ThreadPoolExecutor源码阅读

public class ThreadPoolExecutor extends AbstractExecutorService {
    //线程池的运行状态（整形的高3位）和运行的worker数量（低29位）
    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;

    /**
     * RUNNING     可接受新任务，可处理已经进入阻塞队列的任务
     * SHUTDOWN    不接受新任务，可处理已经进入阻塞队列的任务
     * STOP        不接受新任务，不处理已经进入阻塞队列的任务并且中断正在运行的任务
     * TIDYING     所有的任务都已经终止，workerCount为0， 线程转化为TIDYING状态并且调用terminated钩子函数
     * TERMINATED  terminated钩子函数已经运行完成
     */
    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;


    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; }

    //阻塞队列
    private final BlockingQueue<Runnable> workQueue;
    //可重入锁
    private final ReentrantLock mainLock = new ReentrantLock();
     存放工作线程集合
    private final HashSet<Worker> workers = new HashSet<Worker>();
    // 终止条件
    private final Condition termination = mainLock.newCondition();
    // 最大线程池容量
    private int largestPoolSize;
    // 已完成任务数量
    private long completedTaskCount;
    //线程工厂
    private volatile ThreadFactory threadFactory;
    //拒绝执行处理器
    private volatile RejectedExecutionHandler handler;
    // 线程池中超过corePoolSize数目的空闲线程最大存活时间
    private volatile long keepAliveTime;
    // 是否允许核心线程超时
    private volatile boolean allowCoreThreadTimeOut;
    // 核心池的大小
    private volatile int corePoolSize;
    // 最大线程池大小
    private volatile int maximumPoolSize;
    // 默认拒绝执行处理器
    private static final RejectedExecutionHandler defaultHandler =
        new AbortPolicy();

    private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable
    {
        private static final long serialVersionUID = 6138294804551838833L;

        //worker所对应的线程
        final Thread thread;
        //worker的执行任务
        Runnable firstTask;
        //已完成任务数量
        volatile long completedTasks;

        Worker(Runnable firstTask) {
            setState(-1);
            this.firstTask = firstTask;
            this.thread = getThreadFactory().newThread(this);
        }

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

        protected boolean isHeldExclusively() {
            return getState() != 0;
        }
        //尝试获取锁
        protected boolean tryAcquire(int unused) {
            if (compareAndSetState(0, 1)) {
                setExclusiveOwnerThread(Thread.currentThread());
                return true;
            }
            return false;
        }
        //尝试释放锁
        protected boolean tryRelease(int unused) {
            setExclusiveOwnerThread(null);
            setState(0);
            return true;
        }

        public void lock()        { acquire(1); }
        public boolean tryLock()  { return tryAcquire(1); }
        public void unlock()      { release(1); }
        public boolean isLocked() { return isHeldExclusively(); }

    }

    //尝试终止线程池
    final void tryTerminate() {
        for (;;) {
            int c = ctl.get();
            if (isRunning(c) || //运行状态
                runStateAtLeast(c, TIDYING) || //TIDYIN或者TERMINATED状态
                (runStateOf(c) == SHUTDOWN && ! workQueue.isEmpty())) //SHUTDOWN状态且阻塞任务队列非空
                return;// 不能终止，直接返回
            if (workerCountOf(c) != 0) { // Eligible to terminate
                interruptIdleWorkers(ONLY_ONE);// 仅仅中断一个空闲的worker
                return;
            }

            final ReentrantLock mainLock = this.mainLock;
            mainLock.lock();
            try {
                if (ctl.compareAndSet(c, ctlOf(TIDYING, 0))) {// 比较并设置线程池控制状态为TIDYING
                    try {
                        terminated();
                    } finally {
                        ctl.set(ctlOf(TERMINATED, 0));// 设置线程池控制状态为TERMINATED
                        termination.signalAll();// 释放在termination条件上等待的所有线程
                    }
                    return;
                }
            } finally {
                mainLock.unlock();
            }
            // else retry on failed CAS
        }
    }

    private void interruptIdleWorkers(boolean onlyOne) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            for (Worker w : workers) {
                Thread t = w.thread;
                if (!t.isInterrupted() && w.tryLock()) {// 线程未被中断并且成功获得锁
                    try {
                        t.interrupt();// 中断线程
                    } catch (SecurityException ignore) {
                    } finally {
                        w.unlock();
                    }
                }
                if (onlyOne)// 若只中断一个，则跳出循环
                    break;
            }
        } finally {
            mainLock.unlock();
        }
    }

    private void interruptIdleWorkers() {
        interruptIdleWorkers(false);
    }

    private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        for (;;) {
            int c = ctl.get();//获取线程池控制状态
            int rs = runStateOf(c);//线程池运行状态

            // Check if queue empty only if necessary.
            if (rs >= SHUTDOWN &&
                ! (rs == SHUTDOWN &&
                   firstTask == null &&
                   ! workQueue.isEmpty()))
                return false;

            for (;;) {
                int wc = workerCountOf(c);// worker数量
                if (wc >= CAPACITY || // worker数量大于等于最大容量
                    wc >= (core ? corePoolSize : maximumPoolSize))// worker数量大于等于核心线程池大小或者最大线程池大小
                    return false;
                if (compareAndIncrementWorkerCount(c))// 比较并增加worker的数量
                    break retry;//结束循环
                c = ctl.get();  // Re-read ctl
                if (runStateOf(c) != rs)// 此次的状态与上次获取的状态不相同
                    continue retry;
            }
        }

        boolean workerStarted = false;
        boolean workerAdded = false;
        Worker w = null;
        try {
            w = new Worker(firstTask);//创建一个worker
            final Thread t = w.thread;// 获取worker对应的线程
            if (t != null) {
                final ReentrantLock mainLock = this.mainLock;
                mainLock.lock();
                try {
                    int rs = runStateOf(ctl.get());// 线程池的运行状态
                    if (rs < SHUTDOWN || (rs == SHUTDOWN && firstTask == null)) {
                        if (t.isAlive()) // precheck that t is startable
                            throw new IllegalThreadStateException();
                        workers.add(w);// 将worker添加到worker集合
                        int s = workers.size();// 获取worker集合的大小
                        if (s > largestPoolSize)
                            largestPoolSize = s;
                        workerAdded = true;// 设置worker已被添加标识
                    }
                } finally {
                    mainLock.unlock();
                }
                if (workerAdded) {
                    t.start();// 开始执行worker
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

    private void addWorkerFailed(Worker w) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            if (w != null)
                workers.remove(w);
            decrementWorkerCount();
            tryTerminate();
        } finally {
            mainLock.unlock();
        }
    }

    //worker退出
    private void processWorkerExit(Worker w, boolean completedAbruptly) {
        if (completedAbruptly) // If abrupt, then workerCount wasn't adjusted
            decrementWorkerCount();

        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            // 将worker完成的任务添加到总的完成任务中
            completedTaskCount += w.completedTasks;
            workers.remove(w);// 从workers集合中移除该worker
        } finally {
            mainLock.unlock();
        }
        // 尝试终止线程池
        tryTerminate();

        int c = ctl.get();
        if (runStateLessThan(c, STOP)) {
            if (!completedAbruptly) {
                int min = allowCoreThreadTimeOut ? 0 : corePoolSize;
                if (min == 0 && ! workQueue.isEmpty())// 允许核心线程超时且workQueue阻塞队列不为空
                    min = 1;
                if (workerCountOf(c) >= min)
                    return; // replacement not needed
            }
            addWorker(null, false);// 添加worker
        }
    }

    //从workerQueue阻塞队列中获取Runnable对象
    private Runnable getTask() {
        boolean timedOut = false; // Did the last poll() time out?

        for (;;) {
            int c = ctl.get();// 获取线程池控制状态
            int rs = runStateOf(c);// 线程池运行状态

            // 大于等于SHUTDOWN（表示调用了shutDown）并且（大于等于STOP（调用了shutDownNow）或者worker阻塞队列为空）
            if (rs >= SHUTDOWN && (rs >= STOP || workQueue.isEmpty())) {
                decrementWorkerCount();// 减少worker的数量
                return null;// 返回null，不执行任务
            }

            int wc = workerCountOf(c);// 获取worker数量

            // Are workers subject to culling?
            boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

            if ((wc > maximumPoolSize || (timed && timedOut))
                && (wc > 1 || workQueue.isEmpty())) {
                if (compareAndDecrementWorkerCount(c))
                    return null;// 返回null，不执行任务，该worker会退出
                continue;
            }

            try {
                Runnable r = timed ?
                    workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                    workQueue.take();// 一直等待，直到有任务
                if (r != null)
                    return r;//返回任务
                timedOut = true;// 等待指定时间后，没有获取元素，则超时
            } catch (InterruptedException retry) {
                timedOut = false;
            }
        }
    }

    final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();//获取当前线程
        Runnable task = w.firstTask; 获取w的firstTask
        w.firstTask = null;
        w.unlock(); // allow interrupts
        boolean completedAbruptly = true;//异常标志
        try {// 任务不为null或者阻塞队列还存在任务
            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();// 运行给定的任务
                    } 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);
        }
    }

    public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue,
                              ThreadFactory threadFactory,
                              RejectedExecutionHandler handler) {
        if (corePoolSize < 0 ||
            maximumPoolSize <= 0 ||
            maximumPoolSize < corePoolSize ||
            keepAliveTime < 0)
            throw new IllegalArgumentException();
        if (workQueue == null || threadFactory == null || handler == null)
            throw new NullPointerException();
        this.corePoolSize = corePoolSize;
        this.maximumPoolSize = maximumPoolSize;
        this.workQueue = workQueue;
        this.keepAliveTime = unit.toNanos(keepAliveTime);
        this.threadFactory = threadFactory;
        this.handler = handler;
    }

    /**
    * 进行下面三步
    *
    * 1. 如果运行的线程小于corePoolSize,则尝试使用用户定义的Runnalbe对象创建一个新的线程
    *    调用addWorker函数会原子性的检查runState和workCount，通过返回false来防止在不应
    *    该添加线程时添加了线程.
    * 2. 如果一个任务能够成功入队列，在添加一个线程时仍需要进行双重检查（因为在前一次检查后
    *    该线程死亡了），或者当进入到此方法时，线程池已经shutdown了，所以需要再次检查状态，
    *    若有必要，当停止时还需要回滚入队列操作，或者当线程池没有线程时需要创建一个新线程.
    * 3. 如果无法入队列，那么需要增加一个新线程，如果此操作失败，那么就意味着线程池已经shutdown
    *    或者已经饱和了，所以拒绝任务.
    */
    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        int c = ctl.get();// 获取线程池控制状态
        if (workerCountOf(c) < corePoolSize) {// worker数量小于corePoolSize
            if (addWorker(command, true))// 添加worker,返回
                return;
            c = ctl.get();// 不成功则再次获取线程池控制状态
        }
        // 线程池处于RUNNING状态，将command添加进workQueue队列
        if (isRunning(c) && workQueue.offer(command)) {
            int recheck = ctl.get();// 再次检查，获取线程池控制状态
            // 线程池不处于RUNNING状态，将command从workQueue队列中移除
            if (! isRunning(recheck) && remove(command))
                reject(command);//拒绝执行command
            else if (workerCountOf(recheck) == 0)// worker数量等于0
                addWorker(null, false);
        }
        else if (!addWorker(command, false))// 添加worker失败
            reject(command);
    }

    public void shutdown() {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        try {
            checkShutdownAccess();
            advanceRunState(SHUTDOWN);// 设置线程池控制状态为SHUTDOWN
            interruptIdleWorkers();// 中断空闲worker
            onShutdown(); // hook for ScheduledThreadPoolExecutor
        } finally {
            mainLock.unlock();
        }
        tryTerminate();// 尝试终止线程池
    }

}