Class-local data

Today I ran across a problem that I’ve hit frequently in the past, this time while designing an extension to JBoss Marshallingwhich allows users to annotate classes which are to be specially externalized. The problem is that one wants to “remember” what Externalizer instances go with what classes on a semi-permanent basis; however, one does not wish to leak class references (which can lead to the permanent generation filling up, and your application crashing with an OutOfMemoryError, especially if you’re frequently loading and unloading classes).

The traditional solution to keep a leak-resistant cache is to employ a WeakHashMap like so (usually in conjunction with some concurrency strategy, like a Lock):

  private static final Map<Class, Externalizer> cache = new WeakHashMap<Class, Externalizer>();

The WeakHashMap retains a weak reference to the key values (in this case, the Class instance), so if no strong references remain, the Class is unloaded, and all is well. However, in this particular use case, the Externalizer instance very likely will itself hold a strong reference to the Class. Why? Because in JBoss Marshalling, Externalizer instances may be custom-designed with a single class in mind; thus, it might have a field or a constructor call of that class’ type, which means strong reference. Since WeakHashMap retains a strong reference to its value, this means that there is a strong reference from my cache to the class after all, so all is for naught.

So I got to thinking - what if I could stash my Externalizer instance on the Class itself? Then I wouldn’t even need to keep a cache at all, and the only reference to the Class that I could cause to be would be from the Class itself. That would be awesome! Of course, we wouldn’t want anyone else to be able to get at my data; it could be private to me, after all. And we wouldn’t want it to leak the other way - the Class should not hold a direct strong reference to my class, yet we don’t exactly want it to be a weak reference either (because then the data might disappear before I’ve had a chance to use it).

So my idea is to create a new type of storage: class-local data. You’d access it similarly to thread-local data, except instead of being keyed from the current thread, you’d give it a class:

  // The methods are similar to the corresponding methods on Map and ConcurrentMap  
  public final class ClassLocal<T> {  
     public T get(Class  clazz) { ... }  
     public T put(Class  clazz, T value) { ... }  
     public boolean hasData(Class  clazz) { ... }  
     public T putIfAbsent(Class  clazz, T value) { ... }  
     public T remove(Class  clazz) { ... }  
     public boolean remove(Class  clazz, T oldValue) { ... }  
     public T replace(Class  clazz, T newValue) { ... }  
     public boolean replace(Class  clazz, T oldValue, T newValue) { ... }  
  }  
  private final ClassLocal<Externalizer> classLocal = new ClassLocal<Externalizer>();  
  ...in some method...  
  Externalizer externalizer = classLocal.get(SomeClass.class);

A ClassLocal instance looks and acts a lot like a map, but with one important difference: the data itself is stored on the Class instance, not in the ClassLocal. Implementation-wise this could be accomplished via a WeakHashMap or similar structure on the Class itself (with a concurrency strategy) similar to this:

  public final class Class<T> {  
     ...  
     final Map<ClassLocal, Object> classLocalData = Collections.synchronizedMap(new WeakHashMap<ClassLocal, Object>());  
  }

…which could then be accessed directly by the ClassLocal implementation. Real JDK experts could probably think of a few ways to optimize this - e.g. avoid creating a map instance until someone puts data there, come up with a more clever locking strategy, etc.

Update: As I ought to have expected, I’m nowhere near the first person ever to let their mind wander down this path - there’s a Sun bug (#6493635)about it, as well as articles by such clever folks as Crazy Boband Jacob Hookomabout this very topic.

Written on February 25, 2009