Spring HandlerInterceptor

1.Spring HandlerInterceptor 可以组成一个chain。 

这个接口有三个方法：

public interface HandlerInterceptor {

    /**
     * Intercept the execution of a handler. Called after HandlerMapping determined
     * an appropriate handler object, but before HandlerAdapter invokes the handler.
     * <p>DispatcherServlet processes a handler in an execution chain, consisting
     * of any number of interceptors, with the handler itself at the end.
     * With this method, each interceptor can decide to abort the execution chain,
     * typically sending a HTTP error or writing a custom response.
     * @param request current HTTP request
     * @param response current HTTP response
     * @param handler chosen handler to execute, for type and/or instance evaluation
     * @return {@code true} if the execution chain should proceed with the
     * next interceptor or the handler itself. Else, DispatcherServlet assumes
     * that this interceptor has already dealt with the response itself.
     * @throws Exception in case of errors
     */
    boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
        throws Exception;

    /**
     * Intercept the execution of a handler. Called after HandlerAdapter actually
     * invoked the handler, but before the DispatcherServlet renders the view.
     * Can expose additional model objects to the view via the given ModelAndView.
     * <p>DispatcherServlet processes a handler in an execution chain, consisting
     * of any number of interceptors, with the handler itself at the end.
     * With this method, each interceptor can post-process an execution,
     * getting applied in inverse order of the execution chain.
     * @param request current HTTP request
     * @param response current HTTP response
     * @param handler handler (or {@link HandlerMethod}) that started async
     * execution, for type and/or instance examination
     * @param modelAndView the {@code ModelAndView} that the handler returned
     * (can also be {@code null})
     * @throws Exception in case of errors
     */
    void postHandle(
            HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView)
            throws Exception;

    /**
     * Callback after completion of request processing, that is, after rendering
     * the view. Will be called on any outcome of handler execution, thus allows
     * for proper resource cleanup.
     * <p>Note: Will only be called if this interceptor's {@code preHandle}
     * method has successfully completed and returned {@code true}!
     * <p>As with the {@code postHandle} method, the method will be invoked on each
     * interceptor in the chain in reverse order, so the first interceptor will be
     * the last to be invoked.
     * @param request current HTTP request
     * @param response current HTTP response
     * @param handler handler (or {@link HandlerMethod}) that started async
     * execution, for type and/or instance examination
     * @param ex exception thrown on handler execution, if any
     * @throws Exception in case of errors
     */
    void afterCompletion(
            HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex)
            throws Exception;

}

preHandle 是在方法执行前被执行，可以用来取消执行
postHandle是在方法执行之后执行，返回视图之前执行
afterCompletion则是在最后执行，并且它只有在preHandle成功执行，并且返回true才会被执行。
               同时此方法执行的顺序：the first interceptor will be the last to be invoked。
　　　　　　　　 先执行的拦截器的这个方法会最后执行。