Question

Interesting Quiz

Asked by: testn

Why does this cause memory leak and How to solve it?


class ChildThread  extends Thread
{
     public void run() {
          System.out.print("a");
     }

};

class ParentThread extends Thread
{

     public void run() {
          for(int i=0; i<10000; i++) {
               Thread a = new ChildThread();
               a.run();
               try
               {
                    a.join();
               }
               catch (Exception e)
               {
               }
          }
     }

     public static void main(String[] args) {
          for (int i=0; i<9000; i++ )
          {
               try
               {
               Thread a = new ParentThread();
               a.start();
               a.join();
               a = null;
               }
               catch (Exception e)
               {
               }
               System.gc();
          }
     }
};

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Asked On
2003-03-19 at 04:48:42ID20556006
Tags

quiz

,

interesting

,

run

,

threads

Topic

Java Programming Language

Participating Experts
3
Points
50
Comments
9

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Answers

 

by: TimYatesPosted on 2003-03-19 at 04:53:04ID: 8166160

Call

a.start()

instead of a.run() in the run() method of ParentThread

 

by: mayankeaglePosted on 2003-03-19 at 04:54:29ID: 8166172

>>  a.run();

Please don't call run () this way. Please replace this by:

a.start () ;

Mayank.

 

by: mayankeaglePosted on 2003-03-19 at 04:55:49ID: 8166185

Oh, Tim Yates gave the same suggestion.... almost same time, so that's why it wasn't there when I loaded this page!

Mayank.

 

by: TimYatesPosted on 2003-03-19 at 05:04:08ID: 8166243

I don't think it will cause a memory leak tho,

the problem is more likely to be this:

-------------------

public void run()
{
  for(int i=0; i<10000; i++)
  {
    Thread a = new ChildThread();
    // CALLING RUN DIRECTLY, INSTEAD OF STARTING THIS THREAD
    a.run();
    try
    {
      // NOT SURE WHAT HAPPENS HERE...  I GUESS IT WILL JUST LOCK UP FOREVER, AS THE THREAD IS NOT YET STARTED...
      a.join();
    }
    catch (Exception e)
    {
    }
  }
}


 

by: Venci75Posted on 2003-03-19 at 05:09:56ID: 8166275

If you create a Thread object without calling its start method - this object will not be garbage collected. This is a java bug/problem and exists in all java versions.

If you don't want to call the start method - use:
class ChildThread implements Runnable
...
and call:
ChildThread ch = new ChildThread();
ch.run();
or
new Thread(ch).start();

 

by: Venci75Posted on 2003-03-19 at 05:12:34ID: 8166290

 

by: testnPosted on 2003-03-19 at 06:17:56ID: 8166706

You guys figured it out so fast!!!! :)

My point is to trick you guys about the things... about Thread. There is obviously a long time bug that hasn't been solved by Sun. This shouldn't be happened since sometimes.... you might create some Thread and maybe an exception is thrown before you have a chance to call start(). Your thread will stay there forever until JVM quits and you will have no chance to refer to that thread anymore (except you refer to it via ThreadGroup if it is in the same ThreadGroup as your current thread. Weird, huh?

 

by: TimYatesPosted on 2003-03-19 at 06:45:16ID: 8166901

> You guys figured it out so fast!!!! :)

It was glaringly obvious ;-)

 

by: testnPosted on 2003-03-20 at 05:13:36ID: 8173534

Hehe, I should hide it out somewhere :P

20120131-EE-VQP-002

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