SpringBoot 默认json解析器详解和字段序列化自定义

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前言

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

SpringBoot默认的json解析方案

我们知道在springboot中有默认的json解析器，Spring Boot 中默认使用的 Json 解析技术框架是 jackson。我们点开 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>

Spring Boot 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 Spring Boot 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controller中返回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

org.springframework.boot.autoconfigure.http.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) {return new MappingJackson2HttpMessageConverter(objectMapper);}}

JacksonAutoConfiguration

org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration

@Configuration(proxyBeanMethods = false)@ConditionalOnClass(Jackson2ObjectMapperBuilder.class)static class JacksonObjectMapperConfiguration {@Bean@Primary@ConditionalOnMissingBeanObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {return builder.createXmlMapper(false).build();}}

Gson的自动化配置类

org.springframework.boot.autoconfigure.http.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);return converter;}}

自定义SprinBoot的JSON解析

日期格式解析

默认返回的是时间戳类型格式，但是时间戳会少一天需要在数据库连接url上加上时区如：

spring.datasource.url=jdbc:p6spy:mysql://47.100.78.146:3306/mall?zeroDateTimeBehavior=convertToNull&useUnicode=true&characterEncoding=UTF-8&serverTimezone=GMT%2B8&autoReconnect=true

1. 使用@JsonFormat注解自定义格式

	@JsonFormat(pattern = "yyyy-MM-dd")private Date birthday;

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

2. 全局添加
   在配置文件中直接添加spring.jackson.date-format=yyyy-MM-dd

NULL字段不返回

1. 在接口中如果不需要返回null字段可以使用@JsonInclude注解

    @JsonInclude(JsonInclude.Include.NON_NULL)private String title;

但是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

2. 全局添加 在配置文件中直接添加spring.jackson.default-property-inclusion=non_null

自定义字段序列化

自定义null字符串类型字段返回空字符NullStringJsonSerializer序列化

public class NullStringJsonSerializer extends JsonSerializer {@Overridepublic void 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默认值NullIntegerJsonSerializer序列化

public class NullIntegerJsonSerializer extends JsonSerializer {@Overridepublic void 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DoubleJsonSerialize序列化

public class DoubleJsonSerialize extends JsonSerializer {private DecimalFormat df = new DecimalFormat("##.00");@Overridepublic void serialize(Object value, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {if (value != null) {jsonGenerator.writeString(NumberUtil.roundStr(value.toString(), 2));}else{jsonGenerator.writeString("0.00");}}
}

自定义NullArrayJsonSerializer序列化

public class NullArrayJsonSerializer extends JsonSerializer {@Overridepublic void serialize(Object o, JsonGenerator jsonGenerator, SerializerProvider serializerProvider) throws IOException {if(o==null){jsonGenerator.writeStartArray();}else {jsonGenerator.writeObject(o);}}
}

自定义BeanSerializerModifier使用我们自己的序列化器进行bean序列化

public class MyBeanSerializerModifier extends BeanSerializerModifier {private JsonSerializer _nullArrayJsonSerializer = new NullArrayJsonSerializer();private JsonSerializer _nullStringJsonSerializer = new NullStringJsonSerializer();private JsonSerializer _nullIntegerJsonSerializer = new NullIntegerJsonSerializer();private JsonSerializer _doubleJsonSerializer = new DoubleJsonSerialize();@Overridepublic List changeProperties(SerializationConfig config, BeanDescription beanDesc,List beanProperties) { // 循环所有的beanPropertyWriterfor (int i = 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());}}return beanProperties;} // 判断是什么类型protected boolean isArrayType(BeanPropertyWriter writer) {Class clazz = writer.getPropertyType();return clazz.isArray() || clazz.equals(List.class) || clazz.equals(Set.class);}protected boolean isStringType(BeanPropertyWriter writer) {Class clazz = writer.getPropertyType();return clazz.equals(String.class);}protected boolean isIntegerType(BeanPropertyWriter writer) {Class clazz = writer.getPropertyType();return clazz.equals(Integer.class) || clazz.equals(int.class) || clazz.equals(Long.class);}protected boolean isDoubleType(BeanPropertyWriter writer) {Class clazz = writer.getPropertyType();return clazz.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 提供MappingJackson2HttpMessageConverter类
在配置类中提供MappingJackson2HttpMessageConverter类，使用ObjectMapper 做全局的序列化

@Configuration
public class ClassJsonConfiguration {@Beanpublic MappingJackson2HttpMessageConverter 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()));return converter;}
}

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。

 @Beanpublic ObjectMapper 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()));return mapper;}

通过上面方式自定义序列化，还可以通过注解@JsonSerialize序列化自定义如：

@Component
public class DoubleSerialize extends JsonSerializer<Double> {private DecimalFormat df = new DecimalFormat("##.00");  @Overridepublic void serialize(Double value, JsonGenerator gen, SerializerProvider serializers)throws IOException, JsonProcessingException {if(value != null) {gen.writeString(df.format(value));  }}
}

然后再需要使用字段上面加上

 @JsonSerialize(using = DoubleJsonSerialize.class)private BigDecimal price;

配置文件jackson详细配置

  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

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