public interface Executor {

    /**
     * 在不远的将来执行用户提供的command。
     * command或许在一个新的线程中执行，或者在线程池中执行，或许在调用的线程中。
     * 具体的执行方式在Executor的实现类实现
     *
     * @param command runnable任务
     * @throws RejectedExecutionException 如果command不能被Execute接受
     * @throws NullPointerException comman是null
     */
    void execute(Runnable command);
}

public interface ExecutorService extends Executor {

    void shutdown();

    List<Runnable> shutdownNow();
    //执行器是否关闭
    boolean isShutdown();
		//在调用shutdown之后，所有任务都已经完成，则返回true.
    //如果不提前调用shutdown或者shutdownNow，该方法不可能返回true
    boolean isTerminated();

    boolean awaitTermination(long timeout, TimeUnit unit)
        throws InterruptedException;
    //提交一个Callable任务，有返回值并返回一个Future对象代表任务执行的结果。Future的get方法将等待任务执行完毕。
    <T> Future<T> submit(Callable<T> task);
    //提交一个Runnable任务，并返回Futrue对象
    <T> Future<T> submit(Runnable task, T result);

    Future<?> submit(Runnable task);

    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)
        throws InterruptedException;

    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks,
                                  long timeout, TimeUnit unit)
        throws InterruptedException;

    <T> T invokeAny(Collection<? extends Callable<T>> tasks)
        throws InterruptedException, ExecutionException;

    <T> T invokeAny(Collection<? extends Callable<T>> tasks,
                    long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

public interface Future<V> {

    /**
     * 尝试取消执行这个任务.任务完成或者任务取消或者一些其他的原因会导致尝试失败。
     * 如果任务还未开始执行，取消成功，那么这个任务将永远不会执行。
     * 如果任务已经开始执行，取消成功，则mayInterruptIfRunning参数来决定是不是中断线程
     */
    boolean cancel(boolean mayInterruptIfRunning);
    /**
    *  如果任务在执行完成前被取消，则返回true
    */
    boolean isCancelled();

    /**
     * 如果任务正常执行完毕则返回true
     */
    boolean isDone();

    /**
     * 等待执行结果
     *
     * @return the computed result
     * @throws CancellationException 如果任务被取消
     * @throws ExecutionException 执行过程中出现异常
     * @throws InterruptedException 如果任务被中断
     */
    V get() throws InterruptedException, ExecutionException;

    /**
     * 在有限时间内等待执行结果
     *
     * @throws CancellationException 如果任务被取消
     * @throws ExecutionException 执行过程中出现异常
     * @throws InterruptedException 如果任务被中断
     * @throws TimeoutException 如果任务超时
     */
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

public class FutureTask<V> implements RunnableFuture<V> {
    /**
     * 任务的运行状态,初始状态为NEW.  
     *
     * 可能的状态变换为:
     * NEW -> COMPLETING -> NORMAL
     * NEW -> COMPLETING -> EXCEPTIONAL
     * NEW -> CANCELLED
     * NEW -> INTERRUPTING -> INTERRUPTED
     */
    private volatile int state;
    private static final int NEW          = 0;
    private static final int COMPLETING   = 1;
    private static final int NORMAL       = 2;
    private static final int EXCEPTIONAL  = 3;
    private static final int CANCELLED    = 4;
    private static final int INTERRUPTING = 5;
    private static final int INTERRUPTED  = 6;

    /** The underlying callable; nulled out after running */
    private Callable<V> callable;
    /** 任务执行结果，get()方法获取的就是这个对象 */
    private Object outcome; // non-volatile, protected by state reads/writes
    /** The thread running the callable; CASed during run() */
    private volatile Thread runner;
    /** Treiber stack of waiting threads */
    private volatile WaitNode waiters; 
}

//接受新的任务和处理队列中的任务
private static final int RUNNING    = -1 << COUNT_BITS;
//拒绝新的任务，但是处理队列中的任务
private static final int SHUTDOWN   =  0 << COUNT_BITS;
//拒绝新的任务，不处理队列中的任务，并且中断在执行的任务
private static final int STOP       =  1 << COUNT_BITS;
//所有任务已经终止（terminated），workerCount=0，线程
private static final int TIDYING    =  2 << COUNT_BITS;
//terminated已完成
private static final int TERMINATED =  3 << COUNT_BITS;

public class ThreadPoolExecutor extends AbstractExecutorService {
  //控制变量-存放状态和线程数
	private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
  //线程池阻塞队列
  private final BlockingQueue<Runnable> workQueue;
  //工作线程集合，存放（活跃的）工作线程，只有持有mainLock才能访问这个集合。
  private final HashSet<Worker> workers = new HashSet<>();
  //全局锁
  private final ReentrantLock mainLock = new ReentrantLock();
  //awaitTermination方法使用的等待条件变量
	private final Condition termination = mainLock.newCondition();
  //记录峰值线程数，只有持有全局锁才能访问
  private int largestPoolSize;
  //记录完成任务数，只有持有全局锁才能访问
  private long completedTaskCount;
  //线程工厂
  private volatile ThreadFactory threadFactory;
  //拒绝执行处理器
  private volatile RejectedExecutionHandler handler;
  //任务超时时间
  private volatile long keepAliveTime;
  //是否允许coreSize超时，默认不允许
  private volatile boolean allowCoreThreadTimeOut;
  //线程池容量
  private volatile int maximumPoolSize;
  
  private final class Worker extends AbstractQueuedSynchronizer implements Runnable {
			//工作线程
      final Thread thread;
      //初始要执行的任务，有可能是null
    	Runnable firstTask;
      //每一个线程完成的任务数量
    	volatile long completedTasks;

  }
  
  /**
     * 创建线程池
     *
     * @param corePoolSize 核心线程数量
     * @param maximumPoolSize 最大线程容量
     * @param keepAliveTime 超过CoreSize的工作线程保持的时间，超时即终止
     * @param unit keepAliveTime的单位
     * @param workQueue 阻塞工作队列
     * @param threadFactory 线程工厂，默认Executors.defaultThreadFactory()
     * @param handler 拒绝执行处理回调
     * @throws IllegalArgumentException if one of the following holds:<br>
     *         {@code corePoolSize < 0}<br>
     *         {@code keepAliveTime < 0}<br>
     *         {@code maximumPoolSize <= 0}<br>
     *         {@code maximumPoolSize < corePoolSize}
     * @throws NullPointerException if {@code workQueue}
     *         or {@code threadFactory} or {@code handler} is null
     */
    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.acc = System.getSecurityManager() == null ?
                null :
                AccessController.getContext();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }
}

//ctl的初始值是1110 0000 0000 0000 0000 0000 0000 0000，即状态为RUNNING，worker数量为0
private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));  	

//Java中Integer是32位，所以COUNT_BITS等于29
private static final int COUNT_BITS = Integer.SIZE - 3;
  //0001 1111 1111 1111 1111 1111 1111 1111
  private static final int CAPACITY   = (1 << COUNT_BITS) - 1;    

//-1左移29位 1110 0000 0000 0000 0000 0000 0000 0000 高3位 111代表运行中
private static final int RUNNING    = -1 << COUNT_BITS;
  //0 左移 29位  高三位000代表SHUTDOWN
  private static final int SHUTDOWN   =  0 << COUNT_BITS;
  //1 左移 29位  高三位001代表STOP
  private static final int STOP       =  1 << COUNT_BITS;
  //2 左移 29位  高三位010代表TIDYING
  private static final int TIDYING    =  2 << COUNT_BITS;
  //3 左移 29位  高三位011代表TERMINATED
  private static final int TERMINATED =  3 << COUNT_BITS;

  //获取线程池状态，CAPACITY取反是 1110 0000 0000 0000 0000 0000 0000 0000
  //c & ~CAPACITY ,c代表ctl值，因为低29位都是0，所以与操作得出的值只与高三位相关
  //这个算法证明：ctl高三位决定了线程池的状态
private static int runStateOf(int c)     { return c & ~CAPACITY; }
  //获取worder数量，低29位于ctl与操作
  private static int workerCountOf(int c)  { return c & CAPACITY; }

  private static int ctlOf(int rs, int wc) { return rs | wc; }

// ctl和状态常量比较，判断是否小于
private static boolean runStateLessThan(int c, int s) {
    return c < s;
}

// ctl和状态常量比较，判断是否小于或等于
private static boolean runStateAtLeast(int c, int s) {
    return c >= s;
}

// ctl和状态常量SHUTDOWN比较，判断是否处于RUNNING状态
private static boolean isRunning(int c) {
    return c < SHUTDOWN;
}

/**
  * 异步执行给定的task，即可能用新的线程执行任务，也可以用线程池中已存在的线程执行任务
  * 如果线程池已经shutdown，那么会拒绝执行任务并抛出异常，由RejectedExecutionHandler处理
  *
  * @param command 要执行的任务
  * @throws RejectedExecutionException at discretion of
  *         {@code RejectedExecutionHandler}, if the task
  *         cannot be accepted for execution
  * @throws NullPointerException if {@code command} is null
  */
 public void execute(Runnable command) {
     if (command == null)
         throw new NullPointerException();
     /*
      * Proceed in 3 steps:
      *
      * 1. 如果工作线程小于corePoolSize，ThreadPoolExecutor会把command当成firstTask，
      * 交给新创建的线程。addWorker函数会校验线程池状态和线程池数量.
      *
      * 2. 如果任务能够入队, 我们还是要进行double-check是否新加线程还是用线程池中的工作线程,因为线程可能在最后一次check的时候死掉. 
      *
      * 3. 如果任务没有进入队列，就尝试创建一个新的线程。如果创建失败了，则线程池shutdown或者饱和，拒绝了这次请求
      */
     int c = ctl.get();
     //如果工作线程数量小于corePoolSize则创建一个core worker
     if (workerCountOf(c) < corePoolSize) {
         //默认把command当做firstTask
         if (addWorker(command, true))
             return;
         c = ctl.get();
     }
     //说明创建新的核心线程失败，也就是当前工作线程数大于等于corePoolSize
     //判定线程池是否是RUNNING,工作队列添加command
     if (isRunning(c) && workQueue.offer(command)) {
         //再次确认状态，如果线程池状态不是RUNNING并且移除成功则拒绝掉command
         int recheck = ctl.get();
         if (! isRunning(recheck) && remove(command))
             reject(command);
         else if (workerCountOf(recheck) == 0)
             addWorker(null, false);
     }
     else if (!addWorker(command, false))
         reject(command);
 }

//检验线程池状态和线程池数量边界，通过校验则新增一个线程到线程池中
//firstTask是新创建的worker第一次要执行的。
//当线程数量少于corePoolSize线程或队列已满（这种情况下需要绕过队列），firstTask不为空
private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();
            //获取线程池状态
            int rs = runStateOf(c);
            //这个判断是线程池拒绝任务的逻辑，比较绕，包含如下两个逻辑
            //1、线程池状态不再是RUNNING则拒绝创建worker
            //2、线程池状态如果是SHUTDOWN并且firstTask为Null且任务队列中有其他任务则拒绝新的任务
            //其实这个判定的具体业务时：在线程池状态为STOP，TIDYING,TERMINATED状态下，不会再接受新的任务。如果状态是SHUTDOWN，firstTask为空并且workQueue不为空，则可以创建新的工作线程。当队列为空了，就不能继续提交任务了。
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;
            //自旋检测状态
            for (;;) {
                //获取worker的数量
                int wc = workerCountOf(c);
                //判定数量是否大于边界，大于的话则拒绝新建线程
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                //CAS增加工作线程数量
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                //如果CAS失败，则需要再次读取ctl
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }
        }

        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            //创建工作线程
            w = new Worker(firstTask);
            final Thread t = w.thread;
            if (t != null) {
                //获取全局锁
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();
                try {
                    // Recheck while holding lock.
                    // Back out on ThreadFactory failure or if
                    // shut down before lock acquired.
                    //获取全局锁之后再次校验避免线程池shutdown
                    int rs = runStateOf(ctl.get());
                    //线程池状态是RUNNING或者状态是SHUTDOWN但是core已满且队列未满
                    if (rs < SHUTDOWN ||
                        (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                        //把线程添加进workers容器中
                        workers.add(w);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                //如果worker添加完毕，则需要启动线程
                if (workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable
    {
        final Thread thread;
        Runnable firstTask;
        volatile long completedTasks;

        /**
         * 默认构造函数
         */
        Worker(Runnable firstTask) {
            //设置state初始值为-1，阻止中断工作线程
            setState(-1);
            this.firstTask = firstTask;
            //线程是通过线程工厂创建的.
            this.thread = getThreadFactory().newThread(this);
        }

        public void run() {
            //下文重点讲解
            runWorker(this);
        }

        // Lock methods
        //
        // The value 0 represents the unlocked state.
        // The value 1 represents the locked state.

        protected boolean isHeldExclusively() {
            return getState() != 0;
        }

        protected boolean tryAcquire(int unused) {
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }

        protected boolean tryRelease(int unused) {
            setExclusiveOwnerThread(null);
            setState(0);
            return true;
        }

        public void lock()        { acquire(1); }
        public boolean tryLock()  { return tryAcquire(1); }
        public void unlock()      { release(1); }
        public boolean isLocked() { return isHeldExclusively(); }

        void interruptIfStarted() {
            Thread t;
            if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
                try {
                    t.interrupt();
                } catch (SecurityException ignore) {
                }
            }
        }
    }

//自旋的从队列中获取任务并执行
final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // 允许中断线程
    //突然停止标识位
    boolean completedAbruptly = true;
    try {
        //task不为空或者从队列中获取task，如果队列为空则阻塞while
        while (task != null || (task = getTask()) != null) {
            //
            w.lock();
            // If pool is stopping, ensure thread is interrupted;
            // 如果线程池正在停止（STOPING），请确保线程是中断的。
            // 否则，要确保线程不是中断的
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                //钩子方法，在执行任务前执行
                beforeExecute(wt, task);
                Throwable thrown = null;
                try {
                    //执行任务，此处调用应用实现代码
                    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 {
                    //钩子方法，在任务执行之后执行
                    afterExecute(task, thrown);
                }
            } finally {
                task = null;
                //递增completedTasks，由于worker.lock，所以这个递增是安全的
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        //
        processWorkerExit(w, completedAbruptly);
    }
}

private void processWorkerExit(Worker w, boolean completedAbruptly) {
    if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
      decrementWorkerCount();

    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
      completedTaskCount += w.completedTasks;
      workers.remove(w);
    } finally {
      mainLock.unlock();
    }

    tryTerminate();

    int c = ctl.get();
    if (runStateLessThan(c, STOP)) {
      if (!completedAbruptly) {
        int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
        if (min == 0 && ! workQueue.isEmpty())
          min = 1;
        if (workerCountOf(c) >= min)
          return; // replacement not needed
      }
      addWorker(null, false);
    }
}

//getTask返回NULL代表工作线程将要终止
private Runnable getTask() {
  //记录上次一poll时是否超时
  boolean timedOut = false; // Did the last poll() time out?

  for (;;) {
    int c = ctl.get();
    //获取线程池状态
    int rs = runStateOf(c);

    //线程池状态为SHUTDOWN时，如果工作队列为空，则decrementWorkerCount，返回NULL
    //线程池状态大于SHUTDOWN时，decrementWorkerCount,返回NULL
    if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
      //原子的减少线程池数量，ctl低29位
      decrementWorkerCount();
      return null;
    }
    // 获取工作线程数量
    // 到这里说明线程池还处于RUNNING状态
    int wc = workerCountOf(c);

    // 允许core线程超时或者线程数量大于coreSize
    boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
    // 
    if ((wc > maximumPoolSize || (timed && timedOut))
        && (wc > 1 || workQueue.isEmpty())) {
      if (compareAndDecrementWorkerCount(c))
        return null;
      continue;
    }

    try {
      Runnable r = timed ?
        workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
      workQueue.take();
      // 只有非null的时候才返回，null的情况下会进入下一轮循环
      if (r != null)
        return r;
      timedOut = true;
    } catch (InterruptedException retry) {
      timedOut = false;
    }
  }
}

复制// 工作线程总数大于maximumPoolSize，说明了通过setMaximumPoolSize()方法减少了线程池容量
boolean b1 = wc > maximumPoolSize;
// 允许线程超时同时上一轮通过poll()方法从任务队列中拉取任务为null
boolean b2 = timed && timedOut;
// 工作线程总数大于1
boolean b3 = wc > 1;
// 任务队列为空
boolean b4 = workQueue.isEmpty();
boolean r = (b1 || b2) && (b3 || b4);
if (r) {
    if (compareAndDecrementWorkerCount(c)){
        return null;
    }else{
        continue;
    }
}

private void processWorkerExit(Worker w, boolean completedAbruptly) {
    //突然结束，是因为线程抛出用户异常，直接使工作线程数减1
    if (completedAbruptly)
      decrementWorkerCount();
		
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
      //全局锁锁定情况下，线程安全的方式更新完成的任务数量并移除Worker
      completedTaskCount += w.completedTasks;
      workers.remove(w);
    } finally {
      mainLock.unlock();
    }
    //用于根据当前线程池的状态判断是否需要进行线程池terminate处理
    tryTerminate();

    int c = ctl.get();
    //如果状态小于STOP，即RUNNING 或 SHUTDOWN
    if (runStateLessThan(c, STOP)) {
      //并且不是线程抛出用户异常
      if (!completedAbruptly) {
        int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
        //如果允许核心线程超时且队列不为空，则至少保证一个工作线程
        if (min == 0 && ! workQueue.isEmpty())
          min = 1;
        //工作线程大于等于最小值，直接返回不新增非核心线程
        if (workerCountOf(c) >= min)
          return; // replacement not needed
      }
      //工作线程小于最小值，需要新增非核心线程
      addWorker(null, false);
    }
}

//满足两种情况就会把线程池状态置为TERMINATED
//1、状态为SHUTDOWN切队列数量为0，工作线程数量为0，对应shutdown关闭的情况
//2、状态为STOP且工作线程数量为0，对应shutdownNow关闭的情况
final void tryTerminate() {
    for (;;) {
      int c = ctl.get();
      //1、线程池状态是RUNNING，不做任何操作直接返回
      //2、线程池状态是TIDYING，不做任何操作直接返回
      //3、线程池状态是SHUTDOWN，且队列不为空不做任何操作直接返回
      if (isRunning(c) ||
          runStateAtLeast(c, TIDYING) ||
          (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
        return;
      // 工作线程数不为0，则中断工作线程集合中的第一个空闲的工作线程
      if (workerCountOf(c) != 0) { // Eligible to terminate
        interruptIdleWorkers(ONLY_ONE);
        return;
      }

      final ReentrantLock mainLock = this.mainLock;
      mainLock.lock();
      try {
        //CAS设置状态为TIDYING状态
        if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
          try {
            //执行terminated方法
            terminated();
          } finally {
            //执行terminated方法完毕之后，设置线程池状态为TERMINATED
            ctl.set(ctlOf(TERMINATED, 0));
            // 唤醒阻塞在termination条件的所有线程，这个变量的await()方法在awaitTermination()中调用
            termination.signalAll();
          }
          return;
        }
      } finally {
        mainLock.unlock();
      }
      // else retry on failed CAS
    }
}

//中断空闲的工作线程，以便它们可以检查终止或配置更改
//如果onlyOne未true，中断最多一个线程。
private void interruptIdleWorkers(boolean onlyOne) {
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
      for (Worker w : workers) {
        Thread t = w.thread;
        // 这里判断线程不是中断状态并且尝试获取锁成功的时候才进行线程中断
        if (!t.isInterrupted() && w.tryLock()) {
          try {
            t.interrupt();
          } catch (SecurityException ignore) {
          } finally {
            w.unlock();
          }
        }
        if (onlyOne)
          break;
      }
    } finally {
      mainLock.unlock();
    }
}

//已提交的任务还会继续执行，但是新的任务将被拒绝,shutdown不会影响用户的正常使用 
public void shutdown() {
     final ReentrantLock mainLock = this.mainLock;
     mainLock.lock();
     try {
       //校验权限，安全策略 SecurityManager
       checkShutdownAccess();
       //CAS更新状态为SHUTDOWN
       advanceRunState(SHUTDOWN);
       //中断所有空闲的工作线程
       interruptIdleWorkers();
       //钩子方法
       onShutdown(); // hook for ScheduledThreadPoolExecutor
     } finally {
       mainLock.unlock();
     }
     //尝试把线程池状态置为TERMINATED
     tryTerminate();
 }


public List<Runnable> shutdownNow() {
    List<Runnable> tasks;
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
      //校验权限，安全策略 SecurityManager
      checkShutdownAccess();
      //CAS更新状态为STOP
      advanceRunState(STOP);
      //中断所有空闲的工作线程
      interruptWorkers();
      //清空工作队列，并返回未执行的任务列表
      tasks = drainQueue();
    } finally {
      mainLock.unlock();
    }
    //尝试把线程池状态置为TERMINATED
    tryTerminate();
    //返回任务列表
    return tasks;
}


public boolean awaitTermination(long timeout, TimeUnit unit)
    throws InterruptedException {
    long nanos = unit.toNanos(timeout);
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
      for (;;) {
        //如果发现状态已经变成TERMINATED，则返回true
        if (runStateAtLeast(ctl.get(), TERMINATED))
          return true;
        if (nanos <= 0)
          return false;
        //阻塞直到超时，如果未超时，tryTerminate会signalAll，这时停止阻塞
        nanos = termination.awaitNanos(nanos);
      }
    } finally {
      //释放全局锁
      mainLock.unlock();
    }
}

final void reject(Runnable command) {
    handler.rejectedExecution(command, this);
}