源码解读之SpringMVC（一）

vlambda
2022-04-23

源码解读之SpringMVC（一）

内容简介：

1.CharacterEncodingFilter

2.HiddenHttpMethodFilter

3. HandlerInterceptor

一、CharacterEncodingFilter

（i）用途介绍

主要用途简单理解为解决中文乱码，该过滤器允许指定字符集处理用户请求或者响应。

（ii）直观使用

在web.xml配置文件添加如下代码

   <filter>       <filter-name>char-filter</filter-name>       <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>       <init-param>           <param-name>encoding</param-name>           <param-value>UTF-8</param-value>       </init-param>       <init-param>           <param-name>forceResponseEncoding</param-name>           <param-value>true</param-value>       </init-param>   </filter>   <filter-mapping>       <filter-name>char-filter</filter-name>       <url-pattern>/*</url-pattern>   </filter-mapping>

目的：request和response字符集设置为'UTF-8'

注：filter/servlet都需要mapping来指定过滤成分/“路由”

（iii）理解性使用

需要进入源码（Spring Web MVC » 5.3.1）

org.springframework.web.filter.CharacterEncodingFilter

思路：既然是过滤器，肯定需要找与doFilter相关内容

public class CharacterEncodingFilter extends OncePerRequestFilter { @Nullable private String encoding; private boolean forceRequestEncoding; private boolean forceResponseEncoding;
 protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException { String encoding = this.getEncoding(); if (encoding != null) { /* *request.getCharacterEncoding() == null *本条件在无修改情况为空，所以条件成立能进入到if *request.setCharacterEncoding(encoding) *通过servlet原生api，设置encoding */ if (this.isForceRequestEncoding() || request.getCharacterEncoding() == null) { request.setCharacterEncoding(encoding); } /* * 对response设置encoding需要满足forceResponseEncoding是true */ if (this.isForceResponseEncoding()) { response.setCharacterEncoding(encoding); } } // 进行请求/响应的放行 filterChain.doFilter(request, response); }
 public boolean isForceRequestEncoding() { return this.forceRequestEncoding; }
 public boolean isForceResponseEncoding() { return this.forceResponseEncoding; }}

综上所述，当前我们需要在web.xml 创建包含参数encoding（为了request）以及包含参数forceResponseEncoding （为了response）的 filter

 <init-param>        <param-name>encoding</param-name>        <param-value>UTF-8</param-value> </init-param> <init-param>        <param-name>forceResponseEncoding</param-name>        <param-value>true</param-value> </init-param>

二、HiddenHttpMethodFilter

（i）用途

当浏览器只支持GET/POST的请求时，能使用它发出且不局限于PUT/DELETE

（ii）详细用法

1.首先需要在web.xml添加对应过滤器

<!--配置HiddenHttpMethodFilter过滤器--> <filter> <filter-name>hiddenHttpMethodFilter</filter-name> <filter-class>org.springframework.web.filter.HiddenHttpMethodFilter</filter-class> </filter>
 <filter-mapping> <filter-name>hiddenHttpMethodFilter</filter-name> <url-pattern>/*</url-pattern>    </filter-mapping>

2.以更改举例（PUT），代码略去了bootstrap&js

没有以DELETE举例是因为单纯的以表单形式需要结合Vue来创造点击事件以及需要取消元素默认行为，但是如果使用Ajax则不用考虑这些小问题

<!DOCTYPE html><html lang="en" xmlns:th="http://www.thymeleaf.org"><head> <meta charset="UTF-8"> <title>update</title></head><body><div id="app"> <div class="container"> <div class="row"> <div class="col-md-12"> <h1>Update</h1> </div> </div> <br> <div class="row"> <div class="col-md-12"> <form th:action="@{/users}" method="post"> <input type="hidden" name="_method" value="put"> <input type="hidden" th:value="${user.userId}"> <div class="form-group"> <label th:for="userId">Current ID <i>(You cannot change your ID) </i></label> <input type="text" class="form-control" id="userId" name="userId" th:value="${user.userId}" readonly> </div> <br> <div class="form-group"> <label th:for="userName">New Name</label> <input type="text" class="form-control" th:id="userName" name="userName" th:value="${user.userName}" placeholder="please enter new Username"> </div> <br> <div class="form-group"> <label th:for="userPassword">New Password</label> <input type="password" class="form-control" th:id="userPassword" name="userPassword" th:value="${user.userPassword}" placeholder="please enter new password"> </div> <br> <button type="submit" class="btn btn-primary col-md-1 col-md-offset-5" >Submit</button> <br> <a href="/users" >Cancel</a> </form> </div> </div> </div> </div></body>

显而易见，在表单中添加了隐藏元素

<input type="hidden" name="_method" value="put">

这样就可以做到除了GET/POST以外的请求了

（iii）为什么添加此隐藏元素就能实现功能？

需要进入源码（Spring Web MVC » 5.3.1）

org.springframework.web.filter.HiddenHttpMethodFilter

同样地，需要找doFilter

    // params    private static final List<String> ALLOWED_METHODS; public static final String DEFAULT_METHOD_PARAM = "_method"; private String methodParam = "_method";

protected void doFilterInternal(HttpServletRequest request, HttpServletResponse response, FilterChain filterChain) throws ServletException, IOException {      HttpServletRequest requestToUse = request; // 通过表单的POST进入if      if ("POST".equals(request.getMethod()) && request.getAttribute("javax.servlet.error.exception") == null) {         // 获取name为 "_method"的参数（应为 PUT）          String paramValue = request.getParameter(this.methodParam);         // 确保"_method"的参数存在          if (StringUtils.hasLength(paramValue)) {          // 将"_method"的参数转成大写并本地化              String method = paramValue.toUpperCase(Locale.ENGLISH);           // 检查"_method"的参数存在于ALLOWED_METHODS              if (ALLOWED_METHODS.contains(method)) {              // 将请求重新包装，成为以ALLOWED_METHODS内对应请求方法的请求                  requestToUse = new HiddenHttpMethodFilter.HttpMethodRequestWrapper(request, method);              }          }      }    // 放行      filterChain.doFilter((ServletRequest)requestToUse, response);    }

ALLOWED_METHODS =

Collections.unmodifiableList(

Arrays.asList(HttpMethod.PUT.name(),

HttpMethod.DELETE.name(),

HttpMethod.PATCH.name()));

三、HandlerInterceptor

（i）用途

拦截器

拦截器是Spring MVC中强大的控件，它可以在进入处理器之前做一些操作，或者在处理器完成后进行操作，甚至是在渲染视图后进行操作

Introduction to Spring MVC HandlerInterceptor

https://www.baeldung.com/spring-mvc-handlerinterceptor

（ii）How to use?

SpringMVC的拦截器必须在SpringMVC的配置文件中进行配置：

<bean class="package.MyInterceptor"></bean><ref bean="myInterceptor"></ref><!-- 以上两种配置方式都是对DispatcherServlet所处理的所有的请求进行拦截 --><mvc:interceptor> <!-- ？一个字符*任意个字符**任意层数 --> <mvc:mapping path="/**"/>    <!-- exclude --> <mvc:exclude-mapping path="/test"/> <ref bean="myInterceptor"></ref></mvc:interceptor>

实现HandlerInterceptor

public class MyInterceptor implements HandlerInterceptor { // ctl+o @Override public boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception { return HandlerInterceptor.super.preHandle(request, response, handler); }
 @Override public void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, ModelAndView modelAndView) throws Exception { HandlerInterceptor.super.postHandle(request, response, handler, modelAndView); }
 @Override public void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, Exception ex) throws Exception { HandlerInterceptor.super.afterCompletion(request, response, handler, ex); }}

HandlerInterceptor interface

public interface HandlerInterceptor { default boolean preHandle(HttpServletRequest request, HttpServletResponse response, Object handler) throws Exception { return true; }
 default void postHandle(HttpServletRequest request, HttpServletResponse response, Object handler, @Nullable ModelAndView modelAndView) throws Exception { }
 default void afterCompletion(HttpServletRequest request, HttpServletResponse response, Object handler, @Nullable Exception ex) throws Exception { }}

SpringMVC中的拦截器有三个抽象方法：

1. preHandle：控制器方法执行之前执行preHandle()，其boolean类型的返回值表示是否拦截或放行，返回true为放行，即调用控制器方法；返回false表示拦截，即不调用控制器方法

2. postHandle：控制器方法执行之后执行postHandle()

3. afterCompletion：处理完视图和模型数据，渲染视图完毕之后执行afterCompletion()

（iii）源码解读

需要进入源码（Spring Web MVC » 5.3.1）

为什么要进这儿？Debug打断点找Frames

从doDispatch入手

protected void doDispatch(HttpServletRequest request, HttpServletResponse response) throws Exception { HttpServletRequest processedRequest = request; HandlerExecutionChain mappedHandler = null; boolean multipartRequestParsed = false; WebAsyncManager asyncManager = WebAsyncUtils.getAsyncManager(request);
 try { try { ModelAndView mv = null; Object dispatchException = null;
 try { processedRequest = this.checkMultipart(request); multipartRequestParsed = processedRequest != request; mappedHandler = this.getHandler(processedRequest); if (mappedHandler == null) { this.noHandlerFound(processedRequest, response); return; }
 HandlerAdapter ha = this.getHandlerAdapter(mappedHandler.getHandler()); String method = request.getMethod(); boolean isGet = "GET".equals(method); if (isGet || "HEAD".equals(method)) { long lastModified = ha.getLastModified(request, mappedHandler.getHandler()); if ((new ServletWebRequest(request, response)).checkNotModified(lastModified) && isGet) { return; } }
 if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; }
 mv = ha.handle(processedRequest, response, mappedHandler.getHandler()); if (asyncManager.isConcurrentHandlingStarted()) { return; }
 this.applyDefaultViewName(processedRequest, mv); mappedHandler.applyPostHandle(processedRequest, response, mv); } catch (Exception var20) { dispatchException = var20; } catch (Throwable var21) { dispatchException = new NestedServletException("Handler dispatch failed", var21); }
 this.processDispatchResult(processedRequest, response, mappedHandler, mv, (Exception)dispatchException); } catch (Exception var22) { this.triggerAfterCompletion(processedRequest, response, mappedHandler, var22); } catch (Throwable var23) { this.triggerAfterCompletion(processedRequest, response, mappedHandler, new NestedServletException("Handler processing failed", var23)); }
 } finally { if (asyncManager.isConcurrentHandlingStarted()) { if (mappedHandler != null) { mappedHandler.applyAfterConcurrentHandlingStarted(processedRequest, response); } } else if (multipartRequestParsed) { this.cleanupMultipart(processedRequest); }
 } }

本文只专注于pre的逻辑

因为如果整合并且关注其他点需要基本说明整个DispatchServlet...

希望下一篇关于springMVC的文章能码上所有前端控制器的内容orz

 // mappedHandler.applyPreHandle返回false才能拦截(真正的拦截) if (!mappedHandler.applyPreHandle(processedRequest, response)) { return; }

点击进入applyPreHandle

    /* private final Object handler; *private final List<HandlerInterceptor> interceptorList; *private int interceptorIndex;*/boolean applyPreHandle(HttpServletRequest request, HttpServletResponse response) throws Exception {    for(int i = 0; i < this.interceptorList.size(); this.interceptorIndex = i++) {    // 转型        HandlerInterceptor interceptor = (HandlerInterceptor)this.interceptorList.get(i);        // 这就是接口中的preHandle！！        // preHandle为false的时候才会让applyPreHandle返回false(目的)        if (!interceptor.preHandle(request, response, this.handler)) {            this.triggerAfterCompletion(request, response, (Exception)null);            return false;        }    }    return true;}void applyPostHandle(HttpServletRequest request, HttpServletResponse response, @Nullable ModelAndView mv) throws Exception {    for(int i = this.interceptorList.size() - 1; i >= 0; --i) {        HandlerInterceptor interceptor = (HandlerInterceptor)this.interceptorList.get(i);        interceptor.postHandle(request, response, this.handler, mv);    }}

注：拦截器的执行顺序从此也不难看出，pre的执行为1-2-3，post的执行为3-2-1。applyPostHandle像stack的感觉，因为之前是先进，所以是后出，但是跟这个根本不搭边际，因为本身就是简单的i++和 --i 导致的

23 April 2022

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