线程池源码解析

1.前言

线程池作为java 并发领域的一个重要组成部分，也是面试必问的考点，理论性的东西我这里就不一一说明了，推荐一篇写得很好的文章，来自美团技术团队

https://tech.meituan.com/2020/04/02/java-pooling-pratice-in-meituan.html

我个人觉得理论性的东西可能大家都懂，但是具体的实现细节可能并不是很清楚所以才想记录一下，加深记忆。

2.关键源码解析

（1）ctl

    private final AtomicInteger ctl = new AtomicInteger(ctlOf(RUNNING, 0));
    private static final int COUNT_BITS = Integer.SIZE - 3;
    private static final int CAPACITY   = (1 << COUNT_BITS) - 1;

    // runState is stored in the high-order bits
    private static final int RUNNING    = -1 << COUNT_BITS;
    private static final int SHUTDOWN   =  0 << COUNT_BITS;
    private static final int STOP       =  1 << COUNT_BITS;
    private static final int TIDYING    =  2 << COUNT_BITS;
    private static final int TERMINATED =  3 << COUNT_BITS;

    // Packing and unpacking ctl
    private static int runStateOf(int c)     { return c & ~CAPACITY; }
    private static int workerCountOf(int c)  { return c & CAPACITY; }
    private static int ctlOf(int rs, int wc) { return rs | wc; }

ctl 一共32位，高3位表示线程池的状态，剩下的29位表示运行中的线程数

(2)  execute(Runnable command) 

        int c = ctl.get();
        //当前线程数小于核心线程数
        if (workerCountOf(c) < corePoolSize) {
            //创建核心线程，并执行任务
            if (addWorker(command, true))
                return;
            c = ctl.get();
        }
        //当前线程数大于等于核心线程数
        //检查线程池状态，并将任务添加到队列中，可以添加成功代表队列未满
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();
            if (! isRunning(recheck) && remove(command))
                reject(command);
            //启动非核心线程，处理队列中的任务
            else if (workerCountOf(recheck) == 0)
                addWorker(null, false);
        }
        //队列已满，产生添加非核心线程，执行任务，如果失败则拒绝任务
        else if (!addWorker(command, false))
            reject(command);

(3) addWorker(Runnable firstTask, boolean core)

Worker 这个类继承了aqs 并且实现了Runnable 接口，所以它具有一个锁的功能，它的run()方法最终调用了ThreadPoolExecutor 的 runWorker 方法，runWoorker()中通过getTask（）获取任务，并且执行了任务的run（）方法

 private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();
            int rs = runStateOf(c);

            // 校验线程池状态
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);
                //检查线程数量是否超过限制
                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
            }
        }

        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());
                    
                    //workers 是存储当前运行任务的线程
                    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) {
                    //启动worker ，执行worker的run方法
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

woker 的run方法执行调用 runWorker ，并传入自身

    final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        //第一个任务，如果不为空则立刻执行，否则从队列获取
        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 {
                        //执行task的run方法，这里执行的东西就是我们提交的任务
                        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 {
            //删除非核心线程
            processWorkerExit(w, completedAbruptly);
        }
    }

    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?
            //是否允许所有线程超时，或者当前工作线程大于核心线程数，所有线程都可以超时意思就是核心线程和非核心线程没什么区别
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;
            
            //超时则返回空
            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c))
                    return null;
                continue;
            }

            try {
                //如果可以超时，从队列获取任务，会先阻塞一定时间，超过时间会抛出异常，表示超时，不可超时时时take会一直阻塞，直到有任务加入
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();
                if (r != null)
                    return r;
                timedOut = true;
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }