java - 从 protobuf 描述符获取默认实例

Question

如果我只知道它的描述符，如何获取 protobuf 消息的默认实例:

Descriptor descriptor   = SomeOuterMessage.getDescriptor()
                              .findNestedTypeByName("InnerMessageName");
Message defaultInstance = descriptor. <???> // how do I get an instance of "InnerMessageName"?

Answer 1

您可以通过 DynamicMessage 获取默认实例类。

假设原型(prototype)如下:

Message PhoneBook {
    repeated Person people = 1;
}
Message Person {
    required Name name = 1;
    optional String phone = 2;
}
Message Name {
    required String family_name = 1;
    required String given_name = 2;
}

获取 People 默认实例的代码类似于:

PhoneBook phoneBook = phoneBook.getDefaultInstance();
Descriptors.Descriptor personDescriptor = phoneBook.getDescriptor().findNestedTypeByName("people");
DynamicMessage defaultPeopleInstance = DynamicMessage.newBuilder(personDescriptor).getDefaultInstanceForType().build();
assert defaultPeopleInstance == People.getDefaultInstance();