Provide a way to customize `ObjectReader` / `ObjectWriter` used by end points

[cowtowncoder]

(note: inspired by #32)

---

It would be nice to be able to customize configuration of ObjectReader used for reading content via JAX-RS, and configuration of ObjectWriter used for writing.
This functionality should be accessible from code outside of JAX-RS provider implementation.

[cowtowncoder]

The way this is implemented is as follows:

• Two new classes -- ObjectReaderInjector and ObjectWriterInjector -- can be accessed statically (they use ThreadLocal) to "inject" ObjectReaderModifier and ObjectWriterModifier (respectively)
• JAX-RS provider will construct reader/writer to use, based on regular configuration and annotations; and then call injectors to access modifiers. If a modifier is found, it shall be used to (re)configure reader/writer

This basically allows for specifying a modifier to do any and all customization necessary. Assignment has to be made on per-call basis for now.

[cowtowncoder]

Implemented for 2.3.0.

[apemberton]

Hi Tatu: this is awesome; thanks! Can you cut a branch / mvn release of 2.3 so I can give it a whirl?

[cowtowncoder]

I can do maven deploy of the snapshot tonight.

[cowtowncoder]

Ok I pushed 2.3.0-SNAPSHOT of jackson-databind (jackson-core was already done), this project. So that should work.

[apemberton]

Thanks! Looking great on our end so far. Do you have a plan to do a 2.3 release (ie: non-snapshot)?

[cowtowncoder]

Yes, hope is to get things finalized soon (within a week or two). 2.3.0 has dragged on for quite a while, but all the pieces are in now.

[cowtowncoder]

2.3.0 was just released on November 14th, 2013.

[renaudcerrato]

How does that fit with the caching mechanisms? It seems the key used to cache the writer/reader is computed from annotations and type.

[cowtowncoder]

@renaudcerrato It should work fine as information is passed using ThreadLocal; annotations are only used to create baseline ObjectWriter / ObjectReader, but then modifier can create new instances. Modification call is made on per-request basis. I hope test case shows how this occurs.
It all works as ObjectReader/ObjectWriter are immutable, so concurrent threads are not changing reader/writer instances but constructing new ones (with withXxx() methods), returning ones to use.