/*
 * Copyright (c) 1995, 2007, Oracle and/or its affiliates. All rights reserved.
 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * 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).
 *
 * 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,
 * Inc., 51 Franklin St, Fifth Floor, Boston, MA 02110-1301 USA.
 *
 * Please contact Oracle, 500 Oracle Parkway, Redwood Shores, CA 94065 USA
 * or visit www.oracle.com if you need additional information or have any
 * questions.
 */

package java.lang;

import java.io.PrintStream;
import java.util.Arrays;
import sun.misc.VM;

A thread group represents a set of threads. In addition, a thread
group can also include other thread groups. The thread groups form
a tree in which every thread group except the initial thread group
has a parent.

A thread is allowed to access information about its own thread
group, but not to access information about its thread group's
parent thread group or any other thread groups.
Author:
 unascribed
Since:
  JDK1.0/**
 * A thread group represents a set of threads. In addition, a thread
 * group can also include other thread groups. The thread groups form
 * a tree in which every thread group except the initial thread group
 * has a parent.
 * <p>
 * A thread is allowed to access information about its own thread
 * group, but not to access information about its thread group's
 * parent thread group or any other thread groups.
 *
 * @author  unascribed
 * @since   JDK1.0
 */
/* The locking strategy for this code is to try to lock only one level of the
 * tree wherever possible, but otherwise to lock from the bottom up.
 * That is, from child thread groups to parents.
 * This has the advantage of limiting the number of locks that need to be held
 * and in particular avoids having to grab the lock for the root thread group,
 * (or a global lock) which would be a source of contention on a
 * multi-processor system with many thread groups.
 * This policy often leads to taking a snapshot of the state of a thread group
 * and working off of that snapshot, rather than holding the thread group locked
 * while we work on the children.
 */
public
class ThreadGroup implements Thread.UncaughtExceptionHandler {
    private final ThreadGroup parent;
    String name;
    int maxPriority;
    boolean destroyed;
    boolean daemon;
    boolean vmAllowSuspension;

    int nUnstartedThreads = 0;
    int nthreads;
    Thread threads[];

    int ngroups;
    ThreadGroup groups[];

    
Creates an empty Thread group that is not in any Thread group.
This method is used to create the system Thread group.
/**
     * Creates an empty Thread group that is not in any Thread group.
     * This method is used to create the system Thread group.
     */
    private ThreadGroup() {     // called from C code
        this.name = "system";
        this.maxPriority = Thread.MAX_PRIORITY;
        this.parent = null;
    }

    
Constructs a new thread group. The parent of this new group is
the thread group of the currently running thread.

The checkAccess method of the parent thread group is
called with no arguments; this may result in a security exception.
Params:
name –   the name of the new thread group.
Throws:
SecurityException –  if the current thread cannot create a
              thread in the specified thread group.
See Also:
checkAccess()
Since:
  JDK1.0/**
     * Constructs a new thread group. The parent of this new group is
     * the thread group of the currently running thread.
     * <p>
     * The <code>checkAccess</code> method of the parent thread group is
     * called with no arguments; this may result in a security exception.
     *
     * @param   name   the name of the new thread group.
     * @exception  SecurityException  if the current thread cannot create a
     *               thread in the specified thread group.
     * @see     java.lang.ThreadGroup#checkAccess()
     * @since   JDK1.0
     */
    public ThreadGroup(String name) {
        this(Thread.currentThread().getThreadGroup(), name);
    }

    
Creates a new thread group. The parent of this new group is the
specified thread group.

The checkAccess method of the parent thread group is
called with no arguments; this may result in a security exception.
Params:
parent –   the parent thread group.
name –     the name of the new thread group.
Throws:
NullPointerException –  if the thread group argument is
              null.
SecurityException –  if the current thread cannot create a
              thread in the specified thread group.
See Also:
SecurityException
checkAccess()
Since:
  JDK1.0/**
     * Creates a new thread group. The parent of this new group is the
     * specified thread group.
     * <p>
     * The <code>checkAccess</code> method of the parent thread group is
     * called with no arguments; this may result in a security exception.
     *
     * @param     parent   the parent thread group.
     * @param     name     the name of the new thread group.
     * @exception  NullPointerException  if the thread group argument is
     *               <code>null</code>.
     * @exception  SecurityException  if the current thread cannot create a
     *               thread in the specified thread group.
     * @see     java.lang.SecurityException
     * @see     java.lang.ThreadGroup#checkAccess()
     * @since   JDK1.0
     */
    public ThreadGroup(ThreadGroup parent, String name) {
        this(checkParentAccess(parent), parent, name);
    }

    private ThreadGroup(Void unused, ThreadGroup parent, String name) {
        this.name = name;
        this.maxPriority = parent.maxPriority;
        this.daemon = parent.daemon;
        this.vmAllowSuspension = parent.vmAllowSuspension;
        this.parent = parent;
        parent.add(this);
    }

    /*
     * @throws  NullPointerException  if the parent argument is {@code null}
     * @throws  SecurityException     if the current thread cannot create a
     *                                thread in the specified thread group.
     */
    private static Void checkParentAccess(ThreadGroup parent) {
        parent.checkAccess();
        return null;
    }

    
Returns the name of this thread group.
Returns:
 the name of this thread group.
Since:
  JDK1.0/**
     * Returns the name of this thread group.
     *
     * @return  the name of this thread group.
     * @since   JDK1.0
     */
    public final String getName() {
        return name;
    }

    
Returns the parent of this thread group.

First, if the parent is not null, the
checkAccess method of the parent thread group is
called with no arguments; this may result in a security exception.
Throws:
SecurityException –  if the current thread cannot modify
              this thread group.
See Also:
checkAccess()
SecurityException
RuntimePermission
Returns:
 the parent of this thread group. The top-level thread group
         is the only thread group whose parent is null.
Since:
  JDK1.0/**
     * Returns the parent of this thread group.
     * <p>
     * First, if the parent is not <code>null</code>, the
     * <code>checkAccess</code> method of the parent thread group is
     * called with no arguments; this may result in a security exception.
     *
     * @return  the parent of this thread group. The top-level thread group
     *          is the only thread group whose parent is <code>null</code>.
     * @exception  SecurityException  if the current thread cannot modify
     *               this thread group.
     * @see        java.lang.ThreadGroup#checkAccess()
     * @see        java.lang.SecurityException
     * @see        java.lang.RuntimePermission
     * @since   JDK1.0
     */
    public final ThreadGroup getParent() {
        if (parent != null)
            parent.checkAccess();
        return parent;
    }

    
Returns the maximum priority of this thread group. Threads that are
part of this group cannot have a higher priority than the maximum
priority.
See Also:
setMaxPriority
Returns:
 the maximum priority that a thread in this thread group
         can have.
Since:
  JDK1.0/**
     * Returns the maximum priority of this thread group. Threads that are
     * part of this group cannot have a higher priority than the maximum
     * priority.
     *
     * @return  the maximum priority that a thread in this thread group
     *          can have.
     * @see     #setMaxPriority
     * @since   JDK1.0
     */
    public final int getMaxPriority() {
        return maxPriority;
    }

    
Tests if this thread group is a daemon thread group. A
daemon thread group is automatically destroyed when its last
thread is stopped or its last thread group is destroyed.
Returns:
 true if this thread group is a daemon thread group;
         false otherwise.
Since:
  JDK1.0/**
     * Tests if this thread group is a daemon thread group. A
     * daemon thread group is automatically destroyed when its last
     * thread is stopped or its last thread group is destroyed.
     *
     * @return  <code>true</code> if this thread group is a daemon thread group;
     *          <code>false</code> otherwise.
     * @since   JDK1.0
     */
    public final boolean isDaemon() {
        return daemon;
    }

    
Tests if this thread group has been destroyed.
Returns:
 true if this object is destroyed
Since:
  JDK1.1/**
     * Tests if this thread group has been destroyed.
     *
     * @return  true if this object is destroyed
     * @since   JDK1.1
     */
    public synchronized boolean isDestroyed() {
        return destroyed;
    }

    
Changes the daemon status of this thread group.

First, the checkAccess method of this thread group is
called with no arguments; this may result in a security exception.

A daemon thread group is automatically destroyed when its last
thread is stopped or its last thread group is destroyed.
Params:
daemon –   if true, marks this thread group as
                     a daemon thread group; otherwise, marks this
                     thread group as normal.
Throws:
SecurityException –  if the current thread cannot modify
              this thread group.
See Also:
SecurityException
checkAccess()
Since:
     JDK1.0/**
     * Changes the daemon status of this thread group.
     * <p>
     * First, the <code>checkAccess</code> method of this thread group is
     * called with no arguments; this may result in a security exception.
     * <p>
     * A daemon thread group is automatically destroyed when its last
     * thread is stopped or its last thread group is destroyed.
     *
     * @param      daemon   if <code>true</code>, marks this thread group as
     *                      a daemon thread group; otherwise, marks this
     *                      thread group as normal.
     * @exception  SecurityException  if the current thread cannot modify
     *               this thread group.
     * @see        java.lang.SecurityException
     * @see        java.lang.ThreadGroup#checkAccess()
     * @since      JDK1.0
     */
    public final void setDaemon(boolean daemon) {
        checkAccess();
        this.daemon = daemon;
    }

    
Sets the maximum priority of the group. Threads in the thread
group that already have a higher priority are not affected.

First, the checkAccess method of this thread group is
called with no arguments; this may result in a security exception.

If the pri argument is less than Thread.MIN_PRIORITY or greater than Thread.MAX_PRIORITY, the maximum priority of the group remains unchanged. 

Otherwise, the priority of this ThreadGroup object is set to the
smaller of the specified pri and the maximum permitted
priority of the parent of this thread group. (If this thread group
is the system thread group, which has no parent, then its maximum
priority is simply set to pri.) Then this method is
called recursively, with pri as its argument, for
every thread group that belongs to this thread group.
Params:
pri –   the new priority of the thread group.
Throws:
SecurityException –  if the current thread cannot modify
              this thread group.
See Also:
getMaxPriority
SecurityException
checkAccess()
Since:
     JDK1.0/**
     * Sets the maximum priority of the group. Threads in the thread
     * group that already have a higher priority are not affected.
     * <p>
     * First, the <code>checkAccess</code> method of this thread group is
     * called with no arguments; this may result in a security exception.
     * <p>
     * If the <code>pri</code> argument is less than
     * {@link Thread#MIN_PRIORITY} or greater than
     * {@link Thread#MAX_PRIORITY}, the maximum priority of the group
     * remains unchanged.
     * <p>
     * Otherwise, the priority of this ThreadGroup object is set to the
     * smaller of the specified <code>pri</code> and the maximum permitted
     * priority of the parent of this thread group. (If this thread group
     * is the system thread group, which has no parent, then its maximum
     * priority is simply set to <code>pri</code>.) Then this method is
     * called recursively, with <code>pri</code> as its argument, for
     * every thread group that belongs to this thread group.
     *
     * @param      pri   the new priority of the thread group.
     * @exception  SecurityException  if the current thread cannot modify
     *               this thread group.
     * @see        #getMaxPriority
     * @see        java.lang.SecurityException
     * @see        java.lang.ThreadGroup#checkAccess()
     * @since      JDK1.0
     */
    public final void setMaxPriority(int pri) {
        int ngroupsSnapshot;
        ThreadGroup[] groupsSnapshot;
        synchronized (this) {
            checkAccess();
            if (pri < Thread.MIN_PRIORITY || pri > Thread.MAX_PRIORITY) {
                return;
            }
            maxPriority = (parent != null) ? Math.min(pri, parent.maxPriority) : pri;
            ngroupsSnapshot = ngroups;
            if (groups != null) {
                groupsSnapshot = Arrays.copyOf(groups, ngroupsSnapshot);
            } else {
                groupsSnapshot = null;
            }
        }
        for (int i = 0 ; i < ngroupsSnapshot ; i++) {
            groupsSnapshot[i].setMaxPriority(pri);
        }
    }

    
Tests if this thread group is either the thread group
argument or one of its ancestor thread groups.
Params:
g –   a thread group.
Returns:
 true if this thread group is the thread group
         argument or one of its ancestor thread groups;
         false otherwise.
Since:
  JDK1.0/**
     * Tests if this thread group is either the thread group
     * argument or one of its ancestor thread groups.
     *
     * @param   g   a thread group.
     * @return  <code>true</code> if this thread group is the thread group
     *          argument or one of its ancestor thread groups;
     *          <code>false</code> otherwise.
     * @since   JDK1.0
     */
    public final boolean parentOf(ThreadGroup g) {
        for (; g != null ; g = g.parent) {
            if (g == this) {
                return true;
            }
        }
        return false;
    }

    
Determines if the currently running thread has permission to
modify this thread group.

If there is a security manager, its checkAccess method
is called with this thread group as its argument. This may result
in throwing a SecurityException.
Throws:
SecurityException –  if the current thread is not allowed to
              access this thread group.
See Also:
SecurityManager.checkAccess(ThreadGroup)
Since:
     JDK1.0/**
     * Determines if the currently running thread has permission to
     * modify this thread group.
     * <p>
     * If there is a security manager, its <code>checkAccess</code> method
     * is called with this thread group as its argument. This may result
     * in throwing a <code>SecurityException</code>.
     *
     * @exception  SecurityException  if the current thread is not allowed to
     *               access this thread group.
     * @see        java.lang.SecurityManager#checkAccess(java.lang.ThreadGroup)
     * @since      JDK1.0
     */
    public final void checkAccess() {
        SecurityManager security = System.getSecurityManager();
        if (security != null) {
            security.checkAccess(this);
        }
    }

    
Returns an estimate of the number of active threads in this
thread group.  The result might not reflect concurrent activity,
and might be affected by the presence of certain system threads.

Due to the inherently imprecise nature of the result, it is
recommended that this method only be used for informational purposes.
Returns:
 an estimate of the number of active threads in this thread
         group and in any other thread group that has this thread
         group as an ancestor.
Since:
  JDK1.0/**
     * Returns an estimate of the number of active threads in this
     * thread group.  The result might not reflect concurrent activity,
     * and might be affected by the presence of certain system threads.
     * <p>
     * Due to the inherently imprecise nature of the result, it is
     * recommended that this method only be used for informational purposes.
     *
     * @return  an estimate of the number of active threads in this thread
     *          group and in any other thread group that has this thread
     *          group as an ancestor.
     * @since   JDK1.0
     */
    public int activeCount() {
        int result;
        // Snapshot sub-group data so we don't hold this lock
        // while our children are computing.
        int ngroupsSnapshot;
        ThreadGroup[] groupsSnapshot;
        synchronized (this) {
            if (destroyed) {
                return 0;
            }
            result = nthreads;
            ngroupsSnapshot = ngroups;
            if (groups != null) {
                groupsSnapshot = Arrays.copyOf(groups, ngroupsSnapshot);
            } else {
                groupsSnapshot = null;
            }
        }
        for (int i = 0 ; i < ngroupsSnapshot ; i++) {
            result += groupsSnapshot[i].activeCount();
        }
        return result;
    }

    
Copies into the specified array every active thread in this
thread group and its subgroups.

First, the checkAccess method of this thread group is
called with no arguments; this may result in a security exception.

An application might use the activeCount method to
get an estimate of how big the array should be, however if the
array is too short to hold all the threads, the extra threads are
silently ignored.  If it is critical to obtain every active
thread in this thread group and its subgroups, the caller should
verify that the returned int value is strictly less than the length
of list.

Due to the inherent race condition in this method, it is recommended
that the method only be used for informational purposes.
Params:
list –   an array into which to place the list of threads.
Throws:
SecurityException –  if the current thread does not
              have permission to enumerate this thread group.
See Also:
activeCount()
checkAccess()
Returns:
 the number of threads put into the array.
Since:
  JDK1.0/**
     * Copies into the specified array every active thread in this
     * thread group and its subgroups.
     * <p>
     * First, the <code>checkAccess</code> method of this thread group is
     * called with no arguments; this may result in a security exception.
     * <p>
     * An application might use the <code>activeCount</code> method to
     * get an estimate of how big the array should be, however <i>if the
     * array is too short to hold all the threads, the extra threads are
     * silently ignored.</i>  If it is critical to obtain every active
     * thread in this thread group and its subgroups, the caller should
     * verify that the returned int value is strictly less than the length
     * of <tt>list</tt>.
     * <p>
     * Due to the inherent race condition in this method, it is recommended
     * that the method only be used for informational purposes.
     *
     * @param   list   an array into which to place the list of threads.
     * @return  the number of threads put into the array.
     * @exception  SecurityException  if the current thread does not
     *               have permission to enumerate this thread group.
     * @see     java.lang.ThreadGroup#activeCount()
     * @see     java.lang.ThreadGroup#checkAccess()
     * @since   JDK1.0
     */
    public int enumerate(Thread list[]) {
        checkAccess();
        return enumerate(list, 0, true);
    }

    
Copies into the specified array every active thread in this
thread group. If the recurse flag is
true, references to every active thread in this
thread's subgroups are also included. If the array is too short to
hold all the threads, the extra threads are silently ignored.

First, the checkAccess method of this thread group is
called with no arguments; this may result in a security exception.

An application might use the activeCount method to
get an estimate of how big the array should be, however if the
array is too short to hold all the threads, the extra threads are
silently ignored.  If it is critical to obtain every active thread
in this thread group, the caller should verify that the returned int
value is strictly less than the length of list.

Due to the inherent race condition in this method, it is recommended
that the method only be used for informational purposes.
Params:
list –      an array into which to place the list of threads.
recurse –   a flag indicating whether also to include threads
                   in thread groups that are subgroups of this
                   thread group.
Throws:
SecurityException –  if the current thread does not
              have permission to enumerate this thread group.
See Also:
activeCount()
checkAccess()
Returns:
 the number of threads placed into the array.
Since:
  JDK1.0/**
     * Copies into the specified array every active thread in this
     * thread group. If the <code>recurse</code> flag is
     * <code>true</code>, references to every active thread in this
     * thread's subgroups are also included. If the array is too short to
     * hold all the threads, the extra threads are silently ignored.
     * <p>
     * First, the <code>checkAccess</code> method of this thread group is
     * called with no arguments; this may result in a security exception.
     * <p>
     * An application might use the <code>activeCount</code> method to
     * get an estimate of how big the array should be, however <i>if the
     * array is too short to hold all the threads, the extra threads are
     * silently ignored.</i>  If it is critical to obtain every active thread
     * in this thread group, the caller should verify that the returned int
     * value is strictly less than the length of <tt>list</tt>.
     * <p>
     * Due to the inherent race condition in this method, it is recommended
     * that the method only be used for informational purposes.
     *
     * @param   list      an array into which to place the list of threads.
     * @param   recurse   a flag indicating whether also to include threads
     *                    in thread groups that are subgroups of this
     *                    thread group.
     * @return  the number of threads placed into the array.
     * @exception  SecurityException  if the current thread does not
     *               have permission to enumerate this thread group.
     * @see     java.lang.ThreadGroup#activeCount()
     * @see     java.lang.ThreadGroup#checkAccess()
     * @since   JDK1.0
     */
    public int enumerate(Thread list[], boolean recurse) {
        checkAccess();
        return enumerate(list, 0, recurse);
    }

    private int enumerate(Thread list[], int n, boolean recurse) {
        int ngroupsSnapshot = 0;
        ThreadGroup[] groupsSnapshot = null;
        synchronized (this) {
            if (destroyed) {
                return 0;
            }
            int nt = nthreads;
            if (nt > list.length - n) {
                nt = list.length - n;
            }
            for (int i = 0; i < nt; i++) {
                if (threads[i].isAlive()) {
                    list[n++] = threads[i];
                }
            }
            if (recurse) {
                ngroupsSnapshot = ngroups;
                if (groups != null) {
                    groupsSnapshot = Arrays.copyOf(groups, ngroupsSnapshot);
                } else {
                    groupsSnapshot = null;
                }
            }
        }
        if (recurse) {
            for (int i = 0 ; i < ngroupsSnapshot ; i++) {
                n = groupsSnapshot[i].enumerate(list, n, true);
            }
        }
        return n;
    }

    
Returns an estimate of the number of active groups in this
thread group.  The result might not reflect concurrent activity.

Due to the inherently imprecise nature of the result, it is
recommended that this method only be used for informational purposes.
Returns:
 the number of active thread groups with this thread group as
         an ancestor.
Since:
  JDK1.0/**
     * Returns an estimate of the number of active groups in this
     * thread group.  The result might not reflect concurrent activity.
     * <p>
     * Due to the inherently imprecise nature of the result, it is
     * recommended that this method only be used for informational purposes.
     *
     * @return  the number of active thread groups with this thread group as
     *          an ancestor.
     * @since   JDK1.0
     */
    public int activeGroupCount() {
        int ngroupsSnapshot;
        ThreadGroup[] groupsSnapshot;
        synchronized (this) {
            if (destroyed) {
                return 0;
            }
            ngroupsSnapshot = ngroups;
            if (groups != null) {
                groupsSnapshot = Arrays.copyOf(groups, ngroupsSnapshot);
            } else {
                groupsSnapshot = null;
            }
        }
        int n = ngroupsSnapshot;
        for (int i = 0 ; i < ngroupsSnapshot ; i++) {
            n += groupsSnapshot[i].activeGroupCount();
        }
        return n;
    }

    
Copies into the specified array references to every active
subgroup in this thread group.

First, the checkAccess method of this thread group is
called with no arguments; this may result in a security exception.

An application might use the activeGroupCount method to
get an estimate of how big the array should be, however if the
array is too short to hold all the thread groups, the extra thread
groups are silently ignored.  If it is critical to obtain every
active subgroup in this thread group, the caller should verify that
the returned int value is strictly less than the length of
list.

Due to the inherent race condition in this method, it is recommended
that the method only be used for informational purposes.
Params:
list –   an array into which to place the list of thread groups.
Throws:
SecurityException –  if the current thread does not
              have permission to enumerate this thread group.
See Also:
activeGroupCount()
checkAccess()
Returns:
 the number of thread groups put into the array.
Since:
  JDK1.0/**
     * Copies into the specified array references to every active
     * subgroup in this thread group.
     * <p>
     * First, the <code>checkAccess</code> method of this thread group is
     * called with no arguments; this may result in a security exception.
     * <p>
     * An application might use the <code>activeGroupCount</code> method to
     * get an estimate of how big the array should be, however <i>if the
     * array is too short to hold all the thread groups, the extra thread
     * groups are silently ignored.</i>  If it is critical to obtain every
     * active subgroup in this thread group, the caller should verify that
     * the returned int value is strictly less than the length of
     * <tt>list</tt>.
     * <p>
     * Due to the inherent race condition in this method, it is recommended
     * that the method only be used for informational purposes.
     *
     * @param   list   an array into which to place the list of thread groups.
     * @return  the number of thread groups put into the array.
     * @exception  SecurityException  if the current thread does not
     *               have permission to enumerate this thread group.
     * @see     java.lang.ThreadGroup#activeGroupCount()
     * @see     java.lang.ThreadGroup#checkAccess()
     * @since   JDK1.0
     */
    public int enumerate(ThreadGroup list[]) {
        checkAccess();
        return enumerate(list, 0, true);
    }

    
Copies into the specified array references to every active
subgroup in this thread group. If the recurse flag is
true, references to all active subgroups of the
subgroups and so forth are also included.

First, the checkAccess method of this thread group is
called with no arguments; this may result in a security exception.

An application might use the activeGroupCount method to
get an estimate of how big the array should be, however if the
array is too short to hold all the thread groups, the extra thread
groups are silently ignored.  If it is critical to obtain every
active subgroup in this thread group, the caller should verify that
the returned int value is strictly less than the length of
list.

Due to the inherent race condition in this method, it is recommended
that the method only be used for informational purposes.
Params:
list –      an array into which to place the list of threads.
recurse –   a flag indicating whether to recursively enumerate
                   all included thread groups.
Throws:
SecurityException –  if the current thread does not
              have permission to enumerate this thread group.
See Also:
activeGroupCount()
checkAccess()
Returns:
 the number of thread groups put into the array.
Since:
  JDK1.0/**
     * Copies into the specified array references to every active
     * subgroup in this thread group. If the <code>recurse</code> flag is
     * <code>true</code>, references to all active subgroups of the
     * subgroups and so forth are also included.
     * <p>
     * First, the <code>checkAccess</code> method of this thread group is
     * called with no arguments; this may result in a security exception.
     * <p>
     * An application might use the <code>activeGroupCount</code> method to
     * get an estimate of how big the array should be, however <i>if the
     * array is too short to hold all the thread groups, the extra thread
     * groups are silently ignored.</i>  If it is critical to obtain every
     * active subgroup in this thread group, the caller should verify that
     * the returned int value is strictly less than the length of
     * <tt>list</tt>.
     * <p>
     * Due to the inherent race condition in this method, it is recommended
     * that the method only be used for informational purposes.
     *
     * @param   list      an array into which to place the list of threads.
     * @param   recurse   a flag indicating whether to recursively enumerate
     *                    all included thread groups.
     * @return  the number of thread groups put into the array.
     * @exception  SecurityException  if the current thread does not
     *               have permission to enumerate this thread group.
     * @see     java.lang.ThreadGroup#activeGroupCount()
     * @see     java.lang.ThreadGroup#checkAccess()
     * @since   JDK1.0
     */
    public int enumerate(ThreadGroup list[], boolean recurse) {
        checkAccess();
        return enumerate(list, 0, recurse);
    }

    private int enumerate(ThreadGroup list[], int n, boolean recurse) {
        int ngroupsSnapshot = 0;
        ThreadGroup[] groupsSnapshot = null;
        synchronized (this) {
            if (destroyed) {
                return 0;
            }
            int ng = ngroups;
            if (ng > list.length - n) {
                ng = list.length - n;
            }
            if (ng > 0) {
                System.arraycopy(groups, 0, list, n, ng);
                n += ng;
            }
            if (recurse) {
                ngroupsSnapshot = ngroups;
                if (groups != null) {
                    groupsSnapshot = Arrays.copyOf(groups, ngroupsSnapshot);
                } else {
                    groupsSnapshot = null;
                }
            }
        }
        if (recurse) {
            for (int i = 0 ; i < ngroupsSnapshot ; i++) {
                n = groupsSnapshot[i].enumerate(list, n, true);
            }
        }
        return n;
    }

    
Stops all threads in this thread group.

First, the checkAccess method of this thread group is
called with no arguments; this may result in a security exception.

This method then calls the stop method on all the
threads in this thread group and in all of its subgroups.
Throws:
SecurityException –  if the current thread is not allowed
              to access this thread group or any of the threads in
              the thread group.
See Also:
SecurityException
Thread.stop()
checkAccess()
Since:
     JDK1.0
Deprecated:
 This method is inherently unsafe. See Thread.stop for details./**
     * Stops all threads in this thread group.
     * <p>
     * First, the <code>checkAccess</code> method of this thread group is
     * called with no arguments; this may result in a security exception.
     * <p>
     * This method then calls the <code>stop</code> method on all the
     * threads in this thread group and in all of its subgroups.
     *
     * @exception  SecurityException  if the current thread is not allowed
     *               to access this thread group or any of the threads in
     *               the thread group.
     * @see        java.lang.SecurityException
     * @see        java.lang.Thread#stop()
     * @see        java.lang.ThreadGroup#checkAccess()
     * @since      JDK1.0
     * @deprecated    This method is inherently unsafe.  See
     *     {@link Thread#stop} for details.
     */
    @Deprecated
    public final void stop() {
        if (stopOrSuspend(false))
            Thread.currentThread().stop();
    }

    
Interrupts all threads in this thread group.

First, the checkAccess method of this thread group is
called with no arguments; this may result in a security exception.

This method then calls the interrupt method on all the
threads in this thread group and in all of its subgroups.
Throws:
SecurityException –  if the current thread is not allowed
              to access this thread group or any of the threads in
              the thread group.
See Also:
Thread.interrupt()
SecurityException
checkAccess()
Since:
     1.2/**
     * Interrupts all threads in this thread group.
     * <p>
     * First, the <code>checkAccess</code> method of this thread group is
     * called with no arguments; this may result in a security exception.
     * <p>
     * This method then calls the <code>interrupt</code> method on all the
     * threads in this thread group and in all of its subgroups.
     *
     * @exception  SecurityException  if the current thread is not allowed
     *               to access this thread group or any of the threads in
     *               the thread group.
     * @see        java.lang.Thread#interrupt()
     * @see        java.lang.SecurityException
     * @see        java.lang.ThreadGroup#checkAccess()
     * @since      1.2
     */
    public final void interrupt() {
        int ngroupsSnapshot;
        ThreadGroup[] groupsSnapshot;
        synchronized (this) {
            checkAccess();
            for (int i = 0 ; i < nthreads ; i++) {
                threads[i].interrupt();
            }
            ngroupsSnapshot = ngroups;
            if (groups != null) {
                groupsSnapshot = Arrays.copyOf(groups, ngroupsSnapshot);
            } else {
                groupsSnapshot = null;
            }
        }
        for (int i = 0 ; i < ngroupsSnapshot ; i++) {
            groupsSnapshot[i].interrupt();
        }
    }

    
Suspends all threads in this thread group.

First, the checkAccess method of this thread group is
called with no arguments; this may result in a security exception.

This method then calls the suspend method on all the
threads in this thread group and in all of its subgroups.
Throws:
SecurityException –  if the current thread is not allowed
              to access this thread group or any of the threads in
              the thread group.
See Also:
Thread.suspend()
SecurityException
checkAccess()
Since:
     JDK1.0
Deprecated:
 This method is inherently deadlock-prone. See Thread.suspend for details./**
     * Suspends all threads in this thread group.
     * <p>
     * First, the <code>checkAccess</code> method of this thread group is
     * called with no arguments; this may result in a security exception.
     * <p>
     * This method then calls the <code>suspend</code> method on all the
     * threads in this thread group and in all of its subgroups.
     *
     * @exception  SecurityException  if the current thread is not allowed
     *               to access this thread group or any of the threads in
     *               the thread group.
     * @see        java.lang.Thread#suspend()
     * @see        java.lang.SecurityException
     * @see        java.lang.ThreadGroup#checkAccess()
     * @since      JDK1.0
     * @deprecated    This method is inherently deadlock-prone.  See
     *     {@link Thread#suspend} for details.
     */
    @Deprecated
    public final void suspend() {
        if (stopOrSuspend(true))
            Thread.currentThread().suspend();
    }

    
Helper method: recursively stops or suspends (as directed by the
boolean argument) all of the threads in this thread group and its
subgroups, except the current thread.  This method returns true
if (and only if) the current thread is found to be in this thread
group or one of its subgroups.
/**
     * Helper method: recursively stops or suspends (as directed by the
     * boolean argument) all of the threads in this thread group and its
     * subgroups, except the current thread.  This method returns true
     * if (and only if) the current thread is found to be in this thread
     * group or one of its subgroups.
     */
    private boolean stopOrSuspend(boolean suspend) {
        boolean suicide = false;
        Thread us = Thread.currentThread();
        int ngroupsSnapshot;
        ThreadGroup[] groupsSnapshot = null;
        synchronized (this) {
            checkAccess();
            for (int i = 0 ; i < nthreads ; i++) {
                if (threads[i]==us)
                    suicide = true;
                else if (suspend)
                    threads[i].suspend();
                else
                    threads[i].stop();
            }

            ngroupsSnapshot = ngroups;
            if (groups != null) {
                groupsSnapshot = Arrays.copyOf(groups, ngroupsSnapshot);
            }
        }
        for (int i = 0 ; i < ngroupsSnapshot ; i++)
            suicide = groupsSnapshot[i].stopOrSuspend(suspend) || suicide;

        return suicide;
    }

    
Resumes all threads in this thread group.

First, the checkAccess method of this thread group is
called with no arguments; this may result in a security exception.

This method then calls the resume method on all the
threads in this thread group and in all of its sub groups.
Throws:
SecurityException –  if the current thread is not allowed to
              access this thread group or any of the threads in the
              thread group.
See Also:
SecurityException
Thread.resume()
checkAccess()
Since:
     JDK1.0
Deprecated:
   This method is used solely in conjunction with
     Thread.suspend and ThreadGroup.suspend, both of which have been deprecated, as they are inherently deadlock-prone. See Thread.suspend for details./**
     * Resumes all threads in this thread group.
     * <p>
     * First, the <code>checkAccess</code> method of this thread group is
     * called with no arguments; this may result in a security exception.
     * <p>
     * This method then calls the <code>resume</code> method on all the
     * threads in this thread group and in all of its sub groups.
     *
     * @exception  SecurityException  if the current thread is not allowed to
     *               access this thread group or any of the threads in the
     *               thread group.
     * @see        java.lang.SecurityException
     * @see        java.lang.Thread#resume()
     * @see        java.lang.ThreadGroup#checkAccess()
     * @since      JDK1.0
     * @deprecated    This method is used solely in conjunction with
     *      <tt>Thread.suspend</tt> and <tt>ThreadGroup.suspend</tt>,
     *       both of which have been deprecated, as they are inherently
     *       deadlock-prone.  See {@link Thread#suspend} for details.
     */
    @Deprecated
    public final void resume() {
        int ngroupsSnapshot;
        ThreadGroup[] groupsSnapshot;
        synchronized (this) {
            checkAccess();
            for (int i = 0 ; i < nthreads ; i++) {
                threads[i].resume();
            }
            ngroupsSnapshot = ngroups;
            if (groups != null) {
                groupsSnapshot = Arrays.copyOf(groups, ngroupsSnapshot);
            } else {
                groupsSnapshot = null;
            }
        }
        for (int i = 0 ; i < ngroupsSnapshot ; i++) {
            groupsSnapshot[i].resume();
        }
    }

    
Destroys this thread group and all of its subgroups. This thread
group must be empty, indicating that all threads that had been in
this thread group have since stopped.

First, the checkAccess method of this thread group is
called with no arguments; this may result in a security exception.
Throws:
IllegalThreadStateException –  if the thread group is not
              empty or if the thread group has already been destroyed.
SecurityException –  if the current thread cannot modify this
              thread group.
See Also:
checkAccess()
Since:
     JDK1.0/**
     * Destroys this thread group and all of its subgroups. This thread
     * group must be empty, indicating that all threads that had been in
     * this thread group have since stopped.
     * <p>
     * First, the <code>checkAccess</code> method of this thread group is
     * called with no arguments; this may result in a security exception.
     *
     * @exception  IllegalThreadStateException  if the thread group is not
     *               empty or if the thread group has already been destroyed.
     * @exception  SecurityException  if the current thread cannot modify this
     *               thread group.
     * @see        java.lang.ThreadGroup#checkAccess()
     * @since      JDK1.0
     */
    public final void destroy() {
        int ngroupsSnapshot;
        ThreadGroup[] groupsSnapshot;
        synchronized (this) {
            checkAccess();
            if (destroyed || (nthreads > 0)) {
                throw new IllegalThreadStateException();
            }
            ngroupsSnapshot = ngroups;
            if (groups != null) {
                groupsSnapshot = Arrays.copyOf(groups, ngroupsSnapshot);
            } else {
                groupsSnapshot = null;
            }
            if (parent != null) {
                destroyed = true;
                ngroups = 0;
                groups = null;
                nthreads = 0;
                threads = null;
            }
        }
        for (int i = 0 ; i < ngroupsSnapshot ; i += 1) {
            groupsSnapshot[i].destroy();
        }
        if (parent != null) {
            parent.remove(this);
        }
    }

    
Adds the specified Thread group to this group.
Params:
g – the specified Thread group to be added
Throws:
IllegalThreadStateException – If the Thread group has been destroyed./**
     * Adds the specified Thread group to this group.
     * @param g the specified Thread group to be added
     * @exception IllegalThreadStateException If the Thread group has been destroyed.
     */
    private final void add(ThreadGroup g){
        synchronized (this) {
            if (destroyed) {
                throw new IllegalThreadStateException();
            }
            if (groups == null) {
                groups = new ThreadGroup[4];
            } else if (ngroups == groups.length) {
                groups = Arrays.copyOf(groups, ngroups * 2);
            }
            groups[ngroups] = g;

            // This is done last so it doesn't matter in case the
            // thread is killed
            ngroups++;
        }
    }

    
Removes the specified Thread group from this group.
Params:
g – the Thread group to be removed
Returns:
if this Thread has already been destroyed./**
     * Removes the specified Thread group from this group.
     * @param g the Thread group to be removed
     * @return if this Thread has already been destroyed.
     */
    private void remove(ThreadGroup g) {
        synchronized (this) {
            if (destroyed) {
                return;
            }
            for (int i = 0 ; i < ngroups ; i++) {
                if (groups[i] == g) {
                    ngroups -= 1;
                    System.arraycopy(groups, i + 1, groups, i, ngroups - i);
                    // Zap dangling reference to the dead group so that
                    // the garbage collector will collect it.
                    groups[ngroups] = null;
                    break;
                }
            }
            if (nthreads == 0) {
                notifyAll();
            }
            if (daemon && (nthreads == 0) &&
                (nUnstartedThreads == 0) && (ngroups == 0))
            {
                destroy();
            }
        }
    }


    
Increments the count of unstarted threads in the thread group.
Unstarted threads are not added to the thread group so that they
can be collected if they are never started, but they must be
counted so that daemon thread groups with unstarted threads in
them are not destroyed.
/**
     * Increments the count of unstarted threads in the thread group.
     * Unstarted threads are not added to the thread group so that they
     * can be collected if they are never started, but they must be
     * counted so that daemon thread groups with unstarted threads in
     * them are not destroyed.
     */
    void addUnstarted() {
        synchronized(this) {
            if (destroyed) {
                throw new IllegalThreadStateException();
            }
            nUnstartedThreads++;
        }
    }

    
Adds the specified Thread to this group.
Params:
t – the Thread to be added
Throws:
IllegalThreadStateException – If the Thread group has been destroyed./**
     * Adds the specified Thread to this group.
     * @param t the Thread to be added
     * @exception IllegalThreadStateException If the Thread group has been destroyed.
     */
    void add(Thread t) {
        synchronized (this) {
            if (destroyed) {
                throw new IllegalThreadStateException();
            }
            if (threads == null) {
                threads = new Thread[4];
            } else if (nthreads == threads.length) {
                threads = Arrays.copyOf(threads, nthreads * 2);
            }
            threads[nthreads] = t;

            // This is done last so it doesn't matter in case the
            // thread is killed
            nthreads++;
            nUnstartedThreads--;
        }
    }

    
Removes the specified Thread from this group.
Params:
t – the Thread to be removed
Returns:
if the Thread has already been destroyed./**
     * Removes the specified Thread from this group.
     * @param t the Thread to be removed
     * @return if the Thread has already been destroyed.
     */
    void remove(Thread t) {
        synchronized (this) {
            if (destroyed) {
                return;
            }
            for (int i = 0 ; i < nthreads ; i++) {
                if (threads[i] == t) {
                    System.arraycopy(threads, i + 1, threads, i, --nthreads - i);
                    // Zap dangling reference to the dead thread so that
                    // the garbage collector will collect it.
                    threads[nthreads] = null;
                    break;
                }
            }
            if (nthreads == 0) {
                notifyAll();
            }
            if (daemon && (nthreads == 0) &&
                (nUnstartedThreads == 0) && (ngroups == 0))
            {
                destroy();
            }
        }
    }

    
Prints information about this thread group to the standard
output. This method is useful only for debugging.
Since:
  JDK1.0/**
     * Prints information about this thread group to the standard
     * output. This method is useful only for debugging.
     *
     * @since   JDK1.0
     */
    public void list() {
        list(System.out, 0);
    }
    void list(PrintStream out, int indent) {
        int ngroupsSnapshot;
        ThreadGroup[] groupsSnapshot;
        synchronized (this) {
            for (int j = 0 ; j < indent ; j++) {
                out.print(" ");
            }
            out.println(this);
            indent += 4;
            for (int i = 0 ; i < nthreads ; i++) {
                for (int j = 0 ; j < indent ; j++) {
                    out.print(" ");
                }
                out.println(threads[i]);
            }
            ngroupsSnapshot = ngroups;
            if (groups != null) {
                groupsSnapshot = Arrays.copyOf(groups, ngroupsSnapshot);
            } else {
                groupsSnapshot = null;
            }
        }
        for (int i = 0 ; i < ngroupsSnapshot ; i++) {
            groupsSnapshot[i].list(out, indent);
        }
    }

    
Called by the Java Virtual Machine when a thread in this thread group stops because of an uncaught exception, and the thread does not have a specific UncaughtExceptionHandler installed. 

The uncaughtException method of
ThreadGroup does the following:

If this thread group has a parent thread group, the
    uncaughtException method of that parent is called
    with the same two arguments.
Otherwise, this method checks to see if there is a default
    uncaught exception handler installed, and if so, its uncaughtException method is called with the same
    two arguments.
Otherwise, this method determines if the Throwable argument is an instance of ThreadDeath. If so, nothing special is done. Otherwise, a message containing the thread's name, as returned from the thread's getName method, and a stack backtrace, using the Throwable's printStackTrace method, is printed to the standard error stream. 

Applications can override this method in subclasses of
ThreadGroup to provide alternative handling of
uncaught exceptions.
Params:
t –   the thread that is about to exit.
e –   the uncaught exception.
Since:
  JDK1.0/**
     * Called by the Java Virtual Machine when a thread in this
     * thread group stops because of an uncaught exception, and the thread
     * does not have a specific {@link Thread.UncaughtExceptionHandler}
     * installed.
     * <p>
     * The <code>uncaughtException</code> method of
     * <code>ThreadGroup</code> does the following:
     * <ul>
     * <li>If this thread group has a parent thread group, the
     *     <code>uncaughtException</code> method of that parent is called
     *     with the same two arguments.
     * <li>Otherwise, this method checks to see if there is a
     *     {@linkplain Thread#getDefaultUncaughtExceptionHandler default
     *     uncaught exception handler} installed, and if so, its
     *     <code>uncaughtException</code> method is called with the same
     *     two arguments.
     * <li>Otherwise, this method determines if the <code>Throwable</code>
     *     argument is an instance of {@link ThreadDeath}. If so, nothing
     *     special is done. Otherwise, a message containing the
     *     thread's name, as returned from the thread's {@link
     *     Thread#getName getName} method, and a stack backtrace,
     *     using the <code>Throwable</code>'s {@link
     *     Throwable#printStackTrace printStackTrace} method, is
     *     printed to the {@linkplain System#err standard error stream}.
     * </ul>
     * <p>
     * Applications can override this method in subclasses of
     * <code>ThreadGroup</code> to provide alternative handling of
     * uncaught exceptions.
     *
     * @param   t   the thread that is about to exit.
     * @param   e   the uncaught exception.
     * @since   JDK1.0
     */
    public void uncaughtException(Thread t, Throwable e) {
        if (parent != null) {
            parent.uncaughtException(t, e);
        } else {
            Thread.UncaughtExceptionHandler ueh =
                Thread.getDefaultUncaughtExceptionHandler();
            if (ueh != null) {
                ueh.uncaughtException(t, e);
            } else if (!(e instanceof ThreadDeath)) {
                System.err.print("Exception in thread \""
                                 + t.getName() + "\" ");
                e.printStackTrace(System.err);
            }
        }
    }

    
Used by VM to control lowmem implicit suspension.
Params:
b – boolean to allow or disallow suspension
Returns:
true on success
Since:
  JDK1.1
Deprecated:
The definition of this call depends on suspend, which is deprecated. Further, the behavior of this call was never specified./**
     * Used by VM to control lowmem implicit suspension.
     *
     * @param b boolean to allow or disallow suspension
     * @return true on success
     * @since   JDK1.1
     * @deprecated The definition of this call depends on {@link #suspend},
     *             which is deprecated.  Further, the behavior of this call
     *             was never specified.
     */
    @Deprecated
    public boolean allowThreadSuspension(boolean b) {
        this.vmAllowSuspension = b;
        if (!b) {
            VM.unsuspendSomeThreads();
        }
        return true;
    }

    
Returns a string representation of this Thread group.
Returns:
 a string representation of this thread group.
Since:
  JDK1.0/**
     * Returns a string representation of this Thread group.
     *
     * @return  a string representation of this thread group.
     * @since   JDK1.0
     */
    public String toString() {
        return getClass().getName() + "[name=" + getName() + ",maxpri=" + maxPriority + "]";
    }
}