Android R8混淆致Gson无法实例化NetworkPagingList问题排查

报错信息 #

java.lang.RuntimeException: Unable to invoke no-args constructor for class com.arch.example.network.models.NetworkPagingList. Registering an InstanceCreator with Gson for this type may fix this problem.
                    at com.google.gson.internal.ConstructorConstructor$14.construct(ConstructorConstructor.java:228)
                    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:212)
                    at retrofit2.converter.gson.GsonResponseBodyConverter.convert(GsonResponseBodyConverter.java:40)
                    at retrofit2.converter.gson.GsonResponseBodyConverter.convert(GsonResponseBodyConverter.java:27)
                    at retrofit2.OkHttpCall.parseResponse(OkHttpCall.java:243)
                    at retrofit2.OkHttpCall$1.onResponse(OkHttpCall.java:153)
                    at okhttp3.internal.connection.RealCall$AsyncCall.run(RealCall.kt:519)
                    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1137)
                    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:637)
                    at java.lang.Thread.run(Thread.java:1012)
                 Caused by: java.lang.UnsupportedOperationException: Abstract class can't be instantiated! Class name: com.arch.example.network.models.NetworkPagingList
                    at com.google.gson.internal.UnsafeAllocator.assertInstantiable(UnsafeAllocator.java:120)
                    at com.google.gson.internal.UnsafeAllocator$1.newInstance(UnsafeAllocator.java:49)
                    at com.google.gson.internal.ConstructorConstructor$14.construct(ConstructorConstructor.java:225)
                    at com.google.gson.internal.bind.ReflectiveTypeAdapterFactory$Adapter.read(ReflectiveTypeAdapterFactory.java:212) 
                    at retrofit2.converter.gson.GsonResponseBodyConverter.convert(GsonResponseBodyConverter.java:40) 
                    at retrofit2.converter.gson.GsonResponseBodyConverter.convert(GsonResponseBodyConverter.java:27) 
                    at retrofit2.OkHttpCall.parseResponse(OkHttpCall.java:243) 
                    at retrofit2.OkHttpCall$1.onResponse(OkHttpCall.java:153) 
                    at okhttp3.internal.connection.RealCall$AsyncCall.run(RealCall.kt:519) 
                    at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1137) 
                    at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:637) 
                    at java.lang.Thread.run(Thread.java:1012) 

实体类代码 #

import com.google.gson.annotations.SerializedName

data class NetworkPagingList<T>(
    @SerializedName("data")
    val data: List<T>
)

使用场景 #

@GET("worker/shifts")
suspend fun getShifts(
    @Query("page") page: Int,
    @Query("perPage") perPage: Int
): NetworkPagingList<NetworkShift>

问题原因 #

升级Android Gradle插件（AGP）后，R8混淆器的默认优化规则变得更激进，导致泛型数据类NetworkPagingList<T>被误处理：

• R8可能移除了该类的反射元数据，甚至错误地将其标记为抽象类（从报错的Abstract class can't be instantiated可直接看出）
• Kotlin data类默认只有带参构造函数，Gson原本可以通过反射调用，但R8的优化破坏了这一逻辑，使得Gson找不到合法的构造方式，进而抛出无法实例化的异常

解决方法 #

方案1：添加R8混淆保护规则 #

在项目的proguard-rules.pro文件中添加以下规则，保留NetworkPagingList类的构造函数和反射元数据：

# 保留泛型数据类的构造函数与字段
-keep class com.arch.example.network.models.NetworkPagingList {
    <init>(...);
    <fields>;
}
# 保留Gson反射所需的签名信息
-keepattributes Signature, InnerClasses

方案2：给数据类添加无参构造函数 #

修改Kotlin data类，给字段设置默认值，让编译器自动生成无参构造函数，Gson可以直接调用：

data class NetworkPagingList<T>(
    @SerializedName("data")
    val data: List<T> = emptyList()
)

方案3：注册Gson InstanceCreator #

按照报错提示，手动给NetworkPagingList注册实例创建器，让Gson知道如何生成该类的实例：

// 创建自定义Gson实例
val customGson = GsonBuilder()
    .registerTypeAdapter(NetworkPagingList::class.java, InstanceCreator<NetworkPagingList<*>> {
        NetworkPagingList(emptyList())
    })
    .create()

// 传给Retrofit
val retrofit = Retrofit.Builder()
    .baseUrl("你的基础URL")
    .addConverterFactory(GsonConverterFactory.create(customGson))
    .build()

内容的提问来源于stack exchange，提问作者user924