线程池之阻塞主线程，等子线程全部跑完之后，执行主线程

在spark中有异步多线程的需求，需要阻塞主线程，等所有子线程都执行完成后，主线程继续执行。
如果是用Thread，不太好实现，用Callable+FutrueTask结合线程池，可以快速实现：

final AtomicInteger messagesReceived = new AtomicInteger(0);

// ThreadedListenerAdapter is the class that I'm testing 
// It's not germane to the question other than as a target for a thread pool.
final ThreadedListenerAdapter adapter = 
    new ThreadedListenerAdapter(listener);
int taskCount = 10;

List> taskList = new ArrayList>();

for (int whichTask = 0; whichTask < taskCount; whichTask++) {
    FutureTask futureTask = 
        new FutureTask(new Callable() {
        @Override
        public Integer call() throws Exception {
            // Does useful work that affects messagesSent
            return messagesSent;
        }
    });
    taskList.add(futureTask);
}

for (FutureTask task : taskList) {
    LocalExecutorService.getExecutorService().submit(task);
}

for (FutureTask task : taskList) {
    int result = 0;
    try {
       // 关键是这里
        result = task.get();
    } catch (InterruptedException ex) {
        Thread.currentThread().interrupt();
    } catch (ExecutionException ex) {
        throw new RuntimeException("ExecutionException in task " + task, ex);
    }
    assertEquals(maxMessages, result);
}

int messagesSent = taskCount * maxMessages;
assertEquals(messagesSent, messagesReceived.intValue());