Java ScheduledExecutorService源码分析

Java 定时任务可以用Timer + TimerTask来做，或者使用ScheduledExecutorService，使用ScheduledExecutorService有两个好处：

1. 如果任务执行时间过长，TimerTask会出现延迟执行的情况。比如，第一任务在1000ms执行了4000ms，第二个任务定时在2000ms开始执行。这里由于第一个任务要执行4000，所以第二个任务实际在5000ms开始执行。这是由于Timer是单线程，且顺序执行提交的任务

2. 如果执行任务抛出异常，Timer是不会执行会后面的任务的

使用ScheduledExecutorService可以避免上面两种情况，因为ScheduledExecutorService是线程池，有多个线程执行。

下面是一个使用ScheduledExecutorService实现定时任务的Demo

import java.util.concurrent.ScheduledExecutorService;
import java.util.concurrent.ScheduledThreadPoolExecutor;
import java.util.concurrent.TimeUnit;

/**
 * Created by gxf on 2017/6/26.
 */
public class TestSchedule {
    public static void main(String[] args) {
        ScheduledExecutorService scheduledExecutorService = new ScheduledThreadPoolExecutor(5);
        Task task = new Task();
        scheduledExecutorService.scheduleAtFixedRate(task, -10, 1, TimeUnit.SECONDS);
    }
}

class Task implements Runnable{
    public void run(){
        System.out.println("do task...");
    }
}

这里每隔1秒，控制台会输出do task

ScheduledExecutorService是一个接口

public interface ScheduledExecutorService extends ExecutorService {

其中一个实现是

public class ScheduledThreadPoolExecutor
        extends ThreadPoolExecutor
        implements ScheduledExecutorService {

在Demo中

ScheduledExecutorService scheduledExecutorService = new ScheduledThreadPoolExecutor(5);

跟进ScheduledThreadPoolExecutor

/**
     * Creates a new {@code ScheduledThreadPoolExecutor} with the
     * given core pool size.
     *
     * @param corePoolSize the number of threads to keep in the pool, even
     *        if they are idle, unless {@code allowCoreThreadTimeOut} is set
     * @throws IllegalArgumentException if {@code corePoolSize < 0}
     */
    public ScheduledThreadPoolExecutor(int corePoolSize) {
        super(corePoolSize, Integer.MAX_VALUE, 0, NANOSECONDS,
              new DelayedWorkQueue());
    }

这里Super调用

public ThreadPoolExecutor(int corePoolSize,
                              int maximumPoolSize,
                              long keepAliveTime,
                              TimeUnit unit,
                              BlockingQueue<Runnable> workQueue) {
        this(corePoolSize, maximumPoolSize, keepAliveTime, unit, workQueue,
             Executors.defaultThreadFactory(), defaultHandler);
    }

设定了线程池的各个参数，核心线程数，最大线程数，任务队列等。但这里还有线程创建，有任务提交了，才会创建线程池

继续Demo中

Task task = new Task();

这里只是创建了一个Runable对象

继续Demo

scheduledExecutorService.scheduleAtFixedRate(task, -10, 1, TimeUnit.SECONDS);

这里已经把任务提交给线程池了，进入方法

/**
     * @throws RejectedExecutionException {@inheritDoc}
     * @throws NullPointerException       {@inheritDoc}
     * @throws IllegalArgumentException   {@inheritDoc}
     */
    public ScheduledFuture<?> scheduleAtFixedRate(Runnable command,
                                                  long initialDelay,
                                                  long period,
                                                  TimeUnit unit) {
        if (command == null || unit == null)
            throw new NullPointerException();
        if (period <= 0)
            throw new IllegalArgumentException();
        ScheduledFutureTask<Void> sft =
            new ScheduledFutureTask<Void>(command,
                                          null,
                                          triggerTime(initialDelay, unit),
                                          unit.toNanos(period));
        RunnableScheduledFuture<Void> t = decorateTask(command, sft);
        sft.outerTask = t;
        delayedExecute(t);
        return t;
    }

前面都是一些条件检查和包装，看最后的delayedExecute(t)

/**
     * Main execution method for delayed or periodic tasks.  If pool
     * is shut down, rejects the task. Otherwise adds task to queue
     * and starts a thread, if necessary, to run it.  (We cannot
     * prestart the thread to run the task because the task (probably)
     * shouldn't be run yet.)  If the pool is shut down while the task
     * is being added, cancel and remove it if required by state and
     * run-after-shutdown parameters.
     *
     * @param task the task
     */
    private void delayedExecute(RunnableScheduledFuture<?> task) {
        if (isShutdown())
            reject(task);
        else {
            super.getQueue().add(task);
            if (isShutdown() &&
                !canRunInCurrentRunState(task.isPeriodic()) &&
                remove(task))
                task.cancel(false);
            else
                ensurePrestart();
        }
    }

ok，这里可以看到任务被添加到了一个队列里面。在看最后，ensurePrestart()

/**
     * Same as prestartCoreThread except arranges that at least one
     * thread is started even if corePoolSize is 0.
     */
    void ensurePrestart() {
        int wc = workerCountOf(ctl.get());
        if (wc < corePoolSize)
            addWorker(null, true);
        else if (wc == 0)
            addWorker(null, false);
    }

这里可以看出，如果线程池里面的线程数，小于核心线程数，会继续添加线程。进入addWork(null, true)

private boolean addWorker(Runnable firstTask, boolean core) {
        retry:
        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;

            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
            }
        }

        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());

                    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) {
                    t.start();
                    workerStarted = true;
                }
            }
        } finally {
            if (! workerStarted)
                addWorkerFailed(w);
        }
        return workerStarted;
    }

第32行和33行可以看出，任务已经提交给Work类了，第57行，t.start()这里启动线程，执行提交的任务

到这里，提交的任务已经开始执行了。这里，我们在看一下Worker这个包装类

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;

可以看到有Thread， task，这里，其实就是把task指向要执行的任务，thread作为作为线程执行任务。

在ThreadPoolExecutor中

private final HashSet<Worker> workers = new HashSet<Worker>();

这个保存我们生成的线程，有了这个就不用重新创建和销毁线程了

我们在看一下Worker的run()方法

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

继续跟进

final void runWorker(Worker w) {
        Thread wt = Thread.currentThread();
        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 {
                        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);
        }
    }

第8行getTask()取要执行的任务，第23行执行任务

继续跟进第8行getTask()

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;
            }
        }
    }

这里主要看第27-29，前面我们说过，任务是放到一个队列里面的。其实，是个阻塞队列，在队列为空的时候取，会阻塞线程。这里就用到了这个功能，在29行workQueue.take()如果没有任务，线程就会阻塞。有任务就会取任务进行执行。

简单点：线程池就是一个Set对象 + BlockingQueue对象， Workers + BlockingQueue