to listeners
                AbstractPreferences src=(AbstractPreferences)event.getSource();
                if (event instanceof PreferenceChangeEvent) {
                    PreferenceChangeEvent pce = (PreferenceChangeEvent)event;
                    PreferenceChangeListener[] listeners = src.prefListeners();
                    for (PreferenceChangeListener listener : listeners)
                        listener.preferenceChange(pce);
                } else {
                    NodeChangeEvent nce = (NodeChangeEvent)event;
                    NodeChangeListener[] listeners = src.nodeListeners();
                    if (nce instanceof NodeAddedEvent) {
                        for (NodeChangeListener listener : listeners)
                            listener.childAdded(nce);
                    } else {
                        // assert nce instanceof NodeRemovedEvent;
                        for (NodeChangeListener listener : listeners)
                            listener.childRemoved(nce);
                    }
                }
            }
        }
    }

    private static Thread eventDispatchThread = null;

    
This method starts the event dispatch thread the first time it
is called.  The event dispatch thread will be started only
if someone registers a listener.

/**
     * This method starts the event dispatch thread the first time it
     * is called.  The event dispatch thread will be started only
     * if someone registers a listener.
     */
    private static synchronized void startEventDispatchThreadIfNecessary() {
        if (eventDispatchThread == null) {
            // XXX Log "Starting event dispatch thread"
            eventDispatchThread = new EventDispatchThread();
            eventDispatchThread.setDaemon(true);
            eventDispatchThread.start();
        }
    }

    
Return this node's preference/node change listeners.  Even though
we're using a copy-on-write lists, we use synchronized accessors to
ensure information transmission from the writing thread to the
reading thread.

/**
     * Return this node's preference/node change listeners.  Even though
     * we're using a copy-on-write lists, we use synchronized accessors to
     * ensure information transmission from the writing thread to the
     * reading thread.
     */
    PreferenceChangeListener[] prefListeners() {
        synchronized(lock) {
            return prefListeners;
        }
    }
    NodeChangeListener[] nodeListeners() {
        synchronized(lock) {
            return nodeListeners;
        }
    }

    
Enqueue a preference change event for delivery to registered
preference change listeners unless there are no registered
listeners.  Invoked with this.lock held.

/**
     * Enqueue a preference change event for delivery to registered
     * preference change listeners unless there are no registered
     * listeners.  Invoked with this.lock held.
     */
    private void enqueuePreferenceChangeEvent(String key, String newValue) {
        if (prefListeners.length != 0) {
            synchronized(eventQueue) {
                eventQueue.add(new PreferenceChangeEvent(this, key, newValue));
                eventQueue.notify();
            }
        }
    }

    
Enqueue a "node added" event for delivery to registered node change
listeners unless there are no registered listeners.  Invoked with
this.lock held.

/**
     * Enqueue a "node added" event for delivery to registered node change
     * listeners unless there are no registered listeners.  Invoked with
     * this.lock held.
     */
    private void enqueueNodeAddedEvent(Preferences child) {
        if (nodeListeners.length != 0) {
            synchronized(eventQueue) {
                eventQueue.add(new NodeAddedEvent(this, child));
                eventQueue.notify();
            }
        }
    }

    
Enqueue a "node removed" event for delivery to registered node change
listeners unless there are no registered listeners.  Invoked with
this.lock held.

/**
     * Enqueue a "node removed" event for delivery to registered node change
     * listeners unless there are no registered listeners.  Invoked with
     * this.lock held.
     */
    private void enqueueNodeRemovedEvent(Preferences child) {
        if (nodeListeners.length != 0) {
            synchronized(eventQueue) {
                eventQueue.add(new NodeRemovedEvent(this, child));
                eventQueue.notify();
            }
        }
    }

    
Implements the exportNode method as per the specification in Preferences.exportNode(OutputStream). 
Params:
  • os – the output stream on which to emit the XML document.
Throws:
  • IOException – if writing to the specified output stream results in an IOException.
  • BackingStoreException – if preference data cannot be read from
        backing store.
/**
     * Implements the {@code exportNode} method as per the specification in
     * {@link Preferences#exportNode(OutputStream)}.
     *
     * @param os the output stream on which to emit the XML document.
     * @throws IOException if writing to the specified output stream
     *         results in an {@code IOException}.
     * @throws BackingStoreException if preference data cannot be read from
     *         backing store.
     */
    public void exportNode(OutputStream os)
        throws IOException, BackingStoreException
    {
        XmlSupport.export(os, this, false);
    }

    
Implements the exportSubtree method as per the specification in Preferences.exportSubtree(OutputStream). 
Params:
  • os – the output stream on which to emit the XML document.
Throws:
  • IOException – if writing to the specified output stream results in an IOException.
  • BackingStoreException – if preference data cannot be read from
        backing store.
/**
     * Implements the {@code exportSubtree} method as per the specification in
     * {@link Preferences#exportSubtree(OutputStream)}.
     *
     * @param os the output stream on which to emit the XML document.
     * @throws IOException if writing to the specified output stream
     *         results in an {@code IOException}.
     * @throws BackingStoreException if preference data cannot be read from
     *         backing store.
     */
    public void exportSubtree(OutputStream os)
        throws IOException, BackingStoreException
    {
        XmlSupport.export(os, this, true);
    }
}