Geospatial JDO queries on the Google AppEngine (+ GWT)

From time to time one might feel the urgent need to execute geospatial queries on the Google AppEngine. Thinking in common SQL this is pretty easy, just a combined WHERE clause:

SELECT * FROM t_spatialdata
WHERE latitude >= south AND latitude <= north AND longitude >= west AND longitude <= east

Unfortunately something like this is not possible, when using AppEngine, as you can only filter by one argument (i.e. only by latitude). I think the reason for this is that AppEngine internally sorts the table and then 'cuts' away what doesn't match the filter.

So this problem can basically be solved, by using a algorithm called Geohash, which unfortunately didn't want to return proper results, at least not the implementations I found. (The 'official' implementation uses the BitSet-Class which is not present when using AppEngine/GWT)

The solution I went for is the "javageomodel"-project which follows (afaik) a similar algorithm but worked. http://code.google.com/p/javageomodel/

The problem you will face, when trying to use this in your AppEngine project is that it will be missing (just at runtime!) two classes from the Apache Commons project: StringUtils and Validator. At least the latter one is over 1MByte in size. So I decided to replace the Validator lines with standard "assert"-calls, but adding the jars probably would have worked too.

For your convenience I uploaded the final jar that works fine:

http://rapidshare.com/files/393375528/libjavageomodel.jar.html

A how to use this for actual queries is located here:

http://code.google.com/p/javageomodel/source/browse/trunk/geocell/src/test/java/com/beoui/utils/HowToUseGeocell.java

HttpServlet doGet / doPost throwing unexpected Exception

Ever wondered why your HttpServlet on Google AppEngine didn't work, even if the code looks sooooo right?
You might have (accidentally) called super.doPost(...,...); or super.doGet(...,...); which calls the default implementation of HttpServlet, which throws an Exception!

package org.crazy.servlets;

import java.io.IOException;

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

public class CrazyServlet extends HttpServlet {

 @Override
 protected void doGet(final HttpServletRequest pReq, final HttpServletResponse pResp) throws ServletException, IOException {
  this.doPost(pReq, pResp);
 }

 @Override
 protected void doPost(final HttpServletRequest pReq, final HttpServletResponse pResp) throws ServletException, IOException {
   super.doPost(pReq, pResp); // <-- DON'T !!! Just delete this line
  // Insert actual work here...
 }
}

Took me quite some to figure this out =D

AppEngine + Scala + JDO + GWT + Google Visualization API

Hi guys,

the last days I found myself playing with the Google AppEngine, a very easy to use and powerful cloud-hosting service powered by Google. I felt the need to code the server-side using Scala which resulted in some very neat and small code. (Special thanks to the superb XML-parsing and collection-framework in Scala)

Recently I enjoy playing a game called Heroes Of Newerth that is the successor of the famous Dota. This game as a public XML-API that I daily pull stats from - of user that 'registered' at my site:

http://honstatslogger.appspot.com/

So I parse the XML-data using Scala, make it persistent saving Java Data Objects (JDO) to the Datastore of the AppEngine. Users than can get the Data visualized using a combination of the Google Web Toolkit (GWT 2.0) and the Google Visualization API. I could create a pretty rich user-experience writing only about 1.000 Lines of Code Java/Scala code and a hand full of HTML and CSS.

Best Regards,
Nicolas