final KeyStore keystore = KeyStore.getInstance("PKCS12");
final FileInputStream fis = new FileInputStream("/path/to/key.p12");
keystore.load(fis, "password".toCharArray());
final PrivateKey key = (PrivateKey) keystore.getKey("privatekey", "password".toCharArray());

final AuthConfig ac = AuthConfig.createFor("email@developer.gserviceaccount.com", key);

final DatastoreServiceOptions options = DatastoreServiceOptions.builder()
    .dataset(datasetId)
    .authConfig(ac)
    .build();

final DatastoreService datastore = DatastoreServiceFactory.getDefault(options);

final Credential credential = DatastoreHelper.getServiceAccountCredential(
    "email@developer.gserviceaccount.com", "/path/to/key.p12");
final DatastoreOptions options = DatastoreHelper.getOptionsfromEnv()
  .dataset(datasetId)
  .credential(credential)
  .build();
final Datastore datastore = DatastoreFactory.get().create(options);