SpringBoot Jackson配置项说明及其自定义序列化器

springboot之jackson的两种配置方式

SpringBoot对于Jackson的配置（通过application.yml）

配置属性说明：

spring.jackson.date-format指定日期格式，比如yyyy-MM-dd HH:mm:ss，或者具体的格式化类的全限定名

spring.jackson.deserialization是否开启Jackson的反序列化

spring.jackson.generator是否开启json的generators.

spring.jackson.joda-date-time-format指定Joda date/time的格式，比如yyyy-MM-ddHH:mm:ss). 如果没有配置的话，dateformat会作为backup

spring.jackson.locale指定json使用的Locale.

spring.jackson.mapper是否开启Jackson通用的特性.

spring.jackson.parser是否开启jackson的parser特性.

spring.jackson.property-naming-strategy指定PropertyNamingStrategy(CAMEL_CASE_TO_LOWER_CASE_WITH_UNDERSCORES)或者指定PropertyNamingStrategy子类的全限定类名.

spring.jackson.serialization是否开启jackson的序列化.

spring.jackson.serialization-inclusion指定序列化时属性的inclusion方式，具体查看JsonInclude.Include枚举.

spring.jackson.time-zone指定日期格式化时区，比如America/Los_Angeles或者GMT+10.

常用配置：
spring:
  jackson:
    #日期格式化
    date-format: yyyy-MM-dd HH:mm:ss
    serialization:
       #格式化输出 
      indent_output: true
      #忽略无法转换的对象
      fail_on_empty_beans: false
    #设置空如何序列化
    defaultPropertyInclusion: NON_EMPTY
    deserialization:
      #允许对象忽略json中不存在的属性
      fail_on_unknown_properties: false
    parser:
      #允许出现特殊字符和转义符
      allow_unquoted_control_chars: true
      #允许出现单引号
      allow_single_quotes: true

方式二：自定义序列化器

默认对Null的处理

@Bean
@Primary
@ConditionalOnMissingBean(ObjectMapper.class)
public ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder)
    {
        ObjectMapper objectMapper = builder.createXmlMapper(false).build();

        // 通过该方法对mapper对象进行设置，所有序列化的对象都将按改规则进行系列化
        // Include.Include.ALWAYS 默认
        // Include.NON_DEFAULT 属性为默认值不序列化
        // Include.NON_EMPTY 属性为 空（""） 或者为 NULL 都不序列化，则返回的json是没有这个字段的。这样对移动端会更省流量
        // Include.NON_NULL 属性为NULL 不序列化
        objectMapper.setSerializationInclusion(JsonInclude.Include.NON_EMPTY);
        objectMapper.configure(DeserializationFeature.FAIL_ON_UNKNOWN_PROPERTIES, false);
        // 允许出现特殊字符和转义符
        objectMapper.configure(JsonParser.Feature.ALLOW_UNQUOTED_CONTROL_CHARS, true);
        // 允许出现单引号
        objectMapper.configure(JsonParser.Feature.ALLOW_SINGLE_QUOTES, true);
        // 字段保留，将null值转为""
        objectMapper.getSerializerProvider().setNullValueSerializer(new JsonSerializer<Object>()
        {
            @Override
            public void serialize(Object o, JsonGenerator jsonGenerator,
                                  SerializerProvider serializerProvider)
                    throws IOException
            {
                jsonGenerator.writeString("");
            }
        });
        return objectMapper;
    }

自定义对Map、List、String、Boolean、Integer、Object的处理

1. 自定义null值序列化处理器*

import com.fasterxml.jackson.core.JsonGenerator;
import com.fasterxml.jackson.databind.JsonSerializer;
import com.fasterxml.jackson.databind.SerializerProvider;
import java.io.IOException;

/**
 * 自定义null值序列化处理器
 */
public class CustomizeNullJsonSerializer {

    /**
     * 处理数组集合类型的null值
     */
    public static class NullArrayJsonSerializer extends JsonSerializer<Object> {
        @Override
        public void serialize(Object value, JsonGenerator jsonGenerator, 
        					  SerializerProvider serializerProvider) throws IOException {
            jsonGenerator.writeStartArray();
            jsonGenerator.writeEndArray();
        }
    }

    /**
     * 处理字符串类型的null值
     */
    public static class NullStringJsonSerializer extends JsonSerializer<Object> {
        @Override
        public void serialize(Object value, JsonGenerator jsonGenerator, 
        					  SerializerProvider serializerProvider) throws IOException {
            jsonGenerator.writeString("");
        }
    }

    /**
     * 处理数值类型的null值
     */
    public static class NullNumberJsonSerializer extends JsonSerializer<Object> {
        @Override
        public void serialize(Object value, JsonGenerator jsonGenerator, 
                              SerializerProvider serializerProvider) throws IOException {
            jsonGenerator.writeNumber(0);
        }
    }

    /**
     * 处理boolean类型的null值
     */
    public static class NullBooleanJsonSerializer extends JsonSerializer<Object> {
        @Override
        public void serialize(Object value, JsonGenerator jsonGenerator, 
                              SerializerProvider serializerProvider) throws IOException {
            jsonGenerator.writeBoolean(false);
        }
    }

    /**
     * 处理实体对象类型的null值
     */
    public static class NullObjectJsonSerializer extends JsonSerializer<Object> {
        @Override
        public void serialize(Object value, JsonGenerator jsonGenerator, 
                              SerializerProvider serializerProvider) throws IOException {
            jsonGenerator.writeStartObject();
            jsonGenerator.writeEndObject();
        }
    }

}

3. 序列化程序修改器*

import com.fasterxml.jackson.databind.BeanDescription;
import com.fasterxml.jackson.databind.SerializationConfig;
import com.fasterxml.jackson.databind.ser.BeanPropertyWriter;
import com.fasterxml.jackson.databind.ser.BeanSerializerModifier;

import java.util.Collection;
import java.util.List;

/**
 * <pre>
 * 此modifier主要做的事情为：
 * 1.当序列化类型为数组集合时，当值为null时，序列化成[]
 * 2.String类型值序列化为""
 *
 * </pre>
 */
public class MyBeanSerializerModifier extends BeanSerializerModifier {

    @Override
    public List<BeanPropertyWriter> changeProperties(SerializationConfig config, 
                                                     BeanDescription beanDesc,
                                                     List<BeanPropertyWriter> beanProperties) {
        // 循环所有的beanPropertyWriter
        for (int i = 0; i < beanProperties.size(); i++) {
            BeanPropertyWriter writer = beanProperties.get(i);
            // 判断字段的类型，如果是数组或集合则注册nullSerializer
            if (isArrayType(writer)) {
                // 给writer注册一个自己的nullSerializer   
                writer.assignNullSerializer(new CustomizeNullJsonSerializer.NullArrayJsonSerializer());
            }
            if (isStringType(writer)) {
                writer.assignNullSerializer(new CustomizeNullJsonSerializer.NullStringJsonSerializer());
            }
        }
        return beanProperties;
    }

    /**
     * 是否是数组
     */
    private boolean isArrayType(BeanPropertyWriter writer) {
        Class<?> clazz = writer.getType().getRawClass();
        return clazz.isArray() || Collection.class.isAssignableFrom(clazz);
    }

    /**
     * 是否是String
     */
    private boolean isStringType(BeanPropertyWriter writer) {
        Class<?> clazz = writer.getType().getRawClass();
        return CharSequence.class.isAssignableFrom(clazz) || Character.class.isAssignableFrom(clazz);
    }

    /**
     * 是否是数值类型
     */
    private boolean isNumberType(BeanPropertyWriter writer) {
        Class<?> clazz = writer.getType().getRawClass();
        return Number.class.isAssignableFrom(clazz);
    }

    /**
     * 是否是boolean
     */
    private boolean isBooleanType(BeanPropertyWriter writer) {
        Class<?> clazz = writer.getType().getRawClass();
        return clazz.equals(Boolean.class);
    }

}

3. 配置Jackson实体

import com.fasterxml.jackson.databind.ObjectMapper;
import com.fasterxml.jackson.databind.SerializerProvider;
import org.springframework.boot.autoconfigure.condition.ConditionalOnMissingBean;
import org.springframework.context.annotation.Bean;
import org.springframework.context.annotation.Configuration;
import org.springframework.context.annotation.Primary;
import org.springframework.http.converter.json.Jackson2ObjectMapperBuilder;

/**
 * 配置Jackson实体
 */
@Configuration
public class JacksonConfig {
    @Bean
    @Primary
    @ConditionalOnMissingBean(ObjectMapper.class)
    public ObjectMapper jacksonObjectMapper(Jackson2ObjectMapperBuilder builder) {
        ObjectMapper objectMapper = builder.createXmlMapper(false).build();
        /** 为objectMapper注册一个带有SerializerModifier的Factory */
        objectMapper.setSerializerFactory(objectMapper.getSerializerFactory()
                .withSerializerModifier(new MyBeanSerializerModifier()));

        SerializerProvider serializerProvider = objectMapper.getSerializerProvider();
        serializerProvider.setNullValueSerializer(new CustomizeNullJsonSerializer
        										.NullObjectJsonSerializer());
        return objectMapper;
    }
}