How to use springboot + kotlin + jackson to globally set a custom Long type serializer?

Question

Related code for springboot jackson settings.

@Configuration
class JacksonConfig {

    @Bean
    fun jackson2ObjectMapperBuilderCustomizer(): Jackson2ObjectMapperBuilderCustomizer =
        Jackson2ObjectMapperBuilderCustomizer { builder ->
            builder.serializerByType(Long::class.java, ToStringSerializer.instance)
        }

}

But this custom serializer only works partially.

@RestController
class Controller {

    @GetMapping("/1")
    fun hello(): Person{
        return Person(116391232976064512L)
    }

    @GetMapping("/2")
    fun hello2(): Student{
        return Student(116391232976064512L)
    }

    @GetMapping("/3")
    fun hello3(): Product{
        return Product(116391232976064512L)
    }

}

data class Person(
    val id: Long
)

data class Student(
    val id: Long? = null
)

I created java object again to test.

public class Product {

    private Long id;

    public Product(Long id) {
        this.id = id;
    }

    public Long getId() {
        return id;
    }

    public void setId(Long id) {
        this.id = id;
    }
}

only the /1 api successfully converts Long to String type, and the other two api do not.

Am I setting the global serializer wrong?

output:

// get http://localhost:8080/1
{"id":"116391232976064512"}

// get http://localhost:8080/2
{"id":116391232976064512}

// get http://localhost:8080/3
{"id":116391232976064512}

If you look closely, you will see that only api/1 returns String type as expected.

I am using springboot built-in jackson support:

<parent>
    <groupId>org.springframework.boot</groupId>
    <artifactId>spring-boot-starter-parent</artifactId>
    <version>3.0.6</version>
    <relativePath/> <!-- lookup parent from repository -->
  </parent>

<dependency>
      <groupId>com.fasterxml.jackson.module</groupId>
      <artifactId>jackson-module-kotlin</artifactId>
    </dependency>

result

With the help of @M.Deinum I found the problem, there was something wrong with the way kotlin was set up. reference

The correct settings are as follows:

@Configuration
class JacksonConfig {

    @Bean
    fun jackson2ObjectMapperBuilderCustomizer(): Jackson2ObjectMapperBuilderCustomizer =
        Jackson2ObjectMapperBuilderCustomizer { builder ->
            builder.serializerByType(Long::class.java, ToStringSerializer.instance)
            builder.serializerByType(Long::class.javaObjectType, ToStringSerializer.instance)
        }

}

Answer 1

If you want to write numbers as string in JSON then configure Jackson using the JsonWriteFeature.WRITE_NUMBERS_AS_STRINGS and set the property to true.

@Bean
fun jackson2ObjectMapperBuilderCustomizer(): Jackson2ObjectMapperBuilderCustomizer =
  Jackson2ObjectMapperBuilderCustomizer { builder ->
   builder.featuresToEnable(JsonWriteFeature.WRITE_NUMBERS_AS_STRINGS)
}

Or specify it in your application.properties, then you don't need the customizer.

spring.jackson.generator.WRITE_NUMBERS_AS_STRINGS=true

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). The serializer you registered applies to the first but not the second, for which I suspect the standard Jackson serializer will now kick in.