Java : Thread.sleep() in a thread : no wait

Question

I have a problem with thread not sleeping. I can't put here my whole code. so, to reproduce, here is a basic code waiting 5 seconds. ``` try { int millisec = 5000; System.out.println(new Date()); System.out.println("We wait " + millisec + " milliseconds"); Thread.sleep(millisec); System.out.println(new Date()); } catch (Exception e) { e.printStackTrace(); } ``` output : Thu Aug 22 20:01:42 CEST 2013 We wait 5000 milliseconds Thu Aug 22 20:01:47 CEST 2013 Everything is OK. But when I put this code in a thread, no sleep. Example with this code : ``` try { Thread aThread = new Thread(new Runnable() { @Override public void run() { try { int millisec = 5000; System.out.println(new Date()); System.out.println("We wait " + millisec + " milliseconds"); Thread.sleep(millisec); System.out.println(new Date()); } catch (Exception e) { e.printStackTrace(); } } }); aThread.start(); } catch (Exception e) { e.printStackTrace(); } ``` output : Thu Aug 22 20:07:30 CEST 2013 We wait 5000 milliseconds ..…

Accepted Answer

Edit:

The issue turned out to be how JUnit works. It does not know that background threads were spawned so when the test thread completes, it kills the rest of the threads without waiting for them.

Your code properly spits out both date lines when I try it in a small `main()` class. Here are some possibilities as to why you aren't seeing the output in your application:

The thread that is spawning the `aThread` may be a daemon thread itself. The `aThread` is therefore daemon because its daemon status is gotten from the spawning thread's status. If the JVM finishes before the 2nd `System.out.println(...)` the `aThread` will be killed. If there is a question about it, you should do:

```
Thread aThread = new Thread(new Runnable() {
...
// ensure the deamon flag is off _before_ we start the thread
aThread.setDaemon(false);
aThread.start();
```

Another possibility is that something is actually closing `System.out` before the 2nd `println(...);`. Unlikely but possible. Are you using shutdown hooks at all for cleanup?

A third possibility is that the output is actually being printed but your IDE or your console is somehow not displaying the output.

Another possibility is that the `System.out.println(...);` threw an `IOException`. It might be interesting to check the value of `System.out.checkError()` to see if it is `true` although I'm not sure how you would then display it.

One thing to try is to create a temporary file instead of printing output. Something like:

```
new File("/var/tmp/" + System.currentTimeMillis()).createNewFile();
```

Then you should see 2 files in `"/var/tmp"` (or wherever a temp directory on your OS is).

You should also try changing `System.out` to `System.err` to see if that changes anything. Doubtful but worth a try.

If these don't work as well then something is killing the JVM forcefully so it can't wait for the background threads.

Java : Thread.sleep() in a thread : no wait

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