Generating Java source code after modifing protobuf definition

1. Check to see you have the correct protoc version

    $ protoc --version
   

   Confirm that the result version is compatible with the version specified in pom.xml.

2. Generate Java source code.

    $ protoc topology.proto  --java_out=${ONOS_HOME}/src/main/java/
   

   Replace --java_out path to root Java source directory.
   e.g., If you're in the same directory as this file:

    $ protoc topology.proto  --java_out=../java/