Cache declared fields for class

[purnhar]

Getting the declared fields for a class is an expensive operation.
Therefore the field names are requested per class and cached.

[MrEasy]

I suppose this code may be called multi-threaded, so using a thread-safe Set-implementation would be a must (e.g. java.util.concurrent.ConcurrentHashMap).

How about making both Sets static? Per ClassLoader the checks should be identical for the corresponding classes.

[purnhar]

Using a ConcurrentHashMap would be great but is not possible, because XStream 1.4 has to be compatible with Java 1.4. Thats why i implemented the necessary synchronisation logic by myself. For XStream 1.5 ConcurrentHashMap would be the preffered solution.

[purnhar]

Comment from Jörg Schaible in the XStream User Group:

I've seen the PR and basically it's OK, but I'll try to use an internal
already availabel cache for it.

Which cache do you mean? Is there a way I can help you?

[joehni]

Uses cache of ReflectionProvider now.