springboot项目中jackson-序列化-处理 NULL教程

在项目中有事需要对值为NULL的对象中Field不做序列化输入配置方式如下：

[配置类型]：

源码包中的枚举类：

public static enum Include {
ALWAYS,
NON_NULL,
NON_ABSENT,
NON_EMPTY,
NON_DEFAULT,
USE_DEFAULTS;

private Include() {
}
}

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

方式一：全局配置，处理所有整个应用的实体对象

#对日期类型的转换配置
spring.jackson.date-format=yyyy-MM-dd HH:mm:ss

# 配置 参数如下 always non_absent non_default non_empty non_null use_defaults

spring.jackson.default-property-inclusion=non_null

方式二：在需要序列的话的实体类上加注解 ->[配置类型]所列

@JsonInclude(Include.NON_NULL)

方式三：配置类型

3.1自定义序列化实现类，可以作用在类上 自定义json序列化需要实现StdSerializer<T>或者JsonSerializer<T>

@JsonSerialize(using = ClientObjectSerialize.class)
public class CreditBorrowerRepaymentRequestDto{
}

实现类：对字段类型转换，以及对值为null字段的过滤

public class ClientObjectSerialize extends JsonSerializer<CreditBorrowerRepaymentRequestDto>{

@Override
public void serialize(CreditBorrowerRepaymentRequestDto dto, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException, JsonProcessingException {

jsonGenerator.writeStartObject();
 try {
  Field[] fields = dto.getClass().getDeclaredFields();
  for (Field field : fields) {
   field.setAccessible(true);
   if(null == field.get(dto)){
    continue;
   }
   jsonGenerator.writeFieldName(field.getName());
   jsonGenerator.writeObject(field.get(dto));
  }
 } catch (Exception e) {
  e.printStackTrace();
 }
 jsonGenerator.writeEndObject();
}
}

3.2自定义序列化实现类，可以作用在实体对象字段上，对NULL值的处理，或者转换

@JsonSerialize(using = ClientStringSerialize.class)
private String name;

@JsonSerialize(using = ClientDtaeSerialize.class)
private Date date;
public class ClientStringSerialize extends JsonSerializer<String> {

@Override
public void serialize(String string, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {

if(string == null){
  jsonGenerator.writeString(string + "[NULL]");
 }else{
  jsonGenerator.writeString(string);
 }
}
}
public class ClientDtaeSerialize extends JsonSerializer<Date> {
@Override
public void serialize(Date createDate, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {
 jsonGenerator.writeString(new SimpleDateFormat("yyyy-MM-dd HH:mm:ss").format(createDate));
}
}

实践总结：

一当全局配置了NULL字段过滤的配置，但有的实体对象需要序列化出NULL的字段值，如何处理？

答：

1. 直接在对象上增加 @JsonInclude(JsonInclude.Include.ALWAYS) 类上的注解优先级比较高，会覆盖全局的配置

2.用自定义的类序列化注解（同上）

二直接在字段上加自定义序列化类会覆盖全局配置吗？

答：不会，有默认的 public class NullSerializer extends StdSerializer<Object> 来处理，当值不为Null的时候才会执行自定义字段上的序列化注解实现类

补充知识：SpringBoot中Jackson返回null处理,字符串类型转空串,数组集合转[],对象转{}

SpringBoot返回Json数据中null值处理，将字符串类型null值转换为""，将集合数组类型null值转换为[]，将原始数据类型null值转换为0，将布尔类型null值转换为false，将实体对象null值转换为{}。

1.自定义null值序列化处理器

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import java.io.IOException;

/**
* 自定义null值序列化处理器
*/
public class CustomizeNullJsonSerializer {

/**
 * 处理数组集合类型的null值
 */
public static class NullArrayJsonSerializer extends JsonSerializer<Object> {
 @Override
 public void serialize(Object value, JsonGenerator jsonGenerator,
   SerializerProvider serializerProvider) throws IOException {
  jsonGenerator.writeStartArray();
  jsonGenerator.writeEndArray();
 }
}

/**
 * 处理字符串类型的null值
 */
public static class NullStringJsonSerializer extends JsonSerializer<Object> {
 @Override
 public void serialize(Object value, JsonGenerator jsonGenerator,
   SerializerProvider serializerProvider) throws IOException {
  jsonGenerator.writeString("");
 }
}

/**
 * 处理数值类型的null值
 */
public static class NullNumberJsonSerializer extends JsonSerializer<Object> {
 @Override
 public void serialize(Object value, JsonGenerator jsonGenerator,
       SerializerProvider serializerProvider) throws IOException {
  jsonGenerator.writeNumber(0);
 }
}

/**
 * 处理boolean类型的null值
 */
public static class NullBooleanJsonSerializer extends JsonSerializer<Object> {
 @Override
 public void serialize(Object value, JsonGenerator jsonGenerator,
       SerializerProvider serializerProvider) throws IOException {
  jsonGenerator.writeBoolean(false);
 }
}

/**
 * 处理实体对象类型的null值
 */
public static class NullObjectJsonSerializer extends JsonSerializer<Object> {
 @Override
 public void serialize(Object value, JsonGenerator jsonGenerator,
       SerializerProvider serializerProvider) throws IOException {
  jsonGenerator.writeStartObject();
  jsonGenerator.writeEndObject();
 }
}
}

2.序列化程序修改器

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;

/**
* <pre>
* 此modifier主要做的事情为：
* 1.当序列化类型为数组集合时，当值为null时，序列化成[]
* 2.String类型值序列化为""
*
* </pre>
*/
public class MyBeanSerializerModifier extends BeanSerializerModifier {

@Override
public List<BeanPropertyWriter> changeProperties(SerializationConfig config,
             BeanDescription beanDesc,
             List<BeanPropertyWriter> beanProperties) {
 // 循环所有的beanPropertyWriter
 for (int i = 0; i < beanProperties.size(); i++) {
  BeanPropertyWriter writer = beanProperties.get(i);
  // 判断字段的类型，如果是数组或集合则注册nullSerializer
  if (isArrayType(writer)) {
   // 给writer注册一个自己的nullSerializer
   writer.assignNullSerializer(new CustomizeNullJsonSerializer.NullArrayJsonSerializer());
  }
  if (isStringType(writer)) {
   writer.assignNullSerializer(new CustomizeNullJsonSerializer.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);
}

/**
 * 是否是数值类型
 */
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);
}
}

3.配置Jackson实体

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;

/**
* 配置Jackson实体
*/
@Configuration
public class JacksonConfig {
@Bean
@Primary
@ConditionalOnMissingBean(ObjectMapper.class)
public ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {
 ObjectMapper objectMapper = builder.createXmlMapper(false).build();
 /** 为objectMapper注册一个带有SerializerModifier的Factory */
 objectMapper.setSerializerFactory(objectMapper.getSerializerFactory()
   .withSerializerModifier(new MyBeanSerializerModifier()));

SerializerProvider serializerProvider = objectMapper.getSerializerProvider();
 serializerProvider.setNullValueSerializer(new CustomizeNullJsonSerializer
.NullObjectJsonSerializer());
 return objectMapper;
}
}

来源：https://blog.csdn.net/yhl_woniu/article/details/84892303