Java线程池

线程池的状态

shutdown状态下不接受新的任务，但是已经提交到线程池的任务会被正常执行
stop状态下不接受新的任务且已经提交的任务不再执行，正在执行的任务会被中断

类中是用AtomicInteger(ctl)的高3位表示池的状态的， 后29位计数池中的线程数

image.png

worker的设计

worker继承AQS，实现了Runnable接口 
run方法调用外部类中的runWorker方法
而runWorker方法中涉及到任务队列中任务的调度与执行

runWorker方法

final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        Runnable task = w.firstTask;
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;
        try {
           //task从当前worker或者任务队列获取， 如果不空会一直执行下去
            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);
        }
    }

任务的提交--execute

源码

public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();
       //1. 核心池未满， 添加核心线程执行
        if (workerCountOf(c) < corePoolSize) {
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        //2.尝试加到非阻塞的加到任务队列
        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);
        } 
        //3. new一个非核心线程执行
        else if (!addWorker(command, false))
            reject(command);
}

调用流图

image.png

新增一个工作线程

主要分两步：一是线程数+1， 二是新建一个工作线程，并且开始执行

调用流图

image.png

线程池状态变量的线程数加一

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

            // Check if queue empty only if necessary.
            // 我的理解： 对于 （rs == shutdown && 任务为空 && 任务队列不空）的情况仍然会创建线程， 用来执行任务队列的任务， 这里不与shutdown状态的语义矛盾（不接受新的任务，但已提交的任务会处理）
            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;
                //CAS增加线程数
                if (compareAndIncrementWorkerCount(c))
                    break retry;
                c = ctl.get();  // Re-read ctl
                //如果CAS失败是因为线程池的状态变了， 跳到外层循环，否则内层循环重试。
                if (runStateOf(c) != rs)
                    continue retry;
                // else CAS failed due to workerCount change; retry inner loop
            }
        }

创建一个线程

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());
                    //如果通过了recheck线程池的状态， 就把worker加到集合中
                    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();
                }
                //如果worker被加到集合中，运行线程
                if (workerAdded) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            //如果worker没有被加到集合中， 线程销毁等操作
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;

线程池的keepAlive是如何实现的

主要体现在getTask方法中

当前线程状态判断
是否满足线程超时回收的条件--如果是的话当前线程就被回收。
获取任务（poll OR take 根据是否超时获取）。

源码

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?
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

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

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