Updating entity in Datastore

//https://cloud.google.com/appengine/docs/java/datastore/transactions

import com.google.appengine.api.datastore.DatastoreService;
import com.google.appengine.api.datastore.DatastoreServiceFactory;
import com.google.appengine.api.datastore.Entity;
import com.google.appengine.api.datastore.EntityNotFoundException;
import com.google.appengine.api.datastore.Key;
import com.google.appengine.api.datastore.KeyFactory;
import com.google.appengine.api.datastore.Transaction;
import com.google.appengine.api.datastore.TransactionOptions;

void myTxn() {
 DatastoreService datastore = DatastoreServiceFactory.getDatastoreService();
 TransactionOptions options = TransactionOptions.Builder.withXG(true);
 Transaction txn = datastore.beginTransaction(options);

 Entity a = new Entity("EntityName");
 a.setProperty("Property", "Value");
 datastore.put(txn, a);

 txn.commit();
}