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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