Question

We use protocol buffers for storing data in a database (as blobs). At some point, we read them again, need to modify them and store them again.

The problem is, protocol buffer message objects are immutable. What we had in mind was just creating a new builder object using the protocol buffer message as prototype:

Foo.Builder.newBuilder(prototype)

This basically works. But as we have a nested structure, so the prototype object actually contains attributes which are messages themselves, this does not work. Excerpt from the documentation:

Since embedded message and string objects are immutable, they are shared between the original and the copy.

Is there a way of cloning a whole structure to new builder objects which are mutable?

Answer 1

Well, sort of. I’ve done this using DynamicMessage.Builder, Descriptor and FieldDescriptor. I recursively walk the object graph using a fully qualified name to the property I want to update. Once found, I update it and call build on the DyynamicMessage.Builder(s) back up the stack. It is not straight forward particularly when dealing with repeated fields.