Android线程池实现原理

线程池的好处

  1. 频繁的创建和销毁线程,会带来性能的问题。线程的创建和销毁都需要时间,当有大量的线程创建和销毁时,那么这些时间的消耗则比较明显,将导致性能上的缺失。
  2. 线程池方便管理线程,定时执行线程,间隔执行线程。线程池能控制线程的最大并发数量,避免大量抢占资源导致的阻塞现象。

ThreadPoorExecutor构造方法

ThreadPoorExecutor是线程池的真正实现,它的构造方法的参数会影响线程池的功能。ThreadPoorExecutor的构造方法如下:

   public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler)
  1. corePoolSize:核心线程数,即使在空闲状态,也会在线程池中存在的线程数量,除非设置了allowCoreThreadTimeOut。
  2. maximumPoolSize:允许在线程池中的最大线程数量。
  3. keepAliveTime:非核心线程最长空闲时间,超过这个时间,空闲的非核心线程会被回收,设置allowCoreThreadTimeOut=true,同样也会作用在核心线程中。
  4. unit:时间单位。
  5. workQueue:存储将被execute方法执行的Runnable任务的队列。
  6. threadFactory:线程工厂为线程池创建新线程的功能。
  7. handler:不常用,当线程容量到顶执行被阻塞时,handler被用来通知调用者。

Android中常用几种线程线程池

FixedThreadPool

ExecutorService executor1 = Executors.newFixedThreadPool(3);
核心线程数和最大线程数相同,一个无限的任务队列,就是说线程池一直有固定的线程数处理任务。

public static ExecutorService newFixedThreadPool(int nThreads) {
        return new ThreadPoolExecutor(nThreads, nThreads,
                                      0L, TimeUnit.MILLISECONDS,
                                      new LinkedBlockingQueue());
    }

10个线程加入线程池,查看执行循序:

    for(int i = 0 ; i < 10 ; i++){
        final int index = i;
        executor1.execute(new Runnable() {
            @Override
            public void run() {
                Log.i(TAG, "ExecutorActivity run: 任务 = " + index + ",线程 = " + Thread.currentThread().getName());
                try {
                    Thread.sleep(1000);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                }
            }
        });
    }
Android线程池实现原理_第1张图片
fixthreadpool.gif
CachedThreadPool

Executors.newCachedThreadPool();
没有核心线程,最大线程数为2^31-1,线程空闲60秒后被回收,任务队列SynchronousQueue是一个不存储的,所以这个线程的特点是只要任务一来,马上就有线程去执行。

    public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue());
    }

10个线程加入线程池,查看执行循序:

Android线程池实现原理_第2张图片
cachetheadpool.gif
SingleThreadPool

Executors.newSingleThreadExecutor()
线程池中只有一个核心线程,按顺序执行队列中的任务

    public static ExecutorService newSingleThreadExecutor() {
        return new FinalizableDelegatedExecutorService
            (new ThreadPoolExecutor(1, 1,
                                    0L, TimeUnit.MILLISECONDS,
                                    new LinkedBlockingQueue()));
    }

10个线程加入线程池,查看执行循序:


Android线程池实现原理_第3张图片
singleThreadpool.gif
ScheduledThreadPool

ScheduledExecutorService executor = Executors.newScheduledThreadPool(3)
该线程最大的特点就可以延迟执行

    public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE,
              DEFAULT_KEEPALIVE_MILLIS, MILLISECONDS,
              new DelayedWorkQueue());
    }

ScheduledThreadPool的常用方法:

    public ScheduledFuture schedule(Runnable command,
                                       long delay, TimeUnit unit);
    public ScheduledFuture scheduleAtFixedRate(Runnable command,
                                                  long initialDelay,
                                                  long period,
                                                  TimeUnit unit);

ThreadPoorExecutor执行任务的顺序

(corePoolSize -> workQueue -> maximumPoolSize)

  1. 当未超过核心线程数时,就直接创建一个核心线程去执行任务。
  2. 当超过核心线程数,就将任务加入到workQueue的任务队列中等待
  3. 当任务队列中任务添满时候,在不超过最大线程数的情况下启动线程去处理任务
  4. 当线程数量超过最大线程数时,RejectedExecutionHandler对象通知调用者
执行方法
  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.
         * 如果当前的线程数小于核心线程池的大小,根据现有的线程作为第一个Worker运行的线程,
         * 新建一个Worker,addWorker自动的检查当前线程池的状态和Worker的数量,
         * 防止线程池在不能添加线程的状态下添加线程
         *
         * 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.
         *  如果线程入队成功,然后还是要进行double-check的,因为线程池在入队之后状态是可能会发生变化的
         *
         * 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.
         * 
         * 如果task不能入队(队列满了),这时候尝试增加一个新线程,如果增加失败那么当前的线程池状态变化了或者线程池已经满了
         * 然后拒绝task
         */
        int c = ctl.get();
        //当前的Worker的数量小于核心线程池大小时,新建一个Worker。
        if (workerCountOf(c) < corePoolSize) { 
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))//recheck防止线程池状态的突变,如果突变,那么将reject线程,防止workQueue中增加新线程
                reject(command);
            else if (workerCountOf(recheck) == 0)//上下两个操作都有addWorker的操作,但是如果在workQueue.offer的时候Worker变为0,
                                                //那么将没有Worker执行新的task,所以增加一个Worker.
                addWorker(null, false);
        }
        //如果workQueue满了,那么这时候可能还没到线程池的maxnum,所以尝试增加一个Worker
        else if (!addWorker(command, false))
            reject(command);//如果Worker数量到达上限,那么就拒绝此线程
    }

核心方法:addWorker#####

Worker的增加和Task的获取以及终止都是在此方法中实现的,也就是这一个方法里面包含了很多东西。在addWorker方法中提到了Status的概念,Status是线程池的核心概念,这里我们先看一段关于status的注释:

/**
     * 首先ctl是一个原子量,同时它里面包含了两个field,一个是workerCount,另一个是runState
     * workerCount表示当前有效的线程数,也就是Worker的数量
     * runState表示当前线程池的状态
     * The main pool control state, ctl, is an atomic integer packing
     * two conceptual fields
     *   workerCount, indicating the effective number of threads
     *   runState,    indicating whether running, shutting down etc
     * 
     * 两者是怎么结合的呢?首先workerCount是占据着一个atomic integer的后29位的,而状态占据了前3位
     * 所以,workerCount上限是(2^29)-1。
     * In order to pack them into one int, we limit workerCount to
     * (2^29)-1 (about 500 million) threads rather than (2^31)-1 (2
     * billion) otherwise representable. If this is ever an issue in
     * the future, the variable can be changed to be an AtomicLong,
     * and the shift/mask constants below adjusted. But until the need
     * arises, this code is a bit faster and simpler using an int.
     *
     * The workerCount is the number of workers that have been
     * permitted to start and not permitted to stop.  The value may be
     * transiently different from the actual number of live threads,
     * for example when a ThreadFactory fails to create a thread when
     * asked, and when exiting threads are still performing
     * bookkeeping before terminating. The user-visible pool size is
     * reported as the current size of the workers set.
     *
     * runState是整个线程池的运行生命周期,有如下取值:
     *  1. RUNNING:可以新加线程,同时可以处理queue中的线程。
     *  2. SHUTDOWN:不增加新线程,但是处理queue中的线程。
     *  3.STOP 不增加新线程,同时不处理queue中的线程。
     *  4.TIDYING 所有的线程都终止了(queue中),同时workerCount为0,那么此时进入TIDYING
     *  5.terminated()方法结束,变为TERMINATED
     * The runState provides the main lifecyle control, taking on values:
     *
     *   RUNNING:  Accept new tasks and process queued tasks
     *   SHUTDOWN: Don't accept new tasks, but process queued tasks
     *   STOP:     Don't accept new tasks, don't process queued tasks,
     *             and interrupt in-progress tasks
     *   TIDYING:  All tasks have terminated, workerCount is zero,
     *             the thread transitioning to state TIDYING
     *             will run the terminated() hook method
     *   TERMINATED: terminated() has completed
     *
     * The numerical order among these values matters, to allow
     * ordered comparisons. The runState monotonically increases over
     * time, but need not hit each state. The transitions are:
     * 状态的转化主要是:
     * RUNNING -> SHUTDOWN(调用shutdown())
     *    On invocation of shutdown(), perhaps implicitly in finalize()
     * (RUNNING or SHUTDOWN) -> STOP(调用shutdownNow())
     *    On invocation of shutdownNow()
     * SHUTDOWN -> TIDYING(queue和pool均empty)
     *    When both queue and pool are empty
     * STOP -> TIDYING(pool empty,此时queue已经为empty)
     *    When pool is empty
     * TIDYING -> TERMINATED(调用terminated())
     *    When the terminated() hook method has completed
     *
     * Threads waiting in awaitTermination() will return when the
     * state reaches TERMINATED.
     *
     * Detecting the transition from SHUTDOWN to TIDYING is less
     * straightforward than you'd like because the queue may become
     * empty after non-empty and vice versa during SHUTDOWN state, but
     * we can only terminate if, after seeing that it is empty, we see
     * that workerCount is 0 (which sometimes entails a recheck -- see
     * below).
     */
下面是状态的代码:
//利用ctl来保证当前线程池的状态和当前的线程的数量。ps:低29位为线程池容量,高3位为线程状态。
    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    //设定偏移量
    private static final int COUNT_BITS = Integer.SIZE - 3;
    //确定最大的容量2^29-1
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;
    //几个状态,用Integer的高三位表示
    // runState is stored in the high-order bits
    //111
    private static final int RUNNING    = -1 << COUNT_BITS;
    //000
    private static final int SHUTDOWN   =  0 << COUNT_BITS;
    //001
    private static final int STOP       =  1 << COUNT_BITS;
    //010
    private static final int TIDYING    =  2 << COUNT_BITS;
    //011
    private static final int TERMINATED =  3 << COUNT_BITS;
    //获取线程池状态,取前三位
    // Packing and unpacking ctl
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
    //获取当前正在工作的worker,主要是取后面29位
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    //获取ctl
    private static int ctlOf(int rs, int wc) { return rs | wc; }
接下来贴上addWorker方法看看:
/**
 * Checks if a new worker can be added with respect to current
 * pool state and the given bound (either core or maximum). If so,
 * the worker count is adjusted accordingly, and, if possible, a
 * new worker is created and started running firstTask as its
 * first task. This method returns false if the pool is stopped or
 * eligible to shut down. It also returns false if the thread
 * factory fails to create a thread when asked, which requires a
 * backout of workerCount, and a recheck for termination, in case
 * the existence of this worker was holding up termination.
 *
 * @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.
        /**
         * rs!=Shutdown || fistTask!=null || workCount.isEmpty
         * 如果当前的线程池的状态>SHUTDOWN 那么拒绝Worker的add 如果=SHUTDOWN
         * 那么此时不能新加入不为null的Task,如果在WorkCount为empty的时候不能加入任何类型的Worker,
         * 如果不为empty可以加入task为null的Worker,增加消费的Worker
         */
        if (rs >= SHUTDOWN &&
            ! (rs == SHUTDOWN &&
               firstTask == null &&
               ! workQueue.isEmpty()))
            return false;

        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
        }
    }

    Worker w = new Worker(firstTask);
    Thread t = w.thread;

    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 c = ctl.get();
        int rs = runStateOf(c);
        /**
         * rs!=SHUTDOWN ||firstTask!=null
         * 
         * 同样检测当rs>SHUTDOWN时直接拒绝减小Wc,同时Terminate,如果为SHUTDOWN同时firstTask不为null的时候也要Terminate
         */
        if (t == null ||
            (rs >= SHUTDOWN &&
             ! (rs == SHUTDOWN &&
                firstTask == null))) {
            decrementWorkerCount();
            tryTerminate();
            return false;
        }

        workers.add(w);

        int s = workers.size();
        if (s > largestPoolSize)
            largestPoolSize = s;
    } finally {
        mainLock.unlock();
    }

    t.start();
    // It is possible (but unlikely) for a thread to have been
    // added to workers, but not yet started, during transition to
    // STOP, which could result in a rare missed interrupt,
    // because Thread.interrupt is not guaranteed to have any effect
    // on a non-yet-started Thread (see Thread#interrupt).
    //Stop或线程Interrupt的时候要中止所有的运行的Worker
    if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted())
        t.interrupt();
    return true;
}

addWorker中首先进行了一次线程池状态的检测:

            int c = ctl.get();
            int rs = runStateOf(c);
            // Check if queue empty only if necessary.
            //判断当前线程池的状态是不是已经shutdown,如果shutdown了拒绝线程加入
            //(rs!=SHUTDOWN || first!=null || workQueue.isEmpty())
            //如果rs不为SHUTDOWN,此时状态是STOP、TIDYING或TERMINATED,所以此时要拒绝请求
            //如果此时状态为SHUTDOWN,而传入一个不为null的线程,那么需要拒绝
            //如果状态为SHUTDOWN,同时队列中已经没任务了,那么拒绝掉
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

其实是比较难懂的,主要在线程池状态判断条件这里:
如果是running,那么跳过if。
如果rs>=SHUTDOWN,同时不等于SHUTDOWN,即为SHUTDOWN以上的状态,那么不接受新线程。
如果rs>=SHUTDOWN,同时等于SHUTDOWN,同时first != null,那么拒绝新线程,如果first==null,那么可能是新增加线程消耗Queue中的线程。但是同时还要检测workQueue是否isEmpty(),如果为Empty,那么队列已空,不需要增加消耗线程,如果队列没有空那么运行增加first=null的Worker。

从这里是可以看出一些策略的首先
在rs>SHUTDOWN时,拒绝一切线程的增加,因为STOP是会终止所有的线程,同时移除Queue中所有的待执行的线程的,所以也不需要增加first=null的Worker了。
其次,在SHUTDOWN状态时,是不能增加first!=null的Worker的,同时即使first=null,但是此时Queue为Empty也是不允许增加Worker的,SHUTDOWN下增加的Worker主要用于消耗Queue中的任务。
SHUTDOWN状态时,是不允许向workQueue中增加线程的,isRunning(c) && workQueue.offer(command) 每次在offer之前都要做状态检测,也就是线程池状态变为>=SHUTDOWN时不允许新线程进入线程池了。
            for (;;) {
                int wc = workerCountOf(c);
                //如果当前的数量超过了CAPACITY,或者超过了corePoolSize和maximumPoolSize(试core而定)
                if (wc >= CAPACITY ||
                    wc >= (core ? corePoolSize : maximumPoolSize))
                    return false;
                //CAS尝试增加线程数,如果失败,证明有竞争,那么重新到retry。
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // Re-read ctl
                //判断当前线程池的运行状态
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }

这段代码做了一个兼容,主要是没有到corePoolSize 或maximumPoolSize上限时,那么允许添加线程,CAS增加Worker的数量后,跳出循环。
接下来实例化Worker,实例化Worker其实是很关键的,后面会说。
因为workers是HashSet线程不安全的,那么此时需要加锁,所以mainLock.lock(); 之后重新检查线程池的状态,如果状态不正确,那么减小Worker的数量,为什么tryTerminate()目前不大清楚。如果状态正常,那么添加Worker到workers。最后:

  if (runStateOf(ctl.get()) == STOP && ! t.isInterrupted())
            t.interrupt();

注释说的很清楚,为了能及时的中断此Worker,因为线程存在未Start的情况,此时是不能响应中断的,如果此时status变为STOP,则不能中断线程。此处用作中断线程之用。
接下来我们看Worker的方法:

 /**
         * Creates with given first task and thread from ThreadFactory.
         * @param firstTask the first task (null if none)
         */
        Worker(Runnable firstTask) {
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }

这里可以看出Worker是对firstTask的包装,并且Worker本身就是Runnable的,看上去真心很流氓的感觉~~~
通过ThreadFactory为Worker自己构建一个线程。
因为Worker是Runnable类型的,所以是有run方法的,上面也看到了会调用t.start() 其实就是执行了run方法:

        /** Delegates main run loop to outer runWorker  */
        public void run() {
            runWorker(this);
        }

调用了runWorker:

/**
     * Main worker run loop.  Repeatedly gets tasks from queue and
     * executes them, while coping with a number of issues:
     * 1 Worker可能还是执行一个初始化的task——firstTask。
     *    但是有时也不需要这个初始化的task(可以为null),只要pool在运行,就会
     *   通过getTask从队列中获取Task,如果返回null,那么worker退出。
     *   另一种就是external抛出异常导致worker退出。
     * 1. We may start out with an initial task, in which case we
     * don't need to get the first one. Otherwise, as long as pool is
     * running, we get tasks from getTask. If it returns null then the
     * worker exits due to changed pool state or configuration
     * parameters.  Other exits result from exception throws in
     * external code, in which case completedAbruptly holds, which
     * usually leads processWorkerExit to replace this thread.
     * 
     * 
     * 2 在运行任何task之前,都需要对worker加锁来防止other pool中断worker。
     *   clearInterruptsForTaskRun保证除了线程池stop,那么现场都没有中断标志
     * 2. Before running any task, the lock is acquired to prevent
     * other pool interrupts while the task is executing, and
     * clearInterruptsForTaskRun called to ensure that unless pool is
     * stopping, this thread does not have its interrupt set.
     *
     * 3. Each task run is preceded by a call to beforeExecute, which
     * might throw an exception, in which case we cause thread to die
     * (breaking loop with completedAbruptly true) without processing
     * the task.
     *
     * 4. Assuming beforeExecute completes normally, we run the task,
     * gathering any of its thrown exceptions to send to
     * afterExecute. We separately handle RuntimeException, Error
     * (both of which the specs guarantee that we trap) and arbitrary
     * Throwables.  Because we cannot rethrow Throwables within
     * Runnable.run, we wrap them within Errors on the way out (to the
     * thread's UncaughtExceptionHandler).  Any thrown exception also
     * conservatively causes thread to die.
     *
     * 5. After task.run completes, we call afterExecute, which may
     * also throw an exception, which will also cause thread to
     * die. According to JLS Sec 14.20, this exception is the one that
     * will be in effect even if task.run throws.
     *
     * The net effect of the exception mechanics is that afterExecute
     * and the thread's UncaughtExceptionHandler have as accurate
     * information as we can provide about any problems encountered by
     * user code.
     *
     * @param w the worker
     */
    final void runWorker(Worker w) {
        Runnable task = w.firstTask;
        w.firstTask = null;
        //标识线程是不是异常终止的
        boolean completedAbruptly = true;
        try {
            //task不为null情况是初始化worker时,如果task为null,则去队列中取线程--->getTask()
            while (task != null || (task = getTask()) != null) {
                w.lock();
                //获取woker的锁,防止线程被其他线程中断
                clearInterruptsForTaskRun();//清楚所有中断标记
                try {
                    beforeExecute(w.thread, task);//线程开始执行之前执行此方法,可以实现Worker未执行退出,本类中未实现
                    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);//线程执行后执行,可以实现标识Worker异常中断的功能,本类中未实现
                    }
                } finally {
                    task = null;//运行过的task标null
                    w.completedTasks++;
                    w.unlock();
                }
            }
            completedAbruptly = false;
        } finally {
            //处理worker退出的逻辑
            processWorkerExit(w, completedAbruptly);
        }
    }

从上面代码可以看出,execute的Task是被“包装 ”了一层,线程启动时是内部调用了Task的run方法。
接下来所有的核心集中在getTask()方法上:

/**
     * Performs blocking or timed wait for a task, depending on
     * current configuration settings, or returns null if this worker
     * must exit because of any of:
     * 1. There are more than maximumPoolSize workers (due to
     *    a call to setMaximumPoolSize).
     * 2. The pool is stopped.
     * 3. The pool is shutdown and the queue is empty.
     * 4. This worker timed out waiting for a task, and timed-out
     *    workers are subject to termination (that is,
     *    {@code allowCoreThreadTimeOut || workerCount > corePoolSize})
     *    both before and after the timed wait.
     *
     * @return task, or null if the worker must exit, in which case
     *         workerCount is decremented
     *         
     *         
     *  队列中获取线程
     */
    private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?

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

            // Check if queue empty only if necessary.
            //当前状态为>stop时,不处理workQueue中的任务,同时减小worker的数量所以返回null,如果为shutdown 同时workQueue已经empty了,同样减小worker数量并返回null
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }

            boolean timed;      // Are workers subject to culling?

            for (;;) {
                int wc = workerCountOf(c);
                timed = allowCoreThreadTimeOut || wc > corePoolSize;

                if (wc <= maximumPoolSize && ! (timedOut && timed))
                    break;
                if (compareAndDecrementWorkerCount(c))
                    return null;
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }

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

这段代码十分关键,首先看几个局部变量:
boolean timedOut = false;
主要是判断后面的poll是否要超时
boolean timed;
主要是标识着当前Worker超时是否要退出。wc > corePoolSize时需要减小空闲的Worker数,那么timed为true,但是wc <= corePoolSize时,不能减小核心线程数timed为false。
timedOut初始为false,如果timed为true那么使用poll取线程。如果正常返回,那么返回取到的task。如果超时,证明worker空闲,同时worker超过了corePoolSize,需要删除。返回r=null。则 timedOut = true。此时循环到wc <= maximumPoolSize && ! (timedOut && timed)时,减小worker数,并返回null,导致worker退出。如果线程数<= corePoolSize,那么此时调用 workQueue.take(),没有线程获取到时将一直阻塞,知道获取到线程或者中断,关于中断后面Shutdown的时候会说。

关于终止线程池

我个人认为,如果想了解明白线程池,那么就一定要理解好各个状态之间的转换,想理解转换,线程池的终止机制是很好的一个途径。对于关闭线程池主要有两个方法shutdown()和shutdownNow():
首先从shutdown()方法开始:

/**
 * Initiates an orderly shutdown in which previously submitted
 * tasks are executed, but no new tasks will be accepted.
 * Invocation has no additional effect if already shut down.
 *
 * 

This method does not wait for previously submitted tasks to * complete execution. Use {@link #awaitTermination awaitTermination} * to do that. * * @throws SecurityException {@inheritDoc} */ public void shutdown() { final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { //判断是否可以操作目标线程 checkShutdownAccess(); //设置线程池状态为SHUTDOWN,此处之后,线程池中不会增加新Task advanceRunState(SHUTDOWN); //中断所有的空闲线程 interruptIdleWorkers(); onShutdown(); // hook for ScheduledThreadPoolExecutor } finally { mainLock.unlock(); } //转到Terminate tryTerminate(); }

shutdown做了几件事:

  1. 检查是否能操作目标线程
  2. 将线程池状态转为SHUTDOWN
  3. 中断所有空闲线程
    这里就引发了一个问题,什么是空闲线程?
    这需要接着看看interruptIdleWorkers是怎么回事。
 private void interruptIdleWorkers(boolean onlyOne) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        //这里的意图很简单,遍历workers 对所有worker做中断处理。
        // w.tryLock()对Worker加锁,这保证了正在运行执行Task的Worker不会被中断,那么能中断哪些线程呢?
        try {
            for (Worker w : workers) {
                Thread t = w.thread;
                if (!t.isInterrupted() && w.tryLock()) {
                    try {
                        t.interrupt();
                    } catch (SecurityException ignore) {
                    } finally {
                        w.unlock();
                    }
                }
                if (onlyOne)
                    break;
            }
        } finally {
            mainLock.unlock();
        }
    }

这里主要是为了中断worker,但是中断之前需要先获取锁,这就意味着正在运行的Worker不能中断。但是上面的代码有w.tryLock(),那么获取不到锁就不会中断,shutdown的Interrupt只是对所有的空闲Worker(正在从workQueue中取Task,此时Worker没有加锁)发送中断信号。

            while (task != null || (task = getTask()) != null) {
                w.lock();
                //获取woker的锁,防止线程被其他线程中断
                clearInterruptsForTaskRun();//清楚所有中断标记
                try {
                    beforeExecute(w.thread, task);//线程开始执行之前执行此方法,可以实现Worker未执行退出,本类中未实现
                    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);//线程执行后执行,可以实现标识Worker异常中断的功能,本类中未实现
                    }
                } finally {
                    task = null;//运行过的task标null
                    w.completedTasks++;
                    w.unlock();
                }
            }

在runWorker中,每一个Worker getTask成功之后都要获取Worker的锁之后运行,也就是说运行中的Worker不会中断。因为核心线程一般在空闲的时候会一直阻塞在获取Task上,也只有中断才可能导致其退出。这些阻塞着的Worker就是空闲的线程(当然,非核心线程,并且阻塞的也是空闲线程)。在getTask方法中:

    private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?

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

            // Check if queue empty only if necessary.
            //当前状态为>stop时,不处理workQueue中的任务,同时减小worker的数量所以返回null,如果为shutdown 同时workQueue已经empty了,同样减小worker数量并返回null
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();
                return null;
            }

            boolean timed;      // Are workers subject to culling?

            for (;;) {
                //allowCoreThreadTimeOu是判断CoreThread是否会超时的,true为会超时,false不会超时。默认为false
                int wc = workerCountOf(c);
                timed = allowCoreThreadTimeOut || wc > corePoolSize;

                if (wc <= maximumPoolSize && ! (timedOut && timed))
                    break;
                if (compareAndDecrementWorkerCount(c))
                    return null;
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }

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

会有两阶段的Worker:

刚进入getTask(),还没进行状态判断。
block在poll或者take上的Worker。
当调用ShutDown方法时,首先设置了线程池的状态为ShutDown,此时1阶段的worker进入到状态判断时会返回null,此时Worker退出。
因为getTask的时候是不加锁的,所以在shutdown时可以调用worker.Interrupt.此时会中断退出,Loop到状态判断时,同时workQueue为empty。那么抛出中断异常,导致重新Loop,在检测线程池状态时,Worker退出。如果workQueue不为null就不会退出,此处有些疑问,因为没有看见中断标志位清除的逻辑,那么这里就会不停的循环直到workQueue为Empty退出。
这里也能看出来SHUTDOWN只是清除一些空闲Worker,并且拒绝新Task加入,对于workQueue中的线程还是继续处理的。
对于shutdown中获取mainLock而addWorker中也做了mainLock的获取,这么做主要是因为Works是HashSet类型的,是线程不安全的,我们也看到在addWorker后面也是对线程池状态做了判断,将Worker添加和中断逻辑分离开。
接下来做了tryTerminate()操作,这操作是进行了后面状态的转换,在shutdownNow后面说。
接下来看看shutdownNow:

    /**
     * Attempts to stop all actively executing tasks, halts the
     * processing of waiting tasks, and returns a list of the tasks
     * that were awaiting execution. These tasks are drained (removed)
     * from the task queue upon return from this method.
     *
     * 

This method does not wait for actively executing tasks to * terminate. Use {@link #awaitTermination awaitTermination} to * do that. * *

There are no guarantees beyond best-effort attempts to stop * processing actively executing tasks. This implementation * cancels tasks via {@link Thread#interrupt}, so any task that * fails to respond to interrupts may never terminate. * * @throws SecurityException {@inheritDoc} */ public List shutdownNow() { List tasks; final ReentrantLock mainLock = this.mainLock; mainLock.lock(); try { checkShutdownAccess(); advanceRunState(STOP); interruptWorkers(); tasks = drainQueue(); } finally { mainLock.unlock(); } tryTerminate(); return tasks; }

shutdownNow和shutdown代码类似,但是实现却很不相同。首先是设置线程池状态为STOP,前面的代码我们可以看到,是对SHUTDOWN有一些额外的判断逻辑,但是对于>=STOP,基本都是reject,STOP也是比SHUTDOWN更加严格的一种状态。此时不会有新Worker加入,所有刚执行完一个线程后去GetTask的Worker都会退出。
之后调用interruptWorkers:

    /**
     * Interrupts all threads, even if active. Ignores SecurityExceptions
     * (in which case some threads may remain uninterrupted).
     */
    private void interruptWorkers() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers) {
                try {
                    w.thread.interrupt();
                } catch (SecurityException ignore) {
                }
            }
        } finally {
            mainLock.unlock();
        }
    }

这里可以看出来,此方法目的是中断所有的Worker,而不是像shutdown中那样只中断空闲线程。这样体现了STOP的特点,中断所有线程,同时workQueue中的Task也不会执行了。所以接下来drainQueue:

   /**
     * Drains the task queue into a new list, normally using
     * drainTo. But if the queue is a DelayQueue or any other kind of
     * queue for which poll or drainTo may fail to remove some
     * elements, it deletes them one by one.
     */
    private List drainQueue() {
        BlockingQueue q = workQueue;
        List taskList = new ArrayList();
        q.drainTo(taskList);
        if (!q.isEmpty()) {
            for (Runnable r : q.toArray(new Runnable[0])) {
                if (q.remove(r))
                    taskList.add(r);
            }
        }
        return taskList;
    }

获取所有没有执行的Task,并且返回。
这也体现了STOP的特点:
拒绝所有新Task的加入,同时中断所有线程,WorkerQueue中没有执行的线程全部抛弃。所以此时Pool是空的,WorkerQueue也是空的。
这之后就是进行到TIDYING和TERMINATED的转化了:

    /**
     * Transitions to TERMINATED state if either (SHUTDOWN and pool
     * and queue empty) or (STOP and pool empty).  If otherwise
     * eligible to terminate but workerCount is nonzero, interrupts an
     * idle worker to ensure that shutdown signals propagate. This
     * method must be called following any action that might make
     * termination possible -- reducing worker count or removing tasks
     * from the queue during shutdown. The method is non-private to
     * allow access from ScheduledThreadPoolExecutor.
     */
    final void tryTerminate() {
        for (;;) {
            int c = ctl.get();
            if (isRunning(c) ||
                runStateAtLeast(c, TIDYING) ||
                (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty()))
                return;
            if (workerCountOf(c) != 0) { // Eligible to terminate
                interruptIdleWorkers(ONLY_ONE);
                return;
            }

            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {
                    try {
                        terminated();
                    } finally {
                        ctl.set(ctlOf(TERMINATED, 0));
                        termination.signalAll();
                    }
                    return;
                }
            } finally {
                mainLock.unlock();
            }
            // else retry on failed CAS
        }
    }

上面的代码其实很有意思有几种状态是不能转化到TIDYING的:

RUNNING状态
TIDYING或TERMINATED
SHUTDOWN状态,但是workQueue不为空
也说明了两点:

  1. SHUTDOWN想转化为TIDYING,需要workQueue为空,同时workerCount为0。
  2. STOP转化为TIDYING,需要workerCount为0
    如果满足上面的条件(一般一定时间后都会满足的),那么CAS成TIDYING,TIDYING也只是个过度状态,最终会转化为TERMINATED。

至此,ThreadPoolExecutor一些核心思想就介绍完了,想分析清楚实在是不容易,对于ThreadPoolExecutor我还是有些不懂地方,以上只是我对源码的片面的见解,如果有不正确之处,希望大神能不吝赐教。同时也希望给正在研究ThreadPoolExecutor的童鞋提供一点帮助。

学习参考如下文章

https://www.jianshu.com/p/ade771d2c9c0

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