jackson设置返回null为空字符串的操作

1、将接口返回的字段为null的设置为空字符串：

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import java.io.IOException;
@Configuration
public class CustomConfig {
 @Bean
 public ObjectMapper objectMapper() {
  ObjectMapper objectMapper = new ObjectMapper();
  objectMapper.getSerializerProvider().setNullValueSerializer(new JsonSerializer<Object>() {
   @Override
   public void serialize(Object paramT, JsonGenerator paramJsonGenerator,
         SerializerProvider paramSerializerProvider) throws IOException {
    //设置返回null转为 空字符串""
    paramJsonGenerator.writeString("");
   }
  });
  return objectMapper;
 }
}

2、设置null字段不返回给前端：

//1.实体上
@JsonInclude(Include.NON_NULL)
//将该标记放在属性上，如果该属性为NULL则不参与序列化
//如果放在类上边,那对这个类的全部属性起作用
//Include.Include.ALWAYS 默认
//Include.NON_DEFAULT 属性为默认值不序列化
//Include.NON_EMPTY 属性为 空（“”） 或者为 NULL 都不序列化
//Include.NON_NULL 属性为NULL 不序列化
//2.代码上
ObjectMapper mapper = new ObjectMapper();
mapper.setSerializationInclusion(Include.NON_NULL);
//通过该方法对mapper对象进行设置，所有序列化的对象都将按改规则进行系列化
//Include.Include.ALWAYS 默认
//Include.NON_DEFAULT 属性为默认值不序列化
//Include.NON_EMPTY 属性为 空（“”） 或者为 NULL 都不序列化
//Include.NON_NULL 属性为NULL 不序列化

来验证下结果，很显然，我们成功了，age、address、sex三个字段值为null，现在看到是空字符串：

补充知识：SpringBoot Jackson 将null转字符串"" ，List、Array转[]，Int转0

如下所示：

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.core.JsonProcessingException;
import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializationConfig;
import com.fasterxml.jackson.databind.SerializerProvider;
import com.fasterxml.jackson.databind.ser.BeanPropertyWriter;
import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;
import org.apache.commons.lang3.StringUtils;
import org.springframework.http.converter.json.MappingJackson2HttpMessageConverter;

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

/**
 * 使用官方自带的json格式类库，fastjson因为content type问题时不时控制台报错、无法直接返回二进制等问题
 */
class JacksonHttpMessageConverter extends MappingJackson2HttpMessageConverter {

  /**
   * 处理数组类型的null值
   */
  public class NullArrayJsonSerializer extends JsonSerializer<Object> {

    @Override
    public void serialize(Object value, JsonGenerator jgen, SerializerProvider provider) throws IOException, JsonProcessingException {
      if (value == null) {
        jgen.writeStartArray();
        jgen.writeEndArray();
      }
    }
  } 

  /**
   * 处理字符串类型的null值
   */
  public class NullStringJsonSerializer extends JsonSerializer<Object> {

    @Override
    public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException, JsonProcessingException {
      jsonGenerator.writeString(StringUtils.EMPTY);
    }
  }

  /**
   * 处理数字类型的null值
   */
  public class NullNumberJsonSerializer extends JsonSerializer<Object> {

    @Override
    public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException, JsonProcessingException {
      jsonGenerator.writeNumber(0);
    }
  }

  /**
   * 处理布尔类型的null值
   */
  public class NullBooleanJsonSerializer extends JsonSerializer<Object> {

    @Override
    public void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException, JsonProcessingException {
      jsonGenerator.writeBoolean(false);
    }
  } 

  public class MyBeanSerializerModifier extends BeanSerializerModifier { 

    @Override
    public List<BeanPropertyWriter> changeProperties(SerializationConfig config, BeanDescription beanDesc, List<BeanPropertyWriter> beanProperties) {
      //循环所有的beanPropertyWriter
      for (Object beanProperty : beanProperties) {
        BeanPropertyWriter writer = (BeanPropertyWriter) beanProperty;
        //判断字段的类型，如果是array，list，set则注册nullSerializer
        if (isArrayType(writer)) {
          //给writer注册一个自己的nullSerializer
          writer.assignNullSerializer(new NullArrayJsonSerializer());
        } else if (isNumberType(writer)) {
          writer.assignNullSerializer(new NullNumberJsonSerializer());
        } else if (isBooleanType(writer)) {
          writer.assignNullSerializer(new NullBooleanJsonSerializer());
        } else if (isStringType(writer)) {
          writer.assignNullSerializer(new NullStringJsonSerializer());
        }
      }
      return beanProperties;
    }

    /**
     * 是否是数组
     */
    private boolean isArrayType(BeanPropertyWriter writer) {
      Class<?> clazz = writer.getType().getRawClass();
      return clazz.isArray() || Collection.class.isAssignableFrom(clazz);
    }

    /**
     * 是否是string
     */
    private boolean isStringType(BeanPropertyWriter writer) {
      Class<?> clazz = writer.getType().getRawClass();
      return CharSequence.class.isAssignableFrom(clazz) || Character.class.isAssignableFrom(clazz);
    } 

    /**
     * 是否是int
     */
    private boolean isNumberType(BeanPropertyWriter writer) {
      Class<?> clazz = writer.getType().getRawClass();
      return Number.class.isAssignableFrom(clazz);
    }

    /**
     * 是否是boolean
     */
    private boolean isBooleanType(BeanPropertyWriter writer) {
      Class<?> clazz = writer.getType().getRawClass();
      return clazz.equals(Boolean.class);
    }
  }

  JacksonHttpMessageConverter() {
    getObjectMapper().setSerializerFactory(getObjectMapper().getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));
  }

}

一定要在下面配置消息转换器，不然没效果

@Configuration
public class WebMvcConfig extends WebMvcConfigurationSupport {
  @Override
  protected void configureMessageConverters(List<HttpMessageConverter<?>> converters) {
    super.configureMessageConverters(converters);
    converters.add(new JacksonHttpMessageConverter());
  }

}

如果想要更丰富的可以在：MyBeanSerializerModifier中自定义。

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