Spring Session在不改变原有使用方式的前提下可以管理session。
从注解@EnableSpringHttpSession入手：

@Retention(java.lang.annotation.RetentionPolicy.RUNTIME)
@Target({ java.lang.annotation.ElementType.TYPE })
@Documented
@Import(SpringHttpSessionConfiguration.class)
@Configuration
public @interface EnableSpringHttpSession {
}

发现导入了配置类
SpringHttpSessionConfiguration

@Configuration
public class SpringHttpSessionConfiguration {

  private CookieHttpSessionStrategy defaultHttpSessionStrategy = new CookieHttpSessionStrategy();

  private HttpSessionStrategy httpSessionStrategy = this.defaultHttpSessionStrategy;

  private List<HttpSessionListener> httpSessionListeners = new ArrayList<HttpSessionListener>();

  private ServletContext servletContext;

  @Bean
  public SessionEventHttpSessionListenerAdapter sessionEventHttpSessionListenerAdapter() {
      return new SessionEventHttpSessionListenerAdapter(this.httpSessionListeners);
  }

  @Bean
  public <S extends ExpiringSession> SessionRepositoryFilter<? extends ExpiringSession> springSessionRepositoryFilter(
        SessionRepository<S> sessionRepository) {
      SessionRepositoryFilter<S> sessionRepositoryFilter = new SessionRepositoryFilter<S>(
            sessionRepository);
      sessionRepositoryFilter.setServletContext(this.servletContext);
      if (this.httpSessionStrategy instanceof MultiHttpSessionStrategy) {
        sessionRepositoryFilter.setHttpSessionStrategy(
              (MultiHttpSessionStrategy) this.httpSessionStrategy);
      }
      else {
        sessionRepositoryFilter.setHttpSessionStrategy(this.httpSessionStrategy);
      }
      return sessionRepositoryFilter;
  }

  @Autowired(required = false)
  public void setServletContext(ServletContext servletContext) {
      this.servletContext = servletContext;
  }

  @Autowired(required = false)
  public void setCookieSerializer(CookieSerializer cookieSerializer) {
      this.defaultHttpSessionStrategy.setCookieSerializer(cookieSerializer);
  }

  @Autowired(required = false)
  public void setHttpSessionStrategy(HttpSessionStrategy httpSessionStrategy) {
      this.httpSessionStrategy = httpSessionStrategy;
  }

  @Autowired(required = false)
  public void setHttpSessionListeners(List<HttpSessionListener> listeners) {
      this.httpSessionListeners = listeners;
  }
}

最底下有四个可以配置的Spring Session方式。看名字可以看出他们的作用。
这个配置往Spring容器中配置两个bean。
SessionEventHttpSessionListenerAdapter session事件的监听器适配器
SessionRepositoryFilter session 持久化的过滤器，这是Spring Session的核心。

SessionRepositoryFilter 可以支持两种策略，

1. CookieHttpSessionStrategy是基于Cookie的默认实现。

2. MultiHttpSessionStrategy可以支持多个用户在一个浏览器上。

SessionRepositoryFilter 过滤核心代码

@Override
protected void doFilterInternal(HttpServletRequest request,
      HttpServletResponse response, FilterChain filterChain)
            throws ServletException, IOException {
  request.setAttribute(SESSION_REPOSITORY_ATTR, this.sessionRepository);

  SessionRepositoryRequestWrapper wrappedRequest = new SessionRepositoryRequestWrapper(
        request, response, this.servletContext);
  SessionRepositoryResponseWrapper wrappedResponse = new SessionRepositoryResponseWrapper(
        wrappedRequest, response);

  HttpServletRequest strategyRequest = this.httpSessionStrategy
        .wrapRequest(wrappedRequest, wrappedResponse);
  HttpServletResponse strategyResponse = this.httpSessionStrategy
        .wrapResponse(wrappedRequest, wrappedResponse);

  try {
      filterChain.doFilter(strategyRequest, strategyResponse);
  }
  finally {
      wrappedRequest.commitSession();
  }
  HttpServletRequest strategyRequest = this.httpSessionStrategy
        .wrapRequest(wrappedRequest, wrappedResponse);
  HttpServletResponse strategyResponse = this.httpSessionStrategy
        .wrapResponse(wrappedRequest, wrappedResponse);
}

SessionRepositoryRequestWrapper及
SessionRepositoryResponseWrapper是将
HttpServletRequest、HttpServletResponse包装成的可以提供session持久化功能的类。

HttpServletRequest strategyRequest = this.httpSessionStrategy
      .wrapRequest(wrappedRequest, wrappedResponse);
HttpServletResponse strategyResponse = this.httpSessionStrategy
      .wrapResponse(wrappedRequest, wrappedResponse);

利用多态完成request及response的实现转换。