Android线程池源码阅读记录介绍

今天面试被问到线程池如何复用线程的？当场就懵掉了...于是面试完毕就赶紧打开源码看了看，在此记录下:

我们都知道线程池的用法，一般就是先new一个ThreadPoolExecutor对象，再调用execute(Runnable runnable)传入我们的Runnable，剩下的交给线程池处理就行了，于是这次我就从ThreadPoolExecutor的execute方法看起：

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();
        //1.如果workerCountOf(c)即正在运行的线程数小于核心线程数，就执行addWork
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        //2.如果线程池还在运行状态并且把任务添加到任务队列成功
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            //3.如果线程池不在运行状态并且从任务队列移除任务成功，执行线程池饱和策略（默认直接抛出异常）
            if (! isRunning(recheck) && remove(command))
                reject(command);
            //4.否则如果此时运行线程数==0，就直接调用addWork方法
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        //5.如果2条件不成立，继续判断如果addWork返回false，执行线程池饱和策略
        else if (!addWorker(command, false))
            reject(command);
    }

大致过程就是如果核心线程未满，则直接addWorker（该方法下面会再分析）；如果核心线程已满，则尝试将任务加进消息队列中，并再判断如果此时运行线程数==0则调addWorker方法，否则不做任何处理（因为运行的线程处理完自己的任务后会去消息队列中取任务来执行，下面会分析）；如果任务队列添加任务失败，那么直接addWorker()，如果addWorker返回false，执行饱和策略，下面我们就来看看addWorker里面做了什么

    /**
     * @param firstTask the task the new thread should run first (or
     * null if none). Workers are created with an initial first task
     * (in method execute()) to bypass queuing when there are fewer
     * than corePoolSize threads (in which case we always start one),
     * or when the queue is full (in which case we must bypass queue).
     * Initially idle threads are usually created via
     * prestartCoreThread or to replace other dying workers.
     *
     * @param core if true use corePoolSize as bound, else
     * maximumPoolSize. (A boolean indicator is used here rather than a
     * value to ensure reads of fresh values after checking other pool
     * state).
     * @return true if successful
     */
    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);
                //1.如果正在运行的线程数大于corePoolSize 或 maximumPoolSize（core代表以核心线程数还是最大线程数为边界），return false，表示addWorker失败
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                //2.否则将运行线程数+1，并跳出这个for循环
                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 {
            //3.创建一个Worker对象，传入我们的runnable
            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.add(w);
                        int s = workers.size();
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    //4.开始启动线程
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

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);
        }
final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            //1.当firstTask不为空或getTask不为空时一直循环
            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 {
                        //2.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 {
                        afterExecute(task, thrown);
                    }
                } finally {
                    task = null;
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            processWorkerExit(w, completedAbruptly);
        }
    }

可以看到addWorker方法主要就是先判断正在运行线程数是否超过了最大线程数（具体根据边界取），如果未超过则创建一个worker对象，其中firstTask是我们传入的Runnable，当然根据上面的execute方法可知当4条件满足时，传入的firstTask是null，Thread是用ThreadFactory创建的线程，传入的Runnable是Worker自己，最后开启线程，于是执行Worker这里的run、runWorker方法，在runWorker方法里，开启一个while循环，当firstTask不为空或getTask不为空时，执行task，下面我们接着看看getTask里面做了什么：

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?
            //1.会不会淘汰空闲线程
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
            //2.return null意味着回收一个Worker即淘汰一个线程
            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

            try {
                //3.等待指定时间
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }

可以看1、2注释，allowCoreThreadTimeOut代表存活一定时间是否对核心线程有效（默认为false），先看它为ture的情况，此时不管是核心线程还是非核心线程在3处都会等待一定时间（就是我们传入的线程保活时间），等待时间内如果从任务队列取到任务，则返回执行，否则timeout为true，继续走到2，由于(timed && timedOut)和workQueue.isEmpty()均为true，返回null，代表回收一个线程；如果allowCoreThreadTimeOut为false，代表不回收核心线程，此时如果在3处没有取到任务，继续执行到2处，只有当wc > corePoolSize或wc > maximumPoolSize时才会执行return null，否则一直循环，相当于该线程一直处于运行状态，直到从任务队列拿到新的任务

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