SpringMVC拦截器与Filter过滤器

????????在系统中，经常需要在处理用户请求之前和之后执行一些行为，例如：检测用户的权限、或者将请求的信息记录到日志中，即：平常所说的“权限检测”及“日志记录”。
???????? 当然不仅仅这些，所以需要一种机制，拦截用户的请求，在请求之后添加处理逻辑。
???????? SpringMVC提供了Interceptor拦截器机制，用于请求的预处理和后处理。
???????? 相比过滤器，拦截器作用的范围比较小，而拦截器几乎可以过滤所有请求。
???????? 拦截器的作用：
???????????? [1]用户请求拦截处理；
???????????? [2]针对一个或者多个或者全部的Controller返回结果进行拦截处理；
???????????? [3]页面渲染之后，响应给浏览器之间进行拦截处理。
???????? 过滤器和拦截器的区别？
???????????? [1]过滤器在请求到达项目组间之前工作；拦截器在前端控制器之后工作；
???????????? [2]过滤器依赖Servlet容器；拦截器是SpringMVC的，它不依赖于servlet容器，而是由Spring容器进行初始化；
???????????? Note that a filter gets configured in web.xml, a HandlerInterceptor in the application context.
???????????? [3]拦截器只能对action动态请求起作用（静态请求，例如：不会对js、css、img等静态资源的请求起到拦截作用）；而过滤器可以对所有的请求起作用，无法实现细粒度的请求拦截；
???????????? [4]在action的生命周期中，拦截器可以多次被调用，而过滤器只能在容器初始化时被调用一次。
拦截器可以访问action上下文、值栈里的对象，而过滤器不能访问。
???????????? [5]拦截器可以获取IOC容器中的各个bean，而过滤器就不太方便，这点很重要，在拦截器里注入一个service，可以调用业务逻辑。

Spring MVC拦截器的定义

????需要实现HandlerInterceptor接口。

package com.xwd.interceptor;

import org.springframework.web.servlet.HandlerInterceptor;
import org.springframework.web.servlet.ModelAndView;

import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;


/**
 * @ClassName AppInterceptor
 * @Description: com.xwd.interceptor
 * @Auther: xiwd
 * @Date: 2022/3/1 - 03 - 01 - 22:12
 * @version: 1.0
 */
public class AppInterceptor implements HandlerInterceptor {
    //properties

    //methods

    /**
     * 在请求到达定义的Handler处理器（Controller）之前，工作
     * @param request   请求对象
     * @param response  响应对象
     * @param handler   目标要调用的handler
     * @return  true-继续向前执行,可以到达handler（Controller层）;false-拒绝继续执行
     * @throws Exception
     */
    @Override
    public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception {
        System.out.println("AppInterceptor preHandle");
        /**/
        //解决POST乱码问题


        //判断是否登录


        //用户权限控制


        System.out.println(handler);
        return true;
    }

    /**
     * handler处理单元返回ModelAndView对象时候工作
     * @param request
     * @param response
     * @param handler
     * @param modelAndView
     * @throws Exception
     */
    @Override
    public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception {
        System.out.println("AppInterceptor postHandle");
        //在handler（Controller控制层）返回ModelAndView对象时候进行拦截

        //控制视图-修改

        //控制数据-敏感数据过滤（敏感词）

    }

    /**
     * 页面渲染完毕，但是还没有响应给浏览器客户端之前，工作
     * @param request
     * @param response
     * @param handler
     * @param ex
     * @throws Exception
     */
    @Override
    public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception {
        System.out.println("AppInterceptor afterCompletion");
        //一般用于做资源的释放操作---该方法无论是否出现异常都会执行

    }
}

SpringMVC拦截器的配置

????需要在SpringMVC配置文件中配置HandlerInterceptor接口。
（有别于Filter过滤器，需要在web.xml中使用filter标签配置）

  <!--注册拦截器-->
    <mvc:interceptors>
        <!--[1]拦截所有的URL请求,那么直接指定bean即可-->
<!--        <bean class="com.xwd.interceptor.AppInterceptor"></bean>-->

        <!--[2]拦截特定的请求,需要使用mvc:interceptor标签来限定被拦截的访问路径-->
        <mvc:interceptor>
            <mvc:mapping path="/login.do"/>
            <bean class="com.xwd.interceptor.AppInterceptor"></bean>
        </mvc:interceptor>
    </mvc:interceptors>

SpringMVC拦截器HandlerInterceptor接口

???? HandlerInterceptor接口中定义了3个方法，分别用于对某一个/某一些特定的请求进行预处理或者后处理。
????【1】HandlerInterceptor拦截器
???????? HandlerInterceptor是一个工作流程（workflow）接口，允许自定义request请求的处理执行链（handler execution chains）。对于某一组特定的handler（controller控制器），应用程序可以注册任意数量的自定义拦截器（Interceptor），在不改变原有接口实现类的前提下，实现一组通用处理行为。
???? 【2】HandlerInterceptor拦截器工作流程
???????? 一个HandlerInterceptor（拦截器）在HandlerAdapter（处理器适配器）触发handler（控制器之前）被调用（具体是：default boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)方法被调用——请求预处理）。
???????? 拦截器机制可以实现一组切面（Aspects）的预处理操作，例如：权限检测（Authorization Checks）、或者一组通用的处理行为（例如：区域设置-locale或者主题修改theme changes），拦截器可以分解出重复性的代码，形成一组通用的操作。
???????????? [0]在HandlerAdapter处理器适配器调用handler（Controller控制器）之前，preHandle()方法被调用；
???????????? [1]在异步处理场景下，在没有调用postHandle()方法或者faterCompletion()方法之前（此时Handler/Controller控制器尚未执行完毕），处理器可能在一个独立线程中执行。
???????????? [2]当并发处理器Handler（即：控制器）（concurrent handler）执行完毕，携带处理结果的Model、ModeAndView等对象被返回给HandlerAdapter处理器适配器时， postHandle()方法就会被调用，可以用于实现数据、视图的进一步处理或者数据过滤操作；
???????????? [3]当HandlerAdapter处理器适配器将封装ModeAndView对象返回给DispatcherServlet前端处理器之后，前端处理器就开始调用ViewResolver视图解析器，处理得到View视图对象，在将View视图对象响应给前端浏览器之前，faterCompletion()方法就会被调用。通常用于实现资源清理工作。
??????? 【3】SpringMVC拦截器适用场景：与细粒度预处理任务相关的功能实现，优先考虑通过实现HandlerInterceptor接口，定义拦截器的方式。尤其是：分解出通用处理代码、权限检查等。
????????????????? Filter过滤器适用场景：Filter过滤器更适合用来处理request请求内容、view视图内容处理，例如：multipart表单和GZip压缩处理。

附：HandlerInterceptor处理器拦截器接口源码

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 an HTTP error or writing a custom response.
    * <p><strong>Note:</strong> special considerations apply for asynchronous
    * request processing. For more details see
    * {@link org.springframework.web.servlet.AsyncHandlerInterceptor}.
    * <p>The default implementation returns {@code true}.
    * @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
    */
   default boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler)
         throws Exception {

      return true;
   }

   /**
    * 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.
    * <p><strong>Note:</strong> special considerations apply for asynchronous
    * request processing. For more details see
    * {@link org.springframework.web.servlet.AsyncHandlerInterceptor}.
    * <p>The default implementation is empty.
    * @param request current HTTP request
    * @param response current HTTP response
    * @param handler the handler (or {@link HandlerMethod}) that started asynchronous
    * 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
    */
   default void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler,
         @Nullable 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.
    * <p><strong>Note:</strong> special considerations apply for asynchronous
    * request processing. For more details see
    * {@link org.springframework.web.servlet.AsyncHandlerInterceptor}.
    * <p>The default implementation is empty.
    * @param request current HTTP request
    * @param response current HTTP response
    * @param handler the handler (or {@link HandlerMethod}) that started asynchronous
    * execution, for type and/or instance examination
    * @param ex any exception thrown on handler execution, if any; this does not
    * include exceptions that have been handled through an exception resolver
    * @throws Exception in case of errors
    */
   default void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler,
         @Nullable Exception ex) throws Exception {
   }

}