Easiest way to cause a memory leak in Java [duplicate]

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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.

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