Spring Ajax - @ResponseBody - Returning null response

Question

In my spring webapp, I have an ajax servlet that answer json (using jackson):

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xmlns:context="http://www.springframework.org/schema/context" xmlns:mvc="http://www.springframework.org/schema/mvc"
    xmlns:util="http://www.springframework.org/schema/util" xmlns:tx="http://www.springframework.org/schema/tx"
    xsi:schemaLocation="http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc-3.2.xsd
        http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-3.2.xsd
        http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context-3.2.xsd
        http://www.springframework.org/schema/util http://www.springframework.org/schema/util/spring-util-3.2.xsd
        http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx-3.2.xsd">

    <context:component-scan base-package="com.myapp.ajax" />

    <util:list id="messageConvertersList">
        <bean class="org.springframework.http.converter.json.MappingJacksonHttpMessageConverter">
            <property name="supportedMediaTypes">
                <list>
                    <value>application/json;charset=UTF-8</value>
                </list>
            </property>
        </bean>
    </util:list>

    <bean class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter">
        <property name="messageConverters" ref="messageConvertersList" />
    </bean>
    <bean id="handlerMapping" class="org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping" />

    <mvc:interceptors>
        <mvc:interceptor>
            <mvc:mapping path="/**" />
            <bean id="localeChangeInterceptor" class="org.springframework.web.servlet.i18n.LocaleChangeInterceptor">
                <property name="paramName" value="lang" />
            </bean>
        </mvc:interceptor>
    </mvc:interceptors>

    <bean id="localeResolver" class="org.springframework.web.servlet.i18n.SessionLocaleResolver" />

    <bean id="handlerExceptionResolver" class="com.myapp.ajax.AjaxExceptionHandlerResolver">
        <property name="exceptionHandler">
            <bean class="com.myapp.ajax.AjaxExceptionHandler" />
        </property>
        <property name="messageConverters" ref="messageConvertersList" />
    </bean>

I have the following ajax service:

@RequestMapping(value = "getLoggedUser")
@ResponseBody
public DtoUser getLoggedUser() {
    return authenticationService.getLoggedUser();
}

When the user is logged in it returns something like:

{ userName : "jojo", email : "[email protected]", firstName : "John", lastName : "Doe" }

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

null

But it returns an empty response which is not a valid JSON response (and additionally with a bad Content-type header)

Why is this happening ?
Do I have solutions to obtain the expected behaviour ?

Answer 1

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 Code
null => null
empty 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.