ThreadPoolExecutor中關於keepAliveTime的解讀

先說結論：它是指大於核心線程數的線程空閒多久後被移除。

首先要明白ThreadPoolExecutor中的Worker是幹嘛的？

它其實是一個持有Task工作任務的線程封裝類，也就是它來直接執行你提交的task任務的run()的。

從execute方法開始

    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();
        }
        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))//這裏就是大於核心線程數且阻塞隊列滿了，嘗試直接執行task的地方，這個方法很多地方會調用很明顯是個核心方法。
            reject(command);
    }

進入addWorker（）

    private boolean addWorker(Runnable firstTask, boolean core) {
        //。。。這裏部分代碼省略，主要是線程數和狀態的校驗，主要看下面的方法
        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            w = new Worker(firstTask);
            final Thread t = w.thread;
            if (t != null) {
                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 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();
                }
                if (workerAdded) {
                    t.start();-------------啓動線程Worker持有的線程。讓我們走進run()方法
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

        public void run() {
            runWorker(this);----------run()方法執行runWorker(this)讓我們繼續向下看。
        }


    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) {//getTask()方法是個阻塞方法，也就是keepAliveTime生效的地方。如果超出這個時間沒獲取到任務則繼續向下執行processWorkerExit。
                //這裏其實就是個循環方法，它會不斷嘗試獲取任務來執行，也就保持線程重複利用。
                //省略其他代碼
               task.run();
                //省略其他代碼
            }
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

繼續看getTask()

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

        for (;;) {
             //這裏依然省略部分代碼，主要是些狀態和線程數的校驗。
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
            //當線程數大於核心線程數且獲取任務超時，會將WorkerCount減一，並返回爲null。然後在調用方移除Worker集合。
            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

            try {
                //這個地方就是利用阻塞隊列的超時時間，返回爲null來完成的。
                //超過keepAliveTime還沒有獲取到taask，則timedOut設置爲true.
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }

總結下worker會通過while循環一直getTask()來執行任務，如果getTask()中阻塞隊列沒有在指定時間返回可執行的任務，則移除當前work。