Add support for custom actions on annotations

See the thread: 
http://groups.google.com/group/google-gson/browse_thread/thread/1ea949a5e50cc507

by supporting a custom action on an annotation, we will enable support for 
JPA/Hibernate and other kinds of annotations.

Original issue reported on code.google.com by inder123 on 26 Nov 2010 at 7:00

[amaksoft]

Hi, i want to share my implementation of custom annotation support for gson, which is backwards compatible: new interface ParametrizedTypeAdapterFactory extends TypeAdapterFactory and new method <T> TypeAdapter<T> create(Gson gson, TypeToken<T> type, Annotation[] annotations) only gets called if field is annotated with specific annotation @Parametrized

I mostly wanted it for serializing some dates by specifying custom pattern in annotation like this:

class Person {

  @Expose
  @SerializedName("birth_date")
  @Parametrized
  @CustomDateFormat("MMMM d, yyyy h:mm:ss a z")
  private Date birthDate;

}

Here is a draft implementation: https://github.com/amaksoft/gson/commit/b264334451c487d79241483103853c3fff27cbd4 Can you please review it?

[amaksoft]

Just wanted to explain my position: ParamterizedTypeAdapterFactory is just a special case of TypeAdapterFactory. In most cases Gson don’t even need to know that your factory accepts annotations. But in special cases like @CustomDateFormat on Date or @Id on int, you return a customized TypeAdapter only for this case, (we don’t cache it). No new lists of adapters needed, any of your existing TypeAdapters could be easily transformed into parametrized ones by implementing a new method.