private final class Worker
    extends AbstractQueuedSynchronizer
    implements Runnable
{
    private static final long serialVersionUID = 6138294804551838833L;

    //当前Worker所处于的线程
    final Thread thread;

    //待执行的任务
    Runnable firstTask;

    //任务计数器
    volatile long completedTasks;

    Worker(Runnable firstTask) {
        setState(-1); // inhibit interrupts until runWorker
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this);
    }
  }

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    /*
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn't, by returning false.
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         */
    
   
    int c = ctl.get();
    //通过位运算，获取当前线程数，如果线程数小于corePoolSize，则执行addWorker(),创建新线程执行
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    if (isRunning(c) && workQueue.offer(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);
}

private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));

private boolean addWorker(Runnable firstTask, boolean core) {
    //自旋，判断线程池状态，并对线程数量执行原子+1操作
    retry:
    for (;;) {
        int c = ctl.get();
        
        //获取线程池状态
        int rs = runStateOf(c);

       	//如果线程池已经关闭，则直接返回false
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;

        for (;;) {
            //判断线程数是否已达上限，根据传入参数core的不同，判断corePoolSize或者maximumPoolSize。
            //如果线程数已达上限，直接返回false
            int wc = workerCountOf(c);
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            //执行原子操作，对线程数+1
            if (compareAndIncrementWorkerCount(c))
                break retry;
            c = ctl.get();  
            if (runStateOf(c) != rs)
                continue retry;
        }
    }

    //ctl变量操作成功，执行Worker相关逻辑
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        //创建一个新的Worker，传入待执行的任务
        w = new Worker(firstTask);
        final Thread t = w.thread;
        if (t != null) {
            final ReentrantLock mainLock = this.mainLock;
            
            mainLock.lock();
            try {
                //加锁后，再次判断线程池状态
                int rs = runStateOf(ctl.get());

                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {
                    if (t.isAlive()) // precheck that t is startable
                        throw new IllegalThreadStateException();
                    workers.add(w);
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            //如果创建了新的Worker，则调用其start方法立即执行
            if (workerAdded) {
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    //首先释放锁，允许中断
    w.unlock(); 
    boolean completedAbruptly = true;
    
    try {
        while (task != null || (task = getTask()) != null) {
            //执行任务前先加锁
            w.lock();
            //如果线程池已经终止，则中断该线程。
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                beforeExecute(wt, task);
                Throwable thrown = null;
                try {
                    //执行任务
                    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 {
                    afterExecute(task, thrown);
                }
            } finally {
                task = null;
                //当前Worker计数器+1
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    /*
         * Proceed in 3 steps:
         *
         * 1. If fewer than corePoolSize threads are running, try to
         * start a new thread with the given command as its first
         * task.  The call to addWorker atomically checks runState and
         * workerCount, and so prevents false alarms that would add
         * threads when it shouldn't, by returning false.
         *
         * 2. If a task can be successfully queued, then we still need
         * to double-check whether we should have added a thread
         * (because existing ones died since last checking) or that
         * the pool shut down since entry into this method. So we
         * recheck state and if necessary roll back the enqueuing if
         * stopped, or start a new thread if there are none.
         *
         * 3. If we cannot queue task, then we try to add a new
         * thread.  If it fails, we know we are shut down or saturated
         * and so reject the task.
         */
    
   
    int c = ctl.get();
    //通过位运算，获取当前线程数，如果线程数小于corePoolSize，则执行addWorker(),创建新线程执行
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    
    //针对线程数超过corePoolSize的情况，将任务放入workQueue中
    if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();
        if (! isRunning(recheck) && remove(command))
            reject(command);
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }
    
    //如果workQueue以满，则以maximumPoolSize为上限尝试创建新的线程
    else if (!addWorker(command, false))
        //如果线程数已达maximumPoolSize，则执行拒绝策略
        reject(command);
}