Restrict Spring Security maximum sessions, and force logout when roles change

[naturalprogrammer]

Allow application developers to restrict the number of maximum login sessions for a user by using a property such as lemon.security.max-sessions: 5. A default, say 5, can be set.

Coding this feature will also allow us to go a step further and force logout a user when an admin alters his roles.

References:

http://docs.spring.io/spring-security/site/docs/current/reference/htmlsingle/#concurrent-sessions
spring-projects/spring-boot#1537
https://jira.spring.io/browse/SEC-3069

I think we need to add some code to LemonSecurityConfig, like this:

@Override
protected void configure(HttpSecurity http) throws Exception {

     http
          ...
      .sessionManagement()
        .maximumSessions(10)
        .sessionRegistry(sessionRegistry());
     ...
}

/**
 * Until https://jira.spring.io/browse/SEC-2855
 * is closed, we need to have this custom sessionRegistry
 */
@Bean
public SessionRegistry sessionRegistry() {
    SessionRegistry sessionRegistry = new SessionRegistryImpl();
    return sessionRegistry;
}

/**
 * Register HttpSessionEventPublisher. Note that it is declared
 * static to instantiate it very early, before this configuration
 * class is processed.
 * 
 * See http://docs.spring.io/spring-boot/docs/current/reference/html/howto-embedded-servlet-containers.html
 * for how to add a ServletContextListener.
 * 
 * See http://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/context/annotation/Bean.html
 * for how static instantiation works.
 */
@Bean
public static ServletListenerRegistrationBean<HttpSessionEventPublisher> httpSessionEventPublisher() {
    return new ServletListenerRegistrationBean<HttpSessionEventPublisher>(new HttpSessionEventPublisher());
}

But, for scaling up, won't we need to have our own SessionRegistry implementation, say JPA based, instead of SessionRegistryImpl, which is the in-memory based? I also noticed that SessionRegistryImpl only listens to SessionDestroyedEvent. Should not it be listening to SessionCreatedEvent as well? Need to study more.