使用GSON和Retrofit解析API响应时所有属性值均为null

我的App实体类 #

class App(
    var id: Int,
    var name: String,
    var description: String,
    var url: String,
    var packageName: String,
    var appVersion: String,
    var iconMediaUUID: String?,
    var created: String,
    var createdBy: String
) : Serializable

补充说明 #

我也尝试过使用@SerializedName注解，但没有效果。

Retrofit API调用 #

@GET("/api/app-list")
suspend fun getApplications(): Response<List<App>>

API返回的JSON数据 #

[
    {
        "id": 1,
        "name": "facebook",
        "description": "face book",
        "url": "https://play.google.com/store/apps/details?id=com.facebook.katana",
        "packageName": "com.facebook.katana",
        "appVersion": "1",
        "iconMediaUUID": null,
        "created": "2023-09-05 09:20",
        "createdBy": "ava@mail.hu"
    },
    {
        "id": 3,
        "name": "twitter",
        "description": "X",
        "url": "https://play.google.com/store/apps/details?id=com.twitter.android",
        "packageName": "com.twitter.android",
        "appVersion": "43",
        "iconMediaUUID": "e2913bc1-4963-4a3e-85bf-8244b5b42979",
        "created": "2023-09-05 10:11",
        "createdBy": "ava@mail.hu"
    }
]

Retrofit实例 #

private fun getBackendClient(): Retrofit =
        Retrofit.Builder()
            .client(okHttpClient)
            .baseUrl(BACKEND_BASE_URL)
            .addConverterFactory(ScalarsConverterFactory.create())
            .addConverterFactory(GsonConverterFactory.create())
            .build()

fun callBackend(): Endpoints {
    return getBackendClient().create(Endpoints::class.java)
}

日志打印代码 #

val response: Response<List<App>> = API.callBackend().getApplications()

if (response.isSuccessful) {
    val appList = response.body()
    if (appList != null) {
        Log.i("PARSED_", "size of list: "+appList.size)
        for (app in appList) {
            Log.i("PARSED_", "id: " + app.id + ", name: " + app.name)
        }                        
    } 
}

日志输出 #

PARSED_  size of list: 2
PARSED_  id: null, name: null
PARSED_  id: null, name: null

核心问题 #

解析得到的实体实例所有属性值均为null，为什么无法正常解析？

解决方案 #

1. 将实体类改为data class或添加无参构造函数 #

Gson默认需要类具备无参构造函数才能完成反射赋值，Kotlin普通类默认没有无参构造，而data class会自动生成适配Gson的构造逻辑：

data class App(
    var id: Int,
    var name: String,
    var description: String,
    var url: String,
    var packageName: String,
    var appVersion: String,
    var iconMediaUUID: String?,
    var created: String,
    var createdBy: String
) : Serializable

如果不想用data class，可以手动添加无参构造并给属性设默认值：

class App : Serializable {
    var id: Int = 0
    var name: String = ""
    var description: String = ""
    var url: String = ""
    var packageName: String = ""
    var appVersion: String = ""
    var iconMediaUUID: String? = null
    var created: String = ""
    var createdBy: String = ""

    constructor()
    constructor(
        id: Int,
        name: String,
        description: String,
        url: String,
        packageName: String,
        appVersion: String,
        iconMediaUUID: String?,
        created: String,
        createdBy: String
    ) {
        this.id = id
        this.name = name
        // 其他属性赋值
    }
}

2. 调整ConverterFactory的顺序 #

你当前先添加了ScalarsConverterFactory，它会优先处理文本类型数据，导致Gson转换器没机会解析JSON数组。把GsonConverterFactory放在前面：

private fun getBackendClient(): Retrofit =
        Retrofit.Builder()
            .client(okHttpClient)
            .baseUrl(BACKEND_BASE_URL)
            .addConverterFactory(GsonConverterFactory.create())
            .addConverterFactory(ScalarsConverterFactory.create())
            .build()

3. 确保Gson支持Kotlin类型 #

如果前两步无效，检查是否添加了gson-kotlin依赖，让Gson能正确识别Kotlin属性：

implementation 'com.google.code.gson:gson:2.10.1'
implementation 'com.google.gson:gson-kotlin:2.10.1'

或者手动创建带Kotlin适配器的Gson实例：

val gson = GsonBuilder()
    .registerTypeAdapterFactory(KotlinAdapterFactory())
    .create()

// 替换Retrofit中的GsonConverterFactory
.addConverterFactory(GsonConverterFactory.create(gson))

内容的提问来源于stack exchange，提问作者Adam Varhegyi