Spring Security and JSON Authentication

According with Kevin suggestions,
and after reading this posts: 1, 2, documentation 3, and thanks to this blog post,
I wrote my own FORM_LOGIN_FILTER to directly manage JSON before authentication.
I paste my code for the community.

The goal is to grant both the classical browser form POST authentication with JSON based authentication. Also in JSON authentication I want to avoid the redirect to loginSuccesful.htm

In context:

<security:http use-expressions="true" auto-config="false" entry-point-ref="http403EntryPoint">      
    <security:intercept-url pattern="/logs/**" access="denyAll" />
    <!-- ... All other intercept URL -->

    <security:custom-filter ref="CustomUsernamePasswordAuthenticationFilter" position="FORM_LOGIN_FILTER "/>
    <security:logout
            invalidate-session="true"
            logout-success-url="/LogoutSuccessful.htm"
            delete-cookies="true"
    />
    <security:session-management>
        <security:concurrency-control max-sessions="1" error-if-maximum-exceeded="true" />
    </security:session-management>
    <security:access-denied-handler error-page="/accessDenied.htm" />
</security:http>

<bean id="CustomUsernamePasswordAuthenticationFilter" class="path.to.CustomUsernamePasswordAuthenticationFilter">
    <property name="authenticationManager" ref="authenticationManager" />
    <property name="authenticationSuccessHandler" ref="customSuccessHandler"/>
    <property name="authenticationFailureHandler" ref="failureHandler"/>
    <property name="filterProcessesUrl" value="/j_spring_security_check"/>
    <property name="usernameParameter" value="j_username"/>
    <property name="passwordParameter" value="j_password"/>
</bean>

<bean id="customSuccessHandler" class="path.to.CustomAuthenticationSuccessHandler">
    <property name="defaultTargetUrl" value="/login.htm" />
    <property name="targetUrlParameter" value="/LoginSuccessful.htm" />
</bean>

<bean id="failureHandler" class="org.springframework.security.web.authentication.SimpleUrlAuthenticationFailureHandler">
    <property name="defaultFailureUrl" value="/login.htm" />
</bean>

<bean id="http403EntryPoint" class="org.springframework.security.web.authentication.Http403ForbiddenEntryPoint" />

CustomUsernamePasswordAuthenticationFilter class:

public class CustomUsernamePasswordAuthenticationFilter extends UsernamePasswordAuthenticationFilter{
    private String jsonUsername;
    private String jsonPassword;

    @Override
    protected String obtainPassword(HttpServletRequest request) {
        String password = null; 

        if ("application/json".equals(request.getHeader("Content-Type"))) {
            password = this.jsonPassword;
        }else{
            password = super.obtainPassword(request);
        }

        return password;
    }

    @Override
    protected String obtainUsername(HttpServletRequest request){
        String username = null;

        if ("application/json".equals(request.getHeader("Content-Type"))) {
            username = this.jsonUsername;
        }else{
            username = super.obtainUsername(request);
        }

        return username;
    }

    @Override
    public Authentication attemptAuthentication(HttpServletRequest request, HttpServletResponse response){
        if ("application/json".equals(request.getHeader("Content-Type"))) {
            try {
                /*
                 * HttpServletRequest can be read only once
                 */
                StringBuffer sb = new StringBuffer();
                String line = null;

                BufferedReader reader = request.getReader();
                while ((line = reader.readLine()) != null){
                    sb.append(line);
                }

                //json transformation
                ObjectMapper mapper = new ObjectMapper();
                LoginRequest loginRequest = mapper.readValue(sb.toString(), LoginRequest.class);

                this.jsonUsername = loginRequest.getUsername();
                this.jsonPassword = loginRequest.getPassword();
            } catch (Exception e) {
                e.printStackTrace();
            }
        }

        return super.attemptAuthentication(request, response);
    }
}

CustomAuthenticationSuccessHandler class:

public class CustomAuthenticationSuccessHandler extends SimpleUrlAuthenticationSuccessHandler {

    public void onAuthenticationSuccess(
            HttpServletRequest request,
            HttpServletResponse response,
            Authentication auth
    )throws IOException, ServletException {

        if ("application/json".equals(request.getHeader("Content-Type"))) {
            /*
             * USED if you want to AVOID redirect to LoginSuccessful.htm in JSON authentication
             */         
            response.getWriter().print("{\"responseCode\":\"SUCCESS\"}");
            response.getWriter().flush();
        } else {
            super.onAuthenticationSuccess(request, response, auth);
        }
    }
}