Java-线程池实现线程复用和工作线程的关闭

1.ThreadPoolExecutor.execute

    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
        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))
                reject(command);
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        else if (!addWorker(command, false))
            reject(command);
    }

这里主要做两件事:

  • 小于corePoolSize的时候创建核心线程
  • 当前核心线程都正在执行则入队,判断是否需要创建工作线程

2.ThreadPoolExecutor.addWorker

    private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        // 判断是否可以创建worker
        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);
                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
            }
        }

        // 创建Worker并启动
        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            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) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

这里采用显示锁的方式实现同步创建Worker

3.Worker的run()

Worker其实也是一个Runnable的子类,那么从执行到添加Worker的过程都没有线程复用的逻辑,那么该逻辑就应该是在Worker的run方法中。
在这里可以看到,Worker的runWorker会调用ThreadPoolExecutor的runWorker方法。
而在runWorker方法中,会采用一个while循环从task任务队列中取出待执行的任务,如果任务是没有被中断的,则会调用task.run()方法执行任务。

    final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
            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 {
                        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);
        }
    }

getTask()的内部是采用一个无限for循环的方式进行循环遍历

    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);

            // 判断是否允许核心线程超时或者当前工作线程是否大于核心线程数
            // 如果为true
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

            try {
                // 如果存在的线程数大于核心线程数,则从队列中取任务时没有任务返回null
                // 如果存在的线程数小于核心线程数,则从队列中取任务时没有任务将阻塞队列
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }

而工作线程的关闭,其实可以从上面两种情况知道,在线程通过while无限循环的时候,如果是非核心线程(即工作线程)调用BlockingQueue的poll方法,则会在队列为空的时候返回null而不会阻塞队列,当返回的task是null的时候,那么runWorker中的while循环就会退出,最终就会执行processWorkerExit(w, completedAbruptly)而completedAbruptly=false进行线程的删除;如果是核心线程的话,则会调用BlockingQueue的take()方法,那么在队列为空的时候就会通过take()方法调用而阻塞等待队列结果的返回,而不会返回一个task=null,这样runWorker中的while循环就不会退出,就会一直等待结果。

Runnable r = timed ?
    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
    workQueue.take();

所以工作线程会被关闭,而核心线程不会被关闭,就是因为工作线程的run()方法会通过task=null退出循环而执行完成run()的方法体内容,从而自动关闭;而核心线程因为阻塞等待队列返回的原因而不会导致while循环的结束,所以不会关闭。

BlockingQueue的方法都是成对出现:
add和remove:这两个是非阻塞的,当队列满的时候,add会抛出异常,当队列为空的时候,remove会抛出异常。
offer和poll:使用offer往满的队列里放入元素,会返回false;poll方法往空的队列里拿元素,会返回一个null
put和take:这是真正的阻塞方法,使用put往满的队列里放元素,会被阻塞;使用take往空的队列里拿方法,会被阻塞。

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