线程如何在完成工作后返回值？

Question

假设我们有这个简单的例子:

public Example extends Thread{

    String temp;    

    public Example(){
    }

    @Override
    public void run(){
        .
        .
        .
        .
        temp = "a_value";
    }

    public static void main(String[] args) {

        Example th = new Example();
        th.start();
    }

}

Thread完成工作后怎么能给我返回String temp？

Answer 1

使用(相对)new Callable<T>而不是Runnable(1.5及更新版本):

这是一个(简单)示例:

import java.util.concurrent.Callable;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;


public class Main {

    public static void main(final String[] argv) {
        final ExecutorService service;
        final Future<String>  task;

        service = Executors.newFixedThreadPool(1);        
        task    = service.submit(new Foo());

        try {
            final String str;

            // waits the 10 seconds for the Callable.call to finish.
            str = task.get(); // this raises ExecutionException if thread dies
            System.out.println(str);
        } catch(final InterruptedException ex) {
            ex.printStackTrace();
        } catch(final ExecutionException ex) {
            ex.printStackTrace();
        }

        service.shutdownNow();
    }
}

class Foo implements Callable<String> {
    public String call() {
        try {
            // sleep for 10 seconds
            Thread.sleep(10 * 1000);
        } catch(final InterruptedException ex) {
            ex.printStackTrace();
        }

        return ("Hello, World!");
    }
}

Answer 2

看看Future接口javadoc.它有示例用法,向您展示如何执行此操作.

Answer 3

您可以通过Observer模式实现此目的.在完成线程时通知所有听众它已完成并且他们可以检索值(通过getter).或者它甚至可以发送计算值.

或者您可以使用任务,请参阅FutureTask,一个可运行的(实际上如下所述的Callable),它返回一个结果并可以抛出异常.