这个问题一般出现在使用Spring 4时，因为：
spring3.x是org.springframework.http.converter.json.MappingJacksonHttpMessageConverter
spring4.x是org.springframework.http.converter.json.MappingJackson2HttpMessageConverter
所以Spring3.x配置如下：
<bean class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter">
<property name="messageConverters">
<list>
<ref bean="jsonHttpMessageConverter" />
</list>
</property>
</bean>
<bean id="jsonHttpMessageConverter" class="org.springframework.http.converter.json.MappingJacksonHttpMessageConverter">
<property name="supportedMediaTypes">
<list>
<value>application/json;charset=UTF-8</value>
</list>
</property>
</bean>
Spring 4.x配置如下：
<bean class="org.springframework.web.servlet.mvc.annotation.AnnotationMethodHandlerAdapter">
<property name="messageConverters">
<list>
<ref bean="jsonHttpMessageConverter" />
</list>
</property>
</bean>
<bean id="jsonHttpMessageConverter" class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
<property name="supportedMediaTypes">
<list>
<value>application/json;charset=UTF-8</value>
</list>
</property>
</bean>
