A common problem we all have to deal with is fixing memory leaks. In garbage collecting languages like Java we typically expect not to ever need to worry about memory management. However the limitations of how a garbage collector works means that we can still create leaks. Which defeats the advantage of a garbage collector.

How do leaks get formed?

The Java GC determines which objects to collect by looking for references to the object. If there is no references, then the object is no longer in use and can be safely destroyed without making the application unstable. If an object is now unused but still referenced, a leak occurs. Let's create a leak:

There are four distinct forms of references in the JVM, and indeed many of these apply to other garbage collected languages.

• Strong references
• Soft references
• Weak references
• Phantom references

It's important to know the differences, what affect they have on the collector and when you should be using them.