• 深入理解SpringBoot中默认的json解析方案
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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。

默认的json解析方案
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) {
   return new MappingJackson2HttpMessageConverter(objectMapper);
  }

 }
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;
  }

 }
正文
自定义JSON解析
日期格式解析
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,在配置文件中直接添加spring.jackson.date-format=yyyy-MM-dd

NULL字段不返回
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

自定义字段序列化
自定义null字符串类型字段返回空字符
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
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
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");
        }

    }
}
处理数组类型的null值
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
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
@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;
  }
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

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