gRPC java server use username/password authentication to protect service invocation

Application.java

@SpringBootApplication
public class GrpcDemoApplication implements CommandLineRunner {

    @Autowired
    private HelloServiceGrpc.HelloServiceImplBase helloServiceGrpc;

    @Autowired
    private UsernamePasswordAuthenticator usernamePasswordAuthenticator;

    public static void main(String[] args) {
        SpringApplication.run(GrpcDemoApplication.class, args);
    }

    @Override
    public void run(String... args) throws Exception {
        Server server = ServerBuilder.forPort(8003)
                .intercept(usernamePasswordAuthenticator)
                .addService(helloServiceGrpc)
                .build()
                .start();
        System.out.println("gRPC server started on port 8003");
        server.awaitTermination();
    }
}

UsernamePasswordAuthenticator.java

@Component
public class UsernamePasswordAuthenticator implements ServerInterceptor {


    @Override
    public <ReqT, RespT> ServerCall.Listener<ReqT> interceptCall(ServerCall<ReqT, RespT> serverCall, Metadata metadata, ServerCallHandler<ReqT, RespT> serverCallHandler) {
        String username = metadata.get(Metadata.Key.of("username", new StringMetaDataMarshaller()));
        String passwordMeta = metadata.get(Metadata.Key.of("password", new StringMetaDataMarshaller()));

        // hard coded username and password, just for prototype
        if (Objects.equals("root", username) && Objects.equals("rootpwd", passwordMeta)) {
            return serverCallHandler.startCall(serverCall, metadata);
        } else {
            serverCall.close(Status.UNAUTHENTICATED, metadata);
            return serverCallHandler.startCall(serverCall, metadata);
        }
    }

    class StringMetaDataMarshaller implements Metadata.AsciiMarshaller<String> {

        @Override
        public String toAsciiString(String s) {
            return s;
        }

        @Override
        public String parseAsciiString(String s) {
            return s;
        }
    }
}