詳解Java線程池是如何重復利用空閑線程的

public static ExecutorService newFixedThreadPool(int var0) {
    return new ThreadPoolExecutor(var0, var0, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue());
}
public static ExecutorService newSingleThreadExecutor() {
    return new Executors.FinalizableDelegatedExecutorService(new ThreadPoolExecutor(1, 1, 0L, TimeUnit.MILLISECONDS, new LinkedBlockingQueue()));
}
public static ExecutorService newCachedThreadPool() {
    return new ThreadPoolExecutor(0, 2147483647, 60L, TimeUnit.SECONDS, new SynchronousQueue());
}
public static ScheduledExecutorService newSingleThreadScheduledExecutor() {
    return new Executors.DelegatedScheduledExecutorService(new ScheduledThreadPoolExecutor(1));
}
public static ScheduledExecutorService newScheduledThreadPool(int var0) {
    return new ScheduledThreadPoolExecutor(var0);
}

/**
 * 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;
}

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.
     *
     * 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.
     *
     * 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.
     */
    int c = ctl.get();
    if (workerCountOf(c) < corePoolSize) { //第一步：如果線程數量小於核心線程數
        if (addWorker(command, true))//則啟動一個核心線程執行任務
            return;
        c = ctl.get();
    }
    if (isRunning(c) && workQueue.offer(command)) {//第二步：當前線程數量大於等於核心線程數，加入任務隊列，成功的話會進行二次檢查
        int recheck = ctl.get();
        if (! isRunning(recheck) && remove(command))
            reject(command);
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);//啟動非核心線程執行，註意這裡任務是null，其實裡面會去取任務隊列裡的任務執行
    }
    else if (!addWorker(command, false))//第三步：加入不瞭隊列（即隊列滿瞭），嘗試啟動非核心線程
        reject(command);//如果啟動不瞭非核心線程執行，說明到達瞭最大線程數量的限制，會使用第7個參數拋出異常
}

// 獲取運行狀態
private static int runStateOf(int c)     { return c & ~CAPACITY; }

// 獲取活動線程數
private static int workerCountOf(int c)  { return c & CAPACITY; }

private boolean addWorker(Runnable firstTask, boolean core) {
    retry:
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);

        // Check if queue empty only if necessary.
        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
        }
    }
    //前面都是線程池狀態的判斷，暫時不理會，主要看下面兩個關鍵的地方
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        w = new Worker(firstTask); // 新建一個Worker對象，這個對象包含瞭待執行的任務，並且新建一個線程
        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(); // 啟動剛創建的worker對象裡面的thread執行
                workerStarted = true;
            }
        }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

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;
    /** 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) {
        setState(-1); // inhibit interrupts until runWorker
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this);
    }

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

    // Lock methods
    //
    // The value 0 represents the unlocked state.
    // The value 1 represents the locked state.

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

    protected boolean tryAcquire(int unused) {
        if (compareAndSetState(0, 1)) {
            setExclusiveOwnerThread(Thread.currentThread());
            return true;
        }
        return false;
    }

    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(); }

    void interruptIfStarted() {
        Thread t;
        if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
            try {
                t.interrupt();
            } catch (SecurityException ignore) {
            }
        }
    }
}

Worker(Runnable firstTask) { // worker本身實現瞭Runnable接口
        setState(-1); // inhibit interrupts until runWorker
        this.firstTask = firstTask; // 持有外部傳進來的runnable任務
        //創建瞭一個thread對象，並把自身這個runnable對象給瞭thread，一旦該thread執行start方法，就會執行worker的run方法
        this.thread = getThreadFactory().newThread(this); 
    }
在addWorker方法中執行的t.start會去執行worker的run方法：

public void run() {
        runWorker(this);
    }
run方法又執行瞭ThreadPoolExecutor的runWorker方法，把當前worker對象傳入。

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask; // 取出worker的runnable任務
    w.firstTask = null;
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
        // 循環不斷的判斷任務是否為空，當第一個判斷為false的時候，即task為null，這個task啥時候為null呢？
        // 要麼w.firstTask為null，還記得我們在execute方法第二步的時候，執行addWorker的時候傳進來的runnable是null嗎？
        // 要麼是執行瞭一遍while循環，在下面的finally中執行瞭task=null；
        // 或者執行第二個判斷，一旦不為空就會繼續執行循環裡的代碼。
        while (task != null || (task = getTask()) != null) {
            w.lock();
            // 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
            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(); // 任務不為空，就會執行任務的run方法，也就是runnable的run方法
                } 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; // 執行完成置null，繼續下一個循環
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}

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);

        // timed變量用於判斷是否需要進行超時控制。
        // allowCoreThreadTimeOut默認是false，也就是核心線程不允許進行超時；
        // wc > corePoolSize，表示當前線程池中的線程數量大於核心線程數量；
        // 對於超過核心線程數量的這些線程或者允許核心線程進行超時控制的時候，需要進行超時控制
        // Are workers subject to culling?
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

        // 如果需要進行超時控制，且上次從緩存隊列中獲取任務時發生瞭超時（timedOut開始為false，後面的循環末尾超時時會置為true)
        // 或者當前線程數量已經超過瞭最大線程數量，那麼嘗試將workerCount減1,即當前活動線程數減1，
        if ((wc > maximumPoolSize || (timed && timedOut))
            && (wc > 1 || workQueue.isEmpty())) {
            // 如果減1成功，則返回null，這就意味著runWorker()方法中的while循環會被退出，其對應的線程就要銷毀瞭，也就是線程池中少瞭一個線程瞭
            if (compareAndDecrementWorkerCount(c))
                return null;
            continue;
        }

        try {
            // 註意workQueue中的poll()方法與take()方法的區別
            //poll方式取任務的特點是從緩存隊列中取任務,最長等待keepAliveTime的時長，取不到返回null
            //take方式取任務的特點是從緩存隊列中取任務，若隊列為空,則進入阻塞狀態，直到能取出對象為止
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                workQueue.take();
            if (r != null)
                return r;
            timedOut = true; // 能走到這裡說明已經超時瞭
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}