多线程环境中的Java回调函数

您可以使用ExecutorService在后台执行任务，然后使用返回的方法Future来等待结果。例如：

class Main {
  private static final ExecutorService es = Executors.newCachedThreadPool();

  public static void main(final String... args) throws Throwable {
    List<Future<Integer>> results = new ArrayList<>();
    for (int i = 0; i < 10; i++) {
      results.add(asyncSum(i, i*i));
    }
    // here, in the main thread, you can do whatever you want
    // while the calculations are performed in background threads
    // ...

    // after the main thread finishes what it was doing, it
    // can process the futures
    for (final Future<Integer> result : results) {
      System.out.println(result.get());
    }
  }

  // this method launches a calculation on a worker thread and immediately
  // returns a Future, which is a reference to the result of the calculation
  // once it is completed
  private static Future<Integer> asyncSum(final int a, final int b) {
    return es.submit(new Callable<Integer>() {
      @Override public Integer call() throws Exception {
        return a + b;
      }
    });
  }
}

在上面的示例中，主线程将阻塞，直到完成第一个计算，然后再打印它。然后阻塞，直到完成第二次计算，然后再打印，依此类推。

如果您希望在结果可用时（以未指定的顺序）打印结果，则可以使用CompletionService，而不是使用结果列表并对其进行迭代，您可以通过CompletionService本身的.take()方法来获取其期货。，会阻塞直到计算完成为止；或者.poll()，如果存在已完成的计算则返回Future；或者如果没有完成的计算将返回null，这样您的主线程将永远不会阻塞。

以下示例使用CompletionService。它显示了一个永不阻塞的主线程，使用后台线程进行计算并在结果可用时处理结果：

class Main {
  public static void main(String[] args) throws Throwable {
    final ExecutorService es = Executors.newCachedThreadPool();
    final CompletionService<Integer> cs = new ExecutorCompletionService<>(es);

    submitSomeCalculations(cs);

    while (true) {
      doMainThreadWork();
      processFinishedCalculations(cs);
    }
  }

  private static void submitSomeCalculations(final CompletionService<Integer> cs) {
    for (int i = 0; i < 10; i++) {
      submitAsyncSum(cs, i, i * i);
    }
  }

  private static void submitAsyncSum(final CompletionService<Integer> cs, final int a, final int b) {
    cs.submit(new Callable<Integer>() {
      @Override public Integer call() throws Exception {
        Thread.sleep(100 + (long) (Math.random() * 900));
        return a + b;
      }
    });
  }

  private static void processFinishedCalculations(final CompletionService<Integer> cs) throws ExecutionException, InterruptedException {
    while (true) {
      final Future<Integer> result = cs.poll();
      if (result == null) {
        System.out.println("> no finished results...");
        break;
      } else {
        System.out.println("> result available: " + result.get());
      }
    }
  }

  static void doMainThreadWork() {
    System.out.println("work from main thread...");
  }
}