https://github.com/jruby/jruby/jruby-core: JRuby is the effort to recreate the Ruby (https://www.ruby-lang.org) interpreter in Java. (JRuby)
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  • enebo
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  • BanzaiMan
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package org.jruby.util.collections;

/*
 * Written by Doug Lea with assistance from members of JCP JSR-166
 * Expert Group and released to the public domain, as explained at
 * http://creativecommons.org/licenses/publicdomain
 */

import java.io.IOException;
import java.io.Serializable;
import java.lang.ref.ReferenceQueue;
import java.lang.ref.WeakReference;
import java.util.AbstractCollection;
import java.util.AbstractMap;
import java.util.AbstractSet;
import java.util.Collection;
import java.util.ConcurrentModificationException;
import java.util.Enumeration;
import java.util.HashMap;
import java.util.Hashtable;
import java.util.Iterator;
import java.util.Map;
import java.util.NoSuchElementException;
import java.util.Set;
import java.util.concurrent.locks.ReentrantLock;


A hash table with weak keys, full concurrency of retrievals, and adjustable expected concurrency for updates. Similar to WeakHashMap, entries of this table are periodically removed once their corresponding keys are no longer referenced outside of this table. In other words, this table will not prevent a key from being discarded by the garbage collector. Once a key has been discarded by the collector, the corresponding entry is no longer visible to this table; however, the entry may occupy space until a future table operation decides to reclaim it. For this reason, summary functions such as size and
isEmpty might return a value greater than the observed number of entries. In order to support a high level of concurrency, stale entries are only reclaimed during blocking (usually mutating) operations. While keys in this table are only held using a weak reference, values are held using a normal strong reference. This provides the guarantee that a value will always have at least the same life-span as it's key. For this reason, care should be taken to ensure that a value never refers, either directly or indirectly, to its key, thereby preventing reclamation. If weak values are desired, one can simply use a WeakReference for the value type. Just like ConcurrentHashMap, this class obeys the same functional specification as Hashtable, and includes versions of methods corresponding to each method of Hashtable.
However, even though all operations are thread-safe, retrieval operations do
not entail locking, and there is not any support for
locking the entire table in a way that prevents all access. This class is
fully interoperable with Hashtable in programs that rely on its
thread safety but not on its synchronization details.


Retrieval operations (including get) generally do not block, so
may overlap with update operations (including put and
remove). Retrievals reflect the results of the most recently
completed update operations holding upon their onset. For
aggregate operations such as putAll and clear,
concurrent retrievals may reflect insertion or removal of only some entries.
Similarly, Iterators and Enumerations return elements reflecting the state of
the hash table at some point at or since the creation of the
iterator/enumeration. They do not throw ConcurrentModificationException. However, iterators are designed to be used by only one thread at a time. 

The allowed concurrency among update operations is guided by the optional
concurrencyLevel constructor argument (default 16),
which is used as a hint for internal sizing. The table is internally
partitioned to try to permit the indicated number of concurrent updates
without contention. Because placement in hash tables is essentially random,
the actual concurrency will vary. Ideally, you should choose a value to
accommodate as many threads as will ever concurrently modify the table. Using
a significantly higher value than you need can waste space and time, and a
significantly lower value can lead to thread contention. But overestimates
and underestimates within an order of magnitude do not usually have much
noticeable impact. A value of one is appropriate when it is known that only
one thread will modify and all others will only read. Also, resizing this or
any other kind of hash table is a relatively slow operation, so, when
possible, it is a good idea to provide estimates of expected table sizes in
constructors.


This class and its views and iterators implement all of the optional methods of the Map and Iterator interfaces. 
 Like Hashtable but unlike HashMap, this class does not allow null to be used as a key or value.


This class is a member of the 
Java Collections Framework.

Author:Doug Lea, Jason T. Greene
Type parameters:
  • <K> – the type of keys maintained by this map
  • <V> – the type of mapped values
/**
 * A hash table with <em>weak keys</em>, full concurrency of retrievals, and
 * adjustable expected concurrency for updates. Similar to
 * {@link java.util.WeakHashMap}, entries of this table are periodically
 * removed once their corresponding keys are no longer referenced outside of
 * this table. In other words, this table will not prevent a key from being
 * discarded by the garbage collector. Once a key has been discarded by the
 * collector, the corresponding entry is no longer visible to this table;
 * however, the entry may occupy space until a future table operation decides to
 * reclaim it. For this reason, summary functions such as <tt>size</tt> and
 * <tt>isEmpty</tt> might return a value greater than the observed number of
 * entries. In order to support a high level of concurrency, stale entries are
 * only reclaimed during blocking (usually mutating) operations.
 *
 * While keys in this table are only held using a weak reference, values are
 * held using a normal strong reference. This provides the guarantee that a
 * value will always have at least the same life-span as it's key. For this
 * reason, care should be taken to ensure that a value never refers, either
 * directly or indirectly, to its key, thereby preventing reclamation. If weak
 * values are desired, one can simply use a {@link WeakReference} for the value
 * type.
 *
 * Just like {@link java.util.ConcurrentHashMap}, this class obeys the same
 * functional specification as {@link java.util.Hashtable}, and includes
 * versions of methods corresponding to each method of <tt>Hashtable</tt>.
 * However, even though all operations are thread-safe, retrieval operations do
 * <em>not</em> entail locking, and there is <em>not</em> any support for
 * locking the entire table in a way that prevents all access. This class is
 * fully interoperable with <tt>Hashtable</tt> in programs that rely on its
 * thread safety but not on its synchronization details.
 *
 * <p>
 * Retrieval operations (including <tt>get</tt>) generally do not block, so
 * may overlap with update operations (including <tt>put</tt> and
 * <tt>remove</tt>). Retrievals reflect the results of the most recently
 * <em>completed</em> update operations holding upon their onset. For
 * aggregate operations such as <tt>putAll</tt> and <tt>clear</tt>,
 * concurrent retrievals may reflect insertion or removal of only some entries.
 * Similarly, Iterators and Enumerations return elements reflecting the state of
 * the hash table at some point at or since the creation of the
 * iterator/enumeration. They do <em>not</em> throw
 * {@link ConcurrentModificationException}. However, iterators are designed to
 * be used by only one thread at a time.
 *
 * <p>
 * The allowed concurrency among update operations is guided by the optional
 * <tt>concurrencyLevel</tt> constructor argument (default <tt>16</tt>),
 * which is used as a hint for internal sizing. The table is internally
 * partitioned to try to permit the indicated number of concurrent updates
 * without contention. Because placement in hash tables is essentially random,
 * the actual concurrency will vary. Ideally, you should choose a value to
 * accommodate as many threads as will ever concurrently modify the table. Using
 * a significantly higher value than you need can waste space and time, and a
 * significantly lower value can lead to thread contention. But overestimates
 * and underestimates within an order of magnitude do not usually have much
 * noticeable impact. A value of one is appropriate when it is known that only
 * one thread will modify and all others will only read. Also, resizing this or
 * any other kind of hash table is a relatively slow operation, so, when
 * possible, it is a good idea to provide estimates of expected table sizes in
 * constructors.
 *
 * <p>
 * This class and its views and iterators implement all of the <em>optional</em>
 * methods of the {@link Map} and {@link Iterator} interfaces.
 *
 * <p>
 * Like {@link Hashtable} but unlike {@link HashMap}, this class does
 * <em>not</em> allow <tt>null</tt> to be used as a key or value.
 *
 * <p>
 * This class is a member of the <a href="{@docRoot}/../technotes/guides/collections/index.html">
 * Java Collections Framework</a>.
 *
 * @author Doug Lea
 * @author Jason T. Greene
 * @param <K> the type of keys maintained by this map
 * @param <V> the type of mapped values
 */
public class ConcurrentWeakHashMap<K, V> extends AbstractMap<K, V>
        implements java.util.concurrent.ConcurrentMap<K, V>, Serializable {
    private static final long serialVersionUID = 7249069246763182397L;

    /*
     * The basic strategy is to subdivide the table among Segments,
     * each of which itself is a concurrently readable hash table.
     */

    /* ---------------- Constants -------------- */

    
The default initial capacity for this table,
used when not otherwise specified in a constructor.

/**
     * The default initial capacity for this table,
     * used when not otherwise specified in a constructor.
     */
    static final int DEFAULT_INITIAL_CAPACITY = 16;

    
The default load factor for this table, used when not
otherwise specified in a constructor.

/**
     * The default load factor for this table, used when not
     * otherwise specified in a constructor.
     */
    static final float DEFAULT_LOAD_FACTOR = 0.75f;

    
The default concurrency level for this table, used when not
otherwise specified in a constructor.

/**
     * The default concurrency level for this table, used when not
     * otherwise specified in a constructor.
     */
    static final int DEFAULT_CONCURRENCY_LEVEL = 16;

    
The maximum capacity, used if a higher value is implicitly
specified by either of the constructors with arguments.  MUST
be a power of two <= 1<<30 to ensure that entries are indexable
using ints.

/**
     * The maximum capacity, used if a higher value is implicitly
     * specified by either of the constructors with arguments.  MUST
     * be a power of two <= 1<<30 to ensure that entries are indexable
     * using ints.
     */
    static final int MAXIMUM_CAPACITY = 1 << 30;

    
The maximum number of segments to allow; used to bound
constructor arguments.

/**
     * The maximum number of segments to allow; used to bound
     * constructor arguments.
     */
    static final int MAX_SEGMENTS = 1 << 16; // slightly conservative

    
Number of unsynchronized retries in size and containsValue
methods before resorting to locking. This is used to avoid
unbounded retries if tables undergo continuous modification
which would make it impossible to obtain an accurate result.

/**
     * Number of unsynchronized retries in size and containsValue
     * methods before resorting to locking. This is used to avoid
     * unbounded retries if tables undergo continuous modification
     * which would make it impossible to obtain an accurate result.
     */
    static final int RETRIES_BEFORE_LOCK = 2;

    /* ---------------- Fields -------------- */

    
Mask value for indexing into segments. The upper bits of a
key's hash code are used to choose the segment.

/**
     * Mask value for indexing into segments. The upper bits of a
     * key's hash code are used to choose the segment.
     */
    final int segmentMask;

    
Shift value for indexing within segments.

/**
     * Shift value for indexing within segments.
     */
    final int segmentShift;

    
The segments, each of which is a specialized hash table

/**
     * The segments, each of which is a specialized hash table
     */
    final Segment<K, V>[] segments;

    transient Set<K> keySet;
    transient Set<Map.Entry<K, V>> entrySet;
    transient Collection<V> values;

    /* ---------------- Small Utilities -------------- */

    
Applies a supplemental hash function to a given hashCode, which
defends against poor quality hash functions.  This is critical
because ConcurrentWeakHashMap uses power-of-two length hash tables,
that otherwise encounter collisions for hashCodes that do not
differ in lower or upper bits.

/**
     * Applies a supplemental hash function to a given hashCode, which
     * defends against poor quality hash functions.  This is critical
     * because ConcurrentWeakHashMap uses power-of-two length hash tables,
     * that otherwise encounter collisions for hashCodes that do not
     * differ in lower or upper bits.
     */
    private static int hash(int h) {
        // Spread bits to regularize both segment and index locations,
        // using variant of single-word Wang/Jenkins hash.
        h += (h << 15) ^ 0xffffcd7d;
        h ^= (h >>> 10);
        h += (h << 3);
        h ^= (h >>> 6);
        h += (h << 2) + (h << 14);
        return h ^ (h >>> 16);
    }

    
Returns the segment that should be used for key with given hash

Params:
  • hash – the hash code for the key
Returns:the segment
/**
     * Returns the segment that should be used for key with given hash
     * @param hash the hash code for the key
     * @return the segment
     */
    final Segment<K, V> segmentFor(int hash) {
        return segments[(hash >>> segmentShift) & segmentMask];
    }

    /* ---------------- Inner Classes -------------- */

    
A weak-key reference which stores the key hash needed for reclamation.

/**
     * A weak-key reference which stores the key hash needed for reclamation.
     */
    static final class WeakKeyReference<K> extends WeakReference<K> {
        final int hash;

        WeakKeyReference(K key, int hash, ReferenceQueue<K> refQueue) {
            super(key, refQueue);
            this.hash = hash;
        }
    }

    
ConcurrentWeakHashMap list entry. Note that this is never exported
out as a user-visible Map.Entry.
Because the value field is volatile, not final, it is legal wrt
the Java Memory Model for an unsynchronized reader to see null
instead of initial value when read via a data race.  Although a
reordering leading to this is not likely to ever actually
occur, the Segment.readValueUnderLock method is used as a
backup in case a null (pre-initialized) value is ever seen in
an unsynchronized access method.

/**
     * ConcurrentWeakHashMap list entry. Note that this is never exported
     * out as a user-visible Map.Entry.
     *
     * Because the value field is volatile, not final, it is legal wrt
     * the Java Memory Model for an unsynchronized reader to see null
     * instead of initial value when read via a data race.  Although a
     * reordering leading to this is not likely to ever actually
     * occur, the Segment.readValueUnderLock method is used as a
     * backup in case a null (pre-initialized) value is ever seen in
     * an unsynchronized access method.
     */
    static final class HashEntry<K, V> {
        final WeakReference<K> keyRef;
        final int hash;
        volatile V value;
        final HashEntry<K, V> next;

        HashEntry(K key, int hash, HashEntry<K, V> next, V value, ReferenceQueue<K> refQueue) {
            this.keyRef = new WeakKeyReference<K>(key, hash, refQueue);
            this.hash = hash;
            this.next = next;
            this.value = value;
        }

        @SuppressWarnings("unchecked")
        static final <K, V> HashEntry<K, V>[] newArray(int i) {
            return new HashEntry[i];
        }
    }

    
Segments are specialized versions of hash tables.  This
subclasses from ReentrantLock opportunistically, just to
simplify some locking and avoid separate construction.

/**
     * Segments are specialized versions of hash tables.  This
     * subclasses from ReentrantLock opportunistically, just to
     * simplify some locking and avoid separate construction.
     */
    static final class Segment<K, V> extends ReentrantLock implements Serializable {
        /*
         * Segments maintain a table of entry lists that are ALWAYS
         * kept in a consistent state, so can be read without locking.
         * Next fields of nodes are immutable (final).  All list
         * additions are performed at the front of each bin. This
         * makes it easy to check changes, and also fast to traverse.
         * When nodes would otherwise be changed, new nodes are
         * created to replace them. This works well for hash tables
         * since the bin lists tend to be short. (The average length
         * is less than two for the default load factor threshold.)
         *
         * Read operations can thus proceed without locking, but rely
         * on selected uses of volatiles to ensure that completed
         * write operations performed by other threads are
         * noticed. For most purposes, the "count" field, tracking the
         * number of elements, serves as that volatile variable
         * ensuring visibility.  This is convenient because this field
         * needs to be read in many read operations anyway:
         *
         *   - All (unsynchronized) read operations must first read the
         *     "count" field, and should not look at table entries if
         *     it is 0.
         *
         *   - All (synchronized) write operations should write to
         *     the "count" field after structurally changing any bin.
         *     The operations must not take any action that could even
         *     momentarily cause a concurrent read operation to see
         *     inconsistent data. This is made easier by the nature of
         *     the read operations in Map. For example, no operation
         *     can reveal that the table has grown but the threshold
         *     has not yet been updated, so there are no atomicity
         *     requirements for this with respect to reads.
         *
         * As a guide, all critical volatile reads and writes to the
         * count field are marked in code comments.
         */

        private static final long serialVersionUID = 2249069246763182397L;

        
The number of elements in this segment's region.

/**
         * The number of elements in this segment's region.
         */
        transient volatile int count;

        
Number of updates that alter the size of the table. This is
used during bulk-read methods to make sure they see a
consistent snapshot: If modCounts change during a traversal
of segments computing size or checking containsValue, then
we might have an inconsistent view of state so (usually)
must retry.

/**
         * Number of updates that alter the size of the table. This is
         * used during bulk-read methods to make sure they see a
         * consistent snapshot: If modCounts change during a traversal
         * of segments computing size or checking containsValue, then
         * we might have an inconsistent view of state so (usually)
         * must retry.
         */
        transient int modCount;

        
The table is rehashed when its size exceeds this threshold.
(The value of this field is always (int)(capacity *
loadFactor).)

/**
         * The table is rehashed when its size exceeds this threshold.
         * (The value of this field is always <tt>(int)(capacity *
         * loadFactor)</tt>.)
         */
        transient int threshold;

        
The per-segment table.

/**
         * The per-segment table.
         */
        transient volatile HashEntry<K, V>[] table;

        
The load factor for the hash table.  Even though this value
is same for all segments, it is replicated to avoid needing
links to outer object.

@serial
/**
         * The load factor for the hash table.  Even though this value
         * is same for all segments, it is replicated to avoid needing
         * links to outer object.
         * @serial
         */
        final float loadFactor;

        
The collected weak-key reference queue for this segment.
This should be (re)initialized whenever table is assigned,

/**
         * The collected weak-key reference queue for this segment.
         * This should be (re)initialized whenever table is assigned,
         */
        transient volatile ReferenceQueue<K> refQueue;

        Segment(int initialCapacity, float lf) {
            loadFactor = lf;
            setTable(HashEntry.<K, V> newArray(initialCapacity));
        }

        @SuppressWarnings("unchecked")
        static final <K, V> Segment<K, V>[] newArray(int i) {
            return new Segment[i];
        }

        
Sets table to new HashEntry array.
Call only while holding lock or in constructor.

/**
         * Sets table to new HashEntry array.
         * Call only while holding lock or in constructor.
         */
        void setTable(HashEntry<K, V>[] newTable) {
            threshold = (int) (newTable.length * loadFactor);
            table = newTable;
            refQueue = new ReferenceQueue<K>();
        }

        
Returns properly casted first entry of bin for given hash.

/**
         * Returns properly casted first entry of bin for given hash.
         */
        HashEntry<K, V> getFirst(int hash) {
            HashEntry<K, V>[] tab = table;
            return tab[hash & (tab.length - 1)];
        }

        
Reads value field of an entry under lock. Called if value
field ever appears to be null. This is possible only if a
compiler happens to reorder a HashEntry initialization with
its table assignment, which is legal under memory model
but is not known to ever occur.

/**
         * Reads value field of an entry under lock. Called if value
         * field ever appears to be null. This is possible only if a
         * compiler happens to reorder a HashEntry initialization with
         * its table assignment, which is legal under memory model
         * but is not known to ever occur.
         */
        V readValueUnderLock(HashEntry<K, V> e) {
            lock();
            try {
                removeStale();
                return e.value;
            } finally {
                unlock();
            }
        }

        /* Specialized implementations of map methods */

        V get(Object key, int hash) {
            if(count != 0) { // read-volatile
                HashEntry<K, V> e = getFirst(hash);
                while(e != null) {
                    if(e.hash == hash && key.equals(e.keyRef.get())) {
                        V v = e.value;
                        if(v != null)
                            return v;
                        return readValueUnderLock(e); // recheck
                    }
                    e = e.next;
                }
            }
            return null;
        }

        boolean containsKey(Object key, int hash) {
            if(count != 0) { // read-volatile
                HashEntry<K, V> e = getFirst(hash);
                while(e != null) {
                    if(e.hash == hash && key.equals(e.keyRef.get()))
                        return true;
                    e = e.next;
                }
            }
            return false;
        }

        boolean containsValue(Object value) {
            if(count != 0) { // read-volatile
                HashEntry<K, V>[] tab = table;
                int len = tab.length;
                for(int i = 0; i < len; i++) {
                    for(HashEntry<K, V> e = tab[i]; e != null; e = e.next) {
                        V v = e.value;
                        if(v == null) // recheck
                            v = readValueUnderLock(e);
                        if(value.equals(v))
                            return true;
                    }
                }
            }
            return false;
        }

        boolean replace(K key, int hash, V oldValue, V newValue) {
            lock();
            try {
                removeStale();
                HashEntry<K, V> e = getFirst(hash);
                while(e != null && (e.hash != hash || !key.equals(e.keyRef.get())))
                    e = e.next;

                boolean replaced = false;
                if(e != null && oldValue.equals(e.value)) {
                    replaced = true;
                    e.value = newValue;
                }
                return replaced;
            } finally {
                unlock();
            }
        }

        V replace(K key, int hash, V newValue) {
            lock();
            try {
                removeStale();
                HashEntry<K, V> e = getFirst(hash);
                while(e != null && (e.hash != hash || !key.equals(e.keyRef.get())))
                    e = e.next;

                V oldValue = null;
                if(e != null) {
                    oldValue = e.value;
                    e.value = newValue;
                }
                return oldValue;
            } finally {
                unlock();
            }
        }

        V put(K key, int hash, V value, boolean onlyIfAbsent) {
            lock();
            try {
                removeStale();
                int c = count;
                if(c++ > threshold) {// ensure capacity
                    int reduced = rehash();
                    if(reduced > 0) // adjust from possible weak cleanups
                        count = (c -= reduced) - 1; // write-volatile
                }

                HashEntry<K, V>[] tab = table;
                int index = hash & (tab.length - 1);
                HashEntry<K, V> first = tab[index];
                HashEntry<K, V> e = first;
                while(e != null && (e.hash != hash || !key.equals(e.keyRef.get())))
                    e = e.next;

                V oldValue;
                if(e != null) {
                    oldValue = e.value;
                    if(!onlyIfAbsent)
                        e.value = value;
                } else {
                    oldValue = null;
                    ++modCount;
                    tab[index] = new HashEntry<K, V>(key, hash, first, value, refQueue);
                    count = c; // write-volatile
                }
                return oldValue;
            } finally {
                unlock();
            }
        }

        int rehash() {
            HashEntry<K, V>[] oldTable = table;
            int oldCapacity = oldTable.length;
            if(oldCapacity >= MAXIMUM_CAPACITY)
                return 0;

            /*
             * Reclassify nodes in each list to new Map.  Because we are
             * using power-of-two expansion, the elements from each bin
             * must either stay at same index, or move with a power of two
             * offset. We eliminate unnecessary node creation by catching
             * cases where old nodes can be reused because their next
             * fields won't change. Statistically, at the default
             * threshold, only about one-sixth of them need cloning when
             * a table doubles. The nodes they replace will be garbage
             * collectable as soon as they are no longer referenced by any
             * reader thread that may be in the midst of traversing table
             * right now.
             */

            HashEntry<K, V>[] newTable = HashEntry.newArray(oldCapacity << 1);
            threshold = (int) (newTable.length * loadFactor);
            int sizeMask = newTable.length - 1;
            int reduce = 0;
            for(int i = 0; i < oldCapacity; i++) {
                // We need to guarantee that any existing reads of old Map can
                //  proceed. So we cannot yet null out each bin.
                HashEntry<K, V> e = oldTable[i];

                if(e != null) {
                    HashEntry<K, V> next = e.next;
                    int idx = e.hash & sizeMask;

                    //  Single node on list
                    if(next == null)
                        newTable[idx] = e;

                    else {
                        // Reuse trailing consecutive sequence at same slot
                        HashEntry<K, V> lastRun = e;
                        int lastIdx = idx;
                        for(HashEntry<K, V> last = next; last != null; last = last.next) {
                            int k = last.hash & sizeMask;
                            if(k != lastIdx) {
                                lastIdx = k;
                                lastRun = last;
                            }
                        }
                        newTable[lastIdx] = lastRun;
                        // Clone all remaining nodes
                        for(HashEntry<K, V> p = e; p != lastRun; p = p.next) {
                            // Skip GC'd weak refs
                            K key = p.keyRef.get();
                            if(key == null) {
                                reduce++;
                                continue;
                            }
                            int k = p.hash & sizeMask;
                            HashEntry<K, V> n = newTable[k];
                            newTable[k] = new HashEntry<K, V>(key, p.hash, n, p.value, refQueue);
                        }
                    }
                }
            }
            table = newTable;
            return reduce;
        }

        
Remove; match on key only if value null, else match both.

/**
         * Remove; match on key only if value null, else match both.
         */
        V remove(Object key, int hash, Object value, boolean weakRemove) {
            lock();
            try {
                if(!weakRemove)
                    removeStale();
                int c = count - 1;
                HashEntry<K, V>[] tab = table;
                int index = hash & (tab.length - 1);
                HashEntry<K, V> first = tab[index];
                HashEntry<K, V> e = first;
                // a weak remove operation compares the WeakReference instance
                while(e != null && (!weakRemove || key != e.keyRef)
                        && (e.hash != hash || !key.equals(e.keyRef.get())))
                    e = e.next;

                V oldValue = null;
                if(e != null) {
                    V v = e.value;
                    if(value == null || value.equals(v)) {
                        oldValue = v;
                        // All entries following removed node can stay
                        // in list, but all preceding ones need to be
                        // cloned.
                        ++modCount;
                        HashEntry<K, V> newFirst = e.next;
                        for(HashEntry<K, V> p = first; p != e; p = p.next) {
                            K pKey = p.keyRef.get();
                            if(pKey == null) { // Skip GC'd keys
                                c--;
                                continue;
                            }

                            newFirst = new HashEntry<K, V>(pKey, p.hash, newFirst, p.value, refQueue);
                        }
                        tab[index] = newFirst;
                        count = c; // write-volatile
                    }
                }
                return oldValue;
            } finally {
                unlock();
            }
        }

        @SuppressWarnings("unchecked")
        void removeStale() {
            WeakKeyReference<K> ref;
            while((ref = (WeakKeyReference<K>) refQueue.poll()) != null) {
                remove(ref, ref.hash, null, true);
            }
        }

        void clear() {
            if(count != 0) {
                lock();
                try {
                    HashEntry<K, V>[] tab = table;
                    for(int i = 0; i < tab.length; i++)
                        tab[i] = null;
                    ++modCount;
                    // replace the reference queue to avoid unnecessary stale cleanups
                    refQueue = new ReferenceQueue<K>();
                    count = 0; // write-volatile
                } finally {
                    unlock();
                }
            }
        }
    }

    /* ---------------- Public operations -------------- */

    
Creates a new, empty map with the specified initial
capacity, load factor and concurrency level.

Params:
  • initialCapacity – the initial capacity. The implementation
performs internal sizing to accommodate this many elements.
  • loadFactor –  the load factor threshold, used to control resizing.
Resizing may be performed when the average number of elements per
bin exceeds this threshold.
  • concurrencyLevel – the estimated number of concurrently
updating threads. The implementation performs internal sizing
to try to accommodate this many threads.
Throws:
  • IllegalArgumentException – if the initial capacity is
negative or the load factor or concurrencyLevel are
nonpositive.
/**
     * Creates a new, empty map with the specified initial
     * capacity, load factor and concurrency level.
     *
     * @param initialCapacity the initial capacity. The implementation
     * performs internal sizing to accommodate this many elements.
     * @param loadFactor  the load factor threshold, used to control resizing.
     * Resizing may be performed when the average number of elements per
     * bin exceeds this threshold.
     * @param concurrencyLevel the estimated number of concurrently
     * updating threads. The implementation performs internal sizing
     * to try to accommodate this many threads.
     * @throws IllegalArgumentException if the initial capacity is
     * negative or the load factor or concurrencyLevel are
     * nonpositive.
     */
    public ConcurrentWeakHashMap(int initialCapacity, float loadFactor, int concurrencyLevel) {
        if(!(loadFactor > 0) || initialCapacity < 0 || concurrencyLevel <= 0)
            throw new IllegalArgumentException();

        if(concurrencyLevel > MAX_SEGMENTS)
            concurrencyLevel = MAX_SEGMENTS;

        // Find power-of-two sizes best matching arguments
        int sshift = 0;
        int ssize = 1;
        while(ssize < concurrencyLevel) {
            ++sshift;
            ssize <<= 1;
        }
        segmentShift = 32 - sshift;
        segmentMask = ssize - 1;
        this.segments = Segment.newArray(ssize);

        if(initialCapacity > MAXIMUM_CAPACITY)
            initialCapacity = MAXIMUM_CAPACITY;
        int c = initialCapacity / ssize;
        if(c * ssize < initialCapacity)
            ++c;
        int cap = 1;
        while(cap < c)
            cap <<= 1;

        for(int i = 0; i < this.segments.length; ++i)
            this.segments[i] = new Segment<K, V>(cap, loadFactor);
    }

    
Creates a new, empty map with the specified initial capacity
and load factor and with the default concurrencyLevel (16).

Params:
  • initialCapacity – The implementation performs internal
sizing to accommodate this many elements.
  • loadFactor –  the load factor threshold, used to control resizing.
Resizing may be performed when the average number of elements per
bin exceeds this threshold.
Throws:
Since:1.6
/**
     * Creates a new, empty map with the specified initial capacity
     * and load factor and with the default concurrencyLevel (16).
     *
     * @param initialCapacity The implementation performs internal
     * sizing to accommodate this many elements.
     * @param loadFactor  the load factor threshold, used to control resizing.
     * Resizing may be performed when the average number of elements per
     * bin exceeds this threshold.
     * @throws IllegalArgumentException if the initial capacity of
     * elements is negative or the load factor is nonpositive
     *
     * @since 1.6
     */
    public ConcurrentWeakHashMap(int initialCapacity, float loadFactor) {
        this(initialCapacity, loadFactor, DEFAULT_CONCURRENCY_LEVEL);
    }

    
Creates a new, empty map with the specified initial capacity,
and with default load factor (0.75) and concurrencyLevel (16).

Params:
  • initialCapacity – the initial capacity. The implementation
performs internal sizing to accommodate this many elements.
Throws:
/**
     * Creates a new, empty map with the specified initial capacity,
     * and with default load factor (0.75) and concurrencyLevel (16).
     *
     * @param initialCapacity the initial capacity. The implementation
     * performs internal sizing to accommodate this many elements.
     * @throws IllegalArgumentException if the initial capacity of
     * elements is negative.
     */
    public ConcurrentWeakHashMap(int initialCapacity) {
        this(initialCapacity, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    
Creates a new, empty map with a default initial capacity (16),
load factor (0.75) and concurrencyLevel (16).

/**
     * Creates a new, empty map with a default initial capacity (16),
     * load factor (0.75) and concurrencyLevel (16).
     */
    public ConcurrentWeakHashMap() {
        this(DEFAULT_INITIAL_CAPACITY, DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
    }

    
Creates a new map with the same mappings as the given map.
The map is created with a capacity of 1.5 times the number
of mappings in the given map or 16 (whichever is greater),
and a default load factor (0.75) and concurrencyLevel (16).

Params:
  • m – the map
/**
     * Creates a new map with the same mappings as the given map.
     * The map is created with a capacity of 1.5 times the number
     * of mappings in the given map or 16 (whichever is greater),
     * and a default load factor (0.75) and concurrencyLevel (16).
     *
     * @param m the map
     */
    public ConcurrentWeakHashMap(Map<? extends K, ? extends V> m) {
        this(Math.max((int) (m.size() / DEFAULT_LOAD_FACTOR) + 1, DEFAULT_INITIAL_CAPACITY), DEFAULT_LOAD_FACTOR, DEFAULT_CONCURRENCY_LEVEL);
        putAll(m);
    }

    
Returns true if this map contains no key-value mappings.

Returns:true if this map contains no key-value mappings
/**
     * Returns <tt>true</tt> if this map contains no key-value mappings.
     *
     * @return <tt>true</tt> if this map contains no key-value mappings
     */
    public boolean isEmpty() {
        final Segment<K, V>[] segments = this.segments;
        /*
         * We keep track of per-segment modCounts to avoid ABA
         * problems in which an element in one segment was added and
         * in another removed during traversal, in which case the
         * table was never actually empty at any point. Note the
         * similar use of modCounts in the size() and containsValue()
         * methods, which are the only other methods also susceptible
         * to ABA problems.
         */
        int[] mc = new int[segments.length];
        int mcsum = 0;
        for(int i = 0; i < segments.length; ++i) {
            if(segments[i].count != 0)
                return false;
            else
                mcsum += mc[i] = segments[i].modCount;
        }
        // If mcsum happens to be zero, then we know we got a snapshot
        // before any modifications at all were made.  This is
        // probably common enough to bother tracking.
        if(mcsum != 0) {
            for(int i = 0; i < segments.length; ++i) {
                if(segments[i].count != 0 || mc[i] != segments[i].modCount)
                    return false;
            }
        }
        return true;
    }

    
Returns the number of key-value mappings in this map.  If the
map contains more than Integer.MAX_VALUE elements, returns
Integer.MAX_VALUE.

Returns:the number of key-value mappings in this map
/**
     * Returns the number of key-value mappings in this map.  If the
     * map contains more than <tt>Integer.MAX_VALUE</tt> elements, returns
     * <tt>Integer.MAX_VALUE</tt>.
     *
     * @return the number of key-value mappings in this map
     */
    public int size() {
        final Segment<K, V>[] segments = this.segments;
        long sum = 0;
        long check = 0;
        int[] mc = new int[segments.length];
        // Try a few times to get accurate count. On failure due to
        // continuous async changes in table, resort to locking.
        for(int k = 0; k < RETRIES_BEFORE_LOCK; ++k) {
            check = 0;
            sum = 0;
            int mcsum = 0;
            for(int i = 0; i < segments.length; ++i) {
                sum += segments[i].count;
                mcsum += mc[i] = segments[i].modCount;
            }
            if(mcsum != 0) {
                for(int i = 0; i < segments.length; ++i) {
                    check += segments[i].count;
                    if(mc[i] != segments[i].modCount) {
                        check = -1; // force retry
                        break;
                    }
                }
            }
            if(check == sum)
                break;
        }
        if(check != sum) { // Resort to locking all segments
            sum = 0;
            for(int i = 0; i < segments.length; ++i)
                segments[i].lock();
            for(int i = 0; i < segments.length; ++i)
                sum += segments[i].count;
            for(int i = 0; i < segments.length; ++i)
                segments[i].unlock();
        }
        if(sum > Integer.MAX_VALUE)
            return Integer.MAX_VALUE;
        else
            return (int) sum;
    }

    
Returns the value to which the specified key is mapped, or null if this map contains no mapping for the key. 
More formally, if this map contains a mapping from a key k to a value v such that key.equals(k), then this method returns v; otherwise it returns null. (There can be at most one such mapping.) 
Throws:
/**
     * Returns the value to which the specified key is mapped,
     * or {@code null} if this map contains no mapping for the key.
     *
     * <p>More formally, if this map contains a mapping from a key
     * {@code k} to a value {@code v} such that {@code key.equals(k)},
     * then this method returns {@code v}; otherwise it returns
     * {@code null}.  (There can be at most one such mapping.)
     *
     * @throws NullPointerException if the specified key is null
     */
    public V get(Object key) {
        int hash = hash(key.hashCode());
        return segmentFor(hash).get(key, hash);
    }

    
Tests if the specified object is a key in this table.

Params:
  • key –   possible key
Throws:
Returns:true if and only if the specified object
        is a key in this table, as determined by the
        equals method; false otherwise.
/**
     * Tests if the specified object is a key in this table.
     *
     * @param  key   possible key
     * @return <tt>true</tt> if and only if the specified object
     *         is a key in this table, as determined by the
     *         <tt>equals</tt> method; <tt>false</tt> otherwise.
     * @throws NullPointerException if the specified key is null
     */
    public boolean containsKey(Object key) {
        int hash = hash(key.hashCode());
        return segmentFor(hash).containsKey(key, hash);
    }

    
Returns true if this map maps one or more keys to the
specified value. Note: This method requires a full internal
traversal of the hash table, and so is much slower than
method containsKey.

Params:
  • value – value whose presence in this map is to be tested
Throws:
Returns:true if this map maps one or more keys to the
        specified value
/**
     * Returns <tt>true</tt> if this map maps one or more keys to the
     * specified value. Note: This method requires a full internal
     * traversal of the hash table, and so is much slower than
     * method <tt>containsKey</tt>.
     *
     * @param value value whose presence in this map is to be tested
     * @return <tt>true</tt> if this map maps one or more keys to the
     *         specified value
     * @throws NullPointerException if the specified value is null
     */
    public boolean containsValue(Object value) {
        if(value == null)
            throw new NullPointerException();

        // See explanation of modCount use above

        final Segment<K, V>[] segments = this.segments;
        int[] mc = new int[segments.length];

        // Try a few times without locking
        for(int k = 0; k < RETRIES_BEFORE_LOCK; ++k) {
            int sum = 0;
            int mcsum = 0;
            for(int i = 0; i < segments.length; ++i) {
                int c = segments[i].count;
                mcsum += mc[i] = segments[i].modCount;
                if(segments[i].containsValue(value))
                    return true;
            }
            boolean cleanSweep = true;
            if(mcsum != 0) {
                for(int i = 0; i < segments.length; ++i) {
                    int c = segments[i].count;
                    if(mc[i] != segments[i].modCount) {
                        cleanSweep = false;
                        break;
                    }
                }
            }
            if(cleanSweep)
                return false;
        }
        // Resort to locking all segments
        for(int i = 0; i < segments.length; ++i)
            segments[i].lock();
        boolean found = false;
        try {
            for(int i = 0; i < segments.length; ++i) {
                if(segments[i].containsValue(value)) {
                    found = true;
                    break;
                }
            }
        } finally {
            for(int i = 0; i < segments.length; ++i)
                segments[i].unlock();
        }
        return found;
    }

    
Legacy method testing if some key maps into the specified value in this table. This method is identical in functionality to containsValue, and exists solely to ensure full compatibility with class Hashtable, which supported this method prior to introduction of the Java Collections framework. 
Params:
  • value – a value to search for
Throws:
Returns:true if and only if some key maps to the
        value argument in this table as
        determined by the equals method;
        false otherwise
/**
     * Legacy method testing if some key maps into the specified value
     * in this table.  This method is identical in functionality to
     * {@link #containsValue}, and exists solely to ensure
     * full compatibility with class {@link java.util.Hashtable},
     * which supported this method prior to introduction of the
     * Java Collections framework.

     * @param  value a value to search for
     * @return <tt>true</tt> if and only if some key maps to the
     *         <tt>value</tt> argument in this table as
     *         determined by the <tt>equals</tt> method;
     *         <tt>false</tt> otherwise
     * @throws NullPointerException if the specified value is null
     */
    public boolean contains(Object value) {
        return containsValue(value);
    }

    
Maps the specified key to the specified value in this table.
Neither the key nor the value can be null.

 The value can be retrieved by calling the get method
with a key that is equal to the original key.

Params:
  • key – key with which the specified value is to be associated
  • value – value to be associated with the specified key
Throws:
Returns:the previous value associated with key, or
        null if there was no mapping for key
/**
     * Maps the specified key to the specified value in this table.
     * Neither the key nor the value can be null.
     *
     * <p> The value can be retrieved by calling the <tt>get</tt> method
     * with a key that is equal to the original key.
     *
     * @param key key with which the specified value is to be associated
     * @param value value to be associated with the specified key
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>
     * @throws NullPointerException if the specified key or value is null
     */
    public V put(K key, V value) {
        if(value == null)
            throw new NullPointerException();
        int hash = hash(key.hashCode());
        return segmentFor(hash).put(key, hash, value, false);
    }

    
{@inheritDoc}

Throws:
Returns:the previous value associated with the specified key,
        or null if there was no mapping for the key
/**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     *         or <tt>null</tt> if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */
    public V putIfAbsent(K key, V value) {
        if(value == null)
            throw new NullPointerException();
        int hash = hash(key.hashCode());
        return segmentFor(hash).put(key, hash, value, true);
    }

    
Copies all of the mappings from the specified map to this one.
These mappings replace any mappings that this map had for any of the
keys currently in the specified map.

Params:
  • m – mappings to be stored in this map
/**
     * Copies all of the mappings from the specified map to this one.
     * These mappings replace any mappings that this map had for any of the
     * keys currently in the specified map.
     *
     * @param m mappings to be stored in this map
     */
    public void putAll(Map<? extends K, ? extends V> m) {
        for(Map.Entry<? extends K, ? extends V> e : m.entrySet())
            put(e.getKey(), e.getValue());
    }

    
Removes the key (and its corresponding value) from this map.
This method does nothing if the key is not in the map.

Params:
  • key – the key that needs to be removed
Throws:
Returns:the previous value associated with key, or
        null if there was no mapping for key
/**
     * Removes the key (and its corresponding value) from this map.
     * This method does nothing if the key is not in the map.
     *
     * @param  key the key that needs to be removed
     * @return the previous value associated with <tt>key</tt>, or
     *         <tt>null</tt> if there was no mapping for <tt>key</tt>
     * @throws NullPointerException if the specified key is null
     */
    public V remove(Object key) {
        int hash = hash(key.hashCode());
        return segmentFor(hash).remove(key, hash, null, false);
    }

    
{@inheritDoc}

Throws:
/**
     * {@inheritDoc}
     *
     * @throws NullPointerException if the specified key is null
     */
    public boolean remove(Object key, Object value) {
        int hash = hash(key.hashCode());
        if(value == null)
            return false;
        return segmentFor(hash).remove(key, hash, value, false) != null;
    }

    
{@inheritDoc}

Throws:
/**
     * {@inheritDoc}
     *
     * @throws NullPointerException if any of the arguments are null
     */
    public boolean replace(K key, V oldValue, V newValue) {
        if(oldValue == null || newValue == null)
            throw new NullPointerException();
        int hash = hash(key.hashCode());
        return segmentFor(hash).replace(key, hash, oldValue, newValue);
    }

    
{@inheritDoc}

Throws:
Returns:the previous value associated with the specified key,
        or null if there was no mapping for the key
/**
     * {@inheritDoc}
     *
     * @return the previous value associated with the specified key,
     *         or <tt>null</tt> if there was no mapping for the key
     * @throws NullPointerException if the specified key or value is null
     */
    public V replace(K key, V value) {
        if(value == null)
            throw new NullPointerException();
        int hash = hash(key.hashCode());
        return segmentFor(hash).replace(key, hash, value);
    }

    
Removes all of the mappings from this map.

/**
     * Removes all of the mappings from this map.
     */
    public void clear() {
        for(int i = 0; i < segments.length; ++i)
            segments[i].clear();
    }

    
Returns a Set view of the keys contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. The set supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove, Set.remove,
removeAll, retainAll, and clear
operations.  It does not support the add or
addAll operations.

The view's iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction. 
/**
     * Returns a {@link Set} view of the keys contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  The set supports element
     * removal, which removes the corresponding mapping from this map,
     * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
     * operations.  It does not support the <tt>add</tt> or
     * <tt>addAll</tt> operations.
     *
     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
     * that will never throw {@link ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     */
    public Set<K> keySet() {
        Set<K> ks = keySet;
        return (ks != null) ? ks : (keySet = new KeySet());
    }

    
Returns a Collection view of the values contained in this map. The collection is backed by the map, so changes to the map are reflected in the collection, and vice-versa. The collection supports element removal, which removes the corresponding mapping from this map, via the Iterator.remove,
Collection.remove, removeAll,
retainAll, and clear operations.  It does not
support the add or addAll operations.

The view's iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction. 
/**
     * Returns a {@link Collection} view of the values contained in this map.
     * The collection is backed by the map, so changes to the map are
     * reflected in the collection, and vice-versa.  The collection
     * supports element removal, which removes the corresponding
     * mapping from this map, via the <tt>Iterator.remove</tt>,
     * <tt>Collection.remove</tt>, <tt>removeAll</tt>,
     * <tt>retainAll</tt>, and <tt>clear</tt> operations.  It does not
     * support the <tt>add</tt> or <tt>addAll</tt> operations.
     *
     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
     * that will never throw {@link ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     */
    public Collection<V> values() {
        Collection<V> vs = values;
        return (vs != null) ? vs : (values = new Values());
    }

    
Returns a Set view of the mappings contained in this map. The set is backed by the map, so changes to the map are reflected in the set, and vice-versa. The set supports element removal, which removes the corresponding mapping from the map, via the Iterator.remove, Set.remove,
removeAll, retainAll, and clear
operations.  It does not support the add or
addAll operations.

The view's iterator is a "weakly consistent" iterator that will never throw ConcurrentModificationException, and guarantees to traverse elements as they existed upon construction of the iterator, and may (but is not guaranteed to) reflect any modifications subsequent to construction. 
/**
     * Returns a {@link Set} view of the mappings contained in this map.
     * The set is backed by the map, so changes to the map are
     * reflected in the set, and vice-versa.  The set supports element
     * removal, which removes the corresponding mapping from the map,
     * via the <tt>Iterator.remove</tt>, <tt>Set.remove</tt>,
     * <tt>removeAll</tt>, <tt>retainAll</tt>, and <tt>clear</tt>
     * operations.  It does not support the <tt>add</tt> or
     * <tt>addAll</tt> operations.
     *
     * <p>The view's <tt>iterator</tt> is a "weakly consistent" iterator
     * that will never throw {@link ConcurrentModificationException},
     * and guarantees to traverse elements as they existed upon
     * construction of the iterator, and may (but is not guaranteed to)
     * reflect any modifications subsequent to construction.
     */
    public Set<Map.Entry<K, V>> entrySet() {
        Set<Map.Entry<K, V>> es = entrySet;
        return (es != null) ? es : (entrySet = new EntrySet());
    }

    
Returns an enumeration of the keys in this table.

See Also:
Returns:an enumeration of the keys in this table
/**
     * Returns an enumeration of the keys in this table.
     *
     * @return an enumeration of the keys in this table
     * @see #keySet()
     */
    public Enumeration<K> keys() {
        return new KeyIterator();
    }

    
Returns an enumeration of the values in this table.

See Also:
Returns:an enumeration of the values in this table
/**
     * Returns an enumeration of the values in this table.
     *
     * @return an enumeration of the values in this table
     * @see #values()
     */
    public Enumeration<V> elements() {
        return new ValueIterator();
    }

    /* ---------------- Iterator Support -------------- */

    abstract class HashIterator {
        int nextSegmentIndex;
        int nextTableIndex;
        HashEntry<K, V>[] currentTable;
        HashEntry<K, V> nextEntry;
        HashEntry<K, V> lastReturned;
        K currentKey; // Strong reference to weak key (prevents gc)

        HashIterator() {
            nextSegmentIndex = segments.length - 1;
            nextTableIndex = -1;
            advance();
        }

        public boolean hasMoreElements() {
            return hasNext();
        }

        final void advance() {
            if(nextEntry != null && (nextEntry = nextEntry.next) != null)
                return;

            while(nextTableIndex >= 0) {
                if((nextEntry = currentTable[nextTableIndex--]) != null)
                    return;
            }

            while(nextSegmentIndex >= 0) {
                Segment<K, V> seg = segments[nextSegmentIndex--];
                if(seg.count != 0) {
                    currentTable = seg.table;
                    for(int j = currentTable.length - 1; j >= 0; --j) {
                        if((nextEntry = currentTable[j]) != null) {
                            nextTableIndex = j - 1;
                            return;
                        }
                    }
                }
            }
        }

        public boolean hasNext() {
            while(nextEntry != null) {
                if(nextEntry.keyRef.get() != null)
                    return true;
                advance();
            }

            return false;
        }

        HashEntry<K, V> nextEntry() {
            do {
                if(nextEntry == null)
                    throw new NoSuchElementException();

                lastReturned = nextEntry;
                currentKey = lastReturned.keyRef.get();
                advance();
            } while(currentKey == null); // Skip GC'd keys

            return lastReturned;
        }

        public void remove() {
            if(lastReturned == null)
                throw new IllegalStateException();
            ConcurrentWeakHashMap.this.remove(currentKey);
            lastReturned = null;
        }
    }

    final class KeyIterator extends HashIterator implements Iterator<K>, Enumeration<K> {
        public K next() {
            return super.nextEntry().keyRef.get();
        }

        public K nextElement() {
            return super.nextEntry().keyRef.get();
        }
    }

    final class ValueIterator extends HashIterator implements Iterator<V>, Enumeration<V> {
        public V next() {
            return super.nextEntry().value;
        }

        public V nextElement() {
            return super.nextEntry().value;
        }
    }

    /*
     * This class is needed for JDK5 compatibility.
     */
    static class SimpleEntry<K, V> implements Entry<K, V>, java.io.Serializable {
        private static final long serialVersionUID = -8499721149061103585L;

        private final K key;
        private V value;

        public SimpleEntry(K key, V value) {
            this.key = key;
            this.value = value;
        }

        public SimpleEntry(Entry<? extends K, ? extends V> entry) {
            this.key = entry.getKey();
            this.value = entry.getValue();
        }

        public K getKey() {
            return key;
        }

        public V getValue() {
            return value;
        }

        public V setValue(V value) {
            V oldValue = this.value;
            this.value = value;
            return oldValue;
        }

        public boolean equals(Object o) {
            if(!(o instanceof Map.Entry))
                return false;
            @SuppressWarnings("unchecked")
            Map.Entry e = (Map.Entry) o;
            return eq(key, e.getKey()) && eq(value, e.getValue());
        }

        public int hashCode() {
            return (key == null ? 0 : key.hashCode()) ^ (value == null ? 0 : value.hashCode());
        }

        public String toString() {
            return key + "=" + value;
        }

        private static boolean eq(Object o1, Object o2) {
            return o1 == null ? o2 == null : o1.equals(o2);
        }
    }

    
Custom Entry class used by EntryIterator.next(), that relays setValue
changes to the underlying map.

/**
     * Custom Entry class used by EntryIterator.next(), that relays setValue
     * changes to the underlying map.
     */
    final class WriteThroughEntry extends SimpleEntry<K, V> {
        private static final long serialVersionUID = -7900634345345313646L;

        WriteThroughEntry(K k, V v) {
            super(k, v);
        }

        
Set our entry's value and write through to the map. The
value to return is somewhat arbitrary here. Since a
WriteThroughEntry does not necessarily track asynchronous
changes, the most recent "previous" value could be
different from what we return (or could even have been
removed in which case the put will re-establish). We do not
and cannot guarantee more.

/**
         * Set our entry's value and write through to the map. The
         * value to return is somewhat arbitrary here. Since a
         * WriteThroughEntry does not necessarily track asynchronous
         * changes, the most recent "previous" value could be
         * different from what we return (or could even have been
         * removed in which case the put will re-establish). We do not
         * and cannot guarantee more.
         */
        public V setValue(V value) {
            if(value == null)
                throw new NullPointerException();
            V v = super.setValue(value);
            ConcurrentWeakHashMap.this.put(getKey(), value);
            return v;
        }
    }

    final class EntryIterator extends HashIterator implements Iterator<Entry<K, V>> {
        public Map.Entry<K, V> next() {
            HashEntry<K, V> e = super.nextEntry();
            return new WriteThroughEntry(e.keyRef.get(), e.value);
        }
    }

    final class KeySet extends AbstractSet<K> {
        public Iterator<K> iterator() {
            return new KeyIterator();
        }

        public int size() {
            return ConcurrentWeakHashMap.this.size();
        }

        public boolean isEmpty() {
            return ConcurrentWeakHashMap.this.isEmpty();
        }

        public boolean contains(Object o) {
            return ConcurrentWeakHashMap.this.containsKey(o);
        }

        public boolean remove(Object o) {
            return ConcurrentWeakHashMap.this.remove(o) != null;
        }

        public void clear() {
            ConcurrentWeakHashMap.this.clear();
        }
    }

    final class Values extends AbstractCollection<V> {
        public Iterator<V> iterator() {
            return new ValueIterator();
        }

        public int size() {
            return ConcurrentWeakHashMap.this.size();
        }

        public boolean isEmpty() {
            return ConcurrentWeakHashMap.this.isEmpty();
        }

        public boolean contains(Object o) {
            return ConcurrentWeakHashMap.this.containsValue(o);
        }

        public void clear() {
            ConcurrentWeakHashMap.this.clear();
        }
    }

    final class EntrySet extends AbstractSet<Map.Entry<K, V>> {
        public Iterator<Map.Entry<K, V>> iterator() {
            return new EntryIterator();
        }

        public boolean contains(Object o) {
            if(!(o instanceof Map.Entry))
                return false;
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            V v = ConcurrentWeakHashMap.this.get(e.getKey());
            return v != null && v.equals(e.getValue());
        }

        public boolean remove(Object o) {
            if(!(o instanceof Map.Entry))
                return false;
            Map.Entry<?, ?> e = (Map.Entry<?, ?>) o;
            return ConcurrentWeakHashMap.this.remove(e.getKey(), e.getValue());
        }

        public int size() {
            return ConcurrentWeakHashMap.this.size();
        }

        public boolean isEmpty() {
            return ConcurrentWeakHashMap.this.isEmpty();
        }

        public void clear() {
            ConcurrentWeakHashMap.this.clear();
        }
    }

    /* ---------------- Serialization Support -------------- */

    
Save the state of the ConcurrentWeakHashMap instance to a
stream (i.e., serialize it).

Params:
  • s – the stream
@serialData
the key (Object) and value (Object)
for each key-value mapping, followed by a null pair.
The key-value mappings are emitted in no particular order.
/**
     * Save the state of the <tt>ConcurrentWeakHashMap</tt> instance to a
     * stream (i.e., serialize it).
     * @param s the stream
     * @serialData
     * the key (Object) and value (Object)
     * for each key-value mapping, followed by a null pair.
     * The key-value mappings are emitted in no particular order.
     */
    private void writeObject(java.io.ObjectOutputStream s) throws IOException {
        s.defaultWriteObject();

        for(int k = 0; k < segments.length; ++k) {
            Segment<K, V> seg = segments[k];
            seg.lock();
            try {
                HashEntry<K, V>[] tab = seg.table;
                for(int i = 0; i < tab.length; ++i) {
                    for(HashEntry<K, V> e = tab[i]; e != null; e = e.next) {
                        K key = e.keyRef.get();
                        if(key == null) // Skip GC'd keys
                            continue;

                        s.writeObject(key);
                        s.writeObject(e.value);
                    }
                }
            } finally {
                seg.unlock();
            }
        }
        s.writeObject(null);
        s.writeObject(null);
    }

    
Reconstitute the ConcurrentWeakHashMap instance from a
stream (i.e., deserialize it).

Params:
  • s – the stream
/**
     * Reconstitute the <tt>ConcurrentWeakHashMap</tt> instance from a
     * stream (i.e., deserialize it).
     * @param s the stream
     */
    @SuppressWarnings("unchecked")
    private void readObject(java.io.ObjectInputStream s) throws IOException, ClassNotFoundException {
        s.defaultReadObject();

        // Initialize each segment to be minimally sized, and let grow.
        for(int i = 0; i < segments.length; ++i) {
            segments[i].setTable(new HashEntry[1]);
        }

        // Read the keys and values, and put the mappings in the table
        for(;;) {
            K key = (K) s.readObject();
            V value = (V) s.readObject();
            if(key == null)
                break;
            put(key, value);
        }
    }
}