@Nullable
    @Override
    public ResponseConverterFunction createResponseConverterFunction(
            Type returnType,
            ResponseConverterFunction responseConverter) {
        final Class<?> clazz = typeToClass(returnType);
        if (clazz != null && isSupportedClass(clazz)) {
            ensureNoMoreObservableSource(returnType, returnType);
            return new ObservableResponseConverterFunction(responseConverter);
        }
        return null;
    }
Provides an {@link ObservableResponseConverterFunction} to annotated services.
rxjava2/src/main/java/com/linecorp/armeria/internal/server/rxjava2/ObservableResponseConverterFunctionProvider.java