SpringBoot 日期类OffsetDateTime序列化问题分析解决

在使用Redis时，常用的值序列化器为GenericJackson2JsonRedisSerializer，但是该序列化器默认不支持Java8的日期相关类（java.time.*）；
测试代码如下：

public class Demo {

    public static void main(String[] args) {
        GenericJackson2JsonRedisSerializer serializer = new GenericJackson2JsonRedisSerializer();

        OffsetDateTime now = OffsetDateTime.now();
        System.out.println(now);
        byte[] serialize = serializer.serialize(now);
        OffsetDateTime deserialize = serializer.deserialize(serialize, OffsetDateTime.class);
        System.out.println(deserialize);
    }
}

代码运行过程中，会发生序列化错误，错误简要信息如下：

Exception in thread "main" org.springframework.data.redis.serializer.SerializationException: Could not write JSON: Java 8 date/time type `java.time.OffsetDateTime` not supported by default: add Module "com.fasterxml.jackson.datatype:jackson-datatype-jsr310" to enable handling;

根据提示信息，加入jackson-datatype-jsr310依赖，如下：

    com.fasterxml.jackson.datatype
    jackson-datatype-jsr310

重新运行后，报错依然；
除了需要引入jackson-datatype-jsr310依赖外，还需要增加JavaTimeModule,代码如下：

public class Demo {

    public static void main(String[] args) {
        ObjectMapper objectMapper = new ObjectMapper();
        objectMapper.registerModule(new JavaTimeModule());
        GenericJackson2JsonRedisSerializer serializer = new GenericJackson2JsonRedisSerializer(objectMapper);

        OffsetDateTime now = OffsetDateTime.now();
        System.out.println(now);
        byte[] serialize = serializer.serialize(now);
        OffsetDateTime deserialize = serializer.deserialize(serialize, OffsetDateTime.class);
        System.out.println(deserialize);
    }
}

重新运行后，序列化报错问题解决了，但是发现，两次打印信息不一致，如下：

2022-04-20T20:57:09.910+08:00
2022-04-20T12:57:09.910Z

可以看出，经过序列化和反序列化后，时区信息丢失；
那么，此处就需要分析两个问题：

1. 日期类如何序列化？
2. 时区信息如何丢失？

代码中，使用JavaTimeModule后，序列化器就支持日期类的序列化，那么直接分析JavaTimeModule做了啥，核心代码如下：

public JavaTimeModule()
{
    super(PackageVersion.VERSION);

    // First deserializers

    // // Instant variants:
    addDeserializer(Instant.class, InstantDeserializer.INSTANT);
    addDeserializer(OffsetDateTime.class, InstantDeserializer.OFFSET_DATE_TIME);
    addDeserializer(ZonedDateTime.class, InstantDeserializer.ZONED_DATE_TIME);

    // // Other deserializers
    addDeserializer(Duration.class, DurationDeserializer.INSTANCE);
    addDeserializer(LocalDateTime.class, LocalDateTimeDeserializer.INSTANCE);
    addDeserializer(LocalDate.class, LocalDateDeserializer.INSTANCE);
    addDeserializer(LocalTime.class, LocalTimeDeserializer.INSTANCE);
    addDeserializer(MonthDay.class, MonthDayDeserializer.INSTANCE);
    addDeserializer(OffsetTime.class, OffsetTimeDeserializer.INSTANCE);
    addDeserializer(Period.class, JSR310StringParsableDeserializer.PERIOD);
    addDeserializer(Year.class, YearDeserializer.INSTANCE);
    addDeserializer(YearMonth.class, YearMonthDeserializer.INSTANCE);
    addDeserializer(ZoneId.class, JSR310StringParsableDeserializer.ZONE_ID);
    addDeserializer(ZoneOffset.class, JSR310StringParsableDeserializer.ZONE_OFFSET);

    // then serializers:
    addSerializer(Duration.class, DurationSerializer.INSTANCE);
    addSerializer(Instant.class, InstantSerializer.INSTANCE);
    addSerializer(LocalDateTime.class, LocalDateTimeSerializer.INSTANCE);
    addSerializer(LocalDate.class, LocalDateSerializer.INSTANCE);
    addSerializer(LocalTime.class, LocalTimeSerializer.INSTANCE);
    addSerializer(MonthDay.class, MonthDaySerializer.INSTANCE);
    addSerializer(OffsetDateTime.class, OffsetDateTimeSerializer.INSTANCE);
    addSerializer(OffsetTime.class, OffsetTimeSerializer.INSTANCE);
    addSerializer(Period.class, new ToStringSerializer(Period.class));
    addSerializer(Year.class, YearSerializer.INSTANCE);
    addSerializer(YearMonth.class, YearMonthSerializer.INSTANCE);

    addSerializer(ZonedDateTime.class, ZonedDateTimeSerializer.INSTANCE);
    
    addSerializer(ZoneId.class, new ZoneIdSerializer());
    addSerializer(ZoneOffset.class, new ToStringSerializer(ZoneOffset.class));

    // key serializers
    addKeySerializer(ZonedDateTime.class, ZonedDateTimeKeySerializer.INSTANCE);

    // key deserializers
    addKeyDeserializer(Duration.class, DurationKeyDeserializer.INSTANCE);
    addKeyDeserializer(Instant.class, InstantKeyDeserializer.INSTANCE);
    addKeyDeserializer(LocalDateTime.class, LocalDateTimeKeyDeserializer.INSTANCE);
    addKeyDeserializer(LocalDate.class, LocalDateKeyDeserializer.INSTANCE);
    addKeyDeserializer(LocalTime.class, LocalTimeKeyDeserializer.INSTANCE);
    addKeyDeserializer(MonthDay.class, MonthDayKeyDeserializer.INSTANCE);
    addKeyDeserializer(OffsetDateTime.class, OffsetDateTimeKeyDeserializer.INSTANCE);
    addKeyDeserializer(OffsetTime.class, OffsetTimeKeyDeserializer.INSTANCE);
    addKeyDeserializer(Period.class, PeriodKeyDeserializer.INSTANCE);
    addKeyDeserializer(Year.class, YearKeyDeserializer.INSTANCE);
    addKeyDeserializer(YearMonth.class, YearMonthKeyDeserializer.INSTANCE);
    addKeyDeserializer(ZonedDateTime.class, ZonedDateTimeKeyDeserializer.INSTANCE);
    addKeyDeserializer(ZoneId.class, ZoneIdKeyDeserializer.INSTANCE);
    addKeyDeserializer(ZoneOffset.class, ZoneOffsetKeyDeserializer.INSTANCE);
}

好家伙，简单直接地引入了Java8的日期相关类（java.time.*）的序列化器、反序列化器；其中为OffsetDateTime引入了OffsetDateTimeSerializer和OffsetDateTimeKeyDeserializer；
解析来就分析serializer.serialize(now)如何找到OffsetDateTimeSerializer的？

GenericJackson2JsonRedisSerializer.serialize代码如下：

public byte[] serialize(@Nullable Object source) throws SerializationException {

    if (source == null) {
        return SerializationUtils.EMPTY_ARRAY;
    }

    try {
        // 使用内部的ObjectMapper实例完成序列化
        return mapper.writeValueAsBytes(source);
    } catch (JsonProcessingException e) {
        throw new SerializationException("Could not write JSON: " + e.getMessage(), e);
    }
}

继续跟踪，会进入SerializerProvider.findTypedValueSerializer方法，代码如下：

public JsonSerializer