java protocol buffers - construct sub message with extension field

Question

How do we construct a sub message by adding the extension field in protocol buffers. In C++, one way of doing it is by using MutableMessage(Message, FieldDescriptor) with reflection interface. How do we do the similar thing in Java?

I'm a newbie so don't know a lot about protocol buffers.

Answer 1

Following from jonathons answer, here is and example of creating and accessing extensions in java

Protobuf definition:

message Message
{
    extensions 100 to max;

    required uint64 A = 1;
}

message Event
{
    extend Message { 
        required Event ext = 101; 
        optional string name = 102;
    }

    extensions 100 to max;

    required uint64 B = 1;

}

Using the extension:

Extension01.Message.Builder bm = Extension01.Message.newBuilder();
Extension01.Message.Builder bm1 = Extension01.Message.newBuilder();
Extension01.Event.Builder be = Extension01.Event.newBuilder();
FileOutputStream out = new FileOutputStream(extFileOut);

be.setB(5678);


bm.setA(123);
bm.setExtension(Extension01.Event.ext, be.build()); // set the extension value

bm.build().writeDelimitedTo(out);
out.close();

FileInputStream in = new FileInputStream(extFileOut);

ExtensionRegistry registry = ExtensionRegistry.newInstance(); // create extension registry
Extension01.registerAllExtensions(registry);  // Allocate the extensions 
bm1.mergeDelimitedFrom(in, registry);
in.close();

Event extension = bm1.getExtension(Extension01.Event.ext); // get the extension
if (extension == null) {
    System.out.println("No extension");
} else {
    System.out.println(extension.getB());
}

Edit:

Line setting extension value:

bm.setExtension(Extension01.Event.ext, be.build());

The variable Extension01.Event.ext is generated by protocol buffers and can be used get/put the values of extensions.

Edit 2:

This is how I think extensionInfo is used (I have not tried it my self):

ExtensionInfo extensionInfo = registry.findExtensionByNumber(..., 101);
bm1.getField(extensionInfo.descriptor);
bm1.setField(extensionInfo.descriptor, value)

Answer 2

Use the setExtension method. Given:

message Bar {
  extend Foo {
    optional string foo_string = 1;
  }
}

You can build a Bar message as so:

Foo foo = Foo.newBuilder()
    .setExtension(Bar.fooString, "My foo string")
    .build();

See the Java Generated Code documentation on extensions for Protocol Buffers for more details.