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Wednesday May 31, 2006

Testing memory leaks with assertGC

I was talking about assertSize in one of previous entry about testing. NbTestCase class contains more useful methods. Today I will be talking about assertGC method. The assertGC method tests tries to run garbage collect with different methods and then scan if exists path to reference in memory by using insane library. Look at simple example:

public class LeakTest extends NbTestCase {
    
    public LeakTest(java.lang.String testName) {
        super(testName);
    }
    public void testLeak() {
        String obj = new String("test1");
        WeakReference ref = new WeakReference(obj);
        obj = null; 
        assertGC("pass",ref);
        obj = new String("test2");
        ref = new WeakReference(obj);
        assertGC("fails",ref);
    }
    
    public static Test suite() {
        TestSuite suite = new NbTestSuite(LeakTest.class);
        return suite;
    }
}

First asertGC passed because in JVM exists only WeakReference for object String("test1"). The second assertGC failed because the reference obj holds String("test2").

Posted by xzajo ( May 31 2006, 02:46:35 PM CEST ) Permalink Comments [16]

Comments:

Will AssertGC produce any output, or is there any API from INSANE that will report what objects are holding references after an AssertGC?

It would be nice to see who's holding a reference to what objects before and after a GC. Or, even better; do a GC, make a marking point, do some work, do several GC's (for finalizers), then report any new object references created in the "do some work" step.

And, how does INSANE or assertGC() deal with objects with finalizers and objects which may be queued up in the finalizer queue, (i.e. take more than 1 GC to be collected) ?

Posted by huntch on June 05, 2006 at 08:54 PM CEST #

Some issues are coming to my mind when reading your sample test case. First thing I find suboptimal is the need for hard-encoding a text ("pass","fail") as a parameter with assertGC. Why not offer two methods (assertGCPasses, assertGCFails resp. assertNotGC)? I didn't look into the NBJUnit code, maybe it's there already, but then you should have used it in your sample. Second thing is the title of your blog entry. It makes the reader assume that assertGC is able finding memory leaks within any application without further requirements. As I understand after reading a second time, this is not the case because assertGC tests if the GC strategy used worked properly. Did I understand that correctly? Third aspect is that I like the idea of assertGC! Klaus

Posted by Klaus Meffert on June 08, 2006 at 10:56 AM CEST #

Yes, it takes more GC. assertGC() method implementation:

 public static void assertGC(String text, Reference<?> ref, Set<?> rootsHint) {
        List<byte[]> alloc = new ArrayList<byte[]>();
        int size = 100000;
        for (int i = 0; i < 50; i++) {
            if (ref.get() == null) {
                return;
            }
            System.gc();
            System.runFinalization();
            try {
                alloc.add(new byte[size]);
                size = (int)(((double)size) * 1.3);
            } catch (OutOfMemoryError error) {
                size = size / 2;
            }
            try {
                if (i % 3 == 0) Thread.sleep(321);
            } catch (InterruptedException t) {
                // ignore
            }
        }
        alloc = null;
        fail(text + ":\n" + findRefsFromRoot(ref.get(), rootsHint));
    }

Posted by xzajo on June 08, 2006 at 01:53 PM CEST #

OK, I see. The text is just displayed to the user. But then there should also be the negative form of assertGC, something named assertNotGC (which fails in case a garbage collection runs). Is something like that there?

Posted by Klaus Meffert on June 08, 2006 at 05:50 PM CEST #

I am not sure if assert notGC will be usefull. You can use:

assertNotNull(ref)

Posted by xzajo on June 09, 2006 at 03:06 PM CEST #

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