KMM项目中Ktor解析API JSON响应触发NoTransformationFoundException

问题描述 #

在KMM项目中使用Ktor框架解析API返回的JSON响应时，总是触发io.ktor.client.call.NoTransformationFoundException异常，但将响应转为String类型时可正常工作。

请求方法代码 #

suspend fun getSkoolURL22(code:String, token:String): GetSchoolResponse {
     
    return clientv.submitForm(
            url = "$CONST_SCHOOL_VERIFICATION_URL.",
            formParameters = Parameters.build {
                append("token", token)
                append("skool_code", code)
            }).body() }

HttpClient配置 #

val clientv = HttpClient() {
    install(Logging) {
        logger = Logger.SIMPLE
        level = LogLevel.ALL  
    }
    install(ContentNegotiation) {
        json(Json {
            prettyPrint = true
            isLenient = true
            ignoreUnknownKeys = true
        })
    }
}

数据类定义 #

@Serializable
data class GetSchoolResponse(
    @Serializable
    val code: Int,
    @Serializable
    val data: List<Data>,
    @Serializable
    val message: String) {
    @Serializable
    data class Data(
        @Serializable
        val domain: String,
        @Serializable
        val id: Int,
        @Serializable
        val ip: String,
        @Serializable
        val sk_code: Int,
        @Serializable
        val sk_name: String)
}

注：内部嵌套的Data类无需重复添加@Serializable注解，外层类的注解已可覆盖嵌套类

异常信息 #

io.ktor.client.call.NoTransformationFoundException: No transformation found: class io.ktor.utils.io.ByteBufferChannel (Kotlin reflection is not available) -> class com.example.skoolkmm.data.GetSchoolResponse (Kotlin reflection is not available)

 status: 200 OK
    response headers: 
    content-type: text/html; charset=UTF-8
    , date: Sun, 11 Jun 2023 21:21:11 GMT
    , server: Apache
    , transfer-encoding: chunked
    , vary: Accept-Encoding
    , x-iplb-instance: 38231
    , x-iplb-request-id: C4B34E62:51E4_36245B3E:0050_64863AC7_0DE2:25792
    , x-powered-by: PHP/7.0

服务器响应内容 #

2023-06-11 22:21:10.229 19455-19511/com.example.skoolkmm.android I/System.out: -> content-type: text/html; charset=UTF-8
2023-06-11 22:21:10.229 19455-19511/com.example.skoolkmm.android I/System.out: -> date: Sun, 11 Jun 2023 21:21:11 GMT
2023-06-11 22:21:10.229 19455-19511/com.example.skoolkmm.android I/System.out: -> server: Apache
2023-06-11 22:21:10.229 19455-19511/com.example.skoolkmm.android I/System.out: -> transfer-encoding: chunked
2023-06-11 22:21:10.229 19455-19511/com.example.skoolkmm.android I/System.out: -> vary: Accept-Encoding
2023-06-11 22:21:10.229 19455-19511/com.example.skoolkmm.android I/System.out: -> x-iplb-instance: 38231
2023-06-11 22:21:10.229 19455-19511/com.example.skoolkmm.android I/System.out: -> x-iplb-request-id: C4B34E62:51E4_36245B3E:0050_64863AC7_0DE2:25792
2023-06-11 22:21:10.229 19455-19511/com.example.skoolkmm.android I/System.out: -> x-powered-by: PHP/7.0
2023-06-11 22:21:10.229 19455-19511/com.example.skoolkmm.android I/System.out: BODY Content-Type: text/html; charset=UTF-8
2023-06-11 22:21:10.229 19455-19511/com.example.skoolkmm.android I/System.out: BODY START
2023-06-11 22:21:10.230 19455-19511/com.example.skoolkmm.android I/System.out: {"code":200,"message":"success","data":[{"id":1,"domain":"https://dev.vyzyoschool.com/","ip":"197.13.17.67","sk_code":111111,"sk_name":"VYZYO"}]}
2023-06-11 22:21:10.230 19455-19511/com.example.skoolkmm.android I/System.out: BODY END

问题原因 #

从响应头可以看到，服务器返回的content-type是text/html，而Ktor的ContentNegotiation默认仅对application/json类型的响应进行JSON解析。即使响应内容是标准JSON，因为Content-Type不匹配，Ktor无法找到对应的转换器，从而抛出NoTransformationFoundException。

解决方案 #

方案1：修改ContentNegotiation配置，支持text/html类型解析 #

在安装ContentNegotiation时，为JSON转换器指定额外的ContentType为text/html：

val clientv = HttpClient() {
    install(Logging) {
        logger = Logger.SIMPLE
        level = LogLevel.ALL  
    }
    install(ContentNegotiation) {
        json(Json {
            prettyPrint = true
            isLenient = true
            ignoreUnknownKeys = true
        }, contentType = ContentType.Text.Html) // 添加此行，支持text/html类型的JSON解析
    }
}

方案2：请求时手动指定解析类型 #

在调用body()方法时，明确指定使用JSON解析并适配text/html类型：

suspend fun getSkoolURL22(code:String, token:String): GetSchoolResponse {
    return clientv.submitForm(
            url = "$CONST_SCHOOL_VERIFICATION_URL.",
            formParameters = Parameters.build {
                append("token", token)
                append("skool_code", code)
            }).body<GetSchoolResponse>(JsonReceiveContent(ContentType.Text.Html))
}

方案3（可选）：优化数据类注解 #

内部嵌套的Data类无需重复添加@Serializable注解，外层GetSchoolResponse的注解已可覆盖嵌套类，简化后的数据类如下：

@Serializable
data class GetSchoolResponse(
    val code: Int,
    val data: List<Data>,
    val message: String) {
    data class Data(
        val domain: String,
        val id: Int,
        val ip: String,
        val sk_code: Int,
        val sk_name: String)
}

内容的提问来源于stack exchange，提问作者Big Boss