Using Kotlin to convert a map collection to a JSON array using Jackson

Question

Using Kotlin to convert a map collection to a JSON array using Jackson

Recently started learning Kotlin, I'm attempting to configure a web server using the kotlin spring webserver template

I need assistance with converting a kotlin collection into an array of JSON objects

The endpoint is called test

@PostMapping(value = [ "/test"],produces = [ "application/json" ],headers = [ "Content-Type=application/json" ])
    fun test(@RequestBody request:IOUDATA): ResponseEntity<String>{

      //I want this as a JSON array of objects
      val numbersMap = mapOf("key1" to 1, "key2" to 2, "key3" to 3, "key4" to 1)

      return ResponseEntity.status(HttpStatus.CREATED).body("${numbersMap}");

}

Desired output

 {
   result:[
     {"key1":1},{"key2":2}...
    ]
   }

json spring-mvc kotlin jackson

Answer 1

Answer №1

The problem lies in your controller's return type being specified as a String, leading Jackson to not handle the serialization. Instead, it resorts to using Kotlin's default toString method through string interpolation.

To address this issue and have Jackson serialize the object correctly, you can opt for one of two approaches:

Modify the controller to return a ResponseEntity<String> and manually serialize the object to form a String:

@RestController
class MyController(private val objectMapper: ObjectMapper) {

    @PostMapping(value = ["/test"], produces = ["application/json"], headers = ["Content-Type=application/json"])
    fun test(@RequestBody request: MyInputData): ResponseEntity<String> {

        val numbersMap = mapOf(
            "result" to listOf(
                mapOf("key1" to 1),
                mapOf("key2" to 2),
                mapOf("key3" to 3),
                mapOf("key4" to 1)
            )
        )
        val mySerialisedString = objectMapper.writeValueAsString(numbersMap)

        return ResponseEntity.status(HttpStatus.CREATED).body(mySerialisedString) // Automatic serialization by Jackson

    }
}

Remember that in this scenario, you'll need to acquire an instance of ObjectMapper (autowired by Spring).

Create a custom result object that Jackson will automatically serialize (leveraged by Spring behind the scenes):

@RestController
class MyController {

    @PostMapping(value = ["/test"], produces = ["application/json"], headers = ["Content-Type=application/json"])
    fun test(@RequestBody request: MyInputData): ResponseEntity<MyOutputData> {

        val numbersMap = listOf(mapOf("key1" to 1), mapOf("key2" to 2), mapOf("key3" to 3), mapOf("key4" to 1))

        val myOutputData = MyOutputData(numbersMap)
        return ResponseEntity.status(HttpStatus.CREATED).body(myOutputData) // Automatic serialization by Jackson

    }
}

class MyOutputData(val result: List<Map<String, Int>>)

Both methods will generate the same JSON response upon invocation:

{
    "result": [
    {
        "key1": 1
    },
    {
        "key2": 2
    },
    {
        "key3": 3
    },
    {
        "key4": 1
    }
    ]
}

Answer 2

The problem lies in your controller's return type being specified as a String, leading Jackson to not handle the serialization. Instead, it resorts to using Kotlin's default toString method through string interpolation.

To address this issue and have Jackson serialize the object correctly, you can opt for one of two approaches:

Modify the controller to return a ResponseEntity<String> and manually serialize the object to form a String:

@RestController
class MyController(private val objectMapper: ObjectMapper) {

    @PostMapping(value = ["/test"], produces = ["application/json"], headers = ["Content-Type=application/json"])
    fun test(@RequestBody request: MyInputData): ResponseEntity<String> {

        val numbersMap = mapOf(
            "result" to listOf(
                mapOf("key1" to 1),
                mapOf("key2" to 2),
                mapOf("key3" to 3),
                mapOf("key4" to 1)
            )
        )
        val mySerialisedString = objectMapper.writeValueAsString(numbersMap)

        return ResponseEntity.status(HttpStatus.CREATED).body(mySerialisedString) // Automatic serialization by Jackson

    }
}

Remember that in this scenario, you'll need to acquire an instance of ObjectMapper (autowired by Spring).

Create a custom result object that Jackson will automatically serialize (leveraged by Spring behind the scenes):

@RestController
class MyController {

    @PostMapping(value = ["/test"], produces = ["application/json"], headers = ["Content-Type=application/json"])
    fun test(@RequestBody request: MyInputData): ResponseEntity<MyOutputData> {

        val numbersMap = listOf(mapOf("key1" to 1), mapOf("key2" to 2), mapOf("key3" to 3), mapOf("key4" to 1))

        val myOutputData = MyOutputData(numbersMap)
        return ResponseEntity.status(HttpStatus.CREATED).body(myOutputData) // Automatic serialization by Jackson

    }
}

class MyOutputData(val result: List<Map<String, Int>>)

Both methods will generate the same JSON response upon invocation:

{
    "result": [
    {
        "key1": 1
    },
    {
        "key2": 2
    },
    {
        "key3": 3
    },
    {
        "key4": 1
    }
    ]
}

Using Kotlin to convert a map collection to a JSON array using Jackson

Answer №1

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