java - ThreadPoolExecutor如何实现线程复用及超时销毁

1. 线程复用

我们知道 Thread.start 执行之后，线程就能再次执行了，那 ThreadPoolExecutor 是如何做到线程复用的呢? 原理很简单，在实际执行的线程外部套一个 Thread ，外层 Thread 的 run 方法 while 循环执行实际执行线程的 run 方法，实现线程的复用并且执行之后不销毁。下面是伪代码:

// 任务等待队列
BlockingQueue<Runnable> taskQueue = new LinkedBlockingQueue();
new Thread(() -> {
    for (;;){
        Runnable runnable = taskQueue.poll();//队列里拿
        runnable.run();//同步执行
}).start();// 异步while执行

下面是 ThreadPoolExecutor 的重点代码:

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) { // 阻塞获取等待任务
            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(); // 实际阻塞执行
                } 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);
}

2. 线程销毁

我们知道，在创建线程池的时候有超时参数 keepAliveTime ，那么线程池是如何实现精确的超时销毁呢? 这个是结合 BlockingQueue 的阻塞超时来实现的，下面是源码:

 /**
 * ...
 * @return task, or null if the worker must exit, in which case workerCount is decremented
 * 翻译: 返回task，如果worker必须退出，则返回null，在这种情况下workerCount递减
private Runnable getTask() {
    boolean timedOut = false; // Did the last poll() time out?
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);
        // Check if queue empty only if necessary.
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
            decrementWorkerCount();
            return null;
        int wc = workerCountOf(c);
        // Are workers subject to culling?
        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) : // 重点在这，如果超时没有获取到任务，则返回null，销毁线程。
                workQueue.take();
            if (r != null)
                return r;
            timedOut = true;