Android线程池是什么

本篇内容主要讲解"Android线程池是什么"，感兴趣的朋友不妨来看看。本文介绍的方法操作简单快捷，实用性强。下面就让小编来带大家学习"Android线程池是什么"吧!

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

到此，相信大家对"Android线程池是什么"有了更深的了解，不妨来实际操作一番吧！这里是网站，更多相关内容可以进入相关频道进行查询，关注我们，继续学习！