【Java】线程池ThreadPoolExecutor实现原理

public class ThreadPoolExecutor extends AbstractExecutorService { ... }
public abstract class AbstractExecutorService implements ExecutorService { ... }
public interface ExecutorService extends Executor { ... }
public interface Executor { ... }

public ThreadPoolExecutor(int corePoolSize,
                          int maximumPoolSize,
                          long keepAliveTime,
                          TimeUnit unit,
                          BlockingQueue<Runnable> workQueue,
                          ThreadFactory threadFactory,
                          RejectedExecutionHandler handler) { 
    //...省略...
}

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; }

public void execute(Runnable command) {
    if (command == null)
        throw new NullPointerException();
    /*
     * Proceed in 3 steps:
     *
     * 1. If fewer than corePoolSize threads are running, try to
     * start a new thread with the given command as its first
     * task.  The call to addWorker atomically checks runState and
     * workerCount, and so prevents false alarms that would add
     * threads when it shouldn't, by returning false.
     *
     * 2. If a task can be successfully queued, then we still need
     * to double-check whether we should have added a thread
     * (because existing ones died since last checking) or that
     * the pool shut down since entry into this method. So we
     * recheck state and if necessary roll back the enqueuing if
     * stopped, or start a new thread if there are none.
     *
     * 3. If we cannot queue task, then we try to add a new
     * thread.  If it fails, we know we are shut down or saturated
     * and so reject the task.
     */
    int c = ctl.get();
    //情况1
    if (workerCountOf(c) < corePoolSize) {
        if (addWorker(command, true))
            return;
        c = ctl.get();
    }
    //情况2
    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);
    }
    //情况3
    else if (!addWorker(command, false))
        reject(command);
}

 private boolean addWorker(Runnable firstTask, boolean core) {
    retry:
    //使用CAS机制轮询线程池的状态，如果线程池处于SHUTDOWN及大于它的状态则拒绝执行任务
    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;

        //使用CAS机制尝试将当前线程数+1
        //如果是核心线程当前线程数必须小于corePoolSize 
        //如果是非核心线程则当前线程数必须小于maximumPoolSize
        //如果当前线程数小于线程池支持的最大线程数CAPACITY 也会返回失败
        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
        }
    }

    //这里已经成功执行了CAS操作将线程池数量+1，下面创建线程
    boolean workerStarted = false;
    boolean workerAdded = false;
    Worker w = null;
    try {
        w = new Worker(firstTask);
        //Worker内部有一个Thread，并且执行Worker的run方法，因为Worker实现了Runnable
        final Thread t = w.thread;
        if (t != null) {
            //这里必须同步在状态为运行的情况下将Worker添加到workers中
            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);  //把新建的woker线程放入集合保存，这里使用的是HashSet
                    int s = workers.size();
                    if (s > largestPoolSize)
                        largestPoolSize = s;
                    workerAdded = true;
                }
            } finally {
                mainLock.unlock();
            }
            //如果添加成功则运行线程
            if (workerAdded) {
                t.start();
                workerStarted = true;
            }
        }
    } finally {
        //如果woker启动失败，则进行一些善后工作，比如说修改当前woker数量等等
        if (! workerStarted)
            addWorkerFailed(w);
    }
    return workerStarted;
}

private final class Worker
        extends AbstractQueuedSynchronizer
        implements Runnable
    {
    /**
     * This class will never be serialized, but we provide a
     * serialVersionUID to suppress a javac warning.
     */
    private static final long serialVersionUID = 6138294804551838833L;

    /** Thread this worker is running in.  Null if factory fails. */
    final Thread thread;
    /** Initial task to run.  Possibly null. */
    Runnable firstTask;
    /** Per-thread task counter */
    volatile long completedTasks;

    /**
     * Creates with given first task and thread from ThreadFactory.
     * @param firstTask the first task (null if none)
     */
    Worker(Runnable firstTask) {
        setState(-1); // inhibit interrupts until runWorker
        this.firstTask = firstTask;
        this.thread = getThreadFactory().newThread(this);
    }

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

    // Lock methods
    //
    // The value 0 represents the unlocked state.
    // The value 1 represents the locked state.

    protected boolean isHeldExclusively() {
        return getState() != 0;
    }

    protected boolean tryAcquire(int unused) {
        if (compareAndSetState(0, 1)) {
            setExclusiveOwnerThread(Thread.currentThread());
            return true;
        }
        return false;
    }

    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(); }

    void interruptIfStarted() {
        Thread t;
        if (getState() >= 0 && (t = thread) != null && !t.isInterrupted()) {
            try {
                t.interrupt();
            } catch (SecurityException ignore) {
            }
        }
    }
}

public void run() {
    runWorker(this);
}

//ThreadPoolExecutor类中
final void runWorker(Worker w) {
    Thread wt = Thread.currentThread();
    Runnable task = w.firstTask;
    w.firstTask = null;
    // 因为Worker的构造函数中setState(-1)禁止了中断，这里的unclock用于恢复中断
    w.unlock(); // allow interrupts
    boolean completedAbruptly = true;
    try {
        //一般情况下，task都不会为空（特殊情况上面注释中也说明了），因此会直接进入循环体中
        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 {
                //该方法是个空的实现，如果有需要用户可以自己继承该类进行实现
                beforeExecute(wt, task);
                Throwable thrown = null;
                try {
                    //真正的任务执行逻辑
                    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 {
                    //该方法是个空的实现，如果有需要用户可以自己继承该类进行实现
                    afterExecute(task, thrown);
                }
            } finally {
                //这里设为null，也就是循环体再执行的时候会调用getTask方法
                task = null;
                w.completedTasks++;
                w.unlock();
            }
        }
        completedAbruptly = false;
    } finally {
        //当指定任务执行完成，阻塞队列中也取不到可执行任务时，会进入这里，做一些善后工作
        //比如在corePoolSize跟maximumPoolSize之间的woker会进行回收
        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?
        boolean timed = allowCoreThreadTimeOut || wc > corePoolSize;

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

        try {
            //根据超时配置有两种方法取出任务
            Runnable r = timed ?
                workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS) :
                workQueue.take();
            if (r != null)
                return r;
            timedOut = true;
        } catch (InterruptedException retry) {
            timedOut = false;
        }
    }
}

public static ExecutorService newFixedThreadPool(int nThreads, ThreadFactory threadFactory) {
    //corePoolSize跟maximumPoolSize值一样，同时传入一个无界阻塞队列
    //根据上面分析的woker回收逻辑，该线程池的线程会维持在指定线程数，不会进行回收
    return new ThreadPoolExecutor(nThreads, nThreads,
                                  0L, TimeUnit.MILLISECONDS,
                                  new LinkedBlockingQueue<Runnable>(),
                                  threadFactory);
}

public static ExecutorService newCachedThreadPool() {
    //这个线程池corePoolSize为0，maximumPoolSize为Integer.MAX_VALUE
    //意思也就是说来一个任务就创建一个woker，回收时间是60s
    return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                  60L, TimeUnit.SECONDS,
                                  new SynchronousQueue<Runnable>());
}

public static ExecutorService newSingleThreadExecutor() {
    //线程池中只有一个线程进行任务执行，其他的都放入阻塞队列
    //外面包装的FinalizableDelegatedExecutorService类实现了finalize方法，在JVM垃圾回收的时候会关闭线程池
    return new FinalizableDelegatedExecutorService
        (new ThreadPoolExecutor(1, 1,
                                0L, TimeUnit.MILLISECONDS,
                                new LinkedBlockingQueue<Runnable>()));
}

public static ScheduledExecutorService newScheduledThreadPool(int corePoolSize) {
    return new ScheduledThreadPoolExecutor(corePoolSize);
}