`EnableSpringDataWebSupport.pageSerializationMode` warning and the numerous reported issues

[xDeyan]

With the change added in #3024 there seems to be couple of reported inconveniences for some users (#3264, #3320, #3327)

I am specifically opening this issue to discuss on what was the initial idea of this change (for which an input from @odrotbohm would be much appreciated) and trying to find a different solution for the problem. I completely agree that providing a stable JSON representation is very important and at the same time allow the implementation PageImpl to evolve independently.

Currently it only targets Jackson as a json serialization library, but AFAIK Gson and others are also supported by the Spring Web project.

So I am having the following questions:

• Was Jackson the primary target ? Or, since probably it is used 99+% of the time, the change was implemented for it only ?
• Was it targeting a direct PageImpl serialization only ? Because current implementation also affects a nested property of any object that is of the PageImpl type.

My overall issue is that now, when I opt in for the pageSerializationMode=VIA_DTO, I am breaking the clients of my API. Which is also true, if the PageImpl suddently changes (which was the primary goal of the initial change - stability).

I am currently thinking if we could use ResponseBodyAdvice that will apply the conversion from Page to PagedModel. This will give us the following benefits:

1. The conversion will be independent from the serialization library (Even tho we can still target only Jackson initially by using a AbstractMappingJacksonResponseBodyAdvice)
2. This will not affect serialization of bean properties of type PageImpl, only directly returned objects from @Controller
3. The "converter" can be chosen/configured by the user/dev and the API stability will be transferred away from the maintainers

Potential implementation:

@Configuration(proxyBeanMethods = false)
public class SpringDataWebConfiguration implements WebMvcConfigurer, BeanClassLoaderAware {
.....

      @Bean
      ResponseBodyAdvice<Object> pageResponseBodyAdvice(@Autowired(required = false) SpringDataWebSettings settings,
      		ObjectProvider<PageResponseBodyConverter> pageConverter) {
      	if (settings == null || settings.pageSerializationMode() == DIRECT) {
      		return PageResponseBodyAdvice.NO_OP;
      	}
      
      	return new PageResponseBodyAdvice(pageConverter.getIfAvailable(() -> PagedModel::new));
      }

      public interface PageResponseBodyConverter {
      
          <T> Object convert(Page<T> page);
      }
      
      @ControllerAdvice
      static final class PageResponseBodyAdvice extends AbstractMappingJacksonResponseBodyAdvice {
      
          static final ResponseBodyAdvice<Object> NO_OP = new ResponseBodyAdvice<>() {
      
              @Override
              public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
                  return false;
              }
      
              @Override
              public Object beforeBodyWrite(Object body, MethodParameter returnType, MediaType selectedContentType, Class<? extends HttpMessageConverter<?>> selectedConverterType, ServerHttpRequest request, ServerHttpResponse response) {
                  return body;
              }
          };
      
          private final PageResponseBodyConverter converter;
      
          public PageResponseBodyAdvice(PageResponseBodyConverter converter) {
              this.converter = converter;
          }
      
          @Override
          public void beforeBodyWriteInternal(MappingJacksonValue bodyContainer, MediaType contentType, MethodParameter returnType, ServerHttpRequest request, ServerHttpResponse response) {
              Object model = converter.convert((PageImpl<?>) bodyContainer.getValue());
              bodyContainer.setValue(model);
          }
      
          @Override
          public boolean supports(MethodParameter returnType, Class<? extends HttpMessageConverter<?>> converterType) {
              return super.supports(returnType, converterType) && returnType.getParameterType().isAssignableFrom(PageImpl.class);
          }
      }
}