已注册JodaModule仍无法序列化org.joda.time.Days的问题求助

环境依赖 #

使用Joda Time 2.10.14与Jackson2 2.18.3，依赖如下：

jackson-core-2.18.3.jar
jackson-databind-2.18.3.jar
jackson-datatype-joda-2.18.3.jar
joda-time-2.10.14.jar

复现代码 #

已注册JodaModule但仍抛出序列化异常，代码示例：

public static void main(String[] args) throws JsonProcessingException {
    DateTime dateTime = new DateTime();
    Days days = Days.days(1);
    Map<String, Object> map = new HashMap<>();
    map.put("dateTime", dateTime);
    map.put("days", days);
    ObjectMapper objectMapper = new ObjectMapper();
    objectMapper.registerModule(new JodaModule());
    String json = objectMapper.writeValueAsString(map);
    System.out.println(json);
}

异常信息 #

Exception in thread "main" com.fasterxml.jackson.databind.exc.InvalidDefinitionException: Joda date/time type `org.joda.time.Days` not supported by default: add Module "com.fasterxml.jackson.datatype:jackson-datatype-joda" to enable handling (through reference chain: java.util.HashMap["days"])
at com.fasterxml.jackson.databind.exc.InvalidDefinitionException.from(InvalidDefinitionException.java:77)
at com.fasterxml.jackson.databind.SerializerProvider.reportBadDefinition(SerializerProvider.java:1340)
at com.fasterxml.jackson.databind.ser.impl.UnsupportedTypeSerializer.serialize(UnsupportedTypeSerializer.java:35)
at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeFields(MapSerializer.java:808)
at com.fasterxml.jackson.databind.ser.std.MapSerializer.serializeWithoutTypeInfo(MapSerializer.java:764)
at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:720)
at com.fasterxml.jackson.databind.ser.std.MapSerializer.serialize(MapSerializer.java:35)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider._serialize(DefaultSerializerProvider.java:502)
at com.fasterxml.jackson.databind.ser.DefaultSerializerProvider.serializeValue(DefaultSerializerProvider.java:341)
at com.fasterxml.jackson.databind.ObjectMapper._writeValueAndClose(ObjectMapper.java:4819)
at com.fasterxml.jackson.databind.ObjectMapper.writeValueAsString(ObjectMapper.java:4060)
at azkaban.utils.JSONUtils.main(JSONUtils.java:96)

经查看JodaModule源码，确认其未内置Days类型的序列化器，可通过以下方式解决：

方案1：自定义序列化/反序列化器 #

针对Days类型编写专属的序列化和反序列化逻辑，注册到ObjectMapper：

// 序列化器：将Days转为整数
public class DaysSerializer extends JsonSerializer<Days> {
    @Override
    public void serialize(Days days, JsonGenerator gen, SerializerProvider serializers) throws IOException {
        gen.writeNumber(days.getDays());
    }
}

// 反序列化器：从整数还原为Days
public class DaysDeserializer extends JsonDeserializer<Days> {
    @Override
    public Days deserialize(JsonParser p, DeserializationContext ctxt) throws IOException {
        return Days.days(p.getIntValue());
    }
}

注册到ObjectMapper：

ObjectMapper objectMapper = new ObjectMapper();
objectMapper.registerModule(new JodaModule());

SimpleModule daysModule = new SimpleModule();
daysModule.addSerializer(Days.class, new DaysSerializer());
daysModule.addDeserializer(Days.class, new DaysDeserializer());
objectMapper.registerModule(daysModule);

方案2：手动转换为基础类型（仅序列化场景） #

如果只需要序列化，无需反序列化，可在存入Map前直接将Days转为整数：

map.put("days", days.getDays());

方案3：使用Jackson Mixin特性 #

通过Mixin为Days类添加序列化注解，指定用getDays()方法的返回值作为序列化结果：

public abstract class DaysMixin {
    @JsonValue
    public abstract int getDays();
}

注册Mixin到ObjectMapper：

ObjectMapper objectMapper = new ObjectMapper();
objectMapper.registerModule(new JodaModule());
objectMapper.addMixIn(Days.class, DaysMixin.class);

内容的提问来源于stack exchange，提问作者hehe