线程池源码解读的示例分析

线程池源码解读的示例分析，针对这个问题，这篇文章详细介绍了相对应的分析和解答，希望可以帮助更多想解决这个问题的小伙伴找到更简单易行的方法。

一、execute

ctl作为AtomicInteger类存放了类中的两种信息，在其中由高3位来保存线程池的状态，后29位来保存此时线程池中的Woker类线程数量(由此可知，线程池中的线程数量最高可以接受大约在五亿左右)。由此可见给出的runStateOf()和workerCountOf()方法分别给出了查看线程状态和线程数量的方法。

public void execute(Runnable command) {        if (command == null)            throw new NullPointerException();        int c = ctl.get();        //如果运行的线程数小于corePoolSize，尝试创建一个新线程（Worker），并执行它的第一个command        if (workerCountOf(c) < corePoolSize) {            if (addWorker(command, true))                return;            c = ctl.get();        }        //线程数大于corePoolSize，将线程放入任务队列        //第一次校验线程池在运行状态        if (isRunning(c) && workQueue.offer(command)) {            int recheck = ctl.get();            //第二次校验，防止在第一次校验通过后线程池关闭。如果线程池关闭，在队列中删除task并拒绝task            if (! isRunning(recheck) && remove(command))                reject(command);            //如果线程数=0（线程都死掉了，比如：corePoolSize=0），新建线程且未指定firstTask，仅轮询任务队列            else if (workerCountOf(recheck) == 0)                addWorker(null, false);        }        //任务队列已满，尝试创建新线程执行task，创建失败后拒绝task        //创建失败原因：1.线程池关闭；2.线程数已经达到maxPoolSize        else if (!addWorker(command, false))            reject(command);}

private boolean addWorker(Runnable firstTask, boolean core) {        retry:        //外层循环判断线程池的状态        for (;;) {            int c = ctl.get();            //线程池状态            int rs = runStateOf(c);            //线程池状态:RUNNING = -1、SHUTDOWN = 0、STOP = 1、TIDYING = 2、TERMINATED            if (rs >= SHUTDOWN &&                ! (rs == SHUTDOWN &&                   firstTask == null &&                   ! workQueue.isEmpty()))                return false;            //用CAS的方式对线程数量进行+1操作            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            }        }        boolean workerStarted = false;        boolean workerAdded = false;        Worker w = null;        try {            //worker实现了Runable接口            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是一个worker数组                        workers.add(w);                        int s = workers.size();                        if (s > largestPoolSize)                            largestPoolSize = s;                        workerAdded = true;                    }                } finally {                    mainLock.unlock();                }                if (workerAdded) {                    //启动线程，执行的就是worker中的run()                    t.start();                    workerStarted = true;                }            }        } finally {            if (! workerStarted)                addWorkerFailed(w);        }        return workerStarted;    }

Worker类：

private final class Worker        extends AbstractQueuedSynchronizer        implements Runnable//Worker是一个线程    {        private static final long serialVersionUID = 6138294804551838833L;        final Thread thread;        Runnable firstTask;        volatile long completedTasks;        Worker(Runnable firstTask) {            setState(-1); // inhibit interrupts until runWorker            this.firstTask = firstTask;            //把当前Worker包装成一个thread            this.thread = getThreadFactory().newThread(this);        }        public void run() {            runWorker(this);        }}

final void runWorker(Worker w) {        Thread wt = Thread.currentThread();        Runnable task = w.firstTask;        w.firstTask = null;        w.unlock(); // allow interrupts        boolean completedAbruptly = true;        try {            //如果这个worker还没有执行过在构造方法就传入的任务，那么在这个方法中，会直接执行这一任务，如果没有，则会             //尝试去从任务队列当中去取的新的任务。            //在执行完毕后，工作线程的使命并没有真正宣告段落。在while部分worker仍旧会通过getTask()方法试图取得新             //的任务。下面是getTask()的实现。            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; // 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?            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;            if ((wc > maximumPoolSize || (timed && timedOut))                && (wc > 1 || workQueue.isEmpty())) {                if (compareAndDecrementWorkerCount(c))                    return null;                continue;            }            try {                //从工作队列中取出线程                Runnable r = timed ?                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :                    workQueue.take();                if (r != null)                    return r;                timedOut = true;            } catch (InterruptedException retry) {                timedOut = false;            }        }    }

二、submit

    public Future submit(Runnable task) {        if (task == null) throw new NullPointerException();        RunnableFuture ftask = newTaskFor(task, null);        execute(ftask);        return ftask;    }    /**     * @throws RejectedExecutionException {@inheritDoc}     * @throws NullPointerException       {@inheritDoc}     */    public  Future submit(Runnable task, T result) {        if (task == null) throw new NullPointerException();        RunnableFuture ftask = newTaskFor(task, result);        execute(ftask);        return ftask;    }    /**     * @throws RejectedExecutionException {@inheritDoc}     * @throws NullPointerException       {@inheritDoc}     */    public  Future submit(Callable task) {        if (task == null) throw new NullPointerException();        RunnableFuture ftask = newTaskFor(task);        execute(ftask);        return ftask;    }

    protected  RunnableFuture newTaskFor(Runnable runnable, T value) {        return new FutureTask(runnable, value);    }    protected  RunnableFuture newTaskFor(Callable callable) {        return new FutureTask(callable);    }

可见，submit将普通的runnable包装成FutureTask并返回，再调用execute去执行。

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