<!-- https://mvnrepository.com/artifact/com.alibaba/fastjson -->
  <dependency>
   <groupId>com.alibaba</groupId>
   <artifactId>fastjson</artifactId>
   <version>1.2.58</version>
  </dependency>

 <mvc:annotation-driven>
    <mvc:message-converters register-defaults="true">
      <bean class="com.alibaba.fastjson.support.spring.FastJsonHttpMessageConverter">
        <property name="supportedMediaTypes">
          <list>
            <value>text/html;charset=UTF-8</value>
            <value>application/json</value>
            <value>application/xml;charset=UTF-8</value>
          </list>
        </property>
      </bean>
    </mvc:message-converters>
  </mvc:annotation-driven>

<script>
  $(function () {
    $.ajax({
      url:"${pageContext.servletContext.contextPath}/user/json",
      data:JSON.stringify({username:"tom"}),//JSON.stringify()必須有,否則只會當做表單的格式提交
      dataType:"json",
      type:"post",
      contentType:"application/json;charset=utf-8",//如果想以json格式把數據提交到后臺的話，這個必須有，否則只會當做表單提交
      success:function (data) {
        alert(data.username);
      }
    });
  });
</script>

@RequestMapping("/json")
  public @ResponseBody User jsonToObject(@RequestBody User user){
    System.out.println("--------");
    System.out.println(user);
    return user;
  }