Defaulting message fields in protobuf

Not well documented is the technique for defaulting message fields in protobuf. Say you have a field of message type, named "start" here:

message Complex {
    required int64 real = 1;
    optional int64 imaginary = 2 [default = 0];
}

message Vector {
    required Complex start = 1;
    required int64 length = 2;
}

How to you provide a default value, say at the origin? Like this:

import "google/protobuf/descriptor.proto";

extend google.protobuf.FieldOptions {
    // Pick the field number that is right for you!
    optional Complex complex = 50000;
}

message Vector {
    optional Complex start = 1
    [(complex) = { real: 0 imaginary: 0 }];
    required int64 length = 2;
}

See the Custom Options section for details.

Working with Google Protobuf DynamicMessage

Google Protobuf is an stand out project with mixed quality documentation. A corporate cynic points working with DynamicMessage, an interesting Protobuf feature lacking official documentation beyond skeletal javadoc and a short blurb.

The key trick condenses down to:

public static Descriptor descriptorFor(
        final InputStream in, final String name)
        throws IOException, DescriptorValidationException {
    final FileDescriptorSet fileDescriptorSet
            = FileDescriptorSet.parseFrom(in);
    final FileDescriptorProto fileDescriptorProto
            = fileDescriptorSet.getFile(0);
    final FileDescriptor fileDescriptor = FileDescriptor.
            buildFrom(fileDescriptorProto, new FileDescriptor[0]);
    return fileDescriptor.findMessageTypeByName(name);
}

// Elsewhere

final InputStream in = Class.class.
        getResourceAsStream("... your descriptors from protoc ...");
final Descriptor descriptor = descriptorFor(in,
        Outer.getDescriptor().getName());
final DynamicMessage dynamicMessage = DynamicMessage.
        parseFrom(descriptor, outer.toByteArray());
System.out.println("dynamicMessage = " + dynamicMessage);

Note the baked-in assumption of "getFile(0)", etc. The cynic's example is more forthcoming. Caveat coder.