How to Write a Memory Leak Unit Test

Unit tests are great for ensuring that your functionality is correct. But how do you make sure you don't have leaks in your code, leaks that eventually cause your application to crash when it runs out of memory? Unit tests don't typically don't run into memory problems directly because they tend to start up, run a little bit of code, and shut down -- getting a nice fresh memory environment on each test start.

One great way to help protect yourself from uptime problems is to write leak tests. These are unit tests where you first perform some operations, then do normal cleanup, and finally you assert that the objects you were using during the operation have been cleaned up completely. In other words, that there are no references left anywhere on the heap.

Testing for leaks isn't very hard; the basic trick is to create a weak reference to your to-be-cleaned object (by constructing a WeakReference wrapping your object), then you delete all your local references to the object, then you perform garbage collection, and finally you see whether the weak reference still contains your object. This works because the weak reference is handled specially by the garbage collection system.

It turns out things aren't quite as easy as that, because garbage collectors try to be smart, and simply calling System.gc() doesn't mean it's going to actually perform a complete and final garbage collection. This means that your unit test could incorrectly conclude you have a leak, since the reference is still held.

The NetBeans team has built a really good test infrastructure for this. When I wrote unit tests for NetBeans plugins, I could simply call their assertGC method to assert that the given weak reference should be garbage collected, and it would handle the rest. (Internally, they do things like actually allocating a bunch of memory chunks to really force the garbage collection to run.)

Here's an example:

// The following unit test fragment wants to ensure that a graphics object
// which shows the current selection in the authoring tool, is cleared up after
// the user clears the selection.
view.selectAll();
Node firstHandle = /* Node showing selection highlight, lookup code here */;
assertNotNull(firstHandle);

// Now clear view selection -- then assert that all the handles have disappeared
view.selectNone();

WeakReference<Node> ref = new WeakReference<Node>(firstHandle);
firstHandle = null;
assertGC("Selection handle leaked", ref);

Read more: Tor Norbye's Weblog

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