Java中ExecutorService和CompletionService区别

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());         }               }