Refine *HttpMessageConverter#getContentLength return value null safety

[DamonChen117]

id("org.springframework.boot") version "2.3.9.RELEASE"

I need to extend MappingJackson2HttpMessageConverter and override getContentLength

1. If the return type is Long?, it will not compile
   override
   fun getContentLength(obj:Any, contentType:MediaType?) : Long?

Compile error:
Return type of 'getContentLength' is not a subtype of the return type of the overridden member 'protected/protected and package/ open fun getContentLength(p0: Any, @nullable p1: MediaType?): Long defined in org.springframework.http.converter.json.MappingJackson2HttpMessageConverter'

2. If the return type is Long, it will get NullPointException at runtime.
   override
   fun getContentLength(obj:Any, contentType:MediaType?) : Long

3. In AbstractJackson2HttpMessageConverter the return is not Nullable
   @OverRide
   protected Long getContentLength(Object object, @nullable MediaType contentType)

4. In AbstractHttpMessageConverter the return is Nullable
   @nullable
   protected Long getContentLength(T t, @nullable MediaType contentType)

[sdeleuze]

Looks like AbstractJackson2HttpMessageConverter#getContentLength nullability needs to be refined indeed, as well as ResourceHttpMessageConverter#getContentLength one.