code: public JsonModelCoder<T> doResolve(T obj) {
  		JsonModelCoder<T> coder = resolve(obj);
  		if (coder == null) {
  			throw new NullPointerException("resolve method must return coder.");
  		}
  		return coder;
  	}
docs: Call {@link #resolve(Object)} and checl not null.
  @param obj
  @return custom coder
  @author vvakame