Saturday, September 17, 2011

Race conditions are the result of lazy coders

Today I got a ticket about a race condition my boss identified in a self-written java data class(Java's  GreogorianCalendar class is extremely slow and just crap). The problem is very subtle and hard to reproduce.

Take a look at the following sample Java snippet:

class DateTime {
  MyCache datetime = null;

  private void initDateTime() {
    // Creates a fresh MyCache object
    // and initializes datetime.colonTime to null
    datetime = new MyCache();

  String getColonTime() {
    // datetime is some kind of 
    // lazy caching variable declared 
    // somewhere(does not matter)
    if (datetime == null) {
      //Uses lazy to initlialize variable, takes some time

  // Colon time stores hh:mm as string
  if (datetime.colonTime == null) {
    StringBuilder sb = new StringBuilder();
    //Now do some steps to build the hh:mm string
    //set colon time
    datetime.colonTime = sb.toString();
  return datetime.colonTime;

  public static class MyCache() {
    String colonTime = null;
    public MyCache() {}

The problem is that the getColonTime( ) method sometimes returns null although it always should return a time in the format hh:mm, like 17:49. Your first reaction sure is "No, it will always return something, because datetime.colonTime gets the value of the StringBuilder."
You're - of course - wrong, but it is very normal that you thing this way. The little devil is sitting in the details, waiting with his triton to peak you really hard.

Assume that we have two threads A and B who call getColonTime() at the same time (or nearly, if you only got one CPU). Threads are managed by something called a scheduler, who determines which thread has to pause and which one has to be run next. There are various scheduling algorithms, but most of them act like threads were having a party and the scheduler has to divide the CPU time cake. He does by serving each thread a piece of the cake while the other threads got to wait for their own slice.
So each thread has a defines period of time it is allowed to run before being paused again by the scheduler (e.g. it ate his piece of cake and now enqueues again to get a new one). Oh and our little devil is the boss of the scheduler ;)

Back to the problem:
What happens if two threads A and B call getColonTime() at the same time? First, A enters the first if block, because datetime is null.  As it happens, the scheduler decides to pause A before it can execute initDateTime(). Now B can run, it enters the method, jumps into the first if block and creates a new MyCache instance. Notice that, as the comment says, this also will set datetime.colonTime to null.
It then resumes and executes the contents of the second if block. It's a really widely used lazy pattern - initialize the variable's value when needed once and afterwards only return that value on subsequent calls.
So, B reaches the line datetime.colonTime = sb.toString() and our colon time member now got a value and continues to just before the return statement. Our evil scheduler now might think that this might be a good opportunity to pause A and resume  B.

Thread B now calls initDateTime() the second time. The result is, guess what, a new MyCache instance is being assigned to datetime. As our intention was to initialize datetime.colonTime lazy, the value of the latter is null again. So where's the problem? It will run through the second if block again, gets the value of the StringBuilder and everything is fine again, isn't it?
Surely not. Little devil has one of its really evil days and pauses thread A before it can execute the datetime.colonTime = sb.toString() command. As a result, datetime.colonTime is still null.
Again, thread A is resumed. There's only the return statement left, so the method will return the value of datetime.colonTime, namely null.

I took a lot of efforts to write a unit test which triggers this problem at least sometimes, lets say for one in a hundred calls, but I failed. The probability for this to occur is so low that the result was null only in 5 or 6 in total of all test runs. Each test runs that method about 200 000 times using multiple threads and I did a lot of test runs....

So whats next?
I will have to make sure that I can write a reasonable test which provokes these errors. While browsing the internet, I stumbled upon a really young project called thread weaver which looks really promising Maybe I'll give it a shot on monday.

Saturday, August 20, 2011

Lifting to a new level with Scala

It's project week at my company! So every employee can spend one week on whatever he likes to, as long as it has benefits for the company.

I will spend this week trying to learn ScaLa. This would of course be really boring if I wouln't have a project at my hands that will be the result of this.
So I came up with It is currently written in PHP, Smarty, some MySQLi and JavaScript. Especially PHP and partly Smarty can be a bit painful and aren't really contributing to the fun I normally have while programming.

The goal of the project is to evaluate IDE/Tomcat integration with Scala, getting to know the language and to try the LIFT-Framework. I am looking forward to a great programming experience and a lot of fun, fun and again - fun. will be rewritten from scratch using the existing HTML and JavaScript code and hopefully extended by some features I wanted to have long ago, like authentication and a lot of other awesomeness.
Of course, the results will be published as open source code on a newly created git repository.

I will report about the next steps soon!

Monday, May 02, 2011

Twitter share button with changeable url and no external widgets.js

Yesterday I wanted to add a "share on Twitter" button to the homepage, so users can share their created combo stations via Twitter, The normal button available by Twitter did not work as I wanted it to, it initializes the Button on page load. If you want to change the url afterwards, nothing happens and even the iframe solution did not work. Shame on you, Twitter!

So I created a custom solution which basically can do the same, but without the counter. Best thing is, it does not depend on the widgets.js so the page doesn't take ages to load.

I basically used the link solution for the tweet button. So assume we want to share an url, but use as the count url(in case twitter or I get this fixed).
The is created within the website dynamically, depending on the users input and you are in no way able to put it as your current url.

Put the following code somewhere in your section:

<script type="text/javascript">
function shareOnTwitter(url) {
popupWindow =,'popUpWindow','height=300,width=500,resizable=yes,scrollbars=yes,toolbar=yes,menubar=no,location=no,directories=no,status=yes');

This function is responsible for creating a popup window where you can tweet the shared url.

For the share button use the following:

<a id="shareOnTwitter" title="Tweet!" href="" onclick="shareOnTwitter(this.href);return false;"><img alt="Tweet this!" src=""></a>

The parameters are explained in the tweet button documentation linked above. If the user clicks the link, the shareOnTwitter function will be called with the link's href attribute, which will open a nice popup window that uses Twitters intent page for tweeting.
See that I use a placeholder %%shareUrl%% in the url. We will use jQuery and the JavaScript replace function to insert the actual url:
function setTwitterShareUrl(url) {
$('#shareOnTwitter').attr('href', $('#shareOnTwitter').attr('href').replace(/%%shareUrl%%/,url));

If you want to change the url more than one time, you might want to use the link in a template element and call the replace() function simply on the html. Then replace the old share link attribute with the new one.

Saturday, March 19, 2011

burny|net - My own personal website

The last days I spent parts of the spare time I should have spent learning maths into creating a new personal homepage, including a German blog!
You can reach it at http//
Don't be afraid, I will continue blogging here, too. But my mother tongue just leaves me more freedom to express what I think. A blog is like a public diary, so in consequence it is mainly for my personal record - but too for entertaining you of course :)
burny|net is not German only. I created this page mainly for the purpose to have a central place which bundles all my projects and virtual identities into one website. Plus, as you aready might have seen, has its home there, too :)

I hope at least my dear german readers will visit my homepage from time to time - it is work in progress and an RSS feed will come, of course, but really worth visiting. Just check out my newest blog entry there, I guess you will have a lot to laugh (even those who use Google Translate ;) ).

Tuesday, March 15, 2011 improves your listening experience on

Nearly a year ago I wrote about the radio query language and wrote a Java application called as a proof of concept.
Naturally, at least as far as I'm concerned, proof of concept projects are not maintained very well and buggy and the Java application was no exception. It just had some major backdraws regarding usablitly, portability and layout which made it a no-fun project.

So about two weeks ago I got the idea to re-write the whole thing from scratch using pure HTML, CSS 3, and JavasScript. I did some investigations and as I am a curious guy I tried some new stuff like jQuery, Felix Brun's Javascript API and the 960 grid system. They are both awesome in and of themselves, but in combination they can create a quite nice look and feel.
The result was overwhelming and the project drastically improved my Javascript and CSS knowledge. No wonder Google lets their employees spend 20% of their working time on own projects - at this point they are doing something right.

The new
Enough talking, just see yourself:

I added's new mix radio and also the friends radio. Using cutting edge HTML 5, should work with all major browsers like Google Chrome, Mozialla Firefox and reqkonq.
As I am a person with lots of ideas, the current version is, lets say: A tiny step for me, but a huge one for the community. And now replace "step" with "beer" ;)

Its current features include:

  • Listen to similar artists, tags, your library, mix radio, recommendations, neighbours and the friends radio
  • Combine stations as you like by connecting them with and, or and not
  • Mainstream-ness: Set how much mainstream your music should be
  • Set song repetition rate on a per-hour basis
  • Enable or disable discovery mode
- Sharing stations over social networks like Twitter and (maybe) Facebook
- Shout your station urls - directly from
- Far in the future: player on which displays station's name properly#

Notice: Combined radio stations are still in beta stage. There might be bugs and trouble and I personally ask not to abuse it. Thank you :)

This project needs your support!

Currently the problem is that especially the new is largely unknown. So:
Link it! Tweet it! Like it! Share it! Put in your forums signature! Join the project! 
Spread the word! 
The messias has arrived and it has great news to say: You can combine's radio stations, in any order and as many as you like!

Special Thanks
To the guys. You are doing good and reasonable work! Go on with that!
To all the moderators who tested and reported bugs.
Skiye for the discussions, chats and testing until the late morning.

Thank you
for using! It means very much to me :)

Tobias Brennecke
The creator of

P.S.: Can I have spam, bacon eggs and spam, but without the spam in it?

Friday, March 11, 2011

KDE: Creating a RocketDock style quick launch bar

Since a few days I've been looking for a KDE replacement of RocketDock, which is basically a quicklaunch bar at the top of the screen.
Today I came up with a nice solution, which isnt a perfect replacement, but enough for what I wanted:

Saturday, March 05, 2011

A Spammer who won a prize and two new CD's

As always, it's been a long time since my last post. I didn't pass the test in higher mathmatics, so I have to learn a lot, plus the usual stuff I have to do for working.
I promise to continue the Ireland series and also will report you the new and noteworthy things from the FOSDEM 2011 in Brussels, where I've been with Nik.

This morning I woke up. Pling! New mail on my cell phone. It was a PM notification mail from You might know I'm one of the moderators there. It was something like
Maybe this is your golden chance! Just one click to win a prize! Give it a try
- it is free but if you have some luck... :)
Dear Spammer!
Thanks for being so stupid, that really made my day :D I mean, which intelligent human being either codes a bot who sends spam to mods or, if sending the message manually, checks that I'm not a bot.
Please, next time use a big red "PLEASE BAN ME" sign as avatar and the same text as profile name.
Thanks :D

Other news....I bought two new CD's, namely

I really love them both although one Euro per track is really an exception I made, because I love "Riders On The Storm" :)
And yes, I still buy CDs! I love it to put them into my CD player and read the inlet while the music is playing and most times the inlet contains lyrics, too, so you can sing the track which is playing currently. Plus you never can get a better audio quality than from CD.

Saturday, February 12, 2011

Ireland day 3: The wonderful barn

The third day I didn't do anything special, I was just soo happy to see Isy again <3
Yesterday's bus trip to Leixlip was nearly painless except the bus had to wait for a traffic accident to be cleaned up. Isy and me had a lot of things to talk about and time passed by too fast! Nina, Zoe and the rest of the family were so friendly and lovely as all the other poeple in Ireland, which is maybe a bit unusal for german poeple, but I was happy :D

When she sent me her letter weeks before, I googled her address and saw a strange building called "The Wonderful Barn" just a footstep away.
So now I liked to see it and went there with her and little Tadgh. He looked really sweet in his buggy and was very happy about the walk for fresh air and a shiny blue sky.
So here it is, the Wonderful Barn:

Friday, February 11, 2011

Ireland day 2: Dublin Bay

So here is a new episode about my trip to Ireland last autumn (how time can fly by...).
Dublin isn't that far away from the Irish Sea, it's got a large area of docks and that stuff. You could see the cranes and some red-white chimneys in the far distance and it was clear to me that I wanted to go there.
On my trip, which partly was a long walk down the liffey, I took of course some nice photos which I want to show you. If you like the photos, take a look at my Picasa web album at All picutures have a Creative Commons cc-by-nc-sa 3.0 license :) If you want to have them in full 12 megapixels, just contact me.

Breakfast started with a really funny and interesting article in a news paper lying around at the hostel:

Tuesday, February 01, 2011

KDE 4.6 is out and I love it!

Today I discovered KDE has released version 4.6 of their desktop Suite.
It is really awesome and much faster, although I still have to explore the details a bit further (never used e.g. "Activities"). I was surprised by the really cool login screen, then by the faceted search feature of dolphin, etc....
Desktop and Dolphin

I came to that point when I booted kubuntu on my desktop pc to upgrade it to Kubuntu 10.10 Maverick. When I clicked on the wifi icon, something totally different than on my laptop showed up. Have to admit I have been using Windows 7 on it for a while, but I was astonished how fast Kubuntu (still with old KDE) was. I thought that I might have missed something during the various dist-upgrades on my laptop and stumbled upon the great news. I first updated on my laptop(just add ppa:kubuntu-ppa/backports to your software sources via KPackageKit), which didn't make any effect on that network-manager tray icon, the old one was still there.

Thursday, January 20, 2011

T42p: Making AccessIbm Button launch an application

Under Kubuntu, the "Access IBM" button has no function. I just decided to change that, luckily it's like a key on the normal keyboard so we can assign an action to it.
In my example this will be konsole, the KDE terminal. You can't ever get enough of them ^^
Follow these simple steps:

  • Open KDE system settings dialog
  • Click 'keyboard shortcuts and gesture control'
  • Now "My Shortcuts" on the left should be highlighted. At the bottom center of the dialog, click "Edit", then "New->Global Shortcut->Command/Address"
  • Give the Action a name, e.g. "AccessIbm Button"
  • Go to the "trigger" tab, click on the button and press the "Access IBM" key. If the caption changed, you can succeed, else retry
  • Go to the action tab and type /usr/bin/konsole into the text field
  • Click "Apply"
Now you're done! Press the "Access IBM" key to test it and have fun ;)
From what I've seen you also can assign dbus signals, keystrokes(keyboard macro functions!) , etc. just play around with it a bit.

Ireland day 1: Dublin (part 3) - Dublin at Night

In this post, I will mostly let pictures talk and maybe say only a few words on single ones. Most of them are bridges, which are illuminated by green light from below.Looks really beautiful, doesn't it?

Custom House at Night

Millenium Bridge

Wednesday, January 19, 2011

Ireland day 1: Dublin (part 2)

So, finally I got some spare time to write the rest of the Ireland series and the photos are all uploaded now, too.
The Historical Walking Tour started at the front of Trinity College. After walking through the front gates we had a look at the wonderful courtyard and the

A part of the Trinity College courtyard*

He told us some interesting stuff about the college's history, but I can't remember anymore, too bad!
Here is some nice photo of the Library Square:

Libary Square*

Trinity College had a lot of famous students, like Oscar Wilde, and also is - from what I understood - much involved in the Irish history and the history of dublin. For more details, have a look at Wikipedia.
Walking back to the front gate, you can find the Irish Houses of Parliament, which now inherits the Bank of Ireland.