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
        @ConditionalOnMissingBean
        ObjectMapper 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
        @ConditionalOnMissingBean
        GsonHttpMessageConverter 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

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

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
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

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 {
    @Override
    public 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 {
    @Override
    public 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");

    @Override
    public 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 {


    @Override
    public 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();

    @Override
    public List changeProperties(SerializationConfig config, BeanDescription beanDesc,
                                 List beanProperties) { // 循环所有的beanPropertyWriter
        for (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nullSerializer
            if (isArrayType(writer)) { //给writer注册一个自己的nullSerializer
                writer.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 {
    @Bean
    public 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。

 @Bean
    public 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");  

    @Override
    public 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pattern
      date-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timestamp
        FAIL_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true
        FAIL_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