Retrofit反序列化JSON报错：缺失类鉴别器的多态序列化器未找到

异常信息 #

kotlinx.serialization.json.internal.JsonDecodingException: Polymorphic serializer was not found for missing class discriminator ('null')

接口返回JSON #

{"CustID":101,"DeviceID":81}

相关代码实现 #

数据类 #

@SerialName("NewCustVals")
@Serializable
data class NewCustVals (
    @SerialName("CustID")
    var CustID: Int? = 0,
    @SerialName("DeviceID")
    var DeviceID: Int? = 0
)

Retrofit服务接口 #

interface LTECustomerInterface {
    companion object {
        private var TAG :String = "LTECustomerInterface"
        var BASE_URL = "http://XXXXXXXXXXX:XXXX/"  //test server
        lateinit var callbackApi :ApiCallback

        @OptIn(ExperimentalSerializationApi::class)
        fun create(cb:ApiCallback): LTECustomerInterface {

            callbackApi = cb

            val retrofit = Retrofit.Builder()
                .baseUrl(BASE_URL)
                .addConverterFactory(Json{ignoreUnknownKeys = true; isLenient = true}.asConverterFactory( "application/json".toMediaType()))
             //   .client(client)
                .build()

            return retrofit.create(LTECustomerInterface::class.java)
        }
    }

    @POST("/newCustomer")
    suspend fun newCustomer(@Body data : Customer ) :Call<NewCustVals>
}

请求调用代码 #

fun sendCustomer() {
    viewModelScope.launch(Dispatchers.IO) {
       val call = serverCustomerHttpInterface.newCustomer(_currentCust.value)

           call.enqueue(object : retrofit2.Callback<NewCustVals>{
             override fun onResponse(call: Call<NewCustVals>, response: Response<NewCustVals>) {
               Log.i(TAG,"NewCustomer onResponse called")
             }

             override fun onFailure(call: Call<NewCustVals>, t: Throwable) {
               Log.i("sendCustomer()","NewCustomer onFailure")
             }
          })
    }
}

排查思路 #

• 修正suspend函数返回值：Retrofit中标记为suspend的接口方法，不能返回Call<T>，需直接返回NewCustVals或Response<NewCustVals>；若要使用enqueue回调，需去掉suspend关键字，保持返回Call<NewCustVals>。当前写法会导致序列化逻辑混乱，触发非预期的多态检查。示例修改：

  @POST("/newCustomer")
  suspend fun newCustomer(@Body data : Customer ) : NewCustVals
  

• 检查Customer类序列化配置：作为@Body参数的Customer类必须添加@Serializable注解，且内部无多态类型字段。若该类涉及继承，需正确配置kotlinx序列化多态规则，但根据描述应为简单数据类，先确认注解是否完整。
• 移除类级别的@SerialName("NewCustVals")：该注解多用于多态场景标记类的鉴别器值，你的数据类直接对应返回JSON结构，无需此注解，移除可避免序列化器误判为多态类型。
• 调整JSON转换器配置：可显式补充序列化配置，避免默认多态逻辑干扰，示例：

  Json {
      ignoreUnknownKeys = true
      isLenient = true
      explicitNulls = true
  }
  

内容的提问来源于stack exchange，提问作者userCB