在我们开发项目API接口的时候，一些没有数据的字段会默认返回NULL,数字类型也会是NULL,这个时候前端希望字符串能够统一返回空字符，数字默认返回0，那我们就需要自定义json序列化处理
我们知道在SpringBoot中有默认的Json解析器，SpringBoot 中默认使用的 Json 解析技术框架是 jackson。
点开 pom.xml 中的 spring-boot-starter-web 依赖，可以看到一个 spring-boot-starter-json依赖：
<dependency><groupId>org.springframework.boot</groupId><artifactId>spring-boot-starter-json</artifactId><version>2.4.7</version><scope>compile</scope></dependency>
SpringBoot 中对依赖都做了很好的封装，可以看到很多 spring-boot-starter-xxx系列的依赖，这是 SpringBoot 的特点之一，不需要人为去引入很多相关的依赖了，starter-xxx 系列直接都包含了所必要的依赖，所以我们再次点进去上面这个 spring-boot-starter-json 依赖，可以看到：
<dependency><groupId>com.fasterxml.jackson.core</groupId><artifactId>jackson-databind</artifactId><version>2.11.4</version><scope>compile</scope></dependency><dependency><groupId>com.fasterxml.jackson.datatype</groupId><artifactId>jackson-datatype-jdk8</artifactId><version>2.11.4</version><scope>compile</scope></dependency><dependency><groupId>com.fasterxml.jackson.datatype</groupId><artifactId>jackson-datatype-jsr310</artifactId><version>2.11.4</version><scope>compile</scope></dependency><dependency><groupId>com.fasterxml.jackson.module</groupId><artifactId>jackson-module-parameter-names</artifactId><version>2.11.4</version><scope>compile</scope></dependency>
我们在返回json时候通过注解@ResponseBody就可以自动帮我们将服务端返回的对象序列化成json字符串，在传递json body参数时候 通过在对象参数上@RequestBody注解就可以自动帮我们将前端传过来的json字符串反序列化成java对象，这些功能都是通过HttpMessageConverter这个消息转换工具类来实现的
SpringMVC自动配置了Jackson和Gson的HttpMessageConverter，SpringBoot对此做了自动化配置
JacksonHttpMessageConvertersConfiguration
@Configuration(proxyBeanMethods=false)@ConditionalOnClass(ObjectMapper.class)@ConditionalOnBean(ObjectMapper.class)@ConditionalOnProperty(name=HttpMessageConvertersAutoConfiguration.PREFERRED_MAPPER_PROPERTY,havingValue="jackson",matchIfMissing=true)static class MappingJackson2HttpMessageConverterConfiguration {@Bean@ConditionalOnMissingBean(value=MappingJackson2HttpMessageConverter.class,ignoredType={"org.springframework.hateoas.server.mvc.TypeConstrainedMappingJackson2HttpMessageConverter","org.springframework.data.rest.webmvc.alps.AlpsJsonHttpMessageConverter"})MappingJackson2HttpMessageConverter mappingJackson2HttpMessageConverter(ObjectMapper objectMapper){returnnew MappingJackson2HttpMessageConverter(objectMapper);}
}
GsonHttpMessageConvertersConfiguration
@Configuration(proxyBeanMethods=false)@ConditionalOnBean(Gson.class)@Conditional(PreferGsonOrJacksonAndJsonbUnavailableCondition.class)static class GsonHttpMessageConverterConfiguration {@Bean@ConditionalOnMissingBeanGsonHttpMessageConverter gsonHttpMessageConverter(Gson gson){
GsonHttpMessageConverter converter=new GsonHttpMessageConverter();converter.setGson(gson);returnconverter;}
}
使用@JsonFormat注解自定义格式
@JsonFormat(pattern="yyyy-MM-dd")privateDatebirthday;
但是这种要对每个实体类中的日期字段都需要添加此注解不够灵活，在配置文件中直接添加spring.jackson.date-format=yyyy-MM-dd
使用@JsonInclude注解
@JsonInclude(JsonInclude.Include.NON_NULL)private String title;
这种要对每个实体类中的字段都需要添加此注解不够灵活，在配置文件中直接添加Spring.jackson.default-property-inclusion=non_null
自定义null字符串类型字段返回空字符
publicclass NullStringJsonSerializer extends JsonSerializer {@Overridepublicvoid serialize(Object o,JsonGenerator jsonGenerator,SerializerProvider serializerProvider)throws IOException {if(o==null){
jsonGenerator.writeString("");}
}
}
自定义null数字类型字段返回0
publicclass NullIntegerJsonSerializer extends JsonSerializer {@Overridepublicvoid serialize(Object o,JsonGenerator jsonGenerator,SerializerProvider serializerProvider)throws IOException {if(o==null){
jsonGenerator.writeNumber(0);}
}
}
自定义浮点小数类型4舍5入保留2位小数
publicclass DoubleJsonSerialize extends JsonSerializer {
private DecimalFormat df=new DecimalFormat("##.00");@Overridepublicvoid serialize(Objectvalue,JsonGenerator jsonGenerator,SerializerProvider serializerProvider)throws IOException {if(value!=null){
jsonGenerator.writeString(NumberUtil.roundStr(value.toString(),2));}else{
jsonGenerator.writeString("0.00");}
}
}
处理数组类型的null值
publicclass NullArrayJsonSerializer extends JsonSerializer {@Overridepublicvoid serialize(Object o,JsonGenerator jsonGenerator,SerializerProvider serializerProvider)throws IOException {if(o==null){
jsonGenerator.writeStartArray();}else{
jsonGenerator.writeObject(o);}
}
}
自定义BeanSerializerModifier
publicclass MyBeanSerializerModifier extends BeanSerializerModifier {
private JsonSerializer _nullArrayJsonSerializer=new NullArrayJsonSerializer();private JsonSerializer _nullStringJsonSerializer=new NullStringJsonSerializer();private JsonSerializer _nullIntegerJsonSerializer=new NullIntegerJsonSerializer();private JsonSerializer _doubleJsonSerializer=new DoubleJsonSerialize();@OverridepublicList changeProperties(SerializationConfig config,BeanDescription beanDesc,List beanProperties){// 循环所有的beanPropertyWriterfor(inti=0;i<beanProperties.size();i++){
BeanPropertyWriter writer=(BeanPropertyWriter)beanProperties.get(i);// 判断字段的类型，如果是array，list，set则注册nullSerializerif(isArrayType(writer)){//给writer注册一个自己的nullSerializerwriter.assignNullSerializer(this.defaultNullArrayJsonSerializer());}if(isStringType(writer)){
writer.assignNullSerializer(this.defaultNullStringJsonSerializer());}if(isIntegerType(writer)){
writer.assignNullSerializer(this.defaultNullIntegerJsonSerializer());}if(isDoubleType(writer)){
writer.assignSerializer(this.defaultDoubleJsonSerializer());}
}returnbeanProperties;}// 判断是什么类型protectedbooleanisArrayType(BeanPropertyWriter writer){
Class clazz=writer.getPropertyType();returnclazz.isArray()||clazz.equals(List.class)||clazz.equals(Set.class);}
protectedbooleanisStringType(BeanPropertyWriter writer){
Class clazz=writer.getPropertyType();returnclazz.equals(String.class);}
protectedbooleanisIntegerType(BeanPropertyWriter writer){
Class clazz=writer.getPropertyType();returnclazz.equals(Integer.class)||clazz.equals(int.class)||clazz.equals(Long.class);}
protectedbooleanisDoubleType(BeanPropertyWriter writer){
Class clazz=writer.getPropertyType();returnclazz.equals(Double.class)||clazz.equals(BigDecimal.class);}
protected JsonSerializer defaultNullArrayJsonSerializer(){return_nullArrayJsonSerializer;}
protected JsonSerializer defaultNullStringJsonSerializer(){return_nullStringJsonSerializer;}
protected JsonSerializer defaultNullIntegerJsonSerializer(){return_nullIntegerJsonSerializer;}
protected JsonSerializer defaultDoubleJsonSerializer(){return_doubleJsonSerializer;}
}
应用我们自己bean序列化使其生效
@Configurationpublicclass ClassJsonConfiguration {@BeanpublicMappingJackson2HttpMessageConverter mappingJacksonHttpMessageConverter(){
final MappingJackson2HttpMessageConverter converter=new MappingJackson2HttpMessageConverter();ObjectMapper mapper=converter.getObjectMapper();// 为mapper注册一个带有SerializerModifier的Factory，此modifier主要做的事情为：判断序列化类型，根据类型指定为null时的值mapper.setSerializerFactory(mapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));returnconverter;}
}
此类会代替SpringBoot默认的json解析方案。事实上，此类中起作用的是ObjectMapper 类，因此也可直接配置此类
@BeanpublicObjectMapper om(){
ObjectMapper mapper=new ObjectMapper();// 为mapper注册一个带有SerializerModifier的Factory，此modifier主要做的事情为：判断序列化类型，根据类型指定为null时的值mapper.setSerializerFactory(mapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));returnmapper;}
spring: jackson:# 设置属性命名策略,对应jackson下PropertyNamingStrategy中的常量值，SNAKE_CASE-返回的json驼峰式转下划线，json body下划线传到后端自动转驼峰式property-naming-strategy: SNAKE_CASE# 全局设置@JsonFormat的格式patterndate-format: yyyy-MM-dd HH:mm:ss# 当地时区locale: zh# 设置全局时区time-zone: GMT+8# 常用，全局设置pojo或被@JsonInclude注解的属性的序列化方式default-property-inclusion: NON_NULL#不为空的属性才会序列化,具体属性可看JsonInclude.Include# 常规默认,枚举类SerializationFeature中的枚举属性为key，值为boolean设置jackson序列化特性,具体key请看SerializationFeature源码serialization: WRITE_DATES_AS_TIMESTAMPS:true# 返回的java.util.date转换成timestampFAIL_ON_EMPTY_BEANS:true# 对象为空时是否报错，默认true# 枚举类DeserializationFeature中的枚举属性为key，值为boolean设置jackson反序列化特性,具体key请看DeserializationFeature源码deserialization:# 常用,json中含pojo不存在属性时是否失败报错,默认trueFAIL_ON_UNKNOWN_PROPERTIES:false# 枚举类MapperFeature中的枚举属性为key，值为boolean设置jackson ObjectMapper特性# ObjectMapper在jackson中负责json的读写、json与pojo的互转、json tree的互转,具体特性请看MapperFeature,常规默认即可mapper:# 使用getter取代setter探测属性，如类中含getName()但不包含name属性与setName()，传输的vo json格式模板中依旧含name属性USE_GETTERS_AS_SETTERS:true#默认false# 枚举类JsonParser.Feature枚举类中的枚举属性为key，值为boolean设置jackson JsonParser特性# JsonParser在jackson中负责json内容的读取,具体特性请看JsonParser.Feature，一般无需设置默认即可parser: ALLOW_SINGLE_QUOTES:true# 是否允许出现单引号,默认false# 枚举类JsonGenerator.Feature枚举类中的枚举属性为key，值为boolean设置jackson JsonGenerator特性，一般无需设置默认即可# JsonGenerator在jackson中负责编写json内容,具体特性请看JsonGenerator.Feature