Java并发之ThreadPoolExecutor和FutureTask

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ExecutorService.submit(Runnable task)方法及其重载方法的返回类型为Future，其真实类型为FutureTask。类图如下：

由类图可知，FutureTask是Future和Runnable的合体，并且持有任务Runnable或者Callable。future.get()是一个阻塞方法，直到任务完成才返回。再看ThreadPoolExecutor类图：

当ExecutorService.submit(Runnable task)方法及其重载方法执行时（这些方法由AbstractExecutorService实现），FutureTask会被实例化：

public Future<?> submit(Runnable task) {        if (task == null) throw new NullPointerException();        RunnableFuture<Object> ftask = newTaskFor(task, null);        execute(ftask);        return ftask;    }    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;    }    public <T> Future<T> submit(Callable<T> task) {        if (task == null) throw new NullPointerException();        RunnableFuture<T> ftask = newTaskFor(task);        execute(ftask);        return ftask;    }

这些方法都调用了execute()方法。需要注意的是submit(Runnable task)方法返回的Future调用其get()将返回null。 newTaskFor()方法：

protected <T> RunnableFuture<T> newTaskFor(Runnable runnable, T value) {        return new FutureTask<T>(runnable, value);    }protected <T> RunnableFuture<T> newTaskFor(Callable<T> callable) {        return new FutureTask<T>(callable);    }

FutureTask构造方法：

public FutureTask(Callable<V> callable) {        if (callable == null)            throw new NullPointerException();        sync = new Sync(callable);    } public FutureTask(Runnable runnable, V result) {        sync = new Sync(Executors.callable(runnable, result));    }

callable和runnable都会被包装成Sync。Executors.callable()方法：

public static <T> Callable<T> callable(Runnable task, T result) {        if (task == null)            throw new NullPointerException();        return new RunnableAdapter<T>(task, result);    }

再看看Executors的几个创建线程池的静态方法：

public static ExecutorService newFixedThreadPool(int nThreads) {        return new ThreadPoolExecutor(nThreads, nThreads,                                      0L, TimeUnit.MILLISECONDS,                                      new LinkedBlockingQueue<Runnable>());    }public static ExecutorService newSingleThreadExecutor() {        return new FinalizableDelegatedExecutorService            (new ThreadPoolExecutor(1, 1,                                    0L, TimeUnit.MILLISECONDS,                                    new LinkedBlockingQueue<Runnable>()));    }public static ExecutorService newCachedThreadPool() {        return new ThreadPoolExecutor(0, Integer.MAX_VALUE,                                      60L, TimeUnit.SECONDS,                                      new SynchronousQueue<Runnable>());    }

它们都实例化了一个线程池，并且传入了相应的阻塞队列。

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