FutureTask 不返回结果

Question

ExecutorService pool2 = Executors.newSingleThreadExecutor();
Future result2 = pool2.submit(new FutureTask<String>(
new Callable<String>() {
public String call() throws Exception {
return "TEST";
}
}));String res2 = (String) result2.get();
Thread.sleep(5000);
System.out.println(res2);// 结果怎么是null，我以为是 "TEST" 呢结果为什么是null呢 java线程池

Answer 1

sleep那一行跟前面那行换一下

Answer 2

  public static void main(final String[] args) throws InterruptedException, ExecutionException {
    ExecutorService pool2 = Executors.newSingleThreadExecutor();
    Future<String> result2 = pool2.submit(new Callable<String>() {
      public String call() throws Exception {
        return "TEST";
      }
    });    Thread.sleep(1000);
    String res2 = result2.get();
    System.out.println(res2);
  }
FutureTask要通过execute方法调用，submit直接传入callable。你这个程序如果用FutureTask来实现要这样：
  public static void main(final String[] args) throws InterruptedException, ExecutionException {
    ExecutorService pool2 = Executors.newSingleThreadExecutor();    FutureTask<String> task = new FutureTask<>(new Callable<String>() {
      @Override
      public String call() throws Exception {
        return "TEST";
      }
    });
    pool2.execute(task);
    Thread.sleep(100);
    if (task.isDone())
      System.out.println(task.get());
    else
      System.err.println("Error executing task");
  }