Understanding OutOfMemoryError
In Java, all objects are stored in the heap. They are allocated by the new operator, and discarded when the JVM determines that no program thread can access them. Most of the time, this happens quietly, and programmers don't give it a second thought. And then, usually a day or so before a deadline, the program dies.
Exception in thread "main" java.lang.OutOfMemoryError: Java heap space
OutOfMemoryError is a frustrating exception. It usually means that you're doing something wrong: either holding onto objects too long, or trying to process too much data at a time. Sometimes, it indicates a problem that's out of your control, such as a third-party library that caches strings, or an application server that doesn't clean up after deploys. And sometimes, it has nothing to do with objects on the heap.
This article looks at the different causes of OutOfMemoryError, and what you can do about them. It focuses on the Sun JVM, which is the one that I use. However, much of the material applies to any JVM implementation. It is written based on literature available online and my own experience. I don't work on the JVM internals, so can't speak from a position of authority. But I have faced — and fixed — a lot of memory problems.
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