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

QueryAsyncController.java

package org.sinmetal.controller;

import java.io.*;
import java.util.*;
import java.util.logging.*;

import org.codehaus.jackson.*;
import org.codehaus.jackson.map.*;
import org.sinmetal.meta.*;
import org.sinmetal.model.*;
import org.slim3.controller.*;
import org.slim3.datastore.*;

import com.google.appengine.api.datastore.*;

public class QueryAsyncController extends SimpleController {

	Logger logger = Logger
			.getLogger(QueryAsyncController.class.getSimpleName());

	TweetMeta meta = TweetMeta.get();

	@Override
	protected Navigation run() throws Exception {
		Iterator<Key> iterator1 = Datastore.query(meta)
				.sort(meta.createdAt.desc).limit(100).asKeyIterator();
		Iterator<Key> iterator2 = Datastore.query(meta).limit(100)
				.asKeyIterator();
		Iterator<Key> iterator3 = Datastore.query(meta).limit(100)
				.asKeyIterator();
		Iterator<Key> iterator4 = Datastore.query(meta).limit(100)
				.sort(meta.key.asc).asKeyIterator();
		Iterator<Key> iterator5 = Datastore.query(meta).limit(100)
				.sort(meta.key.asc).asKeyIterator();

		writeResponse(iterator1);
		writeResponse(iterator2);
		writeResponse(iterator3);
		writeResponse(iterator4);
		writeResponse(iterator5);
		return null;
	}

	void writeResponse(Iterator<Key> iterator) throws JsonGenerationException,
			JsonMappingException, IOException {
		List<Key> keys = new ArrayList<Key>();
		while (iterator.hasNext()) {
			keys.add(iterator.next());
		}

		List<Tweet> list = Datastore.get(meta, keys);
		logger.info(String.valueOf(list.size()));
	}
}