While browsing the Java 7 API documentation I stumbled upon the new class java.lang.ClassValue with the following rather minimal documentation:
Lazily associate a computed value with (potentially) every type. For example, if a dynamic language needs to construct a message dispatch table for each class encountered at a message send call site, it can use a
ClassValueto cache information needed to perform the message send quickly, for each class encountered.
Can anyone give a better explanation of what problem this class solves and perhaps some sample code or open source project that already uses this class?
Update: I'm still interested in some actual source code or examples using this new class.
I also found this mail on the mlvm-dev mailing list concerning some implementation improvements. It was apparently changed from using a WeakHashMap to a new private field on java.lang.Class to make it more scalable.
The best explanation of the purpose of this class is that it solves Java Bug 6389107
There are many use cases where one wants to essentially have a Map<Class<?>, T> for some reason, but this causes all sorts of trouble since Class objects will then not be GC-able until the Map is. WeakHashMap<Class<?>, T> doesn't solve the problem because very frequently, T references the class.
The bug above goes into a much more detailed explanation and contains example projects/code that face this problem.
ClassValue is the answer to this problem. A thread-safe, classloader loading/unloading safe way to associate data with a Class.