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AbstractExecutorService

上一篇文章中，主要介绍了AbstractExecutorService的线程执行的核心流程，execute() 这个方法显然是没有返回执行任务的结果，如果我们需要获取任务执行的结果，怎么办？

Callable 就是一个可以获取线程执行的结果。

public abstract class AbstractExecutorService implements ExecutorService {/** 将任务包装成FutureTask任务。带返回值参数的*/protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {return new FutureTask<T>(runnable, value);}/**** 不带返回值的**/protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {return new FutureTask<T>(callable);}/*** @throws RejectedExecutionException {@inheritDoc}* @throws NullPointerException       {@inheritDoc}*/public Future<?> submit(Runnable task) {if (task == null) throw new NullPointerException();//1.将任务包装成RunableFuture对象，由于RunnableFuture是实现Runable类，所以execute的参数是一个可拓展的类型RunnableFuture<Void> ftask = newTaskFor(task, null);//2，交给具体的执行器进行实现execute(ftask);return ftask;}/*** @throws RejectedExecutionException {@inheritDoc}* @throws NullPointerException       {@inheritDoc}*/public <T> Future<T> submit(Runnable task, T result) {if (task == null) throw new NullPointerException();RunnableFuture<T> ftask = newTaskFor(task, result);execute(ftask);return ftask;}/*** @throws RejectedExecutionException {@inheritDoc}* @throws NullPointerException       {@inheritDoc}*/public <T> Future<T> submit(Callable<T> task) {if (task == null) throw new NullPointerException();//将任务装成成一个FutureTask任务RunnableFuture<T> ftask = newTaskFor(task);//执行任务execute(ftask);return ftask;}}

submit其实是一个重载的方法，分别是一个task，以及可以传递获取结果的任务，以及使用callable。

demo

从源码上看三个方法其实都是将任务进行了封装，然后调用线程池执行的核心方法

    public static void main(String[] args) throws ExecutionException, InterruptedException {Callable<Integer> resultCallable = new Callable<Integer>() {@Overridepublic Integer call() throws Exception {return 1 + 1;}};ExecutorService threadPool = Executors.newFixedThreadPool(1);Future<Integer> resultTask = threadPool.submit(resultCallable);System.out.println(resultTask.get());threadPool.shutdown();}

FutureTask

public class FutureTask<V> implements RunnableFuture<V> {/* NEW -> COMPLETING -> NORMAL* NEW -> COMPLETING -> EXCEPTIONAL* NEW -> CANCELLED* NEW -> INTERRUPTING -> INTERRUPTED*/private volatile int state;private static final int NEW          = 0; // 初始化状态private static final int COMPLETING   = 1; // 结果计算完成或响应中断到赋值给返回值的状态private static final int NORMAL       = 2; // 任务正常完成，结果被setprivate static final int EXCEPTIONAL  = 3; // 任务抛出异常private static final int CANCELLED    = 4; // 任务被取消private static final int INTERRUPTING = 5; // 线程中断状态被设置为true 线程未响应中断private static final int INTERRUPTED  = 6; // 线程已被中断/** The underlying callable; nulled out after running */private Callable<V> callable; // 需要执行的任务/** The result to return or exception to throw from get() */// 执行callable的线程，调用FutureTask.run()方法通过CAS设置private Object outcome; // non-volatile, protected by state reads/writes/** The thread running the callable; CASed during run() */// 执行callable的线程，调用FutureTask.run()方法通过CAS设置private volatile Thread runner;/** Treiber stack of waiting threads */private volatile WaitNode waiters;public FutureTask(Callable<V> callable) {if (callable == null)throw new NullPointerException();this.callable = callable;this.state = NEW; // 初始化状态是new      // ensure visibility of callable}
}

 /* 继承了Runnable ，因为线程池中执行的也是Runnbale的任务*/
public interface RunnableFuture<V> extends Runnable, Future<V> {/*** Sets this Future to the result of its computation* unless it has been cancelled.*/void run();
}

FutureTask 实现RunnableFuture，也间接实现了run方法。

重点

我们知道 execute(ftask); 本质就是利用线程池进行执行，而线程执行的时候，其实就是启动对应任务的run方法。

task.run();

		// 这里是什么时候调用的，其实是// execute(ftask)传入的任务 task.run()public void run() {//不是新建状态 直接中止if (state != NEW ||!UNSAFE.compareAndSwapObject(this, runnerOffset,null, Thread.currentThread()))return;try {Callable<V> c = callable;if (c != null && state == NEW) {V result;boolean ran;try {//核心，执行任务的call方法，你看就是调用普通的方法一样。result = c.call();//同步调用获取结果值ran = true;} catch (Throwable ex) {result = null;ran = false;setException(ex);}if (ran)//设置结果值set(result);}} finally {// runner must be non-null until state is settled to// prevent concurrent calls to run()runner = null;// state must be re-read after nulling runner to prevent// leaked interruptsint s = state;//响应中断if (s >= INTERRUPTING)handlePossibleCancellationInterrupt(s);}}

• 判断当前任务状态，非NEW直接返回
• 执行对应c.call() 其实就是执行callable中的call方法。
• 将返回值set进去

    protected void set(V v) {//CAS 去设置当前任务执行状态 new-completingif (UNSAFE.compareAndSwapInt(this, stateOffset, NEW, COMPLETING)) {//返回结果outcomeoutcome = v;UNSAFE.putOrderedInt(this, stateOffset, NORMAL); // final statefinishCompletion();}}

get

    public V get() throws InterruptedException, ExecutionException {int s = state;//如果是在执行中，则等待一会if (s <= COMPLETING)s = awaitDone(false, 0L);//返回结果return report(s);}/*** @throws CancellationException {@inheritDoc}*/public V get(long timeout, TimeUnit unit)throws InterruptedException, ExecutionException, TimeoutException {if (unit == null)throw new NullPointerException();//设置了超时时间，则等待一定的时间，如果还没有获取到返回异常int s = state;if (s <= COMPLETING &&(s = awaitDone(true, unit.toNanos(timeout))) <= COMPLETING)throw new TimeoutException();return report(s);}private V report(int s) throws ExecutionException {Object x = outcome;//执行完成 返回x结果if (s == NORMAL)return (V)x;//如果任务取消，返回异常if (s >= CANCELLED)throw new CancellationException();throw new ExecutionException((Throwable)x);}

awaitDone

    private int awaitDone(boolean timed, long nanos)throws InterruptedException {final long deadline = timed ? System.nanoTime() + nanos : 0L;WaitNode q = null;boolean queued = false;for (;;) {//如果线程执行interrupted，直接抛出异常，并且将任务移除if (Thread.interrupted()) {removeWaiter(q);throw new InterruptedException();}int s = state;//状态大于COMPLETING 说明完成了if (s > COMPLETING) {if (q != null)q.thread = null;return s;}//else if (s == COMPLETING) // cannot time out yetThread.yield();else if (q == null)q = new WaitNode();else if (!queued)queued = UNSAFE.compareAndSwapObject(this, waitersOffset,q.next = waiters, q);else if (timed) {nanos = deadline - System.nanoTime();if (nanos <= 0L) {removeWaiter(q);return state;}LockSupport.parkNanos(this, nanos);}elseLockSupport.park(this);}}

小结

FutureTask是一个支持取消行为的异步任务执行器。该类实现了Future接口的方法。
如：

1. 取消任务执行
2. 查询任务是否执行完成
3. 获取任务执行结果（”get“任务必须得执行完成才能获取结果，否则会阻塞直至任务完成）。

如果在当前线程中需要执行比较耗时的操作，但又不想阻塞当前线程时，可以把这些作业交给FutureTask，另开一个线程在后台完成，当当前线程将来需要时，就可以通过FutureTask对象获得后台作业的计算结果或者执行状态。

Future模式其实是多线程编程中常用的设计模式，主线程向另外一个线程提交任务，无需等待任务执行的结果，返回一个凭证，就是future，通过future.get()去获取结果。这个过程可能是阻塞的。