springBoot jackson 统一 null 值的默认值

在实际的项目开发中，后端返回的json数据里属性如果没有值的一般直接返回的null

就像这样:

{
  "id": 13,
  "name": "王五",
  "list":null //这里是个数组
}

但是有的时候由于增加了前端的处理或者业务的需要,需要把值为null的设置成""或者[]亦或者{}

就像这样:

{
  "id": 3,
  "name": "张三",
  "list": []
}

此时我们可以配置返回值为null的统一处理，配置方法如下:

1:针对空数组的处理

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

import java.io.IOException;


public class NullArrayJsonSerializer extends JsonSerializer {
    
    public static final NullArrayJsonSerializer INSTANCE=new NullArrayJsonSerializer();

    @Override
    public void serialize(Object value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        //这里不需要判断value的值,因为只要能进到这个方法里面,就证明值是null
        gen.writeStartArray();
        gen.writeEndArray();
    }
}

2:针对空map的处理:

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

import java.io.IOException;


public class NullMapJsonSerializer extends JsonSerializer {
    
    public static final NullMapJsonSerializer INSTANCE=new NullMapJsonSerializer();
    @Override
    public void serialize(Object value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        gen.writeStartObject();
        gen.writeEndObject();
    }
}

3:针对其它类型的处理

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

import java.io.IOException;


public class NullObjectJsonSerializer extends JsonSerializer {
    
    public static final NullObjectJsonSerializer INSTANCE=new NullObjectJsonSerializer();
    @Override
    public void serialize(Object value, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        gen.writeString("");
    }
}

4:编写BeanSerializerModifier

import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.SerializationConfig;
import com.fasterxml.jackson.databind.ser.BeanPropertyWriter;
import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;

import java.util.Collection;
import java.util.List;
import java.util.Map;


public class MyBeanSerializerModifier extends BeanSerializerModifier {

    
    @Override
    public List changeProperties(SerializationConfig config, BeanDescription beanDesc, List beanProperties) {
        //循环所有的beanProperties
        for (BeanPropertyWriter writer : beanProperties) {
            //给writer注册一个自己的nullSerializer
            if (isArrayType(writer)) {
                //这里使用单例的原因是每个被序列化的bean都要执行一次这个方法,且这个类里面的方法是线程安全的,如果每次都new,想想就挺可怕的
                writer.assignNullSerializer(NullArrayJsonSerializer.INSTANCE);
            } else if (isMapType(writer)) {
                writer.assignNullSerializer(NullMapJsonSerializer.INSTANCE);

            } else {//除了list和map外其它的都赋值为"",要是还想加别的类型,可以再这里再写else if
         
                writer.assignNullSerializer(NullObjectJsonSerializer.INSTANCE);
            }
        }


        return beanProperties;
    }


    
    private boolean isArrayType(BeanPropertyWriter writer) {
        Class rawClass = writer.getType().getRawClass();
        return rawClass.isArray() || Collection.class.isAssignableFrom(rawClass);

    }

    
    private boolean isMapType(BeanPropertyWriter writer) {
        Class rawClass = writer.getType().getRawClass();
        return Map.class.isAssignableFrom(rawClass);

    }


}

5:把自定义的MyBeanSerializerModifier设置到ObjectMapper里面去,这里有四种方法

方法一:实现WebMvcConfigurer 接口,找到里面的configureMessageConverters方法

import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.extern.slf4j.Slf4j;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;

import java.util.List;


@Configuration
@Slf4j
public class MyWebAutoConfiguration implements WebMvcConfigurer {


    @Override
    public void configureMessageConverters(List> converters) {
        //把自定义的序列化规则设置进入转换器里
        for (HttpMessageConverter converter : converters) {
            if (converter instanceof MappingJackson2HttpMessageConverter) {

                MappingJackson2HttpMessageConverter jackson2Converter = (MappingJackson2HttpMessageConverter) converter;
                ObjectMapper objectMapper = jackson2Converter.getObjectMapper();

                objectMapper.setSerializerFactory(objectMapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));
            }
        }

    }


}

方法二:与方法一类似 继承WebMvcConfigurationSupport 覆写configureMessageConverters方法

import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.extern.slf4j.Slf4j;
import org.springframework.context.annotation.Configuration;
import org.springframework.http.converter.HttpMessageConverter;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.web.servlet.config.annotation.WebMvcConfigurationSupport;

import java.util.List;


@Slf4j
@Configuration
public class MyWebMvcConfigurationSupport extends WebMvcConfigurationSupport {

    @Override
    protected void configureMessageConverters(List> converters) {
        //把自定义的序列化规则设置进入转换器里
        for (HttpMessageConverter converter : converters) {
            if (converter instanceof MappingJackson2HttpMessageConverter) {

                MappingJackson2HttpMessageConverter jackson2Converter = (MappingJackson2HttpMessageConverter) converter;
                ObjectMapper objectMapper = jackson2Converter.getObjectMapper();

                objectMapper.setSerializerFactory(objectMapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));
            }
    }
}

方法三:自己注册一个消息转换器(不推荐,因为没办法保证能覆盖现有的)

import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.extern.slf4j.Slf4j;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;
import org.springframework.context.annotation.Configuration;



@Slf4j
@Configuration
public class JacksonHttpMessageConverter extends MappingJackson2HttpMessageConverter {

    public JacksonHttpMessageConverter() {
        ObjectMapper objectMapper = getObjectMapper();

        objectMapper.setSerializerFactory(objectMapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));

    }
}

方法四:拿到所有已注册的bean,找到objectMapper配置

import com.fasterxml.jackson.databind.ObjectMapper;
import lombok.extern.slf4j.Slf4j;
import org.springframework.beans.BeansException;
import org.springframework.beans.factory.config.BeanPostProcessor;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;


@Slf4j
@Configuration
public class MyJacksonAutoConfiguration {


    @Bean
    public BeanPostProcessor objectMapperBeanPostProcessor() {
        return new BeanPostProcessor() {
            //此方法不仅仅局限于配置jackson,还可对所有的已注册bean进行增强
            @Override
            public Object postProcessAfterInitialization(Object bean, String beanName) throws BeansException {
                //判断是否是ObjectMapper,如果不是可跳过
                if (!(bean instanceof ObjectMapper)) {
                    return bean;
                }
                ObjectMapper objectMapper = (ObjectMapper) bean;
                objectMapper.setSerializerFactory(objectMapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));
                return bean;
            }
        };
    }

}