code: public void runAsync(int startSuccessTimeout) throws MojoExecutionException {
      runMojo
          .getLog()
          .info("Waiting " + startSuccessTimeout + " seconds for the Dev App Server to start.");
      try {
        runMojo
            .getAppEngineFactory()
            .devServerRunAsync(startSuccessTimeout)
            .run(configBuilder.buildRunConfiguration(processServices(), processProjectId()));
      } catch (AppEngineException ex) {
        throw new RuntimeException(ex);
      }
      runMojo.getLog().info("Dev App Server started.");
      runMojo.getLog().info("Use the 'mvn appengine:stop' command to stop the server.");
    }
docs: Run the dev appserver in async mode.