• print a stack trace to the console
• terminate the thread (and thus terminate the program itself)

There are two common kinds of programs where the above behavior is not in effect:

• programs running in a servlet container
• Swing applications

In Swing applications, the default behavior still prints the stack trace to the console, but the Event Dispatch Thread (the main Swing thread) is not terminated. The Event Dispatch Thread always remains alive, in spite of any RuntimeExceptions thrown by your program.

However, simply printing a stack trace to the console in a Swing application is not very informative for the average user. Instead, you should likely override Java's default handling of uncaught exceptions. The benefits are:

• the end user is kept informed of problems more effectively
• new behavior can be added, such as logging or sending emails to support staff

If you're using JDK 5+, then you may define your own UncaughtExceptionHandler. They can be defined at 3 different levels. From the highest level to the lowest level, they are:

• for the whole Java Runtime, call Thread.setDefaultUncaughtExceptionHandler upon startup
• for a ThreadGroup, override ThreadGroup.uncaughtException
• for a single Thread, call Thread.setUncaughtExceptionHandler

Example

Here's an example of such a handler class.

package hirondelle.movies.exception;

import hirondelle.movies.util.Util;
import hirondelle.movies.util.ui.UiUtil;

import java.io.PrintWriter;
import java.io.StringWriter;
import java.io.Writer;
import java.util.logging.Logger;

import javax.swing.JOptionPane;

/**
 Custom handler for any uncaught exceptions.
 
 <P>By default, a Swing app will handle uncaught exceptions simply by 
  printing a stack trace to {@link System#err}. However, the end user will 
  not usually see that, and if they do, they will not likely understand it. 
  This class addresses that problem, by showing the end user a 
  simple error message in a modal dialog. (The dialog's owner is the 
  currently active frame.)
*/
public final class ExceptionHandler implements Thread.UncaughtExceptionHandler {

  /** 
  Custom handler for uncaught exceptions.
  
  <P>Displays a simple model dialog to the user, showing that an error has
   occured. The text of the error includes {@link Throwable#toString()}. 
   The stack trace is logged at a SEVERE level.
 */
  @Override  public void uncaughtException(Thread aThread, Throwable aThrowable) {
    fLogger.severe(getStackTrace(aThrowable));
    JOptionPane.showMessageDialog(
      UiUtil.getActiveFrame(), "Error: " + aThrowable.toString(), 
      "Error", JOptionPane.ERROR_MESSAGE
    );
  }
  
  // PRIVATE 
  private static final Logger fLogger = Util.getLogger(ExceptionHandler.class);
  
  private String getStackTrace(Throwable aThrowable) {
    final Writer result = new StringWriter();
    final PrintWriter printWriter = new PrintWriter(result);
    aThrowable.printStackTrace(printWriter);
    return result.toString();
  }
}
 

For JDK 1.4, the simplest way of overriding the default handler for uncaught exceptions is to use the following undocumented feature (which has actually been removed from JDK 7):

• create a class with a no-argument constructor
• add to it a method public void handle(Throwable aThrowable){...}
• upon startup, add a System property named 'sun.awt.exception.handler', whose value is the fully qualified class name of this new handler class