BackgroundFunctionExecutor.java failes to detect type when sub-classed

Bắt đầu trong BackgroundFunctionExecutor.java tại dòng 140 và kiểm tra backgroundFunctionTypeArgument. Tái hiện cấu trúc phân cấp MyExtendedBackgroundFunction và MyFunctionHandler được mô tả trong issue, sau đó xác minh rằng executor phát hiện MyReturnType đồng thời vẫn giữ nguyên hành vi đối với các kiểu BackgroundFunction được triển khai trực tiếp.

Error Condition

Exception in thread "main" java.lang.RuntimeException: Could not determine the payload type for BackgroundFunction of type com.example.MyFunctionHandler; must implement BackgroundFunction for some T at com.google.cloud.functions.invoker.BackgroundFunctionExecutor.forClass(BackgroundFunctionExecutor.java:140) at com.google.cloud.functions.invoker.BackgroundFunctionExecutor.forClass(BackgroundFunctionExecutor.java:113) at com.google.cloud.functions.invoker.runner.Invoker.startServer(Invoker.java:256) at com.google.cloud.functions.invoker.runner.Invoker.main(Invoker.java:127)

Occurs when BackgroundFunction<T> is subclassed:

public abstract class MyExtendedBackgroundFunction<T> implements BackgroundFunction<T>

and

public class MyFunctionHandler extends MyExtendedBackgroundFunction<MyReturnType>

In the class mentioned, the following function fails to detect sub-classed BackgroundFunction type:

static Optional<Type> backgroundFunctionTypeArgument(Class<? extends BackgroundFunction<?>> functionClass) {    
// If this is BackgroundFunction<Foo> then the user must have implemented a method    
// accept(Foo, Context), so we look for that method and return the type of its first argument.    
// We must be careful because the compiler will also have added a synthetic method    
// accept(Object, Context).    
return Arrays.stream(functionClass.getMethods())        
.filter(m -> m.getName().equals("accept") &amp;&amp; m.getParameterCount() == 2            && m.getParameterTypes()[1] == Context.class            &amp;&amp; m.getParameterTypes()[0] != Object.class)        
.map(m -> m.getGenericParameterTypes()[0])        
.findFirst();  
}

This fails for described MyFunctionHandler.

The intent is to add common boilerplate handling for a couple of functions which is common to all. A separate abstract function is employed, but the function fails to load because the type is not determined.

Temporary workaround is to re-implement accept() with the type in the sub-classed object with the type, but is inelegant.