Easiest way to cause a memory leak in Java

Question

(This question already has answers here): question from:https://stackoverflow.com/questions/4948529/easiest-way-to-cause-a-memory-leak-in-java

Answer 1

You cannot really "leak memory" in Java unless you:

• intern strings
• generate classes
• leak memory in the native code called by JNI
• keep references to things that you do not want in some forgotten or obscure place.

I take it that you are interested in the last case. The common scenarios are:

• listeners, especially done with inner classes
• caches.

A nice example would be to:

• build a Swing GUI that launches a potentially unlimited number of modal windows;
• have the modal window do something like this during its initialization: StaticGuiHelper.getMainApplicationFrame().getOneOfTheButtons().addActionListener(new ActionListener(){ public void actionPerformed(ActionEvent e){ // do nothing... } })

The registered action does nothing, but it will cause the modal window to linger in memory forever, even after closing, causing a leak - since the listeners are never unregistered, and each anonymous inner class object holds a reference (invisible) to its outer object. What's more - any object referenced from the modal windows have a chance of leaking too.

This is why libraries such as EventBus use weak references by default.

Apart from listeners, other typical examples are caches, but I cannot think of a nice example.