java中异步调用注意

Future接口是Java标准API的一部分，在java.util.concurrent包中。Future接口是Java线程Future模式的实现，可以来进行异步计算。

有了Future就可以进行三段式的编程了，1.启动多线程任务2.处理其他事3.收集多线程任务结果。从而实现了非阻塞的任务调用。在途中遇到一个问题，那就是虽然能异步获取结果，但是Future的结果需要通过isdone来判断是否有结果，或者使用get()函数来阻塞式获取执行结果。这样就不能实时跟踪其他线程的结果状态了，所以直接使用get还是要慎用，最好配合isdone来使用。如果直接使用get来获取结果, get是阻塞操作, 相当于获取结果仍然是同步.因此在获取结果的时候需要注意.

import java.util.ArrayList;
import java.util.List;
import java.util.Random;
import java.util.concurrent.Callable;
import java.util.concurrent.ExecutorService;
import java.util.concurrent.Executors;
import java.util.concurrent.Future;

/**
 * 多线程执行，异步获取结果
 *
 * @author i-clarechen
 *
 */
public class AsyncThread {

    public static void main(String[] args) {
        AsyncThread t = new AsyncThread();
        List<Future<String>> futureList = new ArrayList<Future<String>>();
        t.generate(3, futureList);
        t.doOtherThings();
        t.getResult(futureList);
    }

    /**
     * 生成指定数量的线程，都放入future数组
     *
     * @param threadNum
     * @param fList
     */
    public void generate(int threadNum, List<Future<String>> fList) {
        ExecutorService service = Executors.newFixedThreadPool(threadNum);
        for (int i = 0; i < threadNum; i++) {
            Future<String> f = service.submit(getJob(i));
            fList.add(f);
        }
        service.shutdown();
    }

    /**
     * other things
     */
    public void doOtherThings() {
        try {
            for (int i = 0; i < 3; i++) {
                System.out.println("do thing no:" + i);
                Thread.sleep(1000 * (new Random().nextInt(10)));
            }
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
    }

    /**
     * 从future中获取线程结果，打印结果
     *
     * @param fList
     */
    public void getResult(List<Future<String>> fList) {
        ExecutorService service = Executors.newSingleThreadExecutor();
        service.execute(getCollectJob(fList));
        service.shutdown();
    }

    /**
     * 生成指定序号的线程对象
     *
     * @param i
     * @return
     */
    public Callable<String> getJob(final int i) {
        final int time = new Random().nextInt(10);
        return new Callable<String>() {
            @Override
            public String call() throws Exception {
                Thread.sleep(1000 * time);
                return "thread-" + i;
            }
        };
    }

    /**
     * 生成结果收集线程对象
     *
     * @param fList
     * @return
     */
    public Runnable getCollectJob(final List<Future<String>> fList) {
        return new Runnable() {
            public void run() {
                for (Future<String> future : fList) {
                    try {
                        while (true) {
                            if (future.isDone() && !future.isCancelled()) {
                                System.out.println("Future:" + future
                                        + ",Result:" + future.get());
                                break;
                            } else {
                                Thread.sleep(1000);
                            }
                        }
                    } catch (Exception e) {
                        e.printStackTrace();
                    }
                }
            }
        };
    }

}
优化获取方式:

　　　　　　List<Object> results = null;
        int retryLimit = 3;
        long delay = 10;
        int tryTime = 0;
　　　　for(Future future : futures)
        do {
            try {
            //此处原理:
                results = future.get();
            } catch (Exception e) {
                results = null;
            }

            tryTime++;
            if (results == null) {
                try {
                    Thread.sleep(delay << tryTime);
                } catch (InterruptedException e) {

                }
            }
        } while (results == null && tryTime < retryLimit);
　　　　}
        return results;

运行结果打印和future放入列表时的顺序一致，为0，1，2：

do thing no:0
do thing no:1
do thing no:2
Future:java.util.concurrent.FutureTask@68e1ca74,Result:thread-0
Future:java.util.concurrent.FutureTask@3fb2bb77,Result:thread-1
Future:java.util.concurrent.FutureTask@6f31a24c,Result:thread-2

使用CompletionService，它内部添加了阻塞队列，从而获取future中的值，然后根据返回值做对应的处理

下面是先执行完的线程先处理的方案：

import java.util.Random;
import java.util.concurrent.BlockingQueue;
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;
import java.util.concurrent.LinkedBlockingDeque;

public class testCallable {
    public static void main(String[] args) {
        try {
            completionServiceCount();
        } catch (InterruptedException e) {
            e.printStackTrace();
        } catch (ExecutionException e) {
            e.printStackTrace();
        }
    }

    /**
     * 使用completionService收集callable结果
     * @throws ExecutionException
     * @throws InterruptedException
     */
    public static void completionServiceCount() throws InterruptedException, ExecutionException {
        ExecutorService executorService = Executors.newCachedThreadPool();
        CompletionService<Integer> completionService = new ExecutorCompletionService<Integer>(
                executorService);
        int threadNum = 5;
        for (int i = 0; i < threadNum; i++) {
            completionService.submit(getTask(i));
        }
        int sum = 0;
        int temp = 0;
        for(int i=0;i<threadNum;i++){
            temp = completionService.take().get();
            sum += temp;
            System.out.print(temp + "\t");
        }
        System.out.println("CompletionService all is : " + sum);
        executorService.shutdown();
    }

    public static Callable<Integer> getTask(final int no) {
        final Random rand = new Random();
        Callable<Integer> task = new Callable<Integer>() {
            @Override
            public Integer call() throws Exception {
                int time = rand.nextInt(100)*100;
                System.out.println("thead:"+no+" time is:"+time);
                Thread.sleep(time);
                return no;
            }
        };
        return task;
    }
}

运行结果为最先结束的线程结果先被处理：

thead:0 time is:4200
thead:1 time is:6900
thead:2 time is:2900
thead:3 time is:9000
thead:4 time is:7100
2    0    1    4    3    CompletionService all is : 10