Protobuf-来自同一类别的多个消息

Question

I have the following message in a proto file

message User
required string name = 1;
required string password = 2;

• How can I differentiate multiple instances of the same message?
• If I wanted to serialize the registered users of an app for instance
• Is protobuf indicated for that?

Answer 1

This only works with some additional coding. When protobuf reads a message from a stream it does not know the length of one message. It sees simply a stream of fields as name/value pairs and no indicator for start or end of a message.

From the docs :

If you want to write multiple messages to a single file or stream, it is up to you to keep track of where one message ends and the next begins. The Protocol Buffer wire format is not self-delimiting, so protocol buffer parsers cannot determine where a message ends on their own.

Google's implementations contains some helper classes for this usecase. Have a look at CodedInputStream and CodedOutputStream .

One possible way for writing:

• serialize one object to a memory buffer
• use CodedOutputStream to write the length of the buffer as a single number to the output stream
• add the memory buffer containing your serialized object to the output stream
• proceed with the next object

One possible way for reading:

• use CodedInputStream to read one number from the input stream
• take as many bytes from the input stream as this number indicates to a memory buffer
• parse this buffer to regain one object
• if the input stream has more bytes continue with the next object