Java线程池ThreadPoolExecutor使用和分析(二) - execute()原理

private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
private static final int COUNT_BITS = Integer.SIZE - 3;
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;

// Packing and unpacking ctl
private static int runStateOf(int c)     { return c & ~CAPACITY; }
private static int workerCountOf(int c)  { return c & CAPACITY; }
private static int ctlOf(int rs, int wc) { return rs | wc; }

/**
 * Executes the given task sometime in the future.  The task
 * may execute in a new thread or in an existing pooled thread.
 * 在未来的某个时刻执行给定的任务。这个任务用一个新线程执行，或者用一个线程池中已经存在的线程执行
 *
 * If the task cannot be submitted for execution, either because this
 * executor has been shutdown or because its capacity has been reached,
 * the task is handled by the current {@code RejectedExecutionHandler}.
 * 如果任务无法被提交执行，要么是因为这个Executor已经被shutdown关闭，要么是已经达到其容量上限，任务会被当前的RejectedExecutionHandler处理
 *
 * @param command the task to execute
 * @throws RejectedExecutionException at discretion of
 *         {@code RejectedExecutionHandler}, if the task
 *         cannot be accepted for execution                 RejectedExecutionException是一个RuntimeException
 * @throws NullPointerException if {@code command} is null
 */
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.
     * 如果运行的线程少于corePoolSize，尝试开启一个新线程去运行command，command作为这个线程的第一个任务
     *
     * 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.
     * 如果任务成功放入队列，我们仍需要一个双重校验去确认是否应该新建一个线程（因为可能存在有些线程在我们上次检查后死了） 或者 从我们进入这个方法后，pool被关闭了
     * 所以我们需要再次检查state，如果线程池停止了需要回滚入队列，如果池中没有线程了，新开启 一个线程
     * 
     * 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.
     * 如果无法将任务入队列（可能队列满了），需要新开区一个线程（自己：往maxPoolSize发展）
     * 如果失败了，说明线程池shutdown 或者 饱和了，所以我们拒绝任务
     */
    int c = ctl.get();
    
    /**
     * 1、如果当前线程数少于corePoolSize（可能是由于addWorker()操作已经包含对线程池状态的判断，如此处没加，而入workQueue前加了）
     */
    if (workerCountOf(c) < corePoolSize) {
    	//addWorker()成功，返回
        if (addWorker(command, true))
            return;
        
        /**
         * 没有成功addWorker()，再次获取c（凡是需要再次用ctl做判断时，都会再次调用ctl.get()）
         * 失败的原因可能是：
         * 1、线程池已经shutdown，shutdown的线程池不再接收新任务
         * 2、workerCountOf(c) < corePoolSize 判断后，由于并发，别的线程先创建了worker线程，导致workerCount>=corePoolSize
         */
        c = ctl.get();
    }
    
    /**
     * 2、如果线程池RUNNING状态，且入队列成功
     */
    if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();//再次校验位
        
        /**
         * 再次校验放入workerQueue中的任务是否能被执行
         * 1、如果线程池不是运行状态了，应该拒绝添加新任务，从workQueue中删除任务
         * 2、如果线程池是运行状态，或者从workQueue中删除任务失败（刚好有一个线程执行完毕，并消耗了这个任务），确保还有线程执行任务（只要有一个就够了）
         */
        //如果再次校验过程中，线程池不是RUNNING状态，并且remove(command)--workQueue.remove()成功，拒绝当前command
        if (! isRunning(recheck) && remove(command))
            reject(command);
        //如果当前worker数量为0，通过addWorker(null, false)创建一个线程，其任务为null
        //为什么只检查运行的worker数量是不是0呢？？ 为什么不和corePoolSize比较呢？？
        //只保证有一个worker线程可以从queue中获取任务执行就行了？？
        //因为只要还有活动的worker线程，就可以消费workerQueue中的任务
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);  //第一个参数为null，说明只为新建一个worker线程，没有指定firstTask
                                     //第二个参数为true代表占用corePoolSize，false占用maxPoolSize
    }
    /**
     * 3、如果线程池不是running状态 或者 无法入队列
     *   尝试开启新线程，扩容至maxPoolSize，如果addWork(command, false)失败了，拒绝当前command
     */
    else if (!addWorker(command, false))
        reject(command);
}

/**
 * Checks if a new worker can be added with respect to current
 * pool state and the given bound (either core or maximum). If so,
 * the worker count is adjusted accordingly, and, if possible, a
 * new worker is created and started, running firstTask as its
 * first task. This method returns false if the pool is stopped or
 * eligible to shut down. It also returns false if the thread
 * factory fails to create a thread when asked.  If the thread
 * creation fails, either due to the thread factory returning
 * null, or due to an exception (typically OutOfMemoryError in
 * Thread#start), we roll back cleanly.
 * 检查根据当前线程池的状态和给定的边界(core or maximum)是否可以创建一个新的worker
 * 如果是这样的话，worker的数量做相应的调整，如果可能的话，创建一个新的worker并启动，参数中的firstTask作为worker的第一个任务
 * 如果方法返回false，可能因为pool已经关闭或者调用过了shutdown
 * 如果线程工厂创建线程失败，也会失败，返回false
 * 如果线程创建失败，要么是因为线程工厂返回null，要么是发生了OutOfMemoryError
 *
 * @param firstTask the task the new thread should run first (or
 * null if none). Workers are created with an initial first task
 * (in method execute()) to bypass(绕开) queuing when there are fewer
 * than corePoolSize threads (in which case we always start one),
 * or when the queue is full (in which case we must bypass queue).
 * Initially idle threads are usually created via
 * prestartCoreThread or to replace other dying workers.
 *
 * @param core if true use corePoolSize as bound, else
 * maximumPoolSize. (A boolean indicator is used here rather than a
 * value to ensure reads of fresh values after checking other pool
 * state).
 * @return true if successful
 */
private boolean addWorker(Runnable firstTask, boolean core) {
	//外层循环，负责判断线程池状态
	retry:
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c); //状态

        // Check if queue empty only if necessary.
        /**
         * 线程池的state越小越是运行状态，runnbale=-1，shutdown=0,stop=1,tidying=2，terminated=3
         * 1、如果线程池state已经至少是shutdown状态了
         * 2、并且以下3个条件任意一个是false
         *   rs == SHUTDOWN         （隐含：rs>=SHUTDOWN）false情况： 线程池状态已经超过shutdown，可能是stop、tidying、terminated其中一个，即线程池已经终止
         *   firstTask == null      （隐含：rs==SHUTDOWN）false情况： firstTask不为空，rs==SHUTDOWN 且 firstTask不为空，return false，场景是在线程池已经shutdown后，还要添加新的任务，拒绝
         *   ! workQueue.isEmpty()  （隐含：rs==SHUTDOWN，firstTask==null）false情况： workQueue为空，当firstTask为空时是为了创建一个没有任务的线程，再从workQueue中获取任务，如果workQueue已经为空，那么就没有添加新worker线程的必要了
         * return false，即无法addWorker()
         */
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;

        //内层循环，负责worker数量+1
        for (;;) {
            int wc = workerCountOf(c); //worker数量
            
            //如果worker数量>线程池最大上限CAPACITY（即使用int低29位可以容纳的最大值）
            //或者( worker数量>corePoolSize 或  worker数量>maximumPoolSize )，即已经超过了给定的边界
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            
            //调用unsafe CAS操作，使得worker数量+1，成功则跳出retry循环
            if (compareAndIncrementWorkerCount(c))
                break retry;
            
            //CAS worker数量+1失败，再次读取ctl
            c = ctl.get();  // Re-read ctl
            
            //如果状态不等于之前获取的state，跳出内层循环，继续去外层循环判断
            if (runStateOf(c) != rs)
                continue retry;
            // else CAS failed due to workerCount change; retry inner loop
            // else CAS失败时因为workerCount改变了，继续内层循环尝试CAS对worker数量+1
        }
    }

	/**
	 * worker数量+1成功的后续操作
	 * 添加到workers Set集合，并启动worker线程
	 */
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        final ReentrantLock mainLock = this.mainLock; 
        w = new Worker(firstTask); //1、设置worker这个AQS锁的同步状态state=-1
                                   //2、将firstTask设置给worker的成员变量firstTask
                                   //3、使用worker自身这个runnable，调用ThreadFactory创建一个线程，并设置给worker的成员变量thread
        final Thread t = w.thread;
        if (t != null) {
            mainLock.lock();
            try {
            	//--------------------------------------------这部分代码是上锁的
                // Recheck while holding lock.
                // Back out on ThreadFactory failure or if
                // shut down before lock acquired.
            	// 当获取到锁后，再次检查
                int c = ctl.get();
                int rs = runStateOf(c);

                //如果线程池在运行running<shutdown 或者 线程池已经shutdown，且firstTask==null（可能是workQueue中仍有未执行完成的任务，创建没有初始任务的worker线程执行）
                //worker数量-1的操作在addWorkerFailed()
                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {
                    if (t.isAlive()) // precheck that t is startable   线程已经启动，抛非法线程状态异常
                        throw new IllegalThreadStateException();
                    
                    workers.add(w);//workers是一个HashSet<Worker>
                    
                    //设置最大的池大小largestPoolSize，workerAdded设置为true
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
              //--------------------------------------------
            } 
            finally {
                mainLock.unlock();
            }
            
            //如果往HashSet中添加worker成功，启动线程
            if (workerAdded) {
                t.start();
                workerStarted = true;
            }
        }
    } finally {
    	//如果启动线程失败
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

/**
 * Class Worker mainly maintains interrupt control state for
 * threads running tasks, along with other minor bookkeeping.
 * This class opportunistically extends AbstractQueuedSynchronizer
 * to simplify acquiring and releasing a lock surrounding each
 * task execution.  This protects against interrupts that are
 * intended to wake up a worker thread waiting for a task from
 * instead interrupting a task being run.  We implement a simple
 * non-reentrant mutual exclusion lock rather than use
 * ReentrantLock because we do not want worker tasks to be able to
 * reacquire the lock when they invoke pool control methods like
 * setCorePoolSize.  Additionally, to suppress interrupts until
 * the thread actually starts running tasks, we initialize lock
 * state to a negative value, and clear it upon start (in
 * runWorker).
 * 
 * Worker类大体上管理着运行线程的中断状态 和 一些指标
 * Worker类投机取巧的继承了AbstractQueuedSynchronizer来简化在执行任务时的获取、释放锁
 * 这样防止了中断在运行中的任务，只会唤醒(中断)在等待从workQueue中获取任务的线程
 * 解释：
 *   为什么不直接执行execute(command)提交的command，而要在外面包一层Worker呢？？
 *   主要是为了控制中断
 *   用什么控制？？
 *   用AQS锁，当运行时上锁，就不能中断，TreadPoolExecutor的shutdown()方法中断前都要获取worker锁
 *   只有在等待从workQueue中获取任务getTask()时才能中断
 * worker实现了一个简单的不可重入的互斥锁，而不是用ReentrantLock可重入锁
 * 因为我们不想让在调用比如setCorePoolSize()这种线程池控制方法时可以再次获取锁(重入)
 * 解释：
 *   setCorePoolSize()时可能会interruptIdleWorkers()，在对一个线程interrupt时会要w.tryLock()
 *   如果可重入，就可能会在对线程池操作的方法中中断线程，类似方法还有：
 *   setMaximumPoolSize()
 *   setKeppAliveTime()
 *   allowCoreThreadTimeOut()
 *   shutdown()
 * 此外，为了让线程真正开始后才可以中断，初始化lock状态为负值(-1)，在开始runWorker()时将state置为0，而state>=0才可以中断
 * 
 * 
 * Worker继承了AQS，实现了Runnable，说明其既是一个可运行的任务，也是一把锁（不可重入）
 */
private final class Worker
    extends AbstractQueuedSynchronizer
    implements Runnable
{
    /**
     * This class will never be serialized, but we provide a
     * serialVersionUID to suppress a javac warning.
     */
    private static final long serialVersionUID = 6138294804551838833L;

    /** Thread this worker is running in.  Null if factory fails. */
    final Thread thread; //利用ThreadFactory和 Worker这个Runnable创建的线程对象
    
    /** Initial task to run.  Possibly null. */
    Runnable firstTask;
    
    /** Per-thread task counter */
    volatile long completedTasks;

    /**
     * Creates with given first task and thread from ThreadFactory.
     * @param firstTask the first task (null if none)
     */
    Worker(Runnable firstTask) {
    	//设置AQS的同步状态private volatile int state，是一个计数器，大于0代表锁已经被获取
        setState(-1); // inhibit interrupts until runWorker 
                      // 在调用runWorker()前，禁止interrupt中断，在interruptIfStarted()方法中会判断 getState()>=0
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this); //根据当前worker创建一个线程对象
                                                          //当前worker本身就是一个runnable任务，也就是不会用参数的firstTask创建线程，而是调用当前worker.run()时调用firstTask.run()
    }

    /** Delegates main run loop to outer runWorker  */
    public void run() {
        runWorker(this); //runWorker()是ThreadPoolExecutor的方法
    }

    // Lock methods
    //
    // The value 0 represents the unlocked state. 0代表“没被锁定”状态
    // The value 1 represents the locked state. 1代表“锁定”状态

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

    /**
     * 尝试获取锁
     * 重写AQS的tryAcquire()，AQS本来就是让子类来实现的
     */
    protected boolean tryAcquire(int unused) {
    	//尝试一次将state从0设置为1，即“锁定”状态，但由于每次都是state 0->1，而不是+1，那么说明不可重入
    	//且state==-1时也不会获取到锁
        if (compareAndSetState(0, 1)) {
            setExclusiveOwnerThread(Thread.currentThread()); //设置exclusiveOwnerThread=当前线程
            return true;
        }
        return false;
    }

    /**
     * 尝试释放锁
     * 不是state-1，而是置为0
     */
    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(); }

    /**
     * 中断（如果运行）
     * shutdownNow时会循环对worker线程执行
     * 且不需要获取worker锁，即使在worker运行时也可以中断
     */
    void interruptIfStarted() {
        Thread t;
        //如果state>=0、t!=null、且t没有被中断
        //new Worker()时state==-1，说明不能中断
        if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
            try {
                t.interrupt();
            } catch (SecurityException ignore) {
            }
        }
    }
}

/**
 * Main worker run loop.  Repeatedly gets tasks from queue and
 * executes them, while coping with a number of issues:
 * 重复的从队列中获取任务并执行，同时应对一些问题：
 *
 * 1. We may start out with an initial task, in which case we
 * don't need to get the first one. Otherwise, as long as pool is
 * running, we get tasks from getTask. If it returns null then the
 * worker exits due to changed pool state or configuration
 * parameters.  Other exits result from exception throws in
 * external code, in which case completedAbruptly holds, which
 * usually leads processWorkerExit to replace this thread.
 * 我们可能使用一个初始化任务开始，即firstTask为null
 * 然后只要线程池在运行，我们就从getTask()获取任务
 * 如果getTask()返回null，则worker由于改变了线程池状态或参数配置而退出
 * 其它退出因为外部代码抛异常了，这会使得completedAbruptly为true，这会导致在processWorkerExit()方法中替换当前线程
 *
 * 2. Before running any task, the lock is acquired to prevent
 * other pool interrupts while the task is executing, and
 * clearInterruptsForTaskRun called to ensure that unless pool is
 * stopping, this thread does not have its interrupt set.
 * 在任何任务执行之前，都需要对worker加锁去防止在任务运行时，其它的线程池中断操作
 * clearInterruptsForTaskRun保证除非线程池正在stoping，线程不会被设置中断标示
 *
 * 3. Each task run is preceded by a call to beforeExecute, which
 * might throw an exception, in which case we cause thread to die
 * (breaking loop with completedAbruptly true) without processing
 * the task.
 * 每个任务执行前会调用beforeExecute()，其中可能抛出一个异常，这种情况下会导致线程die（跳出循环，且completedAbruptly==true），没有执行任务
 * 因为beforeExecute()的异常没有cache住，会上抛，跳出循环
 *
 * 4. Assuming beforeExecute completes normally, we run the task,
 * gathering any of its thrown exceptions to send to
 * afterExecute. We separately handle RuntimeException, Error
 * (both of which the specs guarantee that we trap) and arbitrary
 * Throwables.  Because we cannot rethrow Throwables within
 * Runnable.run, we wrap them within Errors on the way out (to the
 * thread's UncaughtExceptionHandler).  Any thrown exception also
 * conservatively causes thread to die.
 * 假定beforeExecute()正常完成，我们执行任务
 * 汇总任何抛出的异常并发送给afterExecute(task, thrown)
 * 因为我们不能在Runnable.run()方法中重新上抛Throwables，我们将Throwables包装到Errors上抛（会到线程的UncaughtExceptionHandler去处理）
 * 任何上抛的异常都会导致线程die
 *
 * 5. After task.run completes, we call afterExecute, which may
 * also throw an exception, which will also cause thread to
 * die. According to JLS Sec 14.20, this exception is the one that
 * will be in effect even if task.run throws.
 * 任务执行结束后，调用afterExecute()，也可能抛异常，也会导致线程die
 * 根据JLS Sec 14.20，这个异常（finally中的异常）会生效
 *
 * The net effect of the exception mechanics is that afterExecute
 * and the thread's UncaughtExceptionHandler have as accurate
 * information as we can provide about any problems encountered by
 * user code.
 *
 * @param w the worker
 */
final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // allow interrupts
                // new Worker()是state==-1，此处是调用Worker类的tryRelease()方法，将state置为0， 而interruptIfStarted()中只有state>=0才允许调用中断
    boolean completedAbruptly = true; //是否“突然完成”，如果是由于异常导致的进入finally，那么completedAbruptly==true就是突然完成的
    try {
    	/**
    	 * 如果task不为null，或者从阻塞队列中getTask()不为null
    	 */
        while (task != null || (task = getTask()) != null) {
            w.lock(); //上锁，不是为了防止并发执行任务，为了在shutdown()时不终止正在运行的worker
            
            // 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
            /**
             * clearInterruptsForTaskRun操作
             * 确保只有在线程stoping时，才会被设置中断标示，否则清除中断标示
             * 1、如果线程池状态>=stop，且当前线程没有设置中断状态，wt.interrupt()
             * 2、如果一开始判断线程池状态<stop，但Thread.interrupted()为true，即线程已经被中断，又清除了中断标示，再次判断线程池状态是否>=stop
             *   是，再次设置中断标示，wt.interrupt()
             *   否，不做操作，清除中断标示后进行后续步骤
             */
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt(); //当前线程调用interrupt()中断
            
            try {
            	//执行前（子类实现）
                beforeExecute(wt, task);
                
                Throwable thrown = null;
                try {
                    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 {
                	//执行后（子类实现）
                    afterExecute(task, thrown); //这里就考验catch和finally的执行顺序了，因为要以thrown为参数
                }
            } 
            finally {
                task = null; //task置为null
                w.completedTasks++; //完成任务数+1
                w.unlock(); //解锁
            }
        }
        
        completedAbruptly = false;
    } 
    finally {
    	//处理worker的退出
        processWorkerExit(w, completedAbruptly);
    }
}

/**
 * Performs blocking or timed wait for a task, depending on
 * current configuration settings, or returns null if this worker
 * must exit because of any of:  以下情况会返回null
 * 1. There are more than maximumPoolSize workers (due to
 *    a call to setMaximumPoolSize).
 *    超过了maximumPoolSize设置的线程数量（因为调用了setMaximumPoolSize()）
 * 2. The pool is stopped.
 *    线程池被stop
 * 3. The pool is shutdown and the queue is empty.
 *    线程池被shutdown，并且workQueue空了
 * 4. This worker timed out waiting for a task, and timed-out
 *    workers are subject to termination (that is,
 *    {@code allowCoreThreadTimeOut || workerCount > corePoolSize})
 *    both before and after the timed wait.
 *    线程等待任务超时
 *
 * @return task, or null if the worker must exit, in which case
 *         workerCount is decremented
 *         返回null表示这个worker要结束了，这种情况下workerCount-1
 */
private Runnable getTask() {
    boolean timedOut = false; // Did the last poll() time out?

    /**
     * 外层循环
     * 用于判断线程池状态
     */
    retry:
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);

        // Check if queue empty only if necessary.
        /**
         * 对线程池状态的判断，两种情况会workerCount-1，并且返回null
         * 线程池状态为shutdown，且workQueue为空（反映了shutdown状态的线程池还是要执行workQueue中剩余的任务的）
         * 线程池状态为stop（shutdownNow()会导致变成STOP）（此时不用考虑workQueue的情况）
         */
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
            decrementWorkerCount(); //循环的CAS减少worker数量，直到成功
            return null;
        }

        boolean timed;      // Are workers subject to culling?
                            // 是否需要定时从workQueue中获取
        
        /**
         * 内层循环
         * 要么break去workQueue获取任务
         * 要么超时了，worker count-1
         */
        for (;;) {
            int wc = workerCountOf(c);
            timed = allowCoreThreadTimeOut || wc > corePoolSize; //allowCoreThreadTimeOut默认为false
                                                                 //如果allowCoreThreadTimeOut为true，说明corePoolSize和maximum都需要定时
            
            //如果当前执行线程数<maximumPoolSize，并且timedOut 和 timed 任一为false，跳出循环，开始从workQueue获取任务
            if (wc <= maximumPoolSize && ! (timedOut && timed))
                break;
            
            /**
             * 如果到了这一步，说明要么线程数量超过了maximumPoolSize（可能maximumPoolSize被修改了）
             * 要么既需要计时timed==true，也超时了timedOut==true
             * worker数量-1，减一执行一次就行了，然后返回null，在runWorker()中会有逻辑减少worker线程
             * 如果本次减一失败，继续内层循环再次尝试减一
             */
            if (compareAndDecrementWorkerCount(c))
                return null;
            
            //如果减数量失败，再次读取ctl
            c = ctl.get();  // Re-read ctl
            
            //如果线程池运行状态发生变化，继续外层循环
            //如果状态没变，继续内层循环
            if (runStateOf(c) != rs)
                continue retry;
            // else CAS failed due to workerCount change; retry inner loop
        }

        try {
        	//poll() - 使用  LockSupport.parkNanos(this, nanosTimeout) 挂起一段时间，interrupt()时不会抛异常，但会有中断响应
        	//take() - 使用 LockSupport.park(this) 挂起，interrupt()时不会抛异常，但会有中断响应
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :    //大于corePoolSize
                workQueue.take();                                        //小于等于corePoolSize
            
            //如获取到了任务就返回
            if (r != null)
                return r;
            
            //没有返回，说明超时，那么在下一次内层循环时会进入worker count减一的步骤
            timedOut = true;
        } 
        /**
			  * blockingQueue的take()阻塞使用LockSupport.park(this)进入wait状态的，对LockSupport.park(this)进行interrupt不会抛异常，但还是会有中断响应
			  * 但AQS的ConditionObject的await()对中断状态做了判断，会报告中断状态 reportInterruptAfterWait(interruptMode)
			  * 就会上抛InterruptedException，在此处捕获，重新开始循环
			  * 如果是由于shutdown()等操作导致的空闲worker中断响应，在外层循环判断状态时，可能return null
			  */
        catch (InterruptedException retry) { 
            timedOut = false; //响应中断，重新开始，中断状态会被清除
        }
    }
}

/**
 * Performs cleanup and bookkeeping for a dying worker. Called
 * only from worker threads. Unless completedAbruptly is set,
 * assumes that workerCount has already been adjusted to account
 * for exit.  This method removes thread from worker set, and
 * possibly terminates the pool or replaces the worker if either
 * it exited due to user task exception or if fewer than
 * corePoolSize workers are running or queue is non-empty but
 * there are no workers.
 *
 * @param w the worker
 * @param completedAbruptly if the worker died due to user exception
 */
private void processWorkerExit(Worker w, boolean completedAbruptly) {
	/**
	 * 1、worker数量-1
	 * 如果是突然终止，说明是task执行时异常情况导致，即run()方法执行时发生了异常，那么正在工作的worker线程数量需要-1
	 * 如果不是突然终止，说明是worker线程没有task可执行了，不用-1，因为已经在getTask()方法中-1了
	 */
    if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted 代码和注释正好相反啊
        decrementWorkerCount();

    /**
     * 2、从Workers Set中移除worker
     */
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        completedTaskCount += w.completedTasks; //把worker的完成任务数加到线程池的完成任务数
        workers.remove(w); //从HashSet<Worker>中移除
    } finally {
        mainLock.unlock();
    }

    /**
     * 3、在对线程池有负效益的操作时，都需要“尝试终止”线程池
     * 主要是判断线程池是否满足终止的状态
     * 如果状态满足，但还有线程池还有线程，尝试对其发出中断响应，使其能进入退出流程
     * 没有线程了，更新状态为tidying->terminated
     */
    tryTerminate();

    /**
     * 4、是否需要增加worker线程
     * 线程池状态是running 或 shutdown
     * 如果当前线程是突然终止的，addWorker()
     * 如果当前线程不是突然终止的，但当前线程数量 < 要维护的线程数量，addWorker()
     * 故如果调用线程池shutdown()，直到workQueue为空前，线程池都会维持corePoolSize个线程，然后再逐渐销毁这corePoolSize个线程
     */
    int c = ctl.get();
    //如果状态是running、shutdown，即tryTerminate()没有成功终止线程池，尝试再添加一个worker
    if (runStateLessThan(c, STOP)) {
    	//不是突然完成的，即没有task任务可以获取而完成的，计算min，并根据当前worker数量判断是否需要addWorker()
        if (!completedAbruptly) {
            int min = allowCoreThreadTimeOut ? 0 : corePoolSize; //allowCoreThreadTimeOut默认为false，即min默认为corePoolSize
            
            //如果min为0，即不需要维持核心线程数量，且workQueue不为空，至少保持一个线程
            if (min == 0 && ! workQueue.isEmpty())
                min = 1;
            
            //如果线程数量大于最少数量，直接返回，否则下面至少要addWorker一个
            if (workerCountOf(c) >= min)
                return; // replacement not needed
        }
        
        //添加一个没有firstTask的worker
        //只要worker是completedAbruptly突然终止的，或者线程数量小于要维护的数量，就新添一个worker线程，即使是shutdown状态
        addWorker(null, false);
    }
}