前言

在我們開發(fā)項(xiàng)目API接口的時(shí)候落午，一些沒有數(shù)據(jù)的字段會(huì)默認(rèn)返回NULL,數(shù)字類型也會(huì)是NULL,這個(gè)時(shí)候前端希望字符串能夠統(tǒng)一返回空字符摄乒，數(shù)字默認(rèn)返回0瑰煎，那我們就需要自定義json序列化處理

SpringBoot默認(rèn)的json解析方案

我們知道在springboot中有默認(rèn)的json解析器屹篓，Spring Boot 中默認(rèn)使用的 Json 解析技術(shù)框架是 jackson。我們點(diǎn)開 pom.xml 中的 spring-boot-starter-web 依賴撇吞，可以看到一個(gè) spring-boot-starter-json依賴：

 <dependency>
      <groupId>org.springframework.boot</groupId>
      <artifactId>spring-boot-starter-json</artifactId>
      <version>2.4.7</version>
      <scope>compile</scope>
    </dependency>

Spring Boot 中對(duì)依賴都做了很好的封裝俗冻，可以看到很多 spring-boot-starter-xxx 系列的依賴，這是 Spring Boot 的特點(diǎn)之一牍颈，不需要人為去引入很多相關(guān)的依賴了迄薄，starter-xxx 系列直接都包含了所必要的依賴，所以我們?cè)俅吸c(diǎn)進(jìn)去上面這個(gè) spring-boot-starter-json 依賴煮岁，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>

我們?cè)赾ontroller中返回json時(shí)候通過注解@ResponseBody就可以自動(dòng)幫我們將服務(wù)端返回的對(duì)象序列化成json字符串讥蔽，在傳遞json body參數(shù)時(shí)候 通過在對(duì)象參數(shù)上@RequestBody注解就可以自動(dòng)幫我們將前端傳過來的json字符串反序列化成java對(duì)象

這些功能都是通過HttpMessageConverter這個(gè)消息轉(zhuǎn)換工具類來實(shí)現(xiàn)的

SpringMVC自動(dòng)配置了Jackson和Gson的HttpMessageConverter，SpringBoot對(duì)此做了自動(dòng)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的自動(dòng)化配置類

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;
        }

    }

自定義SprinBoot的JSON解析

日期格式解析

默認(rèn)返回的是時(shí)間戳類型格式画机，但是時(shí)間戳?xí)僖惶煨枰跀?shù)據(jù)庫連接url上加上時(shí)區(qū)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注解自定義格式

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

但是這種要對(duì)每個(gè)實(shí)體類中的日期字段都需要添加此注解不夠靈活

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;

但是這種要對(duì)每個(gè)實(shí)體類中的字段都需要添加此注解不夠靈活

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

自定義字段序列化

自定義null字符串類型字段返回空字符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("");
        }
    }
}

自定義null數(shù)字類型字段返回0默認(rèn)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);
        }
    }
}

自定義浮點(diǎn)小數(shù)類型4舍5入保留2位小數(shù)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");
        }

    }
}

自定義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);
        }
    }
}

自定義BeanSerializerModifier使用我們自己的序列化器進(jìn)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) { // 循環(huán)所有的beanPropertyWriter
        for (int i = 0; i < beanProperties.size(); i++) {
            BeanPropertyWriter writer = (BeanPropertyWriter) beanProperties.get(i);
            // 判斷字段的類型冶伞，如果是array，list步氏，set則注冊(cè)nullSerializer
            if (isArrayType(writer)) { //給writer注冊(cè)一個(gè)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;
    } // 判斷是什么類型

    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;
    }
}

應(yīng)用我們自己bean序列化使其生效 提供MappingJackson2HttpMessageConverter類
在配置類中提供MappingJackson2HttpMessageConverter類碰缔，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();

        // 為mapper注冊(cè)一個(gè)帶有SerializerModifier的Factory，此modifier主要做的事情為：判斷序列化類型戳护，根據(jù)類型指定為null時(shí)的值

        mapper.setSerializerFactory(mapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));

        return converter;
    }
}

此類會(huì)代替SpringBoot默認(rèn)的json解析方案。事實(shí)上瀑焦，此類中起作用的是ObjectMapper 類腌且，因此也可直接配置此類。

 @Bean
    public ObjectMapper om() {
        ObjectMapper mapper = new ObjectMapper();
        // 為mapper注冊(cè)一個(gè)帶有SerializerModifier的Factory榛瓮，此modifier主要做的事情為：判斷序列化類型铺董，根據(jù)類型指定為null時(shí)的值

        mapper.setSerializerFactory(mapper.getSerializerFactory().withSerializerModifier(new MyBeanSerializerModifier()));
        return mapper;
    }

通過上面方式自定義序列化，還可以通過注解@JsonSerialize序列化自定義如：

@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詳細(xì)配置

  spring:
    jackson:
      # 設(shè)置屬性命名策略,對(duì)應(yīng)jackson下PropertyNamingStrategy中的常量值禀晓，SNAKE_CASE-返回的json駝峰式轉(zhuǎn)下劃線精续，json body下劃線傳到后端自動(dòng)轉(zhuǎn)駝峰式
      property-naming-strategy: SNAKE_CASE
      # 全局設(shè)置@JsonFormat的格式pattern
      date-format: yyyy-MM-dd HH:mm:ss
      # 當(dāng)?shù)貢r(shí)區(qū)
      locale: zh
      # 設(shè)置全局時(shí)區(qū)
      time-zone: GMT+8
      # 常用，全局設(shè)置pojo或被@JsonInclude注解的屬性的序列化方式
      default-property-inclusion: NON_NULL #不為空的屬性才會(huì)序列化,具體屬性可看JsonInclude.Include
      # 常規(guī)默認(rèn),枚舉類SerializationFeature中的枚舉屬性為key粹懒，值為boolean設(shè)置jackson序列化特性,具體key請(qǐng)看SerializationFeature源碼
      serialization:
        WRITE_DATES_AS_TIMESTAMPS: true # 返回的java.util.date轉(zhuǎn)換成timestamp
        FAIL_ON_EMPTY_BEANS: true # 對(duì)象為空時(shí)是否報(bào)錯(cuò)重付，默認(rèn)true
      # 枚舉類DeserializationFeature中的枚舉屬性為key，值為boolean設(shè)置jackson反序列化特性,具體key請(qǐng)看DeserializationFeature源碼
      deserialization:
        # 常用,json中含pojo不存在屬性時(shí)是否失敗報(bào)錯(cuò),默認(rèn)true
        FAIL_ON_UNKNOWN_PROPERTIES: false
      # 枚舉類MapperFeature中的枚舉屬性為key凫乖，值為boolean設(shè)置jackson ObjectMapper特性
      # ObjectMapper在jackson中負(fù)責(zé)json的讀寫确垫、json與pojo的互轉(zhuǎn)弓颈、json tree的互轉(zhuǎn),具體特性請(qǐng)看MapperFeature,常規(guī)默認(rèn)即可
      mapper:
        # 使用getter取代setter探測(cè)屬性，如類中含getName()但不包含name屬性與setName()删掀，傳輸?shù)膙o json格式模板中依舊含name屬性
        USE_GETTERS_AS_SETTERS: true #默認(rèn)false
      # 枚舉類JsonParser.Feature枚舉類中的枚舉屬性為key翔冀，值為boolean設(shè)置jackson JsonParser特性
      # JsonParser在jackson中負(fù)責(zé)json內(nèi)容的讀取,具體特性請(qǐng)看JsonParser.Feature，一般無需設(shè)置默認(rèn)即可
      parser:
        ALLOW_SINGLE_QUOTES: true # 是否允許出現(xiàn)單引號(hào),默認(rèn)false
      # 枚舉類JsonGenerator.Feature枚舉類中的枚舉屬性為key披泪，值為boolean設(shè)置jackson JsonGenerator特性纤子，一般無需設(shè)置默認(rèn)即可
      # JsonGenerator在jackson中負(fù)責(zé)編寫json內(nèi)容,具體特性請(qǐng)看JsonGenerator.Feature