System.out.println / System.err.println不起作用

Question

I have the following code:

  @Test
  public void testMultipleUpdatesSameTime() {
    final CyclicBarrier gate = new CyclicBarrier(3);

    PrintStream out = null;
    try {
      out = new PrintStream(new FileOutputStream(
          "C:\\pathToFile\\test2_output.txt"));
    } catch (FileNotFoundException e1) {
      System.err.println(e1);
    }
    System.setErr(out);

    new Thread(() -> {
      try {
        gate.await();
      } catch (InterruptedException e) {
        System.err.println(e);
      } catch (BrokenBarrierException e) {
        System.err.println(e);
      }
      System.err.println("FROM1: Starting at: " + System.currentTimeMillis());
      System.err.println("FROM1: Thread with ID: " + Thread.currentThread().getId());
      try {
        Thread.sleep(1000);
      } catch (InterruptedException e) {
        System.err.println(e);
      }
      System.err.println("FROM1: Ending at: " + System.currentTimeMillis());
    }).start();

    new Thread(() -> {
      try {
        gate.await();
      } catch (InterruptedException e) {
        System.err.println(e);
      } catch (BrokenBarrierException e) {
        System.err.println(e);
      }
      System.err.println("FROM2: Starting at: " + System.currentTimeMillis());
      System.err.println("FROM2: Thread with ID: " + Thread.currentThread().getId());
      try {
        Thread.sleep(1000);
      } catch (InterruptedException e) {
        System.err.println(e);
      }
      System.err.println("FROM2: Ending at: " + System.currentTimeMillis());
    }).start();

    System.err.println("NOTHING YET");

    try {
      Thread.sleep(10000);
      gate.await();
    } catch (InterruptedException e) {
      System.err.println(e);
    } catch (BrokenBarrierException e) {
      System.err.println(e);
    }

    System.err.println("DONE");
  }

The result file is containing only half of the output expected:

• NOTHING YET
• DONE
• FROM1: Starting at: 1521464569722
• FROM2: Starting at: 1521464569722
• FROM1: Thread with ID: 11
• FROM2: Thread with ID: 12

Do you have any idea why the file does not contain the "Ending at" nor exceptions?

Answer 1

It's a common occurrence that a file is missing some contents if you don't close the file after writing. There are finalizer() methods in many OutputStream subclasses that do close the stream, but they don't always have a chance to get called as is the case here.

The main thread releases the gate and quits immediately, the other threads quickly run their course and at that point the VM is closed and you can't trust that things finished correctly.

The original code is convoluted and it's hard to "fix" something that's broken from the ground up. For instance in real code you wouldn't have 2 threads competing to write into the same file. That just doesn't make sense.

In any case the best "fix" for this would be for the main thread to wait for the other threads to finish (instead of sleeping), and then close the file as below.

Thread t1 = new Thread(() -> ...
Thread t2 = new Thread(() -> ...
t1.start();
t2.start();

t1.join();
t2.join();
out.close();