javax.swing.ImageIcon classloader leak

[scholzb-hb]

The class javax.swing.ImageIcon causes a classloader leak which is not covered yet by default (as of classloader-leak-prevention 2.7.0).

Steps to reproduce:

1. Build the attached example project imageicon-leak.zip (mvn clean install)
2. Deploy the .war file into a Tomcat 8.5.65
3. Undeploy it again

Then the classloader is not garbage collected due to the following Path2GC:

Obviously the instance of the anonymous class javax.swing.ImageIcon$3 has a reference to an AccessControlContext retaining the ParallelWebappClassLoader. It is created at ImageIcon.java line 347 (from AdoptOpenJDK jdk8u272-b10):

    private MediaTracker getTracker() {
        Object trackerObj;
        AppContext ac = AppContext.getAppContext();
        // Opt: Only synchronize if trackerObj comes back null?
        // If null, synchronize, re-check for null, and put new tracker
        synchronized(ac) {
            trackerObj = ac.get(TRACKER_KEY);
            if (trackerObj == null) {
                Component comp = new Component() {};   // <--- javax.swing.ImageIcon$3
                trackerObj = new MediaTracker(comp);
                ac.put(TRACKER_KEY, trackerObj);
            }
        }
        return (MediaTracker) trackerObj;
    }

Adding the following clean-up step solves this issue:

public class AwtMediaTrackerCleanUp implements ClassLoaderPreMortemCleanUp {

    @Override
    public void cleanUp(ClassLoaderLeakPreventor preventor) {
        StringBuilder trackerKey = preventor.getStaticFieldValue(ImageIcon.class, "TRACKER_KEY");
        AppContext appContext = AppContext.getAppContext();
        synchronized (appContext) {
            appContext.remove(trackerKey);
        }
    }
}