CounterStore.java

package net.recompile.gae.counter;

import java.util.Iterator;

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.PreparedQuery;
import com.google.appengine.api.datastore.Query;
import com.google.appengine.api.datastore.Transaction;

public class CounterStore {

    private static final String COUNTER_KIND = "Counter";
    private static final String COUNTER_VALUE = "value";
    private static final DatastoreService DATASTORE_SERVICE = DatastoreServiceFactory.getDatastoreService();

    private Key counterKey;

    public CounterStore() {
        setCounterKey();
    }

    public Long getAndIncrement() {
        Transaction tx = DATASTORE_SERVICE.beginTransaction();

        try {
            Long count = getByKey(tx);
            put(tx, count + 1);

            tx.commit();
            return count;
        } catch (RuntimeException e) {
            tx.rollback();
            throw e;
        }
    }

    private Long getByKey(Transaction tx) {
        try {
            Entity entity = DATASTORE_SERVICE.get(tx, counterKey);

            return (Long) entity.getProperty(COUNTER_VALUE);
        } catch (EntityNotFoundException e) {
            throw new RuntimeException(e);
        }
    }

    private Key put(Transaction tx, Long value) {
        try {
            Entity entity = DATASTORE_SERVICE.get(counterKey);
            entity.setProperty(COUNTER_VALUE, value);

            return DATASTORE_SERVICE.put(tx, entity);
        } catch (EntityNotFoundException e) {
            throw new RuntimeException(e);
        }
    }

    private Entity querySingleEntity() {
        Query query = new Query(COUNTER_KIND);

        PreparedQuery prepared = DATASTORE_SERVICE.prepare(query);
        Iterable<Entity> entries = prepared.asIterable();

        Iterator<Entity> iter = entries.iterator();
        if (iter.hasNext()) {
            return iter.next();
        } else {
            return null;
        }
    }

    private void setCounterKey() {
        Entity entry = querySingleEntity();

        if (entry != null) {
            counterKey = entry.getKey();
        } else {
            Entity entity = new Entity(COUNTER_KIND);
            entity.setProperty(COUNTER_VALUE, Long.valueOf(0));
            counterKey = DATASTORE_SERVICE.put(entity);
        }
    }

}