ThreadGroup.java

/*
 * Copyright (c) 1995, 2012, 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;

/**
 * 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.
 */
/*
 * ThreadGroup用来管理一组线程和子线程组，每个ThreadGroup在进程中以树形方式存在。
 * 通常情况下根线程组是system线程组，system线程组下是main线程组，接下来是经由main线程组创建出来的自定义线程组。
 */
public class ThreadGroup implements Thread.UncaughtExceptionHandler {
    
    /*▼ 存储子线程组和线程 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
    
    ThreadGroup[] groups;   // 存储子线程组
    int ngroups;    // 子线程组数量
    
    Thread[] threads;   // 存储当前线程组内包含的线程
    int nthreads;   // 当前线程组内包含的线程数量
    
    int nUnstartedThreads = 0; // 创建了但还未start()的线程
    
    /*▲ 存储子线程组和线程 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
    
    
    /*▼ 线程组属性 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
    
    String name;    // 线程组名称
    int maxPriority; // 线程组优先级
    boolean daemon; // 是否为守护线程
    private final ThreadGroup parent; // 父线程组
    
    /*▲ 线程组属性 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
    
    
    /*▼ 线程组状态 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┓ */
    
    boolean destroyed;  // 当前线程组是否已销毁
    
    /*▲ 线程组状态 ▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓▓┛ */
    
    
    
    /*▼ 构造方法 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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
     */
    // 创建指定名称的子线程组
    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
     */
    // 在指定的父线程组parent和name的情形下创建子线程组
    public ThreadGroup(ThreadGroup parent, String name) {
        this(checkParentAccess(parent), parent, name);
    }
    
    /**
     * Creates an empty Thread group that is not in any Thread group.
     * This method is used to create the system Thread group.
     */
    // 由底层代码调用，创建根线程组
    private ThreadGroup() {
        // called from C code
        this.name = "system";
        this.maxPriority = Thread.MAX_PRIORITY;
        this.parent = null;
    }
    
    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);   // 创建完子线程组后要将其添加到父线程组数组中
    }
    
    /*▲ 构造方法 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 线程组属性 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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 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;
    }
    
    /**
     * 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
     */
    // 递归设置线程组的优先级。子线程组优先级不超过父线程组
    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 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;
    }
    
    /**
     * 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;
    }
    
    /**
     * 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;
    }
    
    /*▲ 线程组属性 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 线程组状态 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    
    /**
     * 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} 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.
     * @throws SecurityException           if the current thread cannot modify this
     *                                     thread group.
     * @see java.lang.ThreadGroup#checkAccess()
     * @since 1.0
     */
    // 销毁已经不包含线程的线程组。需要递归清理当前线程组及其子线程组。
    public final void destroy() {
        int ngroupsSnapshot;
        
        ThreadGroup[] groupsSnapshot;
        
        synchronized(this) {
            checkAccess();
            // 禁止重复销毁，禁止销毁仍然包含线程的线程组
            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);
        }
    }
    
    /**
     * Tests if this thread group has been destroyed.
     *
     * @return true if this object is destroyed
     *
     * @since 1.1
     */
    // 当前线程组及其子线程组是否已销毁
    public synchronized boolean isDestroyed() {
        return destroyed;
    }
    
    /**
     * Interrupts all threads in 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>
     * This method then calls the {@code 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 java.lang.Thread#interrupt()
     * @see java.lang.SecurityException
     * @see java.lang.ThreadGroup#checkAccess()
     * @since 1.2
     */
    // 中断线程。递归中断当前线程组及其子线程组内所有线程
    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();
        }
    }
    
    
    /**
     * Stops all threads in 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>
     * This method then calls the {@code 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 java.lang.SecurityException
     * @see java.lang.Thread#stop()
     * @see java.lang.ThreadGroup#checkAccess()
     * @since 1.0
     * @deprecated This method is inherently unsafe.  See
     * {@link Thread#stop} for details.
     */
    @Deprecated(since = "1.2")
    public final void stop() {
        if(stopOrSuspend(false))
            Thread.currentThread().stop();
    }
    
    /**
     * Suspends all threads in 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>
     * This method then calls the {@code 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 java.lang.Thread#suspend()
     * @see java.lang.SecurityException
     * @see java.lang.ThreadGroup#checkAccess()
     * @since 1.0
     * @deprecated This method is inherently deadlock-prone.  See
     * {@link Thread#suspend} for details.
     */
    @Deprecated(since = "1.2")
    @SuppressWarnings("deprecation")
    public final void suspend() {
        if(stopOrSuspend(true))
            Thread.currentThread().suspend();
    }
    
    /**
     * Used by VM to control lowmem implicit suspension.
     *
     * @param b boolean to allow or disallow suspension
     *
     * @return true on success
     *
     * @since 1.1
     * @deprecated The definition of this call depends on {@link #suspend},
     * which is deprecated.  Further, the behavior of this call
     * was never specified.
     */
    @Deprecated(since = "1.2")
    public boolean allowThreadSuspension(boolean b) {
        return true;
    }
    
    /**
     * Resumes all threads in 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>
     * This method then calls the {@code 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 java.lang.SecurityException
     * @see java.lang.Thread#resume()
     * @see java.lang.ThreadGroup#checkAccess()
     * @since 1.0
     * @deprecated This method is used solely in conjunction with
     * {@code Thread.suspend} and {@code ThreadGroup.suspend},
     * both of which have been deprecated, as they are inherently
     * deadlock-prone.  See {@link Thread#suspend} for details.
     */
    @Deprecated(since = "1.2")
    @SuppressWarnings("deprecation")
    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();
        }
    }
    
    /**
     * 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.
     */
    @SuppressWarnings("deprecation")
    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;
    }
    
    /*▲ 线程组状态 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 统计 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * 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.
     */
    // 计数，统计当前线程组内还未start()的线程数量
    void addUnstarted() {
        synchronized(this) {
            if(destroyed) {
                throw new IllegalThreadStateException();
            }
            nUnstartedThreads++;
        }
    }
    
    /**
     * 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
     */
    // 活跃线程数量的快照。递归统计当前线程组及其子线程组内的线程数量。（线程数量会动态变化）
    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;
    }
    
    /**
     * 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
     */
    // 活跃线程组数量的快照。递归统计当前线程组的子线程组数量。（线程组数量会动态变化）
    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;
    }
    
    /*▲ 统计 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼ 管理 ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Adds the specified thread to this thread group.
     *
     * <p> Note: This method is called from both library code
     * and the Virtual Machine. It is called from VM to add
     * certain system threads to the system thread group.
     *
     * @param t the Thread to be added
     *
     * @throws IllegalThreadStateException if the Thread group has been destroyed
     */
    // 将线程t加入到当前线程组
    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++;
            
            // The thread is now a fully fledged member of the group, even though it may, or may not, have been started yet.
            // It will prevent the group from being destroyed so the unstarted Threads count is decremented.
            nUnstartedThreads--;
        }
    }
    
    /**
     * Adds the specified Thread group to this group.
     *
     * @param g the specified Thread group to be added
     *
     * @throws IllegalThreadStateException If the Thread group has been destroyed.
     */
    // 向当前线程组内添加子线程组g
    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) {
                // 翻倍扩容
                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 from this group. Invoking this method
     * on a thread group that has been destroyed has no effect.
     *
     * @param t the Thread to be removed
     */
    // 将线程t从当前线程组中移除
    private 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;
                }
            }
        }
    }
    
    /**
     * Removes the specified Thread group from this group.
     *
     * @param g the Thread group to be removed
     *
     * @return if this Thread has already been destroyed.
     */
    // 将线程组g从当前线程组中移除
    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();
            }
        }
    }
    
    /*
     * ▶ 1 将当前线程组存活的线程复制到数组list中。
     * 参数n为list的下标，代表存入线程时用到的起始下标。
     * 参数recurse表示是否递归存储。
     * 如果需要递归存储，意味着子线程组中的线程也会被存入数组list
     */
    private int enumerate(Thread[] list, int n, boolean recurse) {
        int ngroupsSnapshot = 0;
        ThreadGroup[] groupsSnapshot = null;
        
        synchronized(this) {
            if(destroyed) {
                return 0;
            }
            
            int nt = nthreads;
            // 如果提供的存储空间不足以存储所有存活线程，那就只存储一部分
            if(nt>list.length - n) {
                nt = list.length - n;
            }
            
            // 将指定数量的存活线程存入数组list
            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;
    }
    
    /**
     * 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
     */
    // ▶ 1-1 将线程组中存活的线程递归遍历后存入数组list
    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
     */
    // ▶ 1-2 将线程组中存活的线程存入数组list，参数recurse指示是否递归存入子线程组所有线程
    public int enumerate(Thread list[], boolean recurse) {
        checkAccess();
        return enumerate(list, 0, recurse);
    }
    
    /*
     * ▶ 2 将当前线程组包含的子线程复制到数组list中。
     * 参数n为list的下标，代表存入线程组时用到的起始下标。
     * 参数recurse表示是否递归存储。
     * 如果需要递归存储，意味着子线程组中的线程组也会被存入数组list
     */
    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;
    }
    
    /**
     * 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
     */
    // ▶ 2-1 将线程组中的子线程组递归遍历后存入数组list
    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 {@code recurse} is
     * {@code true}, this method recursively enumerates all subgroups of this
     * thread group and references to every active thread group in these
     * subgroups are also included.
     *
     * <p> An application might use the
     * {@linkplain #activeGroupCount activeGroupCount} 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
     * {@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 thread groups
     * @param recurse if {@code true}, recursively enumerate all subgroups
     *
     * @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
     */
    // ▶ 2-2 将线程组中的子线程组存入数组list，参数recurse指示是否递归存入子线程组所有线程组
    public int enumerate(ThreadGroup list[], boolean recurse) {
        checkAccess();
        return enumerate(list, 0, recurse);
    }
    
    /*▲ 管理 ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /*▼  ████████████████████████████████████████████████████████████████████████████████┓ */
    
    /**
     * Prints information about this thread group to the standard
     * output. This method is useful only for debugging.
     *
     * @since 1.0
     */
    // 将当前线程组及其子线程组内的线程信息递归输出到标准输出流
    public void list() {
        list(System.out, 0);
    }
    
    // 递归列出当前线程组及其子线程组内的线程信息
    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);
        }
    }
    
    /**
     * Returns a string representation of this Thread group.
     *
     * @return a string representation of this thread group.
     *
     * @since 1.0
     */
    public String toString() {
        return getClass().getName() + "[name=" + getName() + ",maxpri=" + maxPriority + "]";
    }
    
    /*▲  ████████████████████████████████████████████████████████████████████████████████┛ */
    
    
    
    /**
     * 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
     */
    // 判断当前线程组是否为线程组g的祖先
    public final boolean parentOf(ThreadGroup g) {
        for(; g != null; g = g.parent) {
            if(g == this) {
                return true;
            }
        }
        return false;
    }
    
    /**
     * 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} method of
     * {@code ThreadGroup} does the following:
     * <ul>
     * <li>If this thread group has a parent thread group, the
     * {@code uncaughtException} 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} method is called with the same
     * two arguments.
     * <li>Otherwise, this method determines if the {@code Throwable}
     * 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}'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} to provide alternative handling of
     * uncaught exceptions.
     *
     * @param t the thread that is about to exit.
     * @param e the uncaught exception.
     *
     * @since 1.0
     */
    // 处理未捕获的异常
    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);
            }
        }
    }
    
    /**
     * Notifies the group that the thread {@code t} has failed
     * an attempt to start.
     *
     * <p> The state of this thread group is rolled back as if the
     * attempt to start the thread has never occurred. The thread is again
     * considered an unstarted member of the thread group, and a subsequent
     * attempt to start the thread is permitted.
     *
     * @param t the Thread whose start method was invoked
     */
    // 线程启动失败，将其从线程组中删除，未启动线程数量重新加一
    void threadStartFailed(Thread t) {
        synchronized(this) {
            remove(t);
            nUnstartedThreads++;
        }
    }
    
    /**
     * Notifies the group that the thread {@code t} has terminated.
     *
     * <p> Destroy the group if all of the following conditions are
     * true: this is a daemon thread group; there are no more alive
     * or unstarted threads in the group; there are no subgroups in
     * this thread group.
     *
     * @param t the Thread that has terminated
     */
    // 线程结束，将其从线程组中移除
    void threadTerminated(Thread t) {
        synchronized(this) {
            remove(t);
            
            // 线程组中已经没有其他线程
            if(nthreads == 0) {
                notifyAll();
            }
            
            if(daemon && (nthreads == 0) && (nUnstartedThreads == 0) && (ngroups == 0)) {
                destroy();
            }
        }
    }
    
    
    /**
     * 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);
        }
    }
    
    /**
     * @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;
    }
    
}