/*

 * Copyright (c) 1995, 2019, 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

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 */

package java.lang;

import java.io.PrintStream;

import java.util.Arrays;

/**

 * 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   1.0

 */

/* 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.

 */

    private final ThreadGroup parent;

    String name;

    int maxPriority;

    boolean destroyed;

    boolean daemon;

    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.

     */

    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.

     * <p>

     * The {@code checkAccess} 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.

     * @throws  SecurityException  if the current thread cannot create a

     *               thread in the specified thread group.

     * @see     java.lang.ThreadGroup#checkAccess()

     * @since   1.0

     */

    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.

     * <p>

     * The {@code checkAccess} 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.

     * @throws    NullPointerException  if the thread group argument is

     *               {@code null}.

     * @throws    SecurityException  if the current thread cannot create a

     *               thread in the specified thread group.

     * @see     java.lang.SecurityException

     * @see     java.lang.ThreadGroup#checkAccess()

     * @since   1.0

     */

    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.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.

     *

     * @return  the name of this thread group.

     * @since   1.0

     */

    public final String getName() {

        return name;

    }

    /**

     * Returns the parent of this thread group.

     * <p>

     * First, if the parent is not {@code null}, the

     * {@code checkAccess} 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}.

     * @throws  SecurityException  if the current thread cannot modify

     *               this thread group.

     * @see        java.lang.ThreadGroup#checkAccess()

     * @see        java.lang.SecurityException

     * @see        java.lang.RuntimePermission

     * @since   1.0

     */

    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.

     *

     * @return  the maximum priority that a thread in this thread group

     *          can have.

     * @see     #setMaxPriority

     * @since   1.0

     */

    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.

     *

     * @return  {@code true} if this thread group is a daemon thread group;

     *          {@code false} otherwise.

     * @since   1.0

     */

    public final boolean isDaemon() {

        return daemon;

    }

    /**

     * Tests if this thread group has been destroyed.

     *

     * @return  true if this object is destroyed

     * @since   1.1

     */

    public synchronized boolean isDestroyed() {

        return destroyed;

    }

    /**

     * Changes the daemon status of this thread group.

     * <p>

     * First, the {@code checkAccess} 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}, 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        java.lang.SecurityException

     * @see        java.lang.ThreadGroup#checkAccess()

     * @since      1.0

     */

    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.

     * <p>

     * First, the {@code checkAccess} method of this thread group is

     * called with no arguments; this may result in a security exception.

     * <p>

     * If the {@code pri} 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} 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}.) Then this method is

     * called recursively, with {@code pri} as its argument, for

     * every thread group that belongs to this thread group.

     *

     * @param      pri   the new priority of the thread group.

     * @throws     SecurityException  if the current thread cannot modify

     *               this thread group.

     * @see        #getMaxPriority

     * @see        java.lang.SecurityException

     * @see        java.lang.ThreadGroup#checkAccess()

     * @since      1.0

     */

    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.

     *

     * @param   g   a thread group.

     * @return  {@code true} if this thread group is the thread group

     *          argument or one of its ancestor thread groups;

     *          {@code false} otherwise.

     * @since   1.0

     */

    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.

     * <p>

     * If there is a security manager, its {@code checkAccess} method

     * is called with this thread group as its argument. This may result

     * in throwing a {@code SecurityException}.

     *

     * @throws     SecurityException  if the current thread is not allowed to

     *               access this thread group.

     * @see        java.lang.SecurityManager#checkAccess(java.lang.ThreadGroup)

     * @since      1.0

     */

    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 and its subgroups. Recursively iterates over all subgroups in

     * this thread group.

     *

     * <p> The value returned is only an estimate because the number of

     * threads may change dynamically while this method traverses internal

     * data structures, and might be affected by the presence of certain

     * system threads. This method is intended primarily for debugging

     * and monitoring 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   1.0

     */

    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.

     *

     * <p> An invocation of this method behaves in exactly the same

     * way as the invocation

     *

     * <blockquote>

     * {@linkplain #enumerate(Thread[], boolean) enumerate}{@code (list, true)}

     * </blockquote>

     *

     * @param  list

     *         an array into which to put the list of threads

     *

     * @return  the number of threads put into the array

     *

     * @throws  SecurityException

     *          if {@linkplain #checkAccess checkAccess} determines that

     *          the current thread cannot access this thread group

     *

     * @since   1.0

     */

    public int enumerate(Thread list[]) {

        checkAccess();

        return enumerate(list, 0, true);

    }

    /**

     * Copies into the specified array every active thread in this

     * thread group. If {@code recurse} is {@code true},

     * this method recursively enumerates all subgroups of this

     * thread group and references to every active thread in these

     * subgroups are also included. If the array is too short to

     * hold all the threads, the extra threads are silently ignored.

     *

     * <p> An application might use the {@linkplain #activeCount activeCount}

     * 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 {@code list}.

     *

     * <p> Due to the inherent race condition in this method, it is recommended

     * that the method only be used for debugging and monitoring purposes.

     *

     * @param  list

     *         an array into which to put the list of threads

     *

     * @param  recurse

     *         if {@code true}, recursively enumerate all subgroups of this

     *         thread group

     *

     * @return  the number of threads put into the array

     *

     * @throws  SecurityException

     *          if {@linkplain #checkAccess checkAccess} determines that

     *          the current thread cannot access this thread group

     *

     * @since   1.0

     */

    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 n;

            }

            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 and its subgroups. Recursively iterates over

     * all subgroups in this thread group.

     *

     * <p> The value returned is only an estimate because the number of

     * thread groups may change dynamically while this method traverses

     * internal data structures. This method is intended primarily for

     * debugging and monitoring purposes.

     *

     * @return  the number of active thread groups with this thread group as

     *          an ancestor

     *

     * @since   1.0

     */

    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 and its subgroups.

     *

     * <p> An invocation of this method behaves in exactly the same

     * way as the invocation

     *

     * <blockquote>

     * {@linkplain #enumerate(ThreadGroup[], boolean) enumerate}{@code (list, true)}

     * </blockquote>

     *

     * @param  list

     *         an array into which to put the list of thread groups

     *

     * @return  the number of thread groups put into the array

     *

     * @throws  SecurityException

     *          if {@linkplain #checkAccess checkAccess} determines that

     *          the current thread cannot access this thread group

     *

     * @since   1.0

     */

    public int enumerate(ThreadGroup list[]) {

        checkAccess();

        return enumerate(list, 0, true);

    }

    /**

     * Copies into the specified array references to every active