As you are extending ServerResource, you can either call getRequestEntity() to retrieve the representation, or declare a Representation parameter on your @Post annotated Java method.
UPDATE
Looking at your updated question, it seems that you don't use the PUT method properly. The entity that you pass with your PUT request should contain the complete new state for your target resource.
In your case, it seems that you want to do only partial updates. You should rather consider using PATCH method if you really need such level of optimization.