FieldDictionary's use of WeakHashMap should be a strategy.

Well, the WeakHashMap is not there in first place to save memory, but to allow the classloader to be purged. Essential in JEE or OSGi environemnts.

OK, but I'm asking for a pluggable impl not a change. I'm happy for the default situation to be as it is now. I'm just wanting to be able to pass in something like new NonWeakFieldDictionaryMaps();

I had a closer look at the implementation. We could make the init method and the two Map fields protected. However, I am quite convinced, that the current implementation prevents the entries from being purged anyway, because the value is a Field that refers the declaring class used in the key ... :-/

As you may have suspected, I'm having trouble reproducing this as a unit test. I'm going to switch instead to a live run, with some instrumentation around second/subsequent reflection usage.

I'll try to create a test case as soon as possible.

It is as suspected: The classes seem not be garbage collected. Enable the commented line and it fails:

public void testDoesReleaseCachedEntries() throws MalformedURLException, ClassNotFoundException {
    ClassLoader classloader = new URLClassLoader(new URL[]{new URL("file:target/dependency/commons-logging-1.1.1.jar")});
    assertNotSame("Classloaders must not be the same", getClass().getClassLoader(), classloader);
    Class loggerType = classloader.loadClass("org.apache.commons.logging.impl.SimpleLog");
    assertSame("Classloaders must be the same", classloader, loggerType.getClassLoader());
    final WeakReference ref = new WeakReference(loggerType);
//    fieldDictionary.field(loggerType, "logName", loggerType);
    assertNotNull("Weak reference already released", ref.get());
    classloader = null;
    loggerType = null;
    System.gc();
    System.gc();
    System.gc();
    assertNull("Weak reference is not released", ref.get());
}

I dropped now the usage of the WeakHashMap altogether. Since the class used as key is again part of the cached data as field member of a Field instance, the entries are never removed. Using WeakPointer for the Field instances does also not work, because these instances are not referenced anywhere and can vanish at arbitrary times making the comlete cache obolete.