// 继承了AbstractExecutorService，AbstractExecutorService定义了基本的任务提交、执行等方法
public class ThreadPoolExecutor extends AbstractExecutorService {
    // 用于表示线程池的运行状态以及当前活动的线程
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    // 一个Integer是32bit, 3位用于表示运行状态，其余用于表示活动线程数
    private static final int COUNT_BITS = Integer.SIZE - 3;
    // 线程数最大值
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;

    // runState is stored in the high-order bits
    private static final int RUNNING    = -1 << COUNT_BITS;
    private static final int SHUTDOWN   =  0 << COUNT_BITS;
    private static final int STOP       =  1 << COUNT_BITS;
    private static final int TIDYING    =  2 << COUNT_BITS;
    private static final int TERMINATED =  3 << COUNT_BITS;

    // 任务队列
    private final BlockingQueue<Runnable> workQueue;
    // 显示锁，用于控制workers的访问等
    private final ReentrantLock mainLock = new ReentrantLock(); 
    // 存储所有的工作线程，只有在mainLock下才能访问
    private final HashSet<Worker> workers = new HashSet<Worker>();     
    // 等待中断条件
    private final Condition termination = mainLock.newCondition();
    // 用于记录线程池中最大的活动线程数
    private int largestPoolSize;  
    // 任务完成数量
    private long completedTaskCount;    
    // 线程工厂
    private volatile ThreadFactory threadFactory;
    // 拒绝策略
    private volatile RejectedExecutionHandler handler;
    // 空闲线程等待工作时间
    private volatile long keepAliveTime;
    // 为true时使用keepAliveTime作为超时等待，为false所有核心线程（包含空闲的）一直保持存活
    private volatile boolean allowCoreThreadTimeOut;    
    // 核心线程数
    private volatile int corePoolSize;
    // 最大线程数
    private volatile int maximumPoolSize;
    // 默认任务拒绝策略，对拒绝的任务抛出异常
    private static final RejectedExecutionHandler defaultHandler =
        new AbortPolicy();  
    // 安全控制访问（主要用于shutdown和 shutdownNow方法）
    private static final RuntimePermission shutdownPerm =
        new RuntimePermission("modifyThread");
    ...
}  

    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        // 控制参数合法
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        // 任务队列、线程工厂、拒绝策略不能为空
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        // 使用时间单位unit将keepAliveTime转成纳秒
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue) {
        // 使用默认的线程工厂、默认的拒绝策略                             
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), defaultHandler);
    }

    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory) {
        // 使用默认的拒绝策略                
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             threadFactory, defaultHandler);
    }

    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              RejectedExecutionHandler handler) {
        // 使用默认的线程工厂                              
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), handler);
    }
    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
        // 运行的线程少于corePoolSize个
        if (workerCountOf(c) < corePoolSize) {
            // 添加任务
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        // 此时表示线程池处于运行中，并且将任务添加到队列
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            // 再次检查线程池状态
            if (! isRunning(recheck) && remove(command))
                // 如果线程池不处于运行状态，将任务从队列中移除
                // 拒绝任务
                reject(command);
            else if (workerCountOf(recheck) == 0)
                // 运行中的任务数为0,有以下2种情况：
                // 1.线程池处于运行中（加一个空任务是为了保证在队列里等待的任务可以被唤醒后执行）
                // 2.线程池不处于运行中，remove(command)失败
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command); 
    }

    private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable
    {
        // 线程
        final Thread thread;
        // 需要执行的任务
        Runnable firstTask;
        // 当前线程完成的任务数
        volatile long completedTasks;

        // worker构造器
        Worker(Runnable firstTask) {
            // 将AQS的state设置为-1
            setState(-1); // inhibit interrupts until runWorker
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }

        // 实现runnable了run接口
        public void run() {
            // 执行任务
            runWorker(this);
        }

        // 是否锁定
        protected boolean isHeldExclusively() {
            return getState() != 0;
        }

        // 实现AQS的抽象方法，尝试获取锁，将state状态置为1
        protected boolean tryAcquire(int unused) {
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }
        // 实现AQS的抽象方法，尝试释放锁，将state状态置为0
        protected boolean tryRelease(int unused) {
            setExclusiveOwnerThread(null);
            setState(0);
            return true;
        }

        // 获取锁
        public void lock()        { acquire(1); }
        // 尝试获取锁
        public boolean tryLock()  { return tryAcquire(1); }
        // 释放锁
        public void unlock()      { release(1); }
        // 是否锁定
        public boolean isLocked() { return isHeldExclusively(); }
        // 中断已启动线程
        void interruptIfStarted() {
            Thread t;
            // 线程处于活动状态
            if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
                try {
                    t.interrupt();
                } catch (SecurityException ignore) {
                }
            }
        }
    }

    public void execute(Runnable command) {
        // 防止提交null任务
        if (command == null)
            throw new NullPointerException();

        // 获取ctl
        int c = ctl.get();
        // 活动的线程小于核心线程数
        if (workerCountOf(c) < corePoolSize) {
            // 添加新的线程执行任务,第2个参数为true表示活动的线程数不要超过核心线程数corePoolSize
            if (addWorker(command, true))
                return;
            // 再次获取ctl
            c = ctl.get();
        }
        // 执行到这里表示活动的线程数大于等于核心线程数
        // isRunning先判断线程池是否处于运行状态，然后将任务放入任务队列等待
        if (isRunning(c) && workQueue.offer(command)) {
            // 再次获取ctl
            int recheck = ctl.get(); 
            if (! isRunning(recheck) && remove(command))
                // 此时线程池状态不处于运行状态中，将任务从队列中移除
                // 调用任务拒绝策略拒绝任务
                reject(command);
            else if (workerCountOf(recheck) == 0)
                // 执行到线程处于运行状态或者不处于运行状态时任务不在任务队列中，并且线程池的活动线程数为0
                // 添加新的线程来执行一个空的任务，用来唤醒等待中的任务被线程执行
                addWorker(null, false);
        }
        // 执行到这里表示线程池状态不处于运行状态或者任务队列已满
        else if (!addWorker(command, false))
            reject(command);
    }

    // 参数core，为true时表示活动的线程数不能超过核心线程数corePoolSize，反之则不能超过maximumPoolSize
    private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            // 获取ctl
            int c = ctl.get();
            // 当前线程池的状态
            int rs = runStateOf(c);

            /*  此处的表达式可以转化成如下所示的表达式
                rs >= SHUTDOWN && rs != SHUTDOWN || firstTask != null || workQueue.isEmpty()
                此表达式为真的情况有如下几种:
                1. 线程池运行状态大于SHUTDOWN(即为STOP、TIDYING、TERMINATED)
                2. 线程池运行状态为SHUTDOWN且任务不为null
                3. 线程池运行状态为SHUTDOWN且任务为null，任务队列为空

                也就是之前提到的当状态为SHUTDOWN时，不再允许添加新的任务，但是会执行已在任务队列中的任务；
                当状态为STOP、TIDYING、TERMINATED时表示不会再处理任务
            */
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            /*  执行到此处的情况为以下几种：
                1. 线程池状态为RUNNING状态
                2. 运行状态为SHUTDOWN, 任务为null，且任务队列不为空
            */
            // 自旋
            for (;;) {
                // 获取当前活动的线程数
                int wc = workerCountOf(c);
                // 当core为true时，只要当前线程数大于等于核心线程数corePoolSize，就不再往下执行
                // 为core为false，只要当前线程数大于等于核心线程数maximumPoolSize，就不再往下执行
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                // 通过CAS原子操作给ctl增加1
                if (compareAndIncrementWorkerCount(c))
                    //原子操作执行成功后直接跳出外部的循环
                    break retry;
                // 原子操作失败后，重新获取ctl
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)
                    // 由于运行状态变化导致之前的CAS原子操作失败，回到外循环判断此时状态是否需要退出
                    continue retry;
                // 执行到这里表示是其它线程提交任务导致CAS原子操作失败，通过内部循环再次操作
                // else CAS failed due to workerCount change; retry inner loop
            }
        }

        // 执行到这里表示已通过原子操作改变了ctl的值，递增了一次ctl的值
        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            // 构造新的worker对象
            w = new Worker(firstTask);
            final Thread t = w.thread;
            if (t != null) {
                final ReentrantLock mainLock = this.mainLock;
                // 同步
                mainLock.lock();
                try {

                    // Recheck while holding lock.
                    // Back out on ThreadFactory failure or if
                    // shut down before lock acquired.
                    // 获取当前线程池运行状态
                    int rs = runStateOf(ctl.get());

                    /*
                     下列表达式为真的情况：
                     1. 线程池状态为RUNNING
                     2. 线程池状态为SHUTDOWN且提交的任务为null
                    */
                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        // 当前worker中的线程已被启动，则抛出宜昌
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                        // 将worker放入集合中
                        workers.add(w);
                        // 获取当前worker的数量
                        int s = workers.size();
                        // 如果当前worker的数量超过largestPoolSize，则需要更新largestPoolSize的值
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        // 新的worker线程已创建
                        workerAdded = true;
                    }
                } finally {
                    // 释放同步
                    mainLock.unlock();
                }
                // 新的worker线程已创建
                if (workerAdded) {
                    //　启动当前worker线程，会执行worker实现的run方法
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                // 任务启动失败， 回滚操作
                addWorkerFailed(w);
        }
        return workerStarted;
    }

        // worker实现的run方法
        public void run() {
            // 调用线程池的方法runWorker
            runWorker(this);
        }
    // 执行任务
    final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        // 将worker中的任务置为null
        w.firstTask = null;
        // 释放worker的状态，允许被其它线程打断
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            // 执行任务，直到队列中的任务被取完
            while (task != null || (task = getTask()) != null) {
                // work加锁
                w.lock();
                // If pool is stopping, ensure thread is interrupted;
                // if not, ensure thread is not interrupted.  This
                // requires a recheck in second case to deal with
                // shutdownNow race while clearing interrupt
                // 线程池为RUNNING、SHUTDOWN、STOP
                // 线程池为RUNNING、SHUTDOWN、STOP且当前线程已中断
                if ((runStateAtLeast(ctl.get(), STOP) ||
                     (Thread.interrupted() &&
                      runStateAtLeast(ctl.get(), STOP))) &&
                    !wt.isInterrupted())
                    // 中断线程
                    wt.interrupt();
                try {
                    // 在task执行任务前执行的方法，该方法为空方法，需要时由子类来重写实现逻辑
                    beforeExecute(wt, task);
                    Throwable thrown = null;
                    try {
                        // 执行task任务
                        task.run();
                    } catch (RuntimeException x) {
                        thrown = x; throw x;
                    } catch (Error x) {
                        thrown = x; throw x;
                    } catch (Throwable x) {
                        thrown = x; throw new Error(x);
                    } finally {
                        // 在task执行任务后执行的方法，该方法为空方法，需要时由子类来重写实现逻辑
                        afterExecute(task, thrown);
                    }
                } finally {
                    // 任务执行完后将task置为null，用来执行任务队列中的任务
                    task = null;
                    // 该worker线程执行的任务递增一次
                    w.completedTasks++;
                    // 解锁
                    w.unlock();
                }
            }
            // 所有任务执行完
            completedAbruptly = false;
        } finally {
            // 当任务队列没有任务执行或者线程池被关闭shutdown或shutdownNow时调用
            processWorkerExit(w, completedAbruptly);
        }
    }  

    // 从队列中获取任务
    private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?

        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // Check if queue empty only if necessary.
            // 线程池状态为STOP、TIDYING、TERMINATED或者线程为RUNNING、SHUTDOWN且任务队列为空
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                // 递减线程数
                decrementWorkerCount();
                return null;
            }

            int wc = workerCountOf(c);

            // 当线程使用keepAliveTime超时等待获取任务或者活跃线程数已超过核心线程数
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

            /*当下列情况出现时：
                1. 活跃的线程数大于maximumPoolSize且活跃线程大于1或任务队列为空
                2. 从任务队列获取任务超时且活跃线程大于1或者任务队列为空
            */
            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                // 递减活跃线程数
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

            try {
                // timed 为真时
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                // 表示r 为null
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }        

    // 处理
    private void processWorkerExit(Worker w, boolean completedAbruptly) {
        // 线程被中断
        if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
            // 递减活动线程的数量
            decrementWorkerCount();

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            // 计算线程池完成的任务数
            completedTaskCount += w.completedTasks;
            // 从worker集合中移除w
            workers.remove(w);
        } finally {
            mainLock.unlock();
        }
        //尝试终止
        tryTerminate();

        int c = ctl.get();
        // 当线程池状态为TIDYING、TERMINATED
        if (runStateLessThan(c, STOP)) {
            if (!completedAbruptly) {
                // completedAbruptly为false表示任务队列里的任务被执行完
                // allowCoreThreadTimeOut为true则min为0，反之为corePoolSize
                int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
                // min为0，且任务队列不为空，表示还有任务需要执行
                if (min == 0 && ! workQueue.isEmpty())
                    min = 1;
                // 若活动的线程大于min
                if (workerCountOf(c) >= min)
                    return; // replacement not needed
            }
            /* 执行到这里表示
               1. completedAbruptly为true，表示在任务执行时中断过，保证线程执行任务
               2. 活动线程小于min，保证任务队列中的任务被执行或者让核心线程处于空闲等待状态
            */
            addWorker(null, false);
        }
    }

    private void addWorkerFailed(Worker w) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (w != null)
                // worker不为null时，将其从workers集合中移除
                workers.remove(w);
            // 递减活动线程数量
            decrementWorkerCount();
            // 尝试中断
            tryTerminate();
        } finally {
            mainLock.unlock();
        }
    }

    // 通过递减ctl来递减活动线程数量
    private void decrementWorkerCount() {
        do {} while (! compareAndDecrementWorkerCount(ctl.get()));
    }
    private boolean compareAndDecrementWorkerCount(int expect) {
        return ctl.compareAndSet(expect, expect - 1);
    }

    // 尝试终止，执行此方法后线程池的状态会先STOP变为TIDYING，再变为TERMINATED
    final void tryTerminate() {
        // 自旋
        for (;;) {
            int c = ctl.get(); 
            if (isRunning(c) ||
                runStateAtLeast(c, TIDYING) ||
                (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
                // 此时的状态为RUNNING、SHUTDOWN、STOP、TIDYING或状态为SHUTDOWN且任务队列不为空
                return;
            if (workerCountOf(c) != 0) { // Eligible to terminate
                // 活跃的线程大于0时，中断其中一个空闲worker的线程
                interruptIdleWorkers(ONLY_ONE);
                return;
            }

            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                // CAS原子操作，将ctl状态置为TIDYING,且worker数量置为0
                if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
                    try {
                        // 执行terminated,默认是空方法，子类可以重写实现逻辑
                        terminated();
                    } finally {
                        // 将ctl状态置为TERMINATED
                        ctl.set(ctlOf(TERMINATED, 0));
                        termination.signalAll();
                    }
                    return;
                }
            } finally {
                mainLock.unlock();
            }
            // else retry on failed CAS
        }
    }

    public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            // 检查是否有每个worker的访问权限
            checkShutdownAccess();
            // 自旋设置线程池的状态为SHUTDOWN
            advanceRunState(SHUTDOWN);
            // 中断所有空闲的worker线程
            interruptIdleWorkers();
            // 空的方法，由子类实现重写实现逻辑，比如ScheduledThreadPoolExecutor就实现了
            onShutdown(); // hook for ScheduledThreadPoolExecutor
        } finally {
            mainLock.unlock();
        }
        // 尝试终止
        tryTerminate();
    }

    // 设置线程池的状态
    private void advanceRunState(int targetState) {
        for (;;) {
            int c = ctl.get();
            // 当线程的状态大于目标状态或者将当前状态设置成目标状态时跳出循环
            if (runStateAtLeast(c, targetState) ||
                ctl.compareAndSet(c, ctlOf(targetState, workerCountOf(c))))
                break;
        }
    }
interruptIdleWorkers用来中断所有空闲的线程

    private void interruptIdleWorkers() {
        interruptIdleWorkers(false);
    }

    // 根据参数来判断是否中断单个进程或者中断所有进程
    private void interruptIdleWorkers(boolean onlyOne) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            // 遍历所有活动线程
            for (Worker w : workers) {
                Thread t = w.thread;
                // 线程t未中断
                if (!t.isInterrupted() && w.tryLock()) {
                    try {
                        // 中断线程t
                        t.interrupt();
                    } catch (SecurityException ignore) {
                    } finally {
                        w.unlock();
                    }
                }
                // 若为true中断一个线程后直接跳出
                if (onlyOne)
                    break;
            }
        } finally {
            mainLock.unlock();
        }
    }   

    public List<Runnable> shutdownNow() {
        List<Runnable> tasks;
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            // 检查是否有每个worker的访问权限
            checkShutdownAccess();
            // 自旋设置线程池的状态为STOP
            advanceRunState(STOP);
            // 中断所有线程
            interruptWorkers();
            // 清除任务队列
            tasks = drainQueue();
        } finally {
            mainLock.unlock();
        }
        // 尝试终止
        tryTerminate();
        return tasks;
    }

    private List<Runnable> drainQueue() {
        BlockingQueue<Runnable> q = workQueue;
        ArrayList<Runnable> taskList = new ArrayList<Runnable>();
        // 将任务队列中任务清除，然后将其添加到taskList待返回
        q.drainTo(taskList);
        // 这里再次判断下任务队列中是否已为空，主要防止BlockingQueue的实现为延迟队列
        // 此时会将其一个个的遍历清除
        if (!q.isEmpty()) {
            for (Runnable r : q.toArray(new Runnable[0])) {
                if (q.remove(r))
                    taskList.add(r);
            }
        }
        return taskList;
    }

    public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

    public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue<Runnable>());
    }