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    }}