Exception in java thread

Question

Consider the following code:

public class ThreadT implements Runnable {

 public void run() {
 System.out.println("run.");
 throw new RuntimeException("Problem");
 }
 public static void main(String[] args) {
 Thread t = new Thread(new ThreadT());
 t.start();
 System.out.println("End of method.");
 }
}

The output I get is:

End of method.
run.
Exception in thread "Thread-0" java.lang.RuntimeException: Problem

Why the output is not like this:

run.
Exception in thread "Thread-0" java.lang.RuntimeException: Problem
End of method.

Highly relevant.

ChiefTwoPencils
– ChiefTwoPencils

2014-12-22 10:13:48 +00:00
Commented Dec 22, 2014 at 10:13 — ChiefTwoPencils
– ChiefTwoPencils, Commented Dec 22, 2014 at 10:13

Eran · Accepted Answer · 2014-12-22 10:13:02Z

2

When you start a new thread (without adding any synchronization), you have no control which statement will be executed first - the next statement of the main thread or the code of the run method in the second thread.

There is no reason to expect that the run method of the new thread would be executed prior to the next statement of the main method of the main thread.

answered Dec 22, 2014 at 10:13

Eran

395k57 gold badges726 silver badges793 bronze badges

Sign up to request clarification or add additional context in comments.

Comments

Peter Lawrey · Accepted Answer · 2014-12-22 10:13:11Z

2

When you start a thread, it takes time. You are also starting a thread with the sole purpose of executing independently of the main thread. This can mean it runs after your main thread has finished.

Also the error can appear before the message.

You are writing to different streams, one is System.out, the other is System.err and the order these appear on the screen is not determined. You can avoid this with

new RuntimeException("Problem").printStackTrace(System.out);

answered Dec 22, 2014 at 10:13

Peter Lawrey

535k83 gold badges770 silver badges1.2k bronze badges

Comments

MoOoG · Accepted Answer · 2014-12-22 10:21:58Z

1

I believe it's because when you run a new thread beside Main, both threads run at the same time and first it prints out the "End of method", right after it runs the second outprint (which is probably few miliseconds behind.

try this:

public class thread implements Runnable {

public void run() {
System.out.println("run.");
throw new RuntimeException("Problem");
}
public static void main(String[] args) throws InterruptedException {
Thread t = new Thread(new thread());
t.start();
Thread.sleep(3000);
System.out.println("End of method.");
}
}

What I did here is to put Main thread into sleep for 3 seconds and give time to Thread t to run first.

Or you can also write join() to wait the Thread to finish first. Like this:

public class thread implements Runnable {

public void run() {
System.out.println("run.");
throw new RuntimeException("Problem");
}
public static void main(String[] args) throws InterruptedException {
Thread t = new Thread(new thread());
t.start();
t.join();
System.out.println("End of method.");
}
}

edited Dec 22, 2014 at 10:21

answered Dec 22, 2014 at 10:16

MoOoG

4032 gold badges7 silver badges22 bronze badges

2 Comments

MoOoG Over a year ago

Try using join() function - I updated my answer with the code.

Solomon Slow Over a year ago

The join() version is smarter than the sleep() version. There are very few legitimate reasons to call sleep(). In particular, there is no synchronization bug that can be fixed by adding a sleep() call. If you think you fixed a bug by calling sleep(), what you actually did was make the bug much less likely (maybe much, much, much less likely) to happen during testing. But the bug still is there, and if and when it ever does bite, it probably will be in the field, at a customer site, long after everyone has forgotten that it existed, and very hard to find/explain.

Collectives™ on Stack Overflow

Exception in java thread

3 Answers 3

Comments

Comments

2 Comments

Your Answer

Linked

Hot Network Questions

Collectives™ on Stack Overflow

3 Answers 3

Comments

Comments

2 Comments

Your Answer

Sign up or log in

Post as a guest

Linked

Related