CachedThreadPool里的线程是如何被回收的？

线程池创建线程的逻辑图：

我们分析CachedThreadPool线程池里的线程是如何被回收的。

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

//ThreadPoolExecutor
public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue<Runnable> workQueue) {
    this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
         Executors.defaultThreadFactory(), defaultHandler);
}

牢牢记住CachedThreadPool的corePoolSize=0, maximumPoolSize=Integer.MAX_VALUE

工作线程的死循环：

//ThreadPoolExecutor
final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
        while (task != null || (task = getTask()) != null) {
            w.lock();
            if ((runStateAtLeast(ctl.get(), STOP) ||
                 (Thread.interrupted() &&
                  runStateAtLeast(ctl.get(), STOP))) &&
                !wt.isInterrupted())
                wt.interrupt();
            try {
                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 {
                    afterExecute(task, thrown);
                }
            } finally {
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}

当工作线程第二次获取的task等于null时，线程将退出while循环，于是就死掉了。

//ThreadPoolExecutor
private Runnable getTask() {
    // 标记poll()是否超时
    boolean timedOut = false; 

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

        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
            //工作线程数-1
            decrementWorkerCount();
            //返回null，工作线程将退出while循环，即线程会死掉
            return null;
        }

        boolean timed;      // Are workers subject to culling?

        for (;;) {
            int wc = workerCountOf(c);
            // allowCoreThreadTimeOut 默认为 false, newCachedThreadPool的corePoolSize为0
            // 所以 timed = false || true，timed恒为true
            timed = allowCoreThreadTimeOut || wc > corePoolSize;
            // 对于newCachedThreadPool，wc 恒小于 maximumPoolSize 
            // 第一次进for循环 true && !(false && true) = true
            // poll超时后，第二次进for循环 true && !(true && true) = false
            if (wc <= maximumPoolSize && ! (timedOut && timed))
                break;
            if (compareAndDecrementWorkerCount(c))
                //poll超时后，第二次进for循环，
                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 {
            //走poll分支
            //poll会阻塞，直到有人调用workQueue.offer；或者超时返回null
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                workQueue.take();
            if (r != null)
                return r;
            //超时
            timedOut = true;
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}

如图中所示：有2种情况会创建工作线程，1. 工作线程数小于corePoolSize；2. 入队失败，且工作线程数小于maximumPoolSize

//ThreadPoolExecutor
public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();

    int c = ctl.get();
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    //对于CachedThreadPool，如果有工作线程在poll中阻塞，则入队成功
    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);
    }
    //对于CachedThreadPool，如果没有工作线程在poll中阻塞，则入队失败
    //初次调用execute，走这个分支，创建工作线程
    else if (!addWorker(command, false))
        reject(command);
}

CachedThreadPool使用的是SynchronousQueue的

入队 ：offer(E e)

出队：poll(long timeout, TimeUnit unit)

工作线程调用poll阻塞，等待timeout时间，如果超时，则返回null并回收线程；如果在等待期内，有任务入队，则成功返回任务，继续执行线程while循环。