Java线程池

2021-02-20

线程池有多重要#####

合理的使用线程池能够带来3个很明显的好处：

降低资源消耗：通过重用已经创建的线程来降低线程创建和销毁的消耗
提高响应速度：任务到达时不需要等待线程创建就可以立即执行。
提高线程的可管理性：线程池可以统一管理、分配、调优和监控。

java多线程池的支持——ThreadPoolExecutor#####

java的线程池支持主要通过ThreadPoolExecutor来实现，我们使用的ExecutorService的各种线程池策略都是基于ThreadPoolExecutor实现的，所以ThreadPoolExecutor十分重要。要弄明白各种线程池策略，必须先弄明白ThreadPoolExecutor。

1. 实现原理#####

首先看一个线程池的流程图：

Paste_Image.png

step1.调用ThreadPoolExecutor的execute提交线程，首先检查CorePool，如果CorePool内的线程小于CorePoolSize，新创建线程执行任务。

step2.如果当前CorePool内的线程大于等于CorePoolSize，那么将线程加入到BlockingQueue。

step3.如果不能加入BlockingQueue，在小于MaxPoolSize的情况下创建线程执行任务。

step4.如果线程数大于等于MaxPoolSize，那么执行拒绝策略。

2.线程池的创建#####

线程池的创建可以通过ThreadPoolExecutor的构造方法实现：

/**
    * Creates a new {@code ThreadPoolExecutor} with the given initial
    * parameters.
    *
    * @param corePoolSize the number of threads to keep in the pool, even
    *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
    * @param maximumPoolSize the maximum number of threads to allow in the
    *        pool
    * @param keepAliveTime when the number of threads is greater than
    *        the core, this is the maximum time that excess idle threads
    *        will wait for new tasks before terminating.
    * @param unit the time unit for the {@code keepAliveTime} argument
    * @param workQueue the queue to use for holding tasks before they are
    *        executed.  This queue will hold only the {@code Runnable}
    *        tasks submitted by the {@code execute} method.
    * @param threadFactory the factory to use when the executor
    *        creates a new thread
    * @param handler the handler to use when execution is blocked
    *        because the thread bounds and queue capacities are reached
    * @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.corePoolSize = corePoolSize;
       this.maximumPoolSize = maximumPoolSize;
       this.workQueue = workQueue;
       this.keepAliveTime = unit.toNanos(keepAliveTime);
       this.threadFactory = threadFactory;
       this.handler = handler;
   }

具体解释一下上述参数：

corePoolSize 核心线程池大小
maximumPoolSize 线程池最大容量大小
keepAliveTime 线程池空闲时，线程存活的时间
TimeUnit 时间单位
ThreadFactory 线程工厂
BlockingQueue任务队列
RejectedExecutionHandler 线程拒绝策略

3.线程的提交#####

ThreadPoolExecutor的构造方法如上所示，但是只是做一些参数的初始化，ThreadPoolExecutor被初始化好之后便可以提交线程任务，线程的提交方法主要是execute和submit。这里主要说execute，submit会在后续的博文中分析。

/**
 * Executes the given task sometime in the future.  The task
 * may execute in a new thread or in an existing pooled thread.
 *
 * 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}.
 *
 * @param command the task to execute
 * @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. 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.
     * 如果当前的线程数小于核心线程池的大小，根据现有的线程作为第一个Worker运行的线程，
     * 新建一个Worker，addWorker自动的检查当前线程池的状态和Worker的数量，
     * 防止线程池在不能添加线程的状态下添加线程
     *
     * 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.
     *  如果线程入队成功，然后还是要进行double-check的，因为线程池在入队之后状态是可能会发生变化的
     *
     * 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task不能入队(队列满了)，这时候尝试增加一个新线程，如果增加失败那么当前的线程池状态变化了或者线程池已经满了
     * 然后拒绝task
     */
     
    int c = ctl.get();
    //当前的Worker的数量小于核心线程池大小时，新建一个Worker。
    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))//recheck防止线程池状态的突变，如果突变，那么将reject线程，防止workQueue中增加新线程
            reject(command);
        else if (workerCountOf(recheck) == 0)//上下两个操作都有addWorker的操作，但是如果在workQueue.offer的时候Worker变为0，
                                            //那么将没有Worker执行新的task，所以增加一个Worker.
            addWorker(null, false);
    }
    //如果workQueue满了，那么这时候可能还没到线程池的maxnum，所以尝试增加一个Worker
    else if (!addWorker(command, false))
        reject(command);//如果Worker数量到达上限，那么就拒绝此线程
}

这里需要明确几个概念：

Worker和Task的区别，Worker是当前线程池中的线程，而task虽然是runnable，但是并没有真正执行，只是被Worker调用了run方法，后面会看到这部分的实现。
maximumPoolSize和corePoolSize的区别：这个概念很重要，maximumPoolSize为线程池最大容量，也就是说线程池最多能起多少Worker。corePoolSize是核心线程池的大小，当corePoolSize满了时，同时workQueue full（ArrayBolckQueue是可能满的）那么此时允许新建Worker去处理workQueue中的Task，但是不能超过maximumPoolSize。超过corePoolSize之外的线程会在空闲超时后终止。

核心方法：addWorker#####

Worker的增加和Task的获取以及终止都是在此方法中实现的，也就是这一个方法里面包含了很多东西。在addWorker方法中提到了Status的概念，Status是线程池的核心概念，这里我们先看一段关于status的注释：

/**
     * 首先ctl是一个原子量，同时它里面包含了两个field，一个是workerCount，另一个是runState
     * workerCount表示当前有效的线程数，也就是Worker的数量
     * runState表示当前线程池的状态
     * The main pool control state, ctl, is an atomic integer packing
     * two conceptual fields
     *   workerCount, indicating the effective number of threads
     *   runState,    indicating whether running, shutting down etc
     * 
     * 两者是怎么结合的呢？首先workerCount是占据着一个atomic integer的后29位的，而状态占据了前3位
     * 所以，workerCount上限是(2^29)-1。
     * In order to pack them into one int, we limit workerCount to
     * (2^29)-1 (about 500 million) threads rather than (2^31)-1 (2
     * billion) otherwise representable. If this is ever an issue in
     * the future, the variable can be changed to be an AtomicLong,
     * and the shift/mask constants below adjusted. But until the need
     * arises, this code is a bit faster and simpler using an int.
     *
     * The workerCount is the number of workers that have been
     * permitted to start and not permitted to stop.  The value may be
     * transiently different from the actual number of live threads,
     * for example when a ThreadFactory fails to create a thread when
     * asked, and when exiting threads are still performing
     * bookkeeping before terminating. The user-visible pool size is
     * reported as the current size of the workers set.
     *
     * runState是整个线程池的运行生命周期，有如下取值：
     *  1. RUNNING：可以新加线程，同时可以处理queue中的线程。
     *  2. SHUTDOWN：不增加新线程，但是处理queue中的线程。
     *  3.STOP 不增加新线程，同时不处理queue中的线程。
     *  4.TIDYING 所有的线程都终止了（queue中），同时workerCount为0，那么此时进入TIDYING
     *  5.terminated()方法结束，变为TERMINATED
     * The runState provides the main lifecyle control, taking on values:
     *
     *   RUNNING:  Accept new tasks and process queued tasks
     *   SHUTDOWN: Don't accept new tasks, but process queued tasks
     *   STOP:     Don't accept new tasks, don't process queued tasks,
     *             and interrupt in-progress tasks
     *   TIDYING:  All tasks have terminated, workerCount is zero,
     *             the thread transitioning to state TIDYING
     *             will run the terminated() hook method
     *   TERMINATED: terminated() has completed
     *
     * The numerical order among these values matters, to allow
     * ordered comparisons. The runState monotonically increases over
     * time, but need not hit each state. The transitions are:
     * 状态的转化主要是：
     * RUNNING -> SHUTDOWN（调用shutdown()）
     *    On invocation of shutdown(), perhaps implicitly in finalize()
     * (RUNNING or SHUTDOWN) -> STOP(调用shutdownNow())
     *    On invocation of shutdownNow()
     * SHUTDOWN -> TIDYING（queue和pool均empty）
     *    When both queue and pool are empty
     * STOP -> TIDYING（pool empty，此时queue已经为empty）
     *    When pool is empty
     * TIDYING -> TERMINATED(调用terminated())
     *    When the terminated() hook method has completed
     *
     * Threads waiting in awaitTermination() will return when the
     * state reaches TERMINATED.
     *
     * Detecting the transition from SHUTDOWN to TIDYING is less
     * straightforward than you'd like because the queue may become
     * empty after non-empty and vice versa during SHUTDOWN state, but
     * we can only terminate if, after seeing that it is empty, we see
     * that workerCount is 0 (which sometimes entails a recheck -- see
     * below).
     */

下面是状态的代码：

//利用ctl来保证当前线程池的状态和当前的线程的数量。ps：低29位为线程池容量，高3位为线程状态。
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    //设定偏移量
    private static final int COUNT_BITS = Integer.SIZE - 3;
    //确定最大的容量2^29-1
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;
    //几个状态，用Integer的高三位表示
    // runState is stored in the high-order bits
    //111
    private static final int RUNNING    = -1 << COUNT_BITS;
    //000
    private static final int SHUTDOWN   =  0 << COUNT_BITS;
    //001
    private static final int STOP       =  1 << COUNT_BITS;
    //010
    private static final int TIDYING    =  2 << COUNT_BITS;
    //011
    private static final int TERMINATED =  3 << COUNT_BITS;
    //获取线程池状态，取前三位
    // Packing and unpacking ctl
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
    //获取当前正在工作的worker,主要是取后面29位
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    //获取ctl
    private static int ctlOf(int rs, int wc) { return rs | wc; }

接下来贴上addWorker方法看看：

/**
 * 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, which requires a
 * backout of workerCount, and a recheck for termination, in case
 * the existence of this worker was holding up termination.
 *
 * @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 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) {
    retry:
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);
        // Check if queue empty only if necessary.
        /**
         * rs!=Shutdown || fistTask！=null || workCount.isEmpty
         * 如果当前的线程池的状态>SHUTDOWN 那么拒绝Worker的add 如果=SHUTDOWN
         * 那么此时不能新加入不为null的Task，如果在WorkCount为empty的时候不能加入任何类型的Worker，
         * 如果不为empty可以加入task为null的Worker,增加消费的Worker
         */
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;

        for (;;) {
            int wc = workerCountOf(c);
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            if (compareAndIncrementWorkerCount(c))
                break retry;
            c = ctl.get();  // Re-read ctl
            if (runStateOf(c) != rs)
                continue retry;
            // else CAS failed due to workerCount change; retry inner loop
        }
    }

    Worker w = new Worker(firstTask);
    Thread t = w.thread;

    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        // Recheck while holding lock.
        // Back out on ThreadFactory failure or if
        // shut down before lock acquired.
        int c = ctl.get();
        int rs = runStateOf(c);
        /**
         * rs!=SHUTDOWN ||firstTask!=null
         * 
         * 同样检测当rs>SHUTDOWN时直接拒绝减小Wc，同时Terminate，如果为SHUTDOWN同时firstTask不为null的时候也要Terminate
         */
        if (t == null ||
            (rs >= SHUTDOWN &&
             ! (rs == SHUTDOWN &&
                firstTask == null))) {
            decrementWorkerCount();
            tryTerminate();
            return false;
        }

        workers.add(w);

        int s = workers.size();
        if (s > largestPoolSize)
            largestPoolSize = s;
    } finally {
        mainLock.unlock();
    }

    t.start();
    // It is possible (but unlikely) for a thread to have been
    // added to workers, but not yet started, during transition to
    // STOP, which could result in a rare missed interrupt,
    // because Thread.interrupt is not guaranteed to have any effect
    // on a non-yet-started Thread (see Thread#interrupt).
    //Stop或线程Interrupt的时候要中止所有的运行的Worker
    if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted())
        t.interrupt();
    return true;
}

addWorker中首先进行了一次线程池状态的检测：

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

           // Check if queue empty only if necessary.
           //判断当前线程池的状态是不是已经shutdown，如果shutdown了拒绝线程加入
           //(rs!=SHUTDOWN || first!=null || workQueue.isEmpty())
           //如果rs不为SHUTDOWN，此时状态是STOP、TIDYING或TERMINATED，所以此时要拒绝请求
           //如果此时状态为SHUTDOWN，而传入一个不为null的线程，那么需要拒绝
           //如果状态为SHUTDOWN，同时队列中已经没任务了，那么拒绝掉
           if (rs >= SHUTDOWN &&
               ! (rs == SHUTDOWN &&
                  firstTask == null &&
                  ! workQueue.isEmpty()))
               return false;

其实是比较难懂的，主要在线程池状态判断条件这里：

如果是runing，那么跳过if。
如果rs>=SHUTDOWN,同时不等于SHUTDOWN，即为SHUTDOWN以上的状态，那么不接受新线程。
如果rs>=SHUTDOWN，同时等于SHUTDOWN，同时first！=null，那么拒绝新线程，如果first==null，那么可能是新增加线程消耗Queue中的线程。但是同时还要检测workQueue是否isEmpty()，如果为Empty，那么队列已空，不需要增加消耗线程，如果队列没有空那么运行增加first=null的Worker。

**从这里是可以看出一些策略的**
**首先，在rs>SHUTDOWN时，拒绝一切线程的增加，因为STOP是会终止所有的线程，同时移除Queue中所有的待执行的线程的，所以也不需要增加first=null的Worker了**
**其次，在SHUTDOWN状态时，是不能增加first！=null的Worker的，同时即使first=null，但是此时Queue为Empty也是不允许增加Worker的，SHUTDOWN下增加的Worker主要用于消耗Queue中的任务。**
**SHUTDOWN状态时，是不允许向workQueue中增加线程的，isRunning(c) && workQueue.offer(command) 每次在offer之前都要做状态检测，也就是线程池状态变为>=SHUTDOWN时不允许新线程进入线程池了。**

for (;;) {
    int wc = workerCountOf(c);
    //如果当前的数量超过了CAPACITY，或者超过了corePoolSize和maximumPoolSize（试core而定）
    if (wc >= CAPACITY ||
        wc >= (core ? corePoolSize : maximumPoolSize))
        return false;
    //CAS尝试增加线程数，如果失败，证明有竞争，那么重新到retry。
    if (compareAndIncrementWorkerCount(c))
        break retry;
    c = ctl.get();  // Re-read ctl
    //判断当前线程池的运行状态
    if (runStateOf(c) != rs)
        continue retry;
    // else CAS failed due to workerCount change; retry inner loop
}

这段代码做了一个兼容，主要是没有到corePoolSize 或maximumPoolSize上限时，那么允许添加线程，CAS增加Worker的数量后，跳出循环。
接下来实例化Worker,实例化Worker其实是很关键的，后面会说。
因为workers是HashSet线程不安全的，那么此时需要加锁，所以mainLock.lock(); 之后重新检查线程池的状态，如果状态不正确，那么减小Worker的数量，为什么tryTerminate（）目前不大清楚。如果状态正常，那么添加Worker到workers。最后：

1 2	if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted()) t.interrupt();

注释说的很清楚，为了能及时的中断此Worker，因为线程存在未Start的情况，此时是不能响应中断的，如果此时status变为STOP，则不能中断线程。此处用作中断线程之用。
接下来我们看Worker的方法：

/**
        * Creates with given first task and thread from ThreadFactory.
        * @param firstTask the first task (null if none)
        */
       Worker(Runnable firstTask) {
           this.firstTask = firstTask;
           this.thread = getThreadFactory().newThread(this);
       }

这里可以看出Worker是对firstTask的包装，并且Worker本身就是Runnable的，看上去真心很流氓的感觉~
通过ThreadFactory为Worker自己构建一个线程。
因为Worker是Runnable类型的，所以是有run方法的,上面也看到了会调用t.start() 其实就是执行了run方法：

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

调用了runWorker:

/**
     * Main worker run loop.  Repeatedly gets tasks from queue and
     * executes them, while coping with a number of issues:
     * 1 Worker可能还是执行一个初始化的task——firstTask。
     *    但是有时也不需要这个初始化的task（可以为null）,只要pool在运行，就会
     *   通过getTask从队列中获取Task，如果返回null，那么worker退出。
     *   另一种就是external抛出异常导致worker退出。
     * 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.
     * 
     * 
     * 2 在运行任何task之前，都需要对worker加锁来防止other pool中断worker。
     *   clearInterruptsForTaskRun保证除了线程池stop，那么现场都没有中断标志
     * 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.
     *
     * 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.
     *
     * 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.
     *
     * 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.
     *
     * 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) {
        Runnable task = w.firstTask;
        w.firstTask = null;
        //标识线程是不是异常终止的
        boolean completedAbruptly = true;
        try {
            //task不为null情况是初始化worker时，如果task为null，则去队列中取线程--->getTask()
            while (task != null || (task = getTask()) != null) {
                w.lock();
                //获取woker的锁，防止线程被其他线程中断
                clearInterruptsForTaskRun();//清楚所有中断标记
                try {
                    beforeExecute(w.thread, task);//线程开始执行之前执行此方法，可以实现Worker未执行退出，本类中未实现
                    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 {
                        afterExecute(task, thrown);//线程执行后执行，可以实现标识Worker异常中断的功能，本类中未实现
                    }
                } finally {
                    task = null;//运行过的task标null
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            //处理worker退出的逻辑
            processWorkerExit(w, completedAbruptly);
        }
    }

从上面代码可以看出，execute的Task是被“包装 ”了一层，线程启动时是内部调用了Task的run方法。
接下来所有的核心集中在getTask()方法上：

/**
     * 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:
     * 1. There are more than maximumPoolSize workers (due to
     *    a call to setMaximumPoolSize).
     * 2. The pool is stopped.
     * 3. The pool is shutdown and the queue is empty.
     * 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.
     *
     * @return task, or null if the worker must exit, in which case
     *         workerCount is decremented
     *         
     *         
     *  队列中获取线程
     */
    private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?

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

            // Check if queue empty only if necessary.
            //当前状态为>stop时，不处理workQueue中的任务，同时减小worker的数量所以返回null，如果为shutdown 同时workQueue已经empty了，同样减小worker数量并返回null
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }

            boolean timed;      // Are workers subject to culling?

            for (;;) {
                int wc = workerCountOf(c);
                timed = allowCoreThreadTimeOut || wc > corePoolSize;

                if (wc <= maximumPoolSize && ! (timedOut && timed))
                    break;
                if (compareAndDecrementWorkerCount(c))
                    return null;
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }

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

这段代码十分关键，首先看几个局部变量：
boolean timedOut = false;
主要是判断后面的poll是否要超时
boolean timed;
主要是标识着当前Worker超时是否要退出。wc > corePoolSize时需要减小空闲的Worker数，那么timed为true，但是wc <= corePoolSize时，不能减小核心线程数timed为false。
timedOut初始为false，如果timed为true那么使用poll取线程。如果正常返回，那么返回取到的task。如果超时，证明worker空闲，同时worker超过了corePoolSize，需要删除。返回r=null。则 timedOut = true。此时循环到wc <= maximumPoolSize && ! (timedOut && timed)时，减小worker数，并返回null，导致worker退出。如果线程数<= corePoolSize，那么此时调用 workQueue.take()，没有线程获取到时将一直阻塞，知道获取到线程或者中断，关于中断后面Shutdown的时候会说。

至此线程执行过程就分析完了~

关于终止线程池#####

我个人认为，如果想了解明白线程池，那么就一定要理解好各个状态之间的转换，想理解转换，线程池的终止机制是很好的一个途径。对于关闭线程池主要有两个方法shutdown()和shutdownNow():
首先从shutdown()方法开始：

/**
 * Initiates an orderly shutdown in which previously submitted
 * tasks are executed, but no new tasks will be accepted.
 * Invocation has no additional effect if already shut down.
 *
 * <p>This method does not wait for previously submitted tasks to
 * complete execution.  Use {@link #awaitTermination awaitTermination}
 * to do that.
 *
 * @throws SecurityException {@inheritDoc}
 */
public void shutdown() {
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        //判断是否可以操作目标线程
        checkShutdownAccess();
        //设置线程池状态为SHUTDOWN,此处之后，线程池中不会增加新Task
        advanceRunState(SHUTDOWN);
        //中断所有的空闲线程
        interruptIdleWorkers();
        onShutdown(); // hook for ScheduledThreadPoolExecutor
    } finally {
        mainLock.unlock();
    }
    //转到Terminate
    tryTerminate();
}

shutdown做了几件事：
1. 检查是否能操作目标线程
2. 将线程池状态转为SHUTDOWN
3. 中断所有空闲线程
这里就引发了一个问题，什么是空闲线程？
这需要接着看看interruptIdleWorkers是怎么回事。

private void interruptIdleWorkers(boolean onlyOne) {
       final ReentrantLock mainLock = this.mainLock;
       mainLock.lock();
       //这里的意图很简单，遍历workers 对所有worker做中断处理。
       // w.tryLock()对Worker加锁，这保证了正在运行执行Task的Worker不会被中断，那么能中断哪些线程呢？
       try {
           for (Worker w : workers) {
               Thread t = w.thread;
               if (!t.isInterrupted() && w.tryLock()) {
                   try {
                       t.interrupt();
                   } catch (SecurityException ignore) {
                   } finally {
                       w.unlock();
                   }
               }
               if (onlyOne)
                   break;
           }
       } finally {
           mainLock.unlock();
       }
   }

这里主要是为了中断worker，但是中断之前需要先获取锁，这就意味着正在运行的Worker不能中断。但是上面的代码有w.tryLock()，那么获取不到锁就不会中断，shutdown的Interrupt只是对所有的空闲Worker（正在从workQueue中取Task，此时Worker没有加锁）发送中断信号。

while (task != null || (task = getTask()) != null) {
    w.lock();
    //获取woker的锁，防止线程被其他线程中断
    clearInterruptsForTaskRun();//清楚所有中断标记
    try {
        beforeExecute(w.thread, task);//线程开始执行之前执行此方法，可以实现Worker未执行退出，本类中未实现
        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 {
            afterExecute(task, thrown);//线程执行后执行，可以实现标识Worker异常中断的功能，本类中未实现
        }
    } finally {
        task = null;//运行过的task标null
        w.completedTasks++;
        w.unlock();
    }
}

在runWorker中，每一个Worker getTask成功之后都要获取Worker的锁之后运行，也就是说运行中的Worker不会中断。因为核心线程一般在空闲的时候会一直阻塞在获取Task上，也只有中断才可能导致其退出。这些阻塞着的Worker就是空闲的线程（当然，非核心线程，并且阻塞的也是空闲线程）。在getTask方法中：

private Runnable getTask() {
    boolean timedOut = false; // Did the last poll() time out?

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

        // Check if queue empty only if necessary.
        //当前状态为>stop时，不处理workQueue中的任务，同时减小worker的数量所以返回null，如果为shutdown 同时workQueue已经empty了，同样减小worker数量并返回null
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
            decrementWorkerCount();
            return null;
        }

        boolean timed;      // Are workers subject to culling?

        for (;;) {
            //allowCoreThreadTimeOu是判断CoreThread是否会超时的，true为会超时，false不会超时。默认为false
            int wc = workerCountOf(c);
            timed = allowCoreThreadTimeOut || wc > corePoolSize;

            if (wc <= maximumPoolSize && ! (timedOut && timed))
                break;
            if (compareAndDecrementWorkerCount(c))
                return null;
            c = ctl.get();  // Re-read ctl
            if (runStateOf(c) != rs)
                continue retry;
            // else CAS failed due to workerCount change; retry inner loop
        }

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

会有两阶段的Worker：

刚进入getTask()，还没进行状态判断。
block在poll或者take上的Worker。

当调用ShutDown方法时，首先设置了线程池的状态为ShutDown，此时1阶段的worker进入到状态判断时会返回null，此时Worker退出。
因为getTask的时候是不加锁的，所以在shutdown时可以调用worker.Interrupt.此时会中断退出，Loop到状态判断时，同时workQueue为empty。那么抛出中断异常，导致重新Loop，在检测线程池状态时，Worker退出。如果workQueue不为null就不会退出，此处有些疑问，因为没有看见中断标志位清除的逻辑，那么这里就会不停的循环直到workQueue为Empty退出。
这里也能看出来SHUTDOWN只是清除一些空闲Worker，并且拒绝新Task加入，对于workQueue中的线程还是继续处理的。
对于shutdown中获取mainLock而addWorker中也做了mainLock的获取，这么做主要是因为Works是HashSet类型的，是线程不安全的，我们也看到在addWorker后面也是对线程池状态做了判断，将Worker添加和中断逻辑分离开。
接下来做了tryTerminate()操作，这操作是进行了后面状态的转换，在shutdownNow后面说。
接下来看看shutdownNow：

/**
 * Attempts to stop all actively executing tasks, halts the
 * processing of waiting tasks, and returns a list of the tasks
 * that were awaiting execution. These tasks are drained (removed)
 * from the task queue upon return from this method.
 *
 * <p>This method does not wait for actively executing tasks to
 * terminate.  Use {@link #awaitTermination awaitTermination} to
 * do that.
 *
 * <p>There are no guarantees beyond best-effort attempts to stop
 * processing actively executing tasks.  This implementation
 * cancels tasks via {@link Thread#interrupt}, so any task that
 * fails to respond to interrupts may never terminate.
 *
 * @throws SecurityException {@inheritDoc}
 */
public List<Runnable> shutdownNow() {
    List<Runnable> tasks;
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        checkShutdownAccess();
        advanceRunState(STOP);
        interruptWorkers();
        tasks = drainQueue();
    } finally {
        mainLock.unlock();
    }
    tryTerminate();
    return tasks;
}

shutdownNow和shutdown代码类似，但是实现却很不相同。首先是设置线程池状态为STOP，前面的代码我们可以看到，是对SHUTDOWN有一些额外的判断逻辑，但是对于>=STOP,基本都是reject，STOP也是比SHUTDOWN更加严格的一种状态。此时不会有新Worker加入，所有刚执行完一个线程后去GetTask的Worker都会退出。
之后调用interruptWorkers：

/**
 * Interrupts all threads, even if active. Ignores SecurityExceptions
 * (in which case some threads may remain uninterrupted).
 */
private void interruptWorkers() {
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        for (Worker w : workers) {
            try {
                w.thread.interrupt();
            } catch (SecurityException ignore) {
            }
        }
    } finally {
        mainLock.unlock();
    }
}

这里可以看出来，此方法目的是中断所有的Worker，而不是像shutdown中那样只中断空闲线程。这样体现了STOP的特点，中断所有线程，同时workQueue中的Task也不会执行了。所以接下来drainQueue：

/**
  * Drains the task queue into a new list, normally using
  * drainTo. But if the queue is a DelayQueue or any other kind of
  * queue for which poll or drainTo may fail to remove some
  * elements, it deletes them one by one.
  */
 private List<Runnable> drainQueue() {
     BlockingQueue<Runnable> q = workQueue;
     List<Runnable> taskList = new ArrayList<Runnable>();
     q.drainTo(taskList);
     if (!q.isEmpty()) {
         for (Runnable r : q.toArray(new Runnable[0])) {
             if (q.remove(r))
                 taskList.add(r);
         }
     }
     return taskList;
 }

获取所有没有执行的Task，并且返回。
这也体现了STOP的特点：
拒绝所有新Task的加入，同时中断所有线程，WorkerQueue中没有执行的线程全部抛弃。所以此时Pool是空的，WorkerQueue也是空的。
这之后就是进行到TIDYING和TERMINATED的转化了：

/**
 * Transitions to TERMINATED state if either (SHUTDOWN and pool
 * and queue empty) or (STOP and pool empty).  If otherwise
 * eligible to terminate but workerCount is nonzero, interrupts an
 * idle worker to ensure that shutdown signals propagate. This
 * method must be called following any action that might make
 * termination possible -- reducing worker count or removing tasks
 * from the queue during shutdown. The method is non-private to
 * allow access from ScheduledThreadPoolExecutor.
 */
final void tryTerminate() {
    for (;;) {
        int c = ctl.get();
        if (isRunning(c) ||
            runStateAtLeast(c, TIDYING) ||
            (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
            return;
        if (workerCountOf(c) != 0) { // Eligible to terminate
            interruptIdleWorkers(ONLY_ONE);
            return;
        }

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
                try {
                    terminated();
                } finally {
                    ctl.set(ctlOf(TERMINATED, 0));
                    termination.signalAll();
                }
                return;
            }
        } finally {
            mainLock.unlock();
        }
        // else retry on failed CAS
    }
}

上面的代码其实很有意思有几种状态是不能转化到TIDYING的:

RUNNING状态
TIDYING或TERMINATED
SHUTDOWN状态，但是workQueue不为空

也说明了两点：
1. SHUTDOWN想转化为TIDYING，需要workQueue为空，同时workerCount为0。
2. STOP转化为TIDYING，需要workerCount为0
如果满足上面的条件（一般一定时间后都会满足的），那么CAS成TIDYING，TIDYING也只是个过度状态，最终会转化为TERMINATED。

至此，ThreadPoolExecutor一些核心思想就介绍完了，想分析清楚实在是不容易，对于ThreadPoolExecutor我还是有些不懂地方，以上只是我对源码的片面的见解，如果有不正确之处，希望大神能不吝赐教。同时也希望给正在研究ThreadPoolExecutor的童鞋提供一点帮助。

作者：一只小哈
链接：https://www.jianshu.com/p/ade771d2c9c0
来源：简书
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