Java多线程之Executor框架(2)

来源：互联网发布：中国出口贸易数据分析编辑：程序博客网时间：2024/06/05 11:02

在Thread,Runnable传统方式实现多线程执行的时候，默认这些线程是没有返回值的，如果需要返回值怎么处理，这里需要用到Executor框架的Callable接口，这个接口和Thread,Runnable语义相同。具体使用的方法是Callable接口实现任务逻辑，将此接口传递给FutureTask，FutureTask可以取得这个接口的返回值。
代码示例如下：

package com.lenovo.plm.exe;import java.util.concurrent.Callable;import java.util.concurrent.ExecutionException;import java.util.concurrent.FutureTask;public class FutureTaskDemo {    public static void main(String[] args) {        Callable<Integer> task = new Callable<Integer>(){            @Override            public Integer call() throws Exception {                // TODO Auto-generated method stub                System.out.println(Thread.currentThread().getName() + " come in..");                Thread.sleep(6000);                return new Integer(9);            }        };        FutureTask<Integer> futureTask = new FutureTask<Integer>(task);        Thread thread = new Thread(futureTask);        thread.start();        System.out.println("blocking....");        try {            Integer result = futureTask.get();            System.out.println(result);        } catch (InterruptedException | ExecutionException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }        System.out.println("end...");    }}

在task执行的时候，FutureTask的对象处于阻塞状态，一直到Callable接口执行完任务返回。FutureTask对象调用get方法取得返回值。看运行结果：
blocking….
Thread-0 come in..
9
end…

如果有多个Callable需要执行，那么也可以使用线程池的方式来执行。具体用法见下：

package com.lenovo.plm.exe;import java.util.concurrent.Callable;import java.util.concurrent.ExecutionException;import java.util.concurrent.Executor;import java.util.concurrent.Executors;import java.util.concurrent.FutureTask;public class FutureTaskDemo2 {    public static void main(String[] args) {        Callable<String> task1 = new Callable<String>(){            @Override            public String call() throws Exception {                // TODO Auto-generated method stub                System.out.println(Thread.currentThread().getName()+" come in...");                return "hello";            }        };        FutureTask<String> futureTask1 = new FutureTask<String>(task1);        Callable<String> task2 = new Callable<String>(){            @Override            public String call() throws Exception {                // TODO Auto-generated method stub                System.out.println(Thread.currentThread().getName()+" come in...");                return "world";            }        };        FutureTask<String> futureTask2 = new FutureTask<String>(task2);        Executor exec = Executors.newFixedThreadPool(5);        exec.execute(futureTask1);        exec.execute(futureTask2);        try {            System.out.println(futureTask1.get() + ","+futureTask2.get());        } catch (InterruptedException | ExecutionException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }    }}

这里取得返回值比较麻烦的是，我们需要挨个找到FutureTask对象来调用get方法，但是有一个问题是，如果先调用的futureTask没执行完，就必须等待了，这样后调用的执行完了也必须等待。因此这里可以使用CompletionService接口来实现。CompletionService接口会自动遍历所有的futureTask对象，将执行完的先取得，没有执行完的会继续等待。用法如下：

package com.lenovo.plm.exe;import java.util.concurrent.Callable;import java.util.concurrent.CompletionService;import java.util.concurrent.ExecutionException;import java.util.concurrent.Executor;import java.util.concurrent.ExecutorCompletionService;import java.util.concurrent.Executors;public class FutureTaskDemo3 {    public static void main(String[] args) {        Callable<String> task1 = new Callable<String>(){            @Override            public String call() throws Exception {                // TODO Auto-generated method stub                System.out.println(Thread.currentThread().getName()+" come in...hello");                return "hello";            }        };        Callable<String> task2 = new Callable<String>(){            @Override            public String call() throws Exception {                // TODO Auto-generated method stub                System.out.println(Thread.currentThread().getName()+" come in...world");                return "world";            }        };        Executor exec = Executors.newFixedThreadPool(5);        CompletionService<String> cservice = new ExecutorCompletionService<String>(exec);        cservice.submit(task1);        cservice.submit(task2);        try {            System.out.println(cservice.take().get() + " " +cservice.take().get());        } catch (InterruptedException | ExecutionException e) {            // TODO Auto-generated catch block            e.printStackTrace();        }    }}

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