packratting : Java Glossary

Java talks to the underlying GUI which is typically written in C++ which does not have automatic garbage collection. Therefore you see kludges in Java dispose methods, to help it along. If you fail to use dispose properly, Java won’t run out of RAM, but the GUI will. Failing to use dispose create true memory leaks. Here are some common packratting situations:

• There is a gray area where Listeners won’t be garbage collected unless you detach them manually before using dispose. So call dispose on each JFrame once you have finished with it.
• Setting the frame invisible with with setVisible( false ) does not make the Frame (and everything contained in it) eligible for gc. There are many ways to close a Frame.
• Call dispose on any Graphics objects that you create programmatically. Failing to do so will gradually fill memory with peer junk. The underlying OS GUI does not have an automatic garbage collection system. Just as you must manually close files, you must manually dispose Graphic objects.
• Call flush on Images after you have finished with them to free up their associated peer contents before dropping the reference.
• Threads are huge objects. Make sure you nullify pointers to them after you no longer need them. You can’t reuse them for anything after they stop so you might as well nullify the references as soon as possible, even before they stop. Make sure you properly terminate threads or you will fill up RAM with moribund Threads.
• If you create a Thread and fail to start it, it will not be eligible for garbage collection, even if your app drops all references to it. It stays alive through a reference in the parent ThreadGroup. This is fixed in JDK 1.5+. But as a general principle, don’t create Threads until you are ready to start them.
• In Swing make sure you use JFrame. setDefaultCloseOperation ( JFrame. DISPOSE_ON_CLOSE ); instead of the default JFrame.setDefaultCloseOperation ( JFrame.HIDE_ON_CLOSE ); otherwise, when a user closes a window, the frames are just hidden. You don’t recover the resources. Use JFrame. setDefaultCloseOperation ( JFrame. DO_NOTHING_ON_CLOSE ); when you have a WindowListener. windowClosing method. To permanently close a frame programmatically, use

  frame.setVisible( false );
  frame.dispose();
  frame = null;