XgtxServlet.java

package org.sinmetal.xgtx;

import java.io.IOException;
import java.util.ConcurrentModificationException;

import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

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;

@SuppressWarnings("serial")
public class XgtxServlet extends HttpServlet {

  private final DatastoreService datastore = DatastoreServiceFactory
			.getDatastoreService();

	public void doGet(HttpServletRequest req, HttpServletResponse resp)
			throws IOException {

		TransactionOptions options = TransactionOptions.Builder.withXG(true);
		Transaction txn = datastore.beginTransaction(options);

		get(txn, "A", "hoge", resp);
		get(txn, "B", "fuga", resp);
		get(txn, "C", "moga", resp);

		Entity d = new Entity("D");
		try {
			datastore.put(txn, d);
		} catch (Exception e) {
			e.printStackTrace();
			resp.getWriter().println("Kind(d) put. " + e.getMessage());
		}

		if (txn != null && txn.isActive()) {
			try {
				txn.commit();
			} catch (Exception e) {
				e.printStackTrace();
				resp.getWriter().println("tx commit. " + e.getMessage());
			}
		} else {
			resp.getWriter().println("transaction closed.");
		}

		resp.setContentType("text/plain");
		resp.getWriter().println("Hello, XGTransaction");
	}

	private Entity get(Transaction txn, String kind, String key,
			HttpServletResponse resp) throws IOException {
		Key akey = KeyFactory.createKey(kind, key);
		try {
			return datastore.get(txn, akey);
		} catch (EntityNotFoundException e) {
			final String message = String.format(
					"Kind(%s).Key(%s) entity NotFound.", kind, key);
			resp.getWriter().println(message);
			return null;
		} catch (ConcurrentModificationException e) {
			final String message = String
					.format("Kind(%s).Key(%s) entity get ConcurrentModificationException.",
							kind, key);
			resp.getWriter().println(message);
			return null;
		}
	}
}

これを2回以上実行すると、以下の状態になる。

Kind(A).Key(hoge) entity NotFound.
Kind(B).Key(fuga) entity NotFound.
Kind(C).Key(moga) entity get ConcurrentModificationException.
Kind(d) put. transaction closed
tx commit. transaction closed
Hello, XGTransaction

Kind(C).Key(moga)がLockされたままになり、ConcurrentModificationExceptionが解消されない。
しかし、別のTxでKind(C).Key(moga)へアクセスすることはできる。