Java中ExecutorService和CompletionService区别

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ExecutorService:等所有submit对象执行完,才会返回get值;
CompletionService:只要notEmpty,就会返回get值.(阻塞针对的是尾部full会阻塞,头部empty会阻塞)而CompletionService的实现是维护一个保存Future对象的BlockingQueue。只有当这个Future对象状态是结束的时候，才会加入到这个Queue中，take()方法其实就是Producer-Consumer中的Consumer。它会从Queue中取出Future对象，如果Queue是空的，就会阻塞在那里，直到有完成的Future对象加入到Queue中。所以，先完成的必定先被取出。这样就减少了不必要的等待时间

public class CompletionServiceTest {
 
    static class Task implements Callable<String>{
        private int i;
         
        public Task(int i){
            this.i = i;
        }
 
        @Override
        public String call() throws Exception {
            Thread.sleep(10000);
            return Thread.currentThread().getName() + "执行完任务：" + i;
        }  
    }
     
    public static void main(String[] args){
        testUseFuture();
    }
     
    private static void testUseFuture(){
        int numThread = 5;
        ExecutorService executor = Executors.newFixedThreadPool(numThread);
        List<Future<String>> futureList = new ArrayList<Future<String>>();
        for(int i = 0;i<numThread;i++ ){
            Future<String> future = executor.submit(new CompletionServiceTest.Task(i));
            futureList.add(future);
        }
                 
        while(numThread > 0){
            for(Future<String> future : futureList){
                String result = null;
                try {
                    result = future.get(0, TimeUnit.SECONDS);
                } catch (InterruptedException e) {
                    e.printStackTrace();
                } catch (ExecutionException e) {
                    e.printStackTrace();
                } catch (TimeoutException e) {
                    //超时异常直接忽略
                }
                if(null != result){
                    futureList.remove(future);
                    numThread--;
                    System.out.println(result);
                    //此处必须break，否则会抛出并发修改异常。（也可以通过将futureList声明为CopyOnWriteArrayList类型解决）
                    break;
                }
            }
        }
    }
}

通过CompletionService包装ExecutorService，然后调用其take()方法去取Future对象。

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public class CompletionServiceTest {
 
    static class Task implements Callable<String>{
        private int i;
         
        public Task(int i){
            this.i = i;
        }
 
        @Override
        public String call() throws Exception {
            Thread.sleep(10000);
            return Thread.currentThread().getName() + "执行完任务：" + i;
        }  
    }
     
    public static void main(String[] args) throws InterruptedException, ExecutionException{
        testExecutorCompletionService();
    }
     
    private static void testExecutorCompletionService() throws InterruptedException, ExecutionException{
        int numThread = 5;
        ExecutorService executor = Executors.newFixedThreadPool(numThread);
        CompletionService<String> completionService = new ExecutorCompletionService<String>(executor);
        for(int i = 0;i<numThread;i++ ){
            completionService.submit(new CompletionServiceTest.Task(i));
        }
}
         
        for(int i = 0;i<numThread;i++ ){    
            System.out.println(completionService.take().get());
        }
         
    }

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