Google Authentification file read from .jar file.

Utility.java

public class Utility {

	public static void main(String[] args) throws Exception {
		Utility utility = new Utility();
		GoogleCredentials credentials = utility.getGoogleCredentials("./credentials.json");
  	}

	GoogleCredentials getGoogleCredentials(String jsonPath) throws IOException {
		// You can specify a credential file by providing a path to GoogleCredentials.
		// Otherwise credentials are read from the GOOGLE_APPLICATION_CREDENTIALS
		// environment variable.
		URL url = this.getClass().getResource(jsonPath);
		String path = url.getPath();
		GoogleCredentials credentials = GoogleCredentials.fromStream(new FileInputStream(path))
				.createScoped(Lists.newArrayList("https://www.googleapis.com/auth/cloud-platform"));
		return credentials;
	}
}

Googleの認証ファイル（credentials.json）はUtility.javaから見た相対パスで取得する。
"./credentials.json"の場合はUtility.javaと同じ階層にcredentials.jsonを置く。