Jackson在Kotlin下全局自定义序列化器Long转String不生效

Mashiro

2023 年 07 月 23 日

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# Jackson在Kotlin下全局自定义序列化器Long转String不生效

## 起因

雪花算法生成的主键传到前段后发生精度丢失，例如后端传入`1397844263642378242`，前端接收后变成`1397844263642378000`

因为Number的精度是16位，而雪花ID是19位

找了网上很多Java下配自定义序列化器的，改成Kotlin语法，如下

```kotlin
@Configuration
class JacksonConfig {
    @Bean
    fun jacksonObjectMapper(builder: Jackson2ObjectMapperBuilder): ObjectMapper {
        val objectMapper: ObjectMapper = builder.createXmlMapper(false).build()
        val simpleModule = SimpleModule()
            .addSerializer(Long::class.java, ToStringSerializer.instance)
            .addSerializer(java.lang.Long.TYPE, ToStringSerializer.instance)
        objectMapper.registerModule(simpleModule)
        return objectMapper
    }
}
```

在Kotlin下怎么试都不生效，但是在字段上指定就可以

```java
@JsonSerialize(using=ToStringSerializer.class)
val id: Long?;
```

## 解决

中文互联网找了几遍没找到，去SO一下就找到答案了

[How to use springboot + kotlin + jackson to globally set a custom Long type serializer?](https://stackoverflow.com/questions/76259685/how-to-use-springboot-kotlin-jackson-to-globally-set-a-custom-long-type-seri)

答主是这样解释的：The issue with your code is that the `val Long` and `val Long?` from Kotlin are treated differently. The first will become a `long` whereas the second will become a `Long` (as it can be `null`).

但是这个回答提供的构造Bean的方式我不是很喜欢，因此我又找到了另一个帖子

[Custom json serialization java primitives from kotlin by Jackson](https://stackoverflow.com/questions/56428069/custom-json-serialization-java-primitives-from-kotlin-by-jackson)

最后的解决方案：

```kotlin
@Configuration
class JacksonConfig {
    @Bean
    fun jacksonObjectMapper(builder: Jackson2ObjectMapperBuilder): ObjectMapper {
        val objectMapper: ObjectMapper = builder.createXmlMapper(false).build()
        val simpleModule = SimpleModule()
            .addSerializer(Long::class.javaPrimitiveType, ToStringSerializer.instance)
            .addSerializer(Long::class.javaObjectType, ToStringSerializer.instance)
        objectMapper.registerModule(simpleModule)
        return objectMapper
    }
}
```