Why is "java.lang.OutOfMemoryError: Java heap space" not caught?

Question

I have a thread in a Java web application that causes a java.lang.OutOfMemoryError: Java heap space exception, but the try/catch block does not catch the error.

Sample code:

private void doSomeWork()
{
     try
     {
         processData();  //Causes OutOfMemoryError
         System.out.println("This line does not execute");
     }
     catch (Exception e)
     {
         System.out.println("Exception.  This line does not execute.");
         //Log error
     }
     finally
     {
         System.out.println("finally.  This line does execute");
         System.out.println("Thread name: " + Thread.currentThread().getName());

     }
}

Output:

finally.  This line does execute 
Thread name: _Worker-8
Exception in thread "_Worker-8" java.lang.OutOfMemoryError: Java heap space
...

Background:

I recently took over this Java project and I'm trying to get up to speed with Java and this project. I'm a C# developer, so I'm not yet familiar with this project or Java. I know I can fix the error using the -Xmx setting, but I'm interested in catching this error so I can log it. The error is not showing up in any of the logs files, and the output is being shown in the console in debug mode in Eclipse.

Answer 1

Because OutOfMemoryError is an Error, not an Exception. Since OutOfMemoryError isn't a subclass of Exception, the catch (Exception e) doesn't apply.

OutOfMemoryError does extend Throwable, however, so you should be able to catch it. Here's a SO discussion on when (if ever) you should catch Errors. Generally, since you can't do anything about it, the recommendation is to not bother catching Errors in production code. But given a special case where you're trying to debug what's going on, it might be helpful.

Answer 2

java.lang.OutOfMemoryError doesn't extends java.lang.Exception so it's not an Exception. OutOfMemoryError extends java.lang.Error. If you want to catch Error try this:

private void doSomeWork()
{
     try
     {
         processData();  //Causes OutOfMemoryError
         System.out.println("This line does not execute");
     }
     catch (Error e)
     {
         System.out.println("Exception.  This line does not execute.");
         //Log error
     }
     finally
     {
         System.out.println("finally.  This line does execute");
         System.out.println("Thread name: " + Thread.currentThread().getName());

     }
}

Note: Exception and Error extends Throwable so you you can also use Throwable to catch both.

http://java.sun.com/j2se/1.4.2/docs/api/java/lang/Throwable.html