Question

Exception: Query result sets are not modifiable
i saw a similar issue here but it did not answer my question so i decided to start a new one.

I am trying to work around the consistently getting inconsistent reads issue ( https://groups.google.com/forum/?fromgroups=#!topic/google-appengine/E95wH0RkJUE ) with the High Replication datastore of Google App Engine by temporarily storing the last added object in a static variable.

Whenever the user adds a new flight, instead of just directly persisting the new flight object, it store it in PMF as a static variable.

So when the list of flights is fetched in a query, it stores the result it the list, and the list is compared with the last added object, if the last added object is not in the list, i manually add the last added object to the list.

However using this method, i get the following exception whenever the newly added object is missing from the list and i manually add it.

Oct 6, 2012 2:19:38 PM com.google.apphosting.utils.jetty.JettyLogger warn
WARNING: /
java.lang.UnsupportedOperationException: Query result sets are not modifiable
    at org.datanucleus.store.query.AbstractQueryResult.add(AbstractQueryResult.java:222)
    at com.jimfoo88.helloorm.GetFlights.queryJDO(GetFlights.java:101)
    at com.jimfoo88.helloorm.GetFlights.doGet(GetFlights.java:50)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:617)
    at javax.servlet.http.HttpServlet.service(HttpServlet.java:717)
    at org.mortbay.jetty.servlet.ServletHolder.handle(ServletHolder.java:511)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1166)
    at com.google.appengine.tools.development.HeaderVerificationFilter.doFilter(HeaderVerificationFilter.java:35)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.appengine.api.blobstore.dev.ServeBlobFilter.doFilter(ServeBlobFilter.java:60)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.apphosting.utils.servlet.TransactionCleanupFilter.doFilter(TransactionCleanupFilter.java:43)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.appengine.tools.development.StaticFileFilter.doFilter(StaticFileFilter.java:125)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at com.google.appengine.tools.development.BackendServersFilter.doFilter(BackendServersFilter.java:97)
    at org.mortbay.jetty.servlet.ServletHandler$CachedChain.doFilter(ServletHandler.java:1157)
    at org.mortbay.jetty.servlet.ServletHandler.handle(ServletHandler.java:388)
    at org.mortbay.jetty.security.SecurityHandler.handle(SecurityHandler.java:216)
    at org.mortbay.jetty.servlet.SessionHandler.handle(SessionHandler.java:182)
    at org.mortbay.jetty.handler.ContextHandler.handle(ContextHandler.java:765)
    at org.mortbay.jetty.webapp.WebAppContext.handle(WebAppContext.java:418)
    at com.google.appengine.tools.development.DevAppEngineWebAppContext.handle(DevAppEngineWebAppContext.java:94)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at com.google.appengine.tools.development.JettyContainerService$ApiProxyHandler.handle(JettyContainerService.java:370)
    at org.mortbay.jetty.handler.HandlerWrapper.handle(HandlerWrapper.java:152)
    at org.mortbay.jetty.Server.handle(Server.java:326)
    at org.mortbay.jetty.HttpConnection.handleRequest(HttpConnection.java:542)
    at org.mortbay.jetty.HttpConnection$RequestHandler.headerComplete(HttpConnection.java:923)
    at org.mortbay.jetty.HttpParser.parseNext(HttpParser.java:547)
    at org.mortbay.jetty.HttpParser.parseAvailable(HttpParser.java:212)
    at org.mortbay.jetty.HttpConnection.handle(HttpConnection.java:404)
    at org.mortbay.io.nio.SelectChannelEndPoint.run(SelectChannelEndPoint.java:409)
    at org.mortbay.thread.QueuedThreadPool$PoolThread.run(QueuedThreadPool.java:582)

My code below is based on the google app engine sample, HelloORM http://code.google.com/p/datanucleus-appengine/source/browse/trunk/demos/helloorm/src/com/google/appengine/demos/helloorm/GetFlights.java?r=736

I only show parts that i modify to simplify reading.

AddFlight.java

private void doPostJDO(Flight f) {
    PersistenceManager pm = PMF.get().getPersistenceManager();
    PMF.setLastObjectAdded(f);
    try {
      pm.makePersistent(f);
    } finally {
      pm.close();
    }
  }

PMF.java

// Copyright 2008 Google Inc. All Rights Reserved.
package com.jimfoo88.helloorm;

import javax.jdo.JDOHelper;
import javax.jdo.PersistenceManagerFactory;

/**
 * @author Max Ross <[e-mail]>
 */
public final class PMF {

    private static Object lastObjectAdded;

  private static final PersistenceManagerFactory INSTANCE = JDOHelper.getPersistenceManagerFactory("transactions-optional");

  public static Object getLastObjectAdded() {
    return lastObjectAdded;
}

public static void setLastObjectAdded(Object lastObject) {
    lastObjectAdded = lastObject;
}

public static PersistenceManagerFactory get() {
    return INSTANCE;
  }

  private PMF() {}
}

GetFlights.java

private Collection<Flight> queryJDO(String query) {
        PersistenceManager pm = PMF.get().getPersistenceManager();
        try {
            List<Flight> flights = (List<Flight>) pm.newQuery(query).execute();
            // Force all results to be pulled back before we close the entity
            // manager.
            // We could have also called pm.detachCopyAll()
            flights.size();
            //pm.detachCopyAll(flights);
            boolean found = false;
            try {
                for (Flight f : flights) {
                    if ((Flight) PMF.getLastObjectAdded() == null) {
                        found = true;
                        break;
                    }
                    if (f.getDest() == ((Flight) PMF.getLastObjectAdded())
                            .getDest()
                            && f.getOrig() == ((Flight) PMF
                                    .getLastObjectAdded()).getOrig()) {
                        logger.info("found object " + f.toString()
                                + "in datastore");
                        found = true;
                        break;
                    }
                }
                if (found == false) {
                    logger.info("did not find object "
                            + (Flight) PMF.getLastObjectAdded()
                            + " in datastore, need to add it");
                    flights.add((Flight) PMF.getLastObjectAdded());
                }
            } catch (NullPointerException e) {
                logger.error(e.getMessage());
            }
            return flights;
        } finally {
            pm.close();
        }
    }

Will appreciate if anybody could point out my mistakes. If there is any better way to work around this issue, please let me know. Thanks.

Answer 1

So query results are not modifiable, so don’t attempt to add objects to them. I don’t see anything even slightly incorrect/inconsistent about that (and is in fact what the JDO/JPA specs say)

If you want to make use of those results in a later context you can easily create a new List, do an addAll() with the query results, and use that List