前言

主要介绍自己在学习spring mvc过程中踩到的一些坑，涉及到当时遇到这个错误是如何思考的，对思路进行总结，下次遇到类似的错误能够提供一些思路甚至快速解决。

环境准备

jdk8，spring4.3.3.RELEASE，spring mvc与spring版本一致，maven3.2.5，tomcat7

目标：

1.测试spring mvc的json参数绑定功能

2.测试spring mvc的返回json功能

项目目录(已经推送到github：https://github.com/ComingOnuguys/frankwin)：

测试1相关代码：
index.jsp

 <%@ page language="java" contentType="text/html; charset=utf-8"

     pageEncoding="utf-8"%>

 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

 <html>

 <head>

 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">

 <title>Insert title here</title>

 <script type="text/javascript"

     src="http://ajax.microsoft.com/ajax/jquery/jquery-1.4.min.js"></script>

 </head>

 <body>

     <div>

         id：<input type="text" id="id" value="123456"><br>

         menuName：<input type="text" id="menuName" value="李白"><br>

         <input type="button" id="abc" value="test">

     </div>

     <script type="text/javascript">

         window.onload = function() {

             $("#abc").click(function() {

                 var menu = {

                     "id" : $("#id").val(),

                     "menuName" : $("#menuName").val()

                 };

                 var jsonStr = JSON.stringify(menu);

                 $.ajax({

                     type : "POST",

                     async : false,

                     url : "http://localhost:8080/ssm/getList",

                     dataType : "json",

                     contentType : "application/json;charset=UTF-8",

                     data : jsonStr,

                     success : function(data) {

                         alert("123");

                     }

                 });

             });

         }

     </script>

 </body>

 </html>

getList.jsp（为了返回的时候不报404的错）

 <%@ page language="java" contentType="text/html; charset=utf-8"

     pageEncoding="utf-8"%>

 <!DOCTYPE html PUBLIC "-//W3C//DTD HTML 4.01 Transitional//EN" "http://www.w3.org/TR/html4/loose.dtd">

 <html>

 <head>

 <meta http-equiv="Content-Type" content="text/html; charset=utf-8">

 <title>Insert title here</title>

 <script type="text/javascript" src="http://ajax.microsoft.com/ajax/jquery/jquery-1.4.min.js"></script>

 </head>

 <body>

     <h2>陈公告</h2>

 </body>

 </html>

TestSpringMvcController.java

 package com.dg.action;

 import org.springframework.stereotype.Controller;

 import org.springframework.web.bind.annotation.RequestBody;

 import org.springframework.web.bind.annotation.RequestMapping;

 import com.dg.bean.Menu;

 @Controller

 public class TestSpringMvcController {

     @RequestMapping("getList")

     public void getList(@RequestBody Menu menu){

         System.out.println("menu:" + menu);

     }

 }

Menu.java

 package com.dg.bean;

 import java.io.Serializable;

 public class Menu implements Serializable {

     private static final long serialVersionUID = 410202860691610816L;

     private String id;

     private String menuCode;

     private String menuName;

     private String menuUrl;

     private String parentMenuId;

     public String getId() {

         return id;

     }

     public void setId(String id) {

         this.id = id;

     }

     public String getMenuCode() {

         return menuCode;

     }

     public void setMenuCode(String menuCode) {

         this.menuCode = menuCode;

     }

     public String getMenuName() {

         return menuName;

     }

     public void setMenuName(String menuName) {

         this.menuName = menuName;

     }

     public String getMenuUrl() {

         return menuUrl;

     }

     public void setMenuUrl(String menuUrl) {

         this.menuUrl = menuUrl;

     }

     public String getParentMenuId() {

         return parentMenuId;

     }

     public void setParentMenuId(String parentMenuId) {

         this.parentMenuId = parentMenuId;

     }

     @Override

     public String toString() {

         return "Menu [id=" + id + ", menuCode=" + menuCode + ", menuName=" + menuName + ", menuUrl=" + menuUrl

                 + ", parentMenuId=" + parentMenuId + "]";

     }

 }

spring-config.xml（进行了最大程度精简）

 <?xml version="1.0" encoding="UTF-8"?>

 <beans xmlns="http://www.springframework.org/schema/beans"

     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

     xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans-4.0.xsd">

 </beans>

spring-mvc.xml

 <?xml version="1.0" encoding="UTF-8"?>

 <beans xmlns="http://www.springframework.org/schema/beans"

     xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p"

     xmlns:context="http://www.springframework.org/schema/context"

     xmlns:mvc="http://www.springframework.org/schema/mvc"

     xsi:schemaLocation="http://www.springframework.org/schema/beans

                             http://www.springframework.org/schema/beans/spring-beans-4.0.xsd

                             http://www.springframework.org/schema/context

                             http://www.springframework.org/schema/context/spring-context-4.0.xsd

                             http://www.springframework.org/schema/mvc

                             http://www.springframework.org/schema/mvc/spring-mvc-4.0.xsd

                             ">

     <!-- 自动扫描该包，使SpringMVC认为包下用了@controller注解的类是控制器 -->

     <context:component-scan base-package="com.dg.action" />

     <!-- 启动SpringMVC的注解功能，完成请求和注解POJO的映射 -->

     <mvc:annotation-driven/>

     <!-- 定义跳转的文件的前后缀 ，视图模式配置 -->

      <bean

         class="org.springframework.web.servlet.view.InternalResourceViewResolver">

         <property name="prefix" value="/WEB-INF/jsp/" />

         <property name="suffix" value=".jsp" />

     </bean> 

 </beans>

web.xml

 <?xml version="1.0" encoding="UTF-8"?>

 <web-app xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

     xmlns="http://java.sun.com/xml/ns/javaee"

     xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd"

     version="3.0">

     <display-name>ssm</display-name>

     <!-- Spring和mybatis的配置文件 -->

     <context-param>

         <param-name>contextConfigLocation</param-name>

         <param-value>classpath:spring-config.xml</param-value>

     </context-param>

     <!-- 编码过滤器 -->

     <filter>

         <filter-name>encodingFilter</filter-name>

         <filter-class>org.springframework.web.filter.CharacterEncodingFilter</filter-class>

         <async-supported>true</async-supported>

         <init-param>

             <param-name>encoding</param-name>

             <param-value>UTF-8</param-value>

         </init-param>

     </filter>

     <filter-mapping>

         <filter-name>encodingFilter</filter-name>

         <url-pattern>/*</url-pattern>

     </filter-mapping>

     <!-- Spring监听器 -->

     <listener>

         <listener-class>org.springframework.web.context.ContextLoaderListener</listener-class>

     </listener>

     <!-- 防止Spring内存溢出监听器 -->

     <listener>

         <listener-class>org.springframework.web.util.IntrospectorCleanupListener</listener-class>

     </listener>

     <!-- Spring MVC servlet -->

     <servlet>

         <servlet-name>SpringMVC</servlet-name>

         <servlet-class>org.springframework.web.servlet.DispatcherServlet</servlet-class>

         <init-param>

             <param-name>contextConfigLocation</param-name>

             <param-value>classpath:spring-mvc.xml</param-value>

         </init-param>

         <load-on-startup>1</load-on-startup>

         <async-supported>true</async-supported>

     </servlet>

     <servlet-mapping>

         <servlet-name>SpringMVC</servlet-name>

         <!-- 此处可以可以配置成*.do，对应struts的后缀习惯 -->

         <url-pattern>/</url-pattern>

     </servlet-mapping>

     <!-- 设置静态资源映射 -->

     <servlet-mapping>

         <servlet-name>default</servlet-name>

         <url-pattern>*.jpg</url-pattern>

     </servlet-mapping>

     <servlet-mapping>

         <servlet-name>default</servlet-name>

         <url-pattern>*.gif</url-pattern>

     </servlet-mapping>

     <servlet-mapping>

         <servlet-name>default</servlet-name>

         <url-pattern>*.png</url-pattern>

     </servlet-mapping>

     <servlet-mapping>

         <servlet-name>default</servlet-name>

         <url-pattern>*.js</url-pattern>

     </servlet-mapping>

     <servlet-mapping>

         <servlet-name>default</servlet-name>

         <url-pattern>*.css</url-pattern>

     </servlet-mapping>

     <welcome-file-list>

         <welcome-file>/index.jsp</welcome-file>

     </welcome-file-list>

 </web-app>

pom.xml

 <?xml version="1.0" encoding="UTF-8"?>

 <project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

     xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">

     <modelVersion>4.0.0</modelVersion>

     <packaging>war</packaging>

     <name>ssm</name>

     <groupId>dg</groupId>

     <artifactId>ssm</artifactId>

     <version>0.0.1-SNAPSHOT</version>

     <properties>

         <!-- spring版本号 -->

         <spring.version>4.3.3.RELEASE</spring.version>

         <!-- log4j日志文件管理包版本 -->

         <slf4j.version>1.7.7</slf4j.version>

         <log4j.version>1.2.17</log4j.version>

     </properties>

     <dependencies>

         <!-- spring核心包 -->

         <dependency>

             <groupId>org.springframework</groupId>

             <artifactId>spring-core</artifactId>

             <version>${spring.version}</version>

         </dependency>

         <dependency>

             <groupId>org.springframework</groupId>

             <artifactId>spring-web</artifactId>

             <version>${spring.version}</version>

         </dependency>

         <dependency>

             <groupId>org.springframework</groupId>

             <artifactId>spring-webmvc</artifactId>

             <version>${spring.version}</version>

         </dependency>

         <!-- 日志文件管理包 -->

         <!-- log start -->

         <dependency>

             <groupId>log4j</groupId>

             <artifactId>log4j</artifactId>

             <version>${log4j.version}</version>

         </dependency>

         <dependency>

             <groupId>org.codehaus.jackson</groupId>

             <artifactId>jackson-mapper-asl</artifactId>

             <version>1.9.13</version>

         </dependency>

     </dependencies>

 </project>

坑一：spring mvc的json包
对于测试1，采用以上代码一直返回httpcode415 (Unsupported Media Type)。如下图：

后台报错的日志为：

 [org.springframework.web.servlet.DispatcherServlet] - DispatcherServlet with name 'SpringMVC' processing POST request for [/ssm/getList]

   [org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping] - Looking up handler method for path /getList

   [org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerMapping] - Returning handler method [public void com.dg.action.TestSpringMvcController.getList(com.dg.bean.Menu)]

   [org.springframework.beans.factory.support.DefaultListableBeanFactory] - Returning cached instance of singleton bean 'testSpringMvcController'

   [org.springframework.web.cors.DefaultCorsProcessor] - Skip CORS processing: request is from same origin

   [org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod] - Error resolving argument [0] [type=com.dg.bean.Menu]

 HandlerMethod details:

 Controller [com.dg.action.TestSpringMvcController]

 Method [public void com.dg.action.TestSpringMvcController.getList(com.dg.bean.Menu)]

   org.springframework.web.HttpMediaTypeNotSupportedException: Content type 'application/json;charset=UTF-8' not supported

     at org.springframework.web.servlet.mvc.method.annotation.AbstractMessageConverterMethodArgumentResolver.readWithMessageConverters(AbstractMessageConverterMethodArgumentResolver.java:235)

     at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.readWithMessageConverters(RequestResponseBodyMethodProcessor.java:149)

     at org.springframework.web.servlet.mvc.method.annotation.RequestResponseBodyMethodProcessor.resolveArgument(RequestResponseBodyMethodProcessor.java:127)

     at org.springframework.web.method.support.HandlerMethodArgumentResolverComposite.resolveArgument(HandlerMethodArgumentResolverComposite.java:121)

     at org.springframework.web.method.support.InvocableHandlerMethod.getMethodArgumentValues(InvocableHandlerMethod.java:161)

     at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:128)

     at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:114)

     at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827)

     at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738)

     at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85)

     at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963)

     at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897)

     at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970)

     at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872)

     at javax.servlet.http.HttpServlet.service(HttpServlet.java:646)

     at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846)

     at javax.servlet.http.HttpServlet.service(HttpServlet.java:727)

     at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:303)

     at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)

     at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:52)

     at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)

     at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)

     at org.springframework.web.filter.CharacterEncodingFilter.doFilterInternal(CharacterEncodingFilter.java:197)

     at org.springframework.web.filter.OncePerRequestFilter.doFilter(OncePerRequestFilter.java:107)

     at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:241)

     at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:208)

     at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:220)

     at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:122)

     at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:504)

     at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:170)

     at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:103)

     at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:950)

     at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:116)

     at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:421)

     at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1074)

     at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:611)

     at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:316)

     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1142)

     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:617)

     at org.apache.tomcat.util.threads.TaskThread$WrappingRunnable.run(TaskThread.java:61)

     at java.lang.Thread.run(Thread.java:745)

 [org.springframework.web.servlet.mvc.method.annotation.ExceptionHandlerExceptionResolver] - Resolving exception from handler [public void com.dg.action.TestSpringMvcController.getList(com.dg.bean.Menu)]: org.springframework.web.HttpMediaTypeNotSupportedException: Content type 'application/json;charset=UTF-8' not supported

   [org.springframework.web.servlet.mvc.annotation.ResponseStatusExceptionResolver] - Resolving exception from handler [public void com.dg.action.TestSpringMvcController.getList(com.dg.bean.Menu)]: org.springframework.web.HttpMediaTypeNotSupportedException: Content type 'application/json;charset=UTF-8' not supported

   [org.springframework.web.servlet.mvc.support.DefaultHandlerExceptionResolver] - Resolving exception from handler [public void com.dg.action.TestSpringMvcController.getList(com.dg.bean.Menu)]: org.springframework.web.HttpMediaTypeNotSupportedException: Content type 'application/json;charset=UTF-8' not supported

   [org.springframework.web.servlet.DispatcherServlet] - Null ModelAndView returned to DispatcherServlet with name 'SpringMVC': assuming HandlerAdapter completed request handling

   [org.springframework.web.servlet.DispatcherServlet] - Successfully completed request

看到这个报错提示的第一反应是把前台请求index.jsp的contentType这一行注释掉（注：不写默认值是application/x-www-form-urlencoded;charset=UTF-8）：
//contentType : "application/json;charset=UTF-8",
报错变成了：Content type 'application/x-www-form-urlencoded;charset=UTF-8' not supported
都不支持，这就没法玩了。考虑controller的参数menu不加@RequestBody能不能参数绑定成功，于是去掉@RequestBody，前台请求注释contentType这一行注释掉，发送的参数由jsonStr这个变量改为menu，发现可以正常注入，根据之前学到的知识，String类型和Json的参数绑定是分别由StringHttpMessageConverter和MappingJackson2HttpMessageConverter进行处理，于是把断点分别打在两个类的构造方法上，断点情况如图：

启动tomcat，发现只进到了StringHttpMessageConverter这个类的构造函数里面。报错的原因找到了（没有加载MappingJackson2HttpMessageConverter），但是为什么没有加载，还不知道原因。重新启动tomcat，顺着StringHttpMessageConverter执行下来，发现有这么一段，

看到MappingJackson2HttpMessageConverter非常眼熟，但是很遗憾，这里的jackson2present变量值是false，

看到这里，接着就是去找该变量是false的原因了。Ctrl+Shift+K找到声明这个变量的地方，

选中com.fasterxml.jackson.databind.ObjectMapper，Ctrl+shift+T，没有找到这个类。嗯，问题就处在这了。意思到有可能有什么包没有加进来。查阅资料得知spring mvc从3.2版本开始依赖的json包由原先的jackson-mapper-asl.jar、jackson-core-asl-1.9.13.jar变更为了jackson-core.jar、jackson-dataformat-xml.jar

于是到mavenrepository找到springmvc4.3.3.RELEASE依赖的包的列表，上面的结论得到验证。

接着修改maven依赖json的包，再使用上面的那些代码，终于大功告成~

总结：

1、ajax请求如果不写contentType，则默认是application/x-www-form-urlencoded;charset=UTF-8

2、@RequestBody的作用是将请求参数的json串绑定到实体类上，要实现这个，请求的contentType要设置成contentType : "application/json;charset=UTF-8"

3、更明确spring mvc的这些默认的处理类在tomcat容器启动时就进行初始化。

4、当怀疑是包依赖产生的问题时，可以到mavenrepository找到该框架依赖的jar包。

包依赖的问题解决后，测试2也成功实现目标。

（完）

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