Spring MVC 解决 Could not write JSON: No serializer found for class java.lang.Object

Spring MVC 解决 Could not write JSON: No serializer found for class java.lang.Object

资料参考：​​http://stackoverflow.com/questions/28862483/spring-and-jackson-how-to-disable-fail-on-empty-beans-through-responsebody​​

出问题前的配置

<mvc:annotation-driven>
    <mvc:message-converters>
        <bean class="org.springframework.http.converter.StringHttpMessageConverter">
            <property name="supportedMediaTypes">
                <list>
                    <value>text/plain;charset=UTF-8</value>
                    <value>text/html;charset=UTF-8</value>
                </list>
            </property>
        </bean>
        <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
            <property name="supportedMediaTypes">
                <list>
                    <value>application/json; charset=UTF-8</value>
                    <value>application/x-www-form-urlencoded; charset=UTF-8</value>
                </list>
            </property>
            <property name="objectMapper">
                <bean class="com.fasterxml.jackson.databind.ObjectMapper">
                    <property name="serializationInclusion">
                        <value type="com.fasterxml.jackson.annotation.JsonInclude.Include">NON_NULL</value>
                    </property>
                </bean>
            </property>
        </bean>
    </mvc:message-converters>
</mvc:annotation-driven>

问题描述

SpringMVC的注解@ResponseBody在以下情况使用时：

@ResponseBody
@RequestMapping("/your_url/act.do")
public Map<String, Object> yourMethod(){

    // do your work...

    Map<String, Object> resultMap = new HashMap<>();
    resultMap.put("code", 0);
    resultMap.put("data", new Object());// 序列化 new Obejct()
}

会报如下错误

 DefaultHandlerExceptionResolver:134 : Resolving exception from handler []: org.springframework.http.converter.HttpMessageNotWritableException: 
 Could not write JSON: No serializer found for class java.lang.Object and no properties discovered to create BeanSerializer 
 (to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS) ) ;

意思是没有找到可用于Object的序列化器，也没有找到属性去创建BeanSerializer。

后面的接着提示了解决方法：(to avoid exception, disable SerializationFeature.FAIL_ON_EMPTY_BEANS)

解决方案

为SpringMVC里默认序列化使用的 com.fasterxml.jackson.databind.ObjectMapper 设置其属性 SerializationFeature.FAIL_ON_EMPTY_BEANS 为false。

貌似没办法直接在XML配置里设置。只能自定义 com.fasterxml.jackson.databind.ObjectMapper 类进行设置，然后再配置xml。过程如下：

// 继承 ObjectMapper 类
public class CustomMapper extends ObjectMapper{ 
    public CustomMapper() {
        this.setSerializationInclusion(JsonInclude.Include.NON_NULL);
        // 设置 SerializationFeature.FAIL_ON_EMPTY_BEANS 为 false
        this.configure(SerializationFeature.FAIL_ON_EMPTY_BEANS, false);
    }
}

<!-- Spring MVC 配置 -->
<mvc:annotation-driven>
    <mvc:message-converters>
        <bean class="org.springframework.http.converter.StringHttpMessageConverter">
            <property name="supportedMediaTypes">
                <list>
                    <value>text/plain;charset=UTF-8</value>
                    <value>text/html;charset=UTF-8</value>
                </list>
            </property>
        </bean>
        <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
            <property name="supportedMediaTypes">
                <list>
                    <value>application/json; charset=UTF-8</value>
                    <value>application/x-www-form-urlencoded; charset=UTF-8</value>
                </list>
            </property>
            <!-- 配置 objectMapper 为我们自定义扩展后的 CustomMapper -->
            <property name="objectMapper">
                <bean class="com.king.framework.jackson.CustomMapper">
                </bean>
            </property>
        </bean>
    </mvc:message-converters>
</mvc:annotation-driven>

附

关于属性 SerializationFeature.FAIL_ON_EMPTY_BEANS 的更多内容不在本文探讨范围内，详见 ​​Jackson​​ 文档及源码。