Java多线程之Executor框架(2)

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();
}
}
}