JAVA中FutureTask

在多线程的编程中你是否也为不知道其他线程是否已经结束、得不到其他线程的执行结果而苦恼。今天我们来看的FutureTask就解决了这个问题。
FutureTask：一种可以携带结果的Runnable.它的计算是通过Callable的call方法来实现的。
我们来看一下FutureTask的构造.
public class FutureTask<V> implements RunnableFuture<V>
可以看到它实现了RunnableFuture接口.
public interface RunnableFuture<V> extends Runnable, Future<V>
而RunnableFuture又实现了Runnable和Future。
Runnable接口想必大家都非常熟悉了。我们来看下Future接口

public interface Future<V> {
    boolean cancel(boolean mayInterruptIfRunning);
    boolean isCancelled();
    boolean isDone();
    V get() throws InterruptedException, ExecutionException;
    V get(long timeout, TimeUnit unit)
        throws InterruptedException, ExecutionException, TimeoutException;
}

cancel(boolean mayInterruptIfRunning)	取消任务，如果mayInterruptIfRunning设置为true,标识可以取消正在进行中的任务.返回值分为三种情况：1.任务执行中,当mayInterruptIfRunning为true时，返回true.当mayInterruptIfRunning为false时候返回false.2.任务已经完成.无论mayInterruptIfRunning为true or false 都返回false.3.任务未运行 .无论mayInterruptIfRunning为true or false 都返回true
isCancelled()	任务是否被成功取消
isDone()	任务是否已经完成.取消、异常、完成都属于完成。
get()	阻塞只到有结果返回
get(long timeout, TimeUnit unit)	指定时间内如果没有返回结果则返回NULL

我们再来看下FutureTask的构造器
public FutureTask(Callable<V> callable) 中有一个Callable参数。上面我们说过FutureTask的计算实际是通过Callable的call方法来实现的。

public interface Callable<V> {
    /**
     * Computes a result, or throws an exception if unable to do so.
     *
     * @return computed result
     * @throws Exception if unable to compute a result
     */
    V call() throws Exception;
}

Callable接口中只有一个call方法，而且有返回值,这个返回值就是我们计算所携带的结果。
下面是FutureTask的常见用法：
1.Thread  执行FutureTask

package com.asiainfo.concurrent;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
/**
 * 可携带结果的任务.
 * 
 * 实现了Runnable接口
 */
import java.util.concurrent.FutureTask;
/**
 * Thread 方式启动FutureTask
 * @author fansh
 *
 */
public class FutureTaskTest{
	
	static FutureTask<String> future=new FutureTask<String>(new Callable<String>() {
		public String call() throws Exception {
			return "this is the result!";
		};
	});
	private Thread thread = new Thread(future);
	
	private void start(){
		thread.start();
	}

	public static void main(String[] args) {
		FutureTaskTest test =new FutureTaskTest();
		test.start();
		try {
			String result=future.get();
			System.out.println(result);
		} catch (InterruptedException e) {
			e.printStackTrace();
		} catch (ExecutionException e) {
			e.printStackTrace();
		}
	}
}

2. ExecutorService启动FutureTask

package com.asiainfo.concurrent;

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.FutureTask;
/**
 * ExecutorService 方式启动FutureTask
 * @author fansh
 *
 */
public class FutureTaskTest1 {

	   static FutureTask<String> future = new FutureTask<String>(new Callable<String>(){
	        public String call(){
	            return "this is the result!";
	        }
	    });
	    
	    public static void main(String[] args) throws InterruptedException, ExecutionException{
	       ExecutorService service =Executors.newCachedThreadPool();
	       service.submit(future);
	       service.shutdown();
	       String result=future.get();
	       System.out.println(result);
	    }
	    
}

3.FutureTask 任务未启动时 取消任务

package com.asiainfo.concurrent;

import java.util.concurrent.Callable;
import java.util.concurrent.FutureTask;
/**
 * 1：任务尚未开始
 * @author fansh
 *
 */
public class FutureTaskCanceledTest {

	static FutureTask<String> future=new FutureTask<String>(new Callable<String>() {
		public String call() throws Exception {
			Thread.sleep(1000);
			return "a";
		};
	});
	private Thread thread = new Thread(future);
	
	private void start(){
		thread.start();
	}

	public static void main(String[] args) {
		FutureTaskCanceledTest test =new FutureTaskCanceledTest();
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		//boolean flag =future.cancel(true);
		boolean flag =future.cancel(false);
		System.out.println("任务是否已经取消:"+flag);
		//保证任务取消的时候 线程尚未执行.
		test.start();
	}
}

4.FutureTask 任务完成后 取消任务

package com.asiainfo.concurrent;

import java.util.concurrent.Callable;
import java.util.concurrent.FutureTask;
/**
 * 1：任务已经完成
 * @author fansh
 *
 */
public class FutureTaskCanceledTest1 {

	static FutureTask<String> future=new FutureTask<String>(new Callable<String>() {
		public String call()  {
			return "a";
		};
	});
	private Thread thread = new Thread(future);
	
	private void start(){
		thread.start();
	}

	public static void main(String[] args) {
		FutureTaskCanceledTest1 test =new FutureTaskCanceledTest1();
		test.start();
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		boolean flag=future.cancel(false);
		//boolean flag=future.cancel(true);
		System.out.println("任务是否已经取消:"+flag);
	}
}

5. 任务进行中  响应取消动作

package com.asiainfo.concurrent;

import java.util.concurrent.Callable;
import java.util.concurrent.FutureTask;
/**
 * 1：任务进行中......
 * @author fansh
 *
 */
public class FutureTaskCanceledTest2 {

	static FutureTask<String> future=new FutureTask<String>(new Callable<String>() {
		public String call()  {
			while(!Thread.currentThread().isInterrupted()){
				System.out.println("任务进行中............");
			}
			return "a";
		};
	});
	private Thread thread = new Thread(future);
	
	private void start(){
		thread.start();
	}

	public static void main(String[] args) {
		FutureTaskCanceledTest2 test =new FutureTaskCanceledTest2();
		test.start();
		try {
			Thread.sleep(1000);
		} catch (InterruptedException e) {
			e.printStackTrace();
		}
		boolean flag=future.cancel(true);
		//boolean flag=future.cancel(false);
		System.out.println("任务是否已经取消:"+flag);
	}
}