Java线程池

ExecutorService executorService = new ThreadPoolExecutor(
        1,
        1,
        3,
        TimeUnit.SECONDS,
        new ArrayBlockingQueue<>(2),
        (r) -> new Thread(r),
        (r, executor) -> System.out.println("拒绝"));

for (int i = 0; i < 5; i++) {
    executorService.submit(() -> {
        System.out.println(Thread.currentThread().getName());
        sleep(TimeUnit.MILLISECONDS, 50);
    });
}


----------------------------------------------执行结果----------------------------------------------
拒绝
拒绝
Thread-0
Thread-0
Thread-0

RUNNING -> SHUTDOWN
   On invocation of shutdown(), perhaps implicitly in finalize()
(RUNNING or SHUTDOWN) -> STOP
   On invocation of shutdownNow()
SHUTDOWN -> TIDYING
   When both queue and pool are empty
STOP -> TIDYING
   When pool is empty
TIDYING -> TERMINATED
   When the terminated() hook method has completed

public ThreadPoolExecutor(int corePoolSize,
                            int maximumPoolSize,
                            long keepAliveTime,
                            TimeUnit unit,
                            BlockingQueue<Runnable> workQueue,
                            ThreadFactory threadFactory,
                            RejectedExecutionHandler handler) {
    if (workQueue == null || threadFactory == null || handler == null)
        throw new NullPointerException();
    this.acc = System.getSecurityManager() == null ?null : AccessController.getContext();
    this.corePoolSize = corePoolSize;
    this.maximumPoolSize = maximumPoolSize;
    this.workQueue = workQueue;
    this.keepAliveTime = unit.toNanos(keepAliveTime);
    this.threadFactory = threadFactory;
    this.handler = handler;
}

// AbstractExecutorService#submit
public Future<?> submit(Runnable task) {
    if (task == null) throw new NullPointerException();
    // 创建一个FutureTask对象
    RunnableFuture<Void> ftask = newTaskFor(task, null);
    // 执行 ThreadPoolExecutor#execute
    execute(ftask);
    return ftask;
}

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();
    // 如果`workerCount < corePoolSize`,则尝试创建一个新线程,创建成功就直接返回,失败继续下面的流程
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    
    // 如果线程池正在运行并且将该任务添加到工作队列成功
    if (isRunning(c) && workQueue.offer(command)) {
        int recheck = ctl.get();
        // 再次检查线程池是否正在运行,如果不在运行了就将该任务移除并执行拒绝策略
        if (! isRunning(recheck) && remove(command))
            reject(command);
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }

    // 如果`workerCount >= corePoolSize && 工作队列放不下了`,再次尝试添加一个新线程,如果添加失败则执行拒绝策略
    else if (!addWorker(command, false))
        reject(command);
}

1. `firstTask`: 任务的具体逻辑,这里是一个`FutureTask`对象
2. `core`: 如果为true,这和`corePoolSize`比较,否则和`maximumPoolSize`比较. 因为执行`addWorker`方法只有两种情况:一种是`workerCount<corePoolSize`;一种是工作队列已满,这时需要和`maximumPoolSize`比较
private boolean addWorker(Runnable firstTask, boolean core) {
    retry:
    for (;;) {
        int c = ctl.get();
        int rs = runStateOf(c);

        // Check if queue empty only if necessary.  仅在必要时检查队列是否为空, 状态 第二个括号里的条件估计和SHUTDOWN语义有关,后面再看吧
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
                firstTask == null &&
                ! workQueue.isEmpty()))
            return false;

        // 通过自旋操作更新`workerCount`的值,即:加1
        for (;;) {
            // 获取目前线程池中的线程数
            int wc = workerCountOf(c);

            // 因为执行`addWorker`方法只有两种情况:一种是`workerCount<corePoolSize`,这时需要和`corePoolSize`比较; 一种是工作队列已满,这时需要和`maximumPoolSize`比较
            if (wc >= CAPACITY || wc >= (core ? corePoolSize : maximumPoolSize))
                return false;

            // 通过CAS操作尝试对`workerCount`加1,如果成功就跳出最外层循环
            if (compareAndIncrementWorkerCount(c))
                break retry;

            c = ctl.get();  // Re-read ctl

            // 如果在自旋(内循环更新`workerCount`值)期间,线程池的状态发生变化,重新进入外循环
            if (runStateOf(c) != rs)
                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) {
            // 这里的`ReentrantLock`主要作用是保证添加`Worker`到`workers`时是线程安全的,因为`workers`是`HashSet`结构,其本身不是线程安全的
            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());

                // 如果线程池正在运行
                if (rs < SHUTDOWN || (rs == SHUTDOWN && firstTask == null)) {
                    // 如果t.isAlive()这说明线程已经被启动了,这时候直接抛异常,一般不会出现 
                    if (t.isAlive()) // precheck that t is startable
                        throw new fIllegalThreadStateException();

                    // 将该任务添加到`workers`中,`workers`是一个`HashSet`结构,不过这里通过`ReentrantLock`保证它是线程安全的
                    workers.add(w);

                    int s = workers.size();
                    // 更新`largestPoolSize`,该值用于表示线程池中曾经达到的最大线程数
                    if (s > largestPoolSize)
                        largestPoolSize = s;

                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
                // 最后启动线程
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        // 根据上面的代码来判断,如果线程池运行状态不正常的时候,会添加`Worker`失败,然后执行`addWorkerFailed`方法
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

private void addWorkerFailed(Worker w) {
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        if (w != null)
            // 从`HashSet`中移除
            workers.remove(w);
        // 对`workerCount`减1
        decrementWorkerCount();
        // 尝试停止线程池
        tryTerminate();
    } finally {
        mainLock.unlock();
    }
}

Worker(Runnable firstTask) {
    // 设置state=-1,则无法获取锁, 在runWorker中会先执行unlock方法,然后再执行lock方法获取锁
    setState(-1); // inhibit interrupts until runWorker
    // 最开始执行的那个任务,之后的任务去队列里面拿
    this.firstTask = firstTask;
    // 以自己为参数创建一个线程
    this.thread = getThreadFactory().newThread(this);
}

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    // 先取第一个任务执行,这是在构造函数中传入的
    Runnable task = w.firstTask;
    w.firstTask = null;

    // 因为在Worker构造函数中默认设置了state为-1,需要先执行`Worker#unlock`将state设置为0
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
        // 如果首个任务不为null并且工作队列里面还有任务
        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
            // 有点不太懂
            if ((runStateAtLeast(ctl.get(), STOP) || (Thread.interrupted() && runStateAtLeast(ctl.get(), STOP))) && !wt.isInterrupted())
                wt.interrupt();

            try {
                // 前置处理,默认为空
                beforeExecute(wt, task);

                Throwable thrown = null;
                try {
                    // 执行任务
                    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 {
                    // 后置处理,默认为空
                    afterExecute(task, thrown);
                }
            } finally {
                // 设置task为null, 用于取下一个任务
                task = null;
                // 完成任务数加1
                w.completedTasks++;
                w.unlock();
            }
        }

        // 工作队列里面没任务了,并且在获取任务时在工作队列上阻塞的时候大于空闲时间,并且时正常结束的,即没有发生什么异常
        completedAbruptly = false;
    } finally {
        // 将该Worker从HashSet中移除,执行一些销毁操作
        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);

        // Are workers subject to culling?
        // allowCoreThreadTimeOut表示线程是否永久存货, 默认是永久存活, 结合下面的代码说明在这两种情况下,空闲时间生效: 1.allowCoreThreadTimeOut==true  2.工作线程数大于corePoolSize
        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;
        }
    }
}

public static ExecutorService newSingleThreadExecutor() {
    return new FinalizableDelegatedExecutorService
        (new ThreadPoolExecutor(1, 1,
                                0L, TimeUnit.MILLISECONDS,
                                new LinkedBlockingQueue<Runnable>()));
}

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

public static ExecutorService newFixedThreadPool(int nThreads) {
    return new ThreadPoolExecutor(nThreads, nThreads,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue<Runnable>());
}

public ScheduledThreadPoolExecutor(int corePoolSize, ThreadFactory threadFactory) {
    super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,new DelayedWorkQueue(),threadFactory);
}