Spring Ajax - @ResponseBody - Returning null response

When the user is not logged in, the expected behavior is to return null

That's my expected behaviour because in both Java and Javascript/JSON, null is a valid value, which have a different mean than nothing, empty or error/exception.

I would expect that Spring answer the null response instead of handling it specifically.In that case, the expected convertion for null (Java) would be null (JSON)
My expected conversion table:

Java Exception => HTTP Error Codenull => nullempty map / object => {}void => no response

Why is this happening ?

For Spring, a controller returning null mean "No response" and not "a response which value is null". This rule applies to all controller methods including ones with @ResponseBody annotation.
This allow to write manually to the response without having something appended to the response later:

if (mySpecialCase) {    Writer writer = response.getWriter();    writer.write("my custom response");    return null;} else {     return myObject;}

So when returning null Spring write nothing to the response, nor Content-type header nor body.

Do I have solutions to obtain the expected behaviour ?

I made the following dirty hack: add a filter on my ajax path that write null to the response when no response have been commited.

public class AjaxEmptyResponseFilter implements Filter {    public void doFilter(ServletRequest request, ServletResponse response, FilterChain chain) throws IOException, ServletException {        chain.doFilter(request, response);        if (!response.isCommitted()) {            response.setCharacterEncoding("UTF-8");            response.setContentType("application/json");            Writer writer = response.getWriter();            writer.write("null");            writer.close();            response.flushBuffer();        }    }}

This solution handle methods answering null and method answering nothing (void) the same way.

Do you have a session filter?

I think, you can bind a global ajax event for errors and make your respective validation there.

here is a example on a similar case: How to handle expired session using spring-security and jQuery?