线程池之ThreadPoolExecutor

线程池状态转换

java.util.concurrent.ThreadPoolExecutor为JDK的线程池对象，线程池的状态和状态转换如下图：

使用方式

ThreadPoolExecutor executor = new ThreadPoolExecutor(1, 10, 1, TimeUnit.MINUTES, new ArrayBlockingQueue<>(10));
executor.execute(new Runnable() {
	@Override
	public void run() {
	}
});

java.util.concurrent.Executors工具类下常用的快捷创建线程池的方法newCachedThreadPool()、newFixedThreadPool()、newSingleThreadExecutor实际上都是对ThreadPoolExecutor的封装。

public static ExecutorService newCachedThreadPool() {
    // 实际上newCachedThreadPool创建的线程池的最大线程数量并不是Integer.MAX_VALUE
    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 static ExecutorService newSingleThreadExecutor() {
    return new FinalizableDelegatedExecutorService
        (new ThreadPoolExecutor(1, 1,
                                0L, TimeUnit.MILLISECONDS,
                                new LinkedBlockingQueue<Runnable>()));
}

状态源码常量

private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0)); // 默认值为-536870912,保存了当前线程池状态信息和线程数量信息

private static final int RUNNING    = -1 << COUNT_BITS; // -536870912
private static final int SHUTDOWN   =  0 << COUNT_BITS; // 0
private static final int STOP       =  1 << COUNT_BITS; // 536870912
private static final int TIDYING    =  2 << COUNT_BITS; // 1073741824
private static final int TERMINATED =  3 << COUNT_BITS; // 1610612736

源码解析

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    int c = ctl.get();
    // 判断当前线程池线程数量是否小于corePoolSize
    if (workerCountOf(c) < corePoolSize) {  
        /**
         * 创新新线程执行任务
         * 第二个参数:ture表示通过corePoolSize控制线程数量;false表示通过maximumPoolSize控制线程数量
         */
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    // 线程池RUNNING状态且任务向任务队列添加成功
    if (isRunning(c) && workQueue.offer(command)) { 
        int recheck = ctl.get();
        // 再次判断线程池状态,如果线程池非RUNNING状态则移除新增的任务,最后抛出RejectedExecutionException异常
        if (! isRunning(recheck) && remove(command))
            reject(command);
        // 执行到这一步说明corePoolSize为0,因此向线程池创建添加一个非核心新线程
        else if (workerCountOf(recheck) == 0)
            addWorker(null, false);
    }
    // 执行到这一步说明线程池非RUNNING状态或者核心线程数量和任务队列已满,尝试直接创建新非核心线程执行该任务.
    // 只有当线程池RUNNING状态且线程数量小于maximumPoolSize时执行任务成功;否则抛出RejectedExecutionException异常
    // 由此可见只有当核心线程全部创建且任务队列已满时，才会创建非核心线程
    else if (!addWorker(command, false))
        reject(command);
}

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

        /**
         * 以下的复杂判断可以拆解成下面3种情况
         * rs > SHUTDOWN  或
         * rs = SHUTDOWN && firstTask != null 或
         * rs = SHUTDOWN && workQueue.isEmpty()
         * 即当以下情况该判断不返回false
         * 1: 线程池状态为RUNNING
         * 2: 线程池状态为SHUTDOWN且firstTask为null且任务队列存在未执行任务(该场景为创新一个空任务的线程执行任务队列中等待执行的任务)
         */
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;

        for (;;) {
            int wc = workerCountOf(c);
            // 线程池最大线程数量实际为CAPACITY=536870911,即Executors.newCachedThreadPool()创建的线程池的线程最大数量并不是Integer.MAX_VALUE
            if (wc >= CAPACITY ||
                wc >= (core ? corePoolSize : maximumPoolSize))
                return false;
            if (compareAndIncrementWorkerCount(c))
                break retry;
            c = ctl.get();
            if (runStateOf(c) != rs)
                continue retry;
        }
    }

    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        // 每个Worker对象init时创建并持有一个新线程
        w = new Worker(firstTask);
        final Thread t = w.thread;
        if (t != null) {
            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                int rs = runStateOf(ctl.get());

                /**
                 * 1:线程池RUNNING状态时添加Worker对象
                 * 2:线程池SHUTDOWN状态时只允许添加空任务的Worker对象去执行任务队列中等待执行的任务
                 */
                if (rs < SHUTDOWN ||
                    (rs == SHUTDOWN && firstTask == null)) {
                    if (t.isAlive())
                        throw new IllegalThreadStateException();
                    workers.add(w);
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            if (workerAdded) {
                // 启动Worker对象中的线程开始执行任务
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    w.unlock(); 
    boolean completedAbruptly = true;
    try {
        /**
         * 1:Worker对象持有任务,执行任务;
         * 2:Worker对象未持有任务(任务执行完成或本身创建时就是个空任务线程),从任务队列中获取(任务队列为空则阻塞)任务去执行
         */
        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);
                Throwable thrown = null;
                try {
                    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 {
                    afterExecute(task, thrown);
                }
            } finally {
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        processWorkerExit(w, completedAbruptly);
    }
}

private Runnable getTask() {
    boolean timedOut = false;

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

        /**
         * 1:线程池状态为STOP、TIDYING、TERMINATED返回null
         * 2:线程池状态为SHUTDOWN且任务队列为空返回null
         * 当getTask()返回null时,runWorker()方法退出while循环并销毁该Worker线程
         */
        if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
            decrementWorkerCount();
            return null;
        }

        int wc = workerCountOf(c);

        // 判断该Worker线程是否需要超时回收
        // 当allowCoreThreadTimeOut为true或者该Worker线程为非核心线程时,需要超时回收
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

        if ((wc > maximumPoolSize || (timed && timedOut))
            && (wc > 1 || workQueue.isEmpty())) {
            if (compareAndDecrementWorkerCount(c))
                return null;
            continue;
        }

        try {
            /**
             * 1:不需要超时回收的Worker线程通过take()无限阻塞直到从任务队列获取到新任务
             * 2:需要超时回收的Worker线程如果在keepAliveTime时间范围内未获取到新任务,则返回null然后线程回收
             */
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                workQueue.take();
            if (r != null)
                return r;
            timedOut = true;
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}

当allowCoreThreadTimeOut为true或者该Worker线程为非核心线程时，则Worker线程需要超时回收。

不需要超时回收的Worker线程从工作流程：

需要超时回收的Worker线程从工作流程：

shutdown和shutdownNow

// shutdown()只中断未执行任务的Worker线程
public void shutdown() {
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        checkShutdownAccess();
        advanceRunState(SHUTDOWN);
        // 中断未执行任务的Worker线程
        interruptIdleWorkers();
        onShutdown(); // hook for ScheduledThreadPoolExecutor
    } finally {
        mainLock.unlock();
    }
    tryTerminate();
}

// shutdownNow()会中断所有Worker线程
public List<Runnable> shutdownNow() {
    List<Runnable> tasks;
    final ReentrantLock mainLock = this.mainLock;
    mainLock.lock();
    try {
        checkShutdownAccess();
        advanceRunState(STOP);
        // 中断所有Worker线程
        interruptWorkers();
        tasks = drainQueue();
    } finally {
        mainLock.unlock();
    }
    tryTerminate();
    return tasks;
}

由于shutdown()不会中断正在执行任务的Worker线程，所有存在当任务执行一直完成不了时，线程池始终处于SHUTDOWN状态而无法关闭。

execute和submit

public Future<?> submit(Runnable task) {
    if (task == null) throw new NullPointerException();
    // submit方法把任务包装后再调用execute方法
    RunnableFuture<Void> ftask = newTaskFor(task, null);
    execute(ftask);
    return ftask;
}

submit方法会通过newTaskFor方法创建FutureTask对象来包装任务，而FutureTask的run方法会对任务的执行进行异常处理，保证任务执行中出现的异常不会向上抛出。代码如下：

public void run() {
    if (state != NEW ||
        !UNSAFE.compareAndSwapObject(this, runnerOffset,
                                     null, Thread.currentThread()))
        return;
    try {
        Callable<V> c = callable;
        if (c != null && state == NEW) {
            V result;
            boolean ran;
            try {
                result = c.call();
                ran = true;
            } catch (Throwable ex) {
                result = null;
                ran = false;
                setException(ex);
            }
            if (ran)
                set(result);
        }
    } finally {
        runner = null;
        int s = state;
        if (s >= INTERRUPTING)
            handlePossibleCancellationInterrupt(s);
    }
}

由此可见，execute执行任务时，任务出现异常直接会向上抛，最终在runWorker方法中异常退出导致该Worker线程直接被回收；而submit则处理了任务中出现的异常，即使任务执行出现异常也不会导致Worker线程的回收。