ExecutorService与Executors例子的简单剖析

对于多线程有了一点了解之后，那么来看看java.lang.concurrent包下面的一些东西。在此之前，我们运行一个线程都是显式调用了Thread的start()方法。我们用concurrent下面的类来实现一下线程的运行，而且这将成为以后常用的方法或者实现思路。 

        看一个简单的例子： 

public class CacheThreadPool {
    public static void main(String[] args) {
        ExecutorService exec=Executors.newCachedThreadPool();
        for(int i=0;i<5;i++)
            exec.execute(new LiftOff());
        exec.shutdown();//并不是终止线程的运行，而是禁止在这个Executor中添加新的任务
    }
}

        这个例子其实很容易看懂，ExecutorService中有一个execute方法，这个方法的参数是Runnable类型。也就是说，将一个实现了Runnable类型的类的实例作为参数传入execute方法并执行，那么线程就相应的执行了。 

        一、ExecutorService 
        先看看ExecutorService，这是一个接口，简单的列一下这个接口： 

public interface ExecutorService extends Executor {

    void shutdown();

    List<Runnable> shutdownNow();

    boolean isShutdown();

    boolean isTerminated();

    boolean awaitTermination(long timeout, TimeUnit unit)

    <T> Future<T> submit(Callable<T> task);

    <T> Future<T> submit(Runnable task, T result);

    Future<?> submit(Runnable task);

    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks)

    <T> List<Future<T>> invokeAll(Collection<? extends Callable<T>> tasks, long timeout, TimeUnit unit)

    <T> T invokeAny(Collection<? extends Callable<T>> tasks)

    <T> T invokeAny(Collection<? extends Callable<T>> tasks,
                    long timeout, TimeUnit unit)
}

        ExecuteService继承了Executor，Executor也是一个接口，里面只有一个方法： 

void execute(Runnable command)

        二、Executors 
        Executors是一个类，直接援引JDK文档的说明来说一下这个类的作用： 
       

        Factory and utility methods for Executor, ExecutorService, ScheduledExecutorService, ThreadFactory, and Callable classes defined in this package. This class supports the following kinds of methods: 

       
• Methods that create and return an ExecutorService set up with commonly useful configuration settings.
       
• Methods that create and return a ScheduledExecutorService set up with commonly useful configuration settings.
       
• Methods that create and return a "wrapped" ExecutorService, that disables reconfiguration by making implementation-specific methods inaccessible.
       
• Methods that create and return a ThreadFactory that sets newly created threads to a known state.
       
• Methods that create and return a Callable out of other closure-like forms, so they can be used in execution methods requiring Callable.
       

        在上面的例子中，我们用到了newCachedThreadPool()方法。看一下这个方法： 

public static ExecutorService newCachedThreadPool() {
        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,
                                      60L, TimeUnit.SECONDS,
                                      new SynchronousQueue<Runnable>());
    }

        在源码中我们可以知道两点，1、这个方法返回类型是ExecutorService；2、此方法返回值实际是另一个类的实例。看一下这个类的信息： 

public class ThreadPoolExecutor extends AbstractExecutorService {
    ..........
    private final BlockingQueue<Runnable> workQueue;//这个变量在下面会提到
    ..........
}

        ThreadPoolExecutor继承了AbstractExecutorService,而AbstractExecutorService又实现了ExecutorService接口。所以，根据多态，ThreadPoolExecutor可以看作是ExecutorService类型。 

        线程执行的最关键的一步是执行了executor方法，根据java的动态绑定，实际执行的是ThreadPoolExecutor所实现的executor方法。看看源码： 

public class ThreadPoolExecutor extends AbstractExecutorService {
    ..........
    public void execute(Runnable command) {
        if (command == null)
            throw new NullPointerException();
        if (poolSize >= corePoolSize || !addIfUnderCorePoolSize(command)) {
            if (runState == RUNNING && workQueue.offer(command)) {
                if (runState != RUNNING || poolSize == 0)
                    ensureQueuedTaskHandled(command);
            }
            else if (!addIfUnderMaximumPoolSize(command))
                reject(command); // is shutdown or saturated
        }
    }
    ..........
}

        根据程序正常执行的路线来看，这个方法中比较重要的两个地方分别是： 
        1、workQueue.offer(command) 
        workQueue在上面提到过，是BlockingQueue<Runnable>类型的变量，这条语句就是将Runnable类型的实例加入到队列中。 
        2、ensureQueuedTaskHandled(command) 
        这个是线程执行的关键语句。看看它的源码： 

public class ThreadPoolExecutor extends AbstractExecutorService {
    ..........
    private void ensureQueuedTaskHandled(Runnable command) {
        final ReentrantLock mainLock = this.mainLock;
        mainLock.lock();
        boolean reject = false;
        Thread t = null;
        try {
            int state = runState;
            if (state != RUNNING && workQueue.remove(command))
                reject = true;
            else if (state < STOP &&
                     poolSize < Math.max(corePoolSize, 1) &&
                     !workQueue.isEmpty())
                t = addThread(null);
        } finally {
            mainLock.unlock();
        }
        if (reject)
            reject(command);
        else if (t != null)
            t.start();
    }
    ..........
}

        在这里我们就可以看到最终执行了t.start()方法来运行线程。在这之前的重点是t=addThread(null)方法，看看addThread方法的源码： 

public class ThreadPoolExecutor extends AbstractExecutorService {
    ..........
    private Thread addThread(Runnable firstTask) {
        Worker w = new Worker(firstTask);
        Thread t = threadFactory.newThread(w);
        if (t != null) {
            w.thread = t;
            workers.add(w);
            int nt = ++poolSize;
            if (nt > largestPoolSize)
                largestPoolSize = nt;
        }
        return t;
    }
    ..........
}

        这里两个重点，很明显： 
        1、Worker w = new Worker(firstTask) 
        2、Thread t = threadFactory.newThread(w) 
        先看Worker是个什么结构： 

public class ThreadPoolExecutor extends AbstractExecutorService {
    ..........
    private final class Worker implements Runnable {
        ..........
        Worker(Runnable firstTask) {
            this.firstTask = firstTask;
        }

        private Runnable firstTask;
        ..........

        public void run() {
            try {
                Runnable task = firstTask;
                firstTask = null;
                while (task != null || (task = getTask()) != null) {
                    runTask(task);
                    task = null;
                }
            } finally {
                workerDone(this);
            }
        }
    }

    Runnable getTask() {
        for (;;) {
            try {
                int state = runState;
                if (state > SHUTDOWN)
                    return null;
                Runnable r;
                if (state == SHUTDOWN)  // Help drain queue
                    r = workQueue.poll();
                else if (poolSize > corePoolSize || allowCoreThreadTimeOut)
                    r = workQueue.poll(keepAliveTime, TimeUnit.NANOSECONDS);
                else
                    r = workQueue.take();
                if (r != null)
                    return r;
                if (workerCanExit()) {
                    if (runState >= SHUTDOWN) // Wake up others
                        interruptIdleWorkers();
                    return null;
                }
                // Else retry
            } catch (InterruptedException ie) {
                // On interruption, re-check runState
            }
        }
    }
    }
    ..........
}

        Worker是一个内部类。根据之前可以知道，传入addThread的参数是null，也就是说Work中firstTask为null。
        在看看newThread是一个什么方法： 

public class Executors {
    ..........
    static class DefaultThreadFactory implements ThreadFactory {
        ..........
        public Thread newThread(Runnable r) {
            Thread t = new Thread(group, r,
                                  namePrefix + threadNumber.getAndIncrement(),
                                  0);
            if (t.isDaemon())
                t.setDaemon(false);
            if (t.getPriority() != Thread.NORM_PRIORITY)
                t.setPriority(Thread.NORM_PRIORITY);
            return t;
        }
        ..........
    }
    ..........
}

        通过源码可以得知threadFactory的实际类型是DefaultThreadFactory，而DefaultThreadFactory是Executors的一个嵌套内部类。 

        之前我们提到了t.start()这个方法执行了线程。那么现在从头顺一下，看看到底是执行了谁的run方法。首先知道，t=addThread(null)，而addThread内部执行了下面三步，Worker w = new Worker(null);Thread t = threadFactory.newThread(w);return t;这里两个t是一致的。 
        从这里可以看出，t.start()实际上执行的是Worker内部的run方法。run()内部会在if条件里面使用“短路”：判断firstTask是否为null，若不是null则直接执行firstTask的run方法；如果是null，则调用getTask()方法来获取Runnable类型实例。从哪里获取呢？workQueue！在execute方法中，执行ensureQueuedTaskHandled(command)之前就已经把Runnable类型实例放入到workQueue中了，所以这里可以从workQueue中获取到。