SpringMVC统一转换null值为空字符串的方法

在SpringMVC中，可以通过在<mvc:annotation-driven>中配置<mvc:message-converters>，把null值统一转换为空字符串，解决这个问题。下面以JSon交互的方式为例说明如何实现：

第一步：创建一个ObjectMapper

package com.xjj.anes.mvc.converter;  

import java.io.IOException;  

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;  

/**
 * @description: 转换null对象为空字符串
 */
public class JsonObjectMapper extends ObjectMapper {
    private static final long serialVersionUID = 1L;  

    public JsonObjectMapper() {
        super();
        // 空值处理为空串
        this.getSerializerProvider().setNullValueSerializer(new JsonSerializer<Object>() {
            @Override
            public void serialize(Object value, JsonGenerator jg, SerializerProvider sp) throws IOException, JsonProcessingException {
                jg.writeString("");
            }
        });
    }
}

　　第二步：在SpringMVC配置文件中，把新建的ObjectMapper注入给MappingJackson2HttpMessageConverter

<!-- 注册RequestMappingHandlerMapping 和RequestMappingHandlerAdapter 两个bean。-->
<mvc:annotation-driven>
    <mvc:message-converters>
        <bean class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
            <property name="objectMapper">
                <bean class="com.xjj.anes.mvc.converter.JsonObjectMapper"></bean>
            </property>
        </bean>
    </mvc:message-converters>
</mvc:annotation-driven>