Can there be memory leak in Java

Question

I get this question asked many times. What is a good way to answer

Answer 1

Can there be memory leak in Java?

The answer is that it depends on what kind of memory leak you are talking about.

Classic C / C++ memory leaks occur when an application neglects to free or dispose an object when they are done with it, and it leaks. Cyclic references are a sub-case of this where the application has difficulty knowing when to free / dispose, and neglects to do it as a result. Related problems are where the application uses an object after it has been freed, or attempts to free it twice. (You could call the latter problems memory leaks, or just bugs. Either way ... )

Java and other (fully¹) managed languages mostly don't suffer from these problems because the GC takes care of freeing objects that are no longer reachable. (Certainly, dangling pointer and double-free problems don't exist, and cycles are not problematic as they are for C / C++ "smart pointers" and other reference count schemes.)

But in some cases GC in Java will miss objects that (from the perspective of the programmer) should be garbage collected. This happens when the GC cannot figure out that an object cannot be reached:

• The logic / state of the program might be such that the execution paths that would use some variable cannot occur. The developer can see this as obvious, but the GC cannot be sure, and errs on the side of caution (as it is required to).
• The programmer could be wrong about it, and the GC is avoiding what might otherwise result in a dangling reference.

(Note that the causes of memory leaks in Java can be simple, or quite subtle; see @jonathan.cone's answer for some subtle ones. The last one potentially involves external resources that you shouldn't rely on the GC to deal with anyway.)

Either way, you can have a situation where unwanted objects cannot be garbage collected, and hang around tying up memory ... a memory leak.

Then there is the problem that a Java application or library can allocate off-heap objects via native code that need to be managed manually. If the application / library is buggy or is used incorrectly, you can get a native memory leak. (For example: Android Bitmap memory leak ... noting that this problem is fixed in later versions of Android.)

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^{1 - I'm alluding to a couple of things. Some managed languages allow you to write unmanaged code where you can create classic storage leaks. Some other managed languages (or more precisely language implementations) use reference counting rather than proper garbage collecting. A reference count-based storage manager needs something (i.e. the application) to break cycles ... or else storage leaks will ensue.}

Answer 2

Yes. Memory leaks can still occur even when you have a GC. For example, you might hold on to resources such as database result sets which you must close manually.