Springmvc前端JSON转换器MappingJackson2HttpMessageConverter设置对值为null的处理

Posted 2022-05-31 当以乐

我们在使用springmvc中的 @ResponseBody 注解返回JSON时可以配置Json转换器如下:

<bean id="mappingJacksonHttpMessageConverter"
        class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
        <property name="supportedMediaTypes">
            <list>
                <value>application/json;charset=UTF-8</value>
            </list>
        </property>
    </bean>

有时我们返回的字段存在null的情况我们需要对其进行处理,objectMapper提供了默认的几种处理方式配置如下:

<bean id="mappingJacksonHttpMessageConverter"
		class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
		<property name="supportedMediaTypes">
			<list>
				<value>application/json;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>

这种是将字段为null的清理掉不在结果Json中显示出来,其余的还有以下几种配置:

        ALWAYS,  NON_NULL,   NON_DEFAULT, NON_EMPTY

有些特殊情况,我们不能将null字段直接去除,而是需要给予一个默认的值,则可以设置如下配置:

	<bean id="mappingJacksonHttpMessageConverter"
		class="org.springframework.http.converter.json.MappingJackson2HttpMessageConverter">
		<property name="supportedMediaTypes">
			<list>
				<value>application/json;charset=UTF-8</value>
			</list>
		</property>
		<property name="objectMapper">
			<bean class="com.fasterxml.jackson.databind.ObjectMapper">
				<property name="serializerProvider">
					<bean class="com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.Impl">
						<property name="nullValueSerializer">
							<bean class="cn.com.mx.gome.flash.component.GomeSearchJsonSerializer"></bean>
						</property>
					</bean>
				</property>
			</bean>
		</property>
	</bean>

自定义的null处理类如下;

import java.io.IOException;
import java.util.ArrayList;
import java.util.List;

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

public class GomeSearchJsonSerializer extends JsonSerializer<Object> 

    @Override
    public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider)
            throws IOException, JsonProcessingException 
//        jgen.writeString("");
        jgen.writeObject(new JsonNullDef());
    



class JsonNullDef
    
    private List<String> def = new ArrayList<>();

    public List<String> getDef() 
        return def;
    

    public void setDef(List<String> def) 
        this.def = def;
    
    
    

大家可以根据自己的需求来设置null的自定义值.