可能的原因:没有添加依赖

可以在pom.xml中加入json的相关依赖

    <properties>
      <jackson.version>2.5.4</jackson.version>
    </properties> 

     <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-core</artifactId>
      <version>${jackson.version}</version>
    </dependency>
    <dependency>
      <groupId>com.fasterxml.jackson.core</groupId>
      <artifactId>jackson-databind</artifactId>
      <version>${jackson.version}</version>
    </dependency>

如果加之后,问题还没有解决,可以在springmvc.xml中进行如下的配置:

<mvc:annotation-driven>
     <mvc:message-converters>
        <bean class="org.springframework.http.converter.StringHttpMessageConverter"/>
        <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter"/>
   </mvc:message-converters>
</mvc:annotation-driven>

参考自:https://www.cnblogs.com/hafiz/p/5812873.html