ThreadPoolExecutor源码

大纲

给线程池中添加任务

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))
            reject(command);
    }

新建Worker开始执行或添加任务到队列

private boolean addWorker(Runnable firstTask, boolean core) {
		// Worker数+1
        retry:
        for (int c = ctl.get();;) {
            // 1、当线程池状态为SHUTDOWN并且
            // 2、当线程池状态为STOP或第一个任务不为null或工作队列为空
            if (runStateAtLeast(c, SHUTDOWN)
                && (runStateAtLeast(c, STOP) || firstTask != null || workQueue.isEmpty()))
                return false;

            for (;;) {
                if (workerCountOf(c)
                    >= ((core ? corePoolSize : maximumPoolSize) & COUNT_MASK))
                    return false;
                // CAS 修改Worker数+1
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // Re-read ctl
                if (runStateAtLeast(c, SHUTDOWN))
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }
        }

        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 c = ctl.get();

                    if (isRunning(c) ||
                        (runStateLessThan(c, STOP) && firstTask == null)) {
                        if (t.getState() != Thread.State.NEW)
                            throw new IllegalThreadStateException();
                        workers.add(w);
                        workerAdded = true;
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                	// SharedThreadContainer
                    container.start(t);
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

runWorker

• 执行在队列中等待的任务
• 从队列中获不到待执行的任务时线程退出

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 执行任务，
        	// 第一次循环执行firstTask，
        	// 第二次循环开始从getTask()获取任务
        	// 当getTask()获取不到任务时线程退出
            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);
                    try {
                    	// 执行任务
                        task.run();
                        afterExecute(task, null);
                    } catch (Throwable ex) {
                        afterExecute(task, ex);
                        throw ex;
                    }
                } finally {
                	// 将第一次循环中的任务重置为null
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

getTask

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

        for (;;) {
            int c = ctl.get();

            // Check if queue empty only if necessary.
            if (runStateAtLeast(c, SHUTDOWN)
                && (runStateAtLeast(c, 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) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }