Java多线程调用并获取返回值

Java多线程调用并获取返回值

main1使用Future是最基本的方法，会造成阻塞，适合执行方法少、时间短的；
main2是功能比较强大的，使用了ExecutorCompletionService保存已经执行完的方法，适合执行不确定时间的需求。

import java.util.ArrayList;
import java.util.List;
import java.util.concurrent.Callable;
import java.util.concurrent.CompletionService;
import java.util.concurrent.ExecutionException;
import java.util.concurrent.ExecutorCompletionService;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

public class TestCallable {

    public static void main1(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(5);
        try {
            List<Future<Integer>> list = new ArrayList<>();
            for(int i = 0; i < 10; i++) {
                list.add(executor.submit(new Task(i)));
            }
            /**
             * 用List保存Future
             * 用get来获取返回结果，如果Future没有执行完，再调用get方法时就会阻塞
             * 导致可能执行完的操作没有即时返回
             */
            for(Future<Integer> f : list) {
                int value = f.get();
                System.out.println(value);
            }
            
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }
        executor.shutdown();
    }
    public static void main2(String[] args) {
        ExecutorService executor = Executors.newFixedThreadPool(5);
        /**
         * ExecutorCompletionService获取先执行完的任务返回值，这样就不会造成阻塞
         */
        CompletionService<Integer> completion = new ExecutorCompletionService<>(executor);
        try {
            for(int i = 0; i < 10; i++) {
                completion.submit(new Task(i));
            }
            for(int i = 0 ; i < 10; i++) {
                int ret = completion.take().get();
                System.out.println(ret);
            }
            
        } catch (InterruptedException | ExecutionException e) {
            e.printStackTrace();
        }
        executor.shutdown();
    }

}
class Task implements Callable<Integer>{
    private int id;
    public Task(int id) {
        this.id = id;
    }
    
    @Override
    public Integer call() throws Exception {
        if(id%2==0)
            Thread.sleep((long) (Math.random()*10000));
         System.out.println("run task["+id+"]");
        return id;
    }
    
}