DDevelopment & Design

Red Black Tree Visualization using HTML5 Canvas and GWT

I created a sample app that demonstrates HTML5 Canvas usage from Java through GWT. I think GWT is an excellent tool for migrating desktop apps into Web nowadays, especially for Java developers. So it’s worth giving it a try. Google has made significant progress on migrating desktop apps into Web during the last year. They propagated trend for HTML5 support and Javascript JIT compilers among modern browsers. So it’s possible to run Quake 2 or CAD software directly in browser at decent speed. Red Black Tree is a balanced BST tree. Details are described on Wikipedia. You can run this sample app directly on appspot (it works on iPhone too :-) ) Sample code can be found here: Browse on GitHub. Let’s start from initialization. First, we need to create Canvas element and add it to HTML. getContext2D is called to obtain drawing context. We register a timer to redraw frames frequently: So doUpdate is called every 50 ms and whenever redrawFrame is set to true, it redraws Canvas contents. In autoplay mode, we call processFrame in while loop. So whenever redraw procedure takes too long, we will process frames without redrawing them. This won’t slow down animation on low resources. Then, we need to draw a tree. We use drawTree procedure, which is recursive and draws nodes along with contents and connections between them: The best part is that we can do regular Java unit tests on Red Black tree to verify the correctness of implementation:

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DDevelopment & Design

Grails render as JSON catch

One of a reasons your controller doesn't render a proper response in JSON format might be wrong package name that you use. It is easy to overlook. Import are on top of a file, you look at your code and everything seems to be fine. Except response is still not in JSON format.

Consider this simple controller:

class RestJsonCatchController {
def grailsJson() {
render([first: 'foo', second: 5] as grails.converters.JSON)
    }

def netSfJson() {
render([first: 'foo', second: 5] as net.sf.json.JSON)
    }
}

And now, with finger crossed... We have a winner!

$ curl localhost:8080/example/restJsonCatch/grailsJson
{"first":"foo","second":5}
$ curl localhost:8080/example/restJsonCatch/netSfJson
{first=foo, second=5}

As you can see only grails.converters.JSON converts your response to JSON format. There is no such converter for net.sf.json.JSON, so Grails has no converter to apply and it renders Map normally.

Conclusion: always carefully look at your imports if you're working with JSON in Grails!

Edit: Burt suggested that this is a bug. I've submitted JIRA issue here: GRAILS-9622 render as class that is not a codec should throw exception

DDevelopment & Design

[:en] Operational problems with Zookeeper

This post is a summary of what has been presented by Kathleen Ting on StrangeLoop conference. You can watch the original here: http://www.infoq.com/presentations/Misconfiguration-ZooKeeper I've decided to put this selection here for quick reference. ...This post is a summary of what has been presented by Kathleen Ting on StrangeLoop conference. You can watch the original here: http://www.infoq.com/presentations/Misconfiguration-ZooKeeper I've decided to put this selection here for quick reference. ...