java中FutureTask的使用
FutureTask是java 5引入的一个类,从名字可以看出来FutureTask既是一个Future,又是一个Task。
我们看下FutureTask的定义:
public class FutureTask<V> implements RunnableFuture<V> { ... }
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实现了RunnableFuture接口,RunnableFuture接口是Runnable和Future的综合体。
作为一个Future,FutureTask可以执行异步计算,可以查看异步程序是否执行完毕,并且可以开始和取消程序,并取得程序最终的执行结果。
除此之外,FutureTask还提供了一个runAndReset()的方法, 该方法可以运行task并且重置Future的状态。
我们知道Callable是有返回值的,而Runnable是没有返回值的。
Executors提供了很多有用的方法,将Runnable转换为Callable:
public static <T> Callable<T> callable(Runnable task, T result) { if (task == null) throw new NullPointerException(); return new RunnableAdapter<T>(task, result); }
FutureTask内部包含一个Callable,并且可以接受Callable和Runnable作为构造函数:
public FutureTask(Callable<V> callable) { if (callable == null) throw new NullPointerException(); this.callable = callable; this.state = NEW; // ensure visibility of callable }
public FutureTask(Runnable runnable, V result) { this.callable = Executors.callable(runnable, result); this.state = NEW; // ensure visibility of callable }
它的内部就是调用了Executors.callable(runnable, result);方法进行转换的。
既然是一个Runnable,那么FutureTask就可以以线程的方式执行,我们来看一个例子:
@Test public void convertRunnableToCallable() throws ExecutionException, InterruptedException { FutureTask<Integer> futureTask = new FutureTask<>(new Callable<Integer>() { @Override public Integer call() throws Exception { log.info("inside callable future task ..."); return 0; } }); Thread thread= new Thread(futureTask); thread.start(); log.info(futureTask.get().toString()); }
上面例子是以单个线程来执行的,同样我们也可以将FutureTask提交给线程池来执行:
@Test public void workWithExecutorService() throws ExecutionException, InterruptedException { FutureTask<Integer> futureTask = new FutureTask<>(new Callable<Integer>() { @Override public Integer call() throws Exception { log.info("inside futureTask"); return 1; } }); ExecutorService executor = Executors.newCachedThreadPool(); executor.submit(futureTask); executor.shutdown(); log.info(futureTask.get().toString()); }
本文的例子可参考 https://github.com/ddean2009/learn-java-concurrency/tree/master/futureTask