If you want to use the protocol buffers compiler protoc manually, navigate to the folder of the source code and execute the command below:

protoc -I=./main/resources --java_out=./main/java ./main/resources/adressbook.proto

With the protoc-jar-maven-plugin in the pom.xml the .proto files are searched under the src/main/resources folder and compiled automatically during the generate-sources phase of maven build lifecycle