Introducing OpenLaszlo 4

 in
Toward desktop-like rich Internet applications with OpenLaszlo 4.

Users' expectations on the Web are changing. No longer content to fill in forms and wait for pages to refresh, modern Web users want to interact with the browser in the same way they do with any other desktop application—that is, interactively and instantly. Recently, this has been made easier by the emergence of AJAX and its JavaScript-centred programming paradigm. But, AJAX is not the only technology that can help here. There is, of course, the ubiquitous Flash technology, but its closed-source heritage puts off the community. And, then there's OpenLaszlo.

Originally a Flash-centric technology, OpenLaszlo Release 4 (hereafter referred to as OpenLaszlo) breaks free from its Flash heritage and supports DHTML as an additional deployment platform. This means that applications written in OpenLaszlo can execute in any browser that supports DHTML or Flash's SWF, which practically covers every major browser on every operating system. As the name suggests, OpenLaszlo is an open-source product, released under the Common Public License, and OpenLaszlo's creators, Laszlo Systems, are keen to see a strong open-source developer community form around this main product offering.

OpenLaszlo is billed as a Rich Internet Application (RIA) development platform. Its goal in life is to add desktop-like functionality to browser-based applications, and it accomplishes this in a non-conventional, yet highly productive way. In this article, I explain how to install and configure OpenLaszlo, and then I present a few small example applications showcasing some of what OpenLaszlo has to offer.

Installing and Configuring OpenLaszlo

OpenLaszlo is a Web development platform built on top of release 1.4 of the Java SDK. Packaged as a Java Servlet, OpenLaszlo can be dropped into any compatible Java Servlet container. The Apache Tomcat server comes with the OpenLaszlo distribution and is already configured and ready to go, so I use it in this article. Whether or not you use Tomcat, it still is necessary to install the Java SDK before attempting to install OpenLaszlo (assuming it's not already installed). If you are on a Debian-derived version of GNU/Linux (like me), installing release 1.4 of the SDK is a breeze:

sudo apt-get install j2sdk1.4

Users of non-Debian distributions should check their package repositories for the Java SDK and install appropriately. Once the Java SDK install is complete, edit the /etc/bash.bashrc file as root, adding the following lines to the end of the file:

export JAVA_HOME="/usr/lib/j2se/1.4"
export PATH=$JAVA_HOME/bin:$PATH

These lines effectively allow Java programs to find the Java runtime environment. Be sure to set these environment variables, as without them, nothing works. With the Java SDK configured, it's time to get OpenLaszlo. Download the latest compressed tarball from the OpenLaszlo site (see Resources), then copy it to your /usr/local directory:

sudo cp openlaszlo-4.0.10-unix.tar.gz /usr/local

At the time of this writing, the latest and greatest OpenLaszlo is release 4.0.10. Be sure to adjust the release number within these instructions if you're using a newer release. Change directory to /usr/local, and unpack the distribution:

cd /usr/local
sudo tar zxvf openlaszlo-4.0.10-unix.tar.gz 

This creates an lps-4.0.10 directory under /usr/local with all the OpenLaszlo goodies unpacked in place. Of importance is the existence of the Tomcat server under the newly created Server directory at lps-4.0.10/Server/tomcat-5.0.24/. To start the server with the OpenLaszlo servlet preconfigured, type:

sudo /usr/local/lps-4.0.10/Server/tomcat-5.0.24/bin/startup.sh

which results in the following output:

Using CATALINA_BASE:   /usr/local/lps-4.0.10/Server/tomcat-5.0.24
Using CATALINA_HOME:   /usr/local/lps-4.0.10/Server/tomcat-5.0.24
Using CATALINA_TMPDIR: /usr/local/lps-4.0.10/Server/tomcat-5.0.24/temp
Using JAVA_HOME:       /usr/lib/j2se/1.4

Tomcat and Openlaszlo are now up and running on port 8080.

Testing OpenLaszlo

An OpenLaszlo test page is provided, and you can access it by typing the following URL into the browser: http://localhost:8080/lps-4.0.10/examples/hello.lzx.

This results in the string “Hello Laszlo!” appearing within the browser after a few seconds. (The first time, OpenLaszlo takes a while to load, but subsequent reloads are as quick as a flash.) Ask your browser to view the HTML source, and a perfectly formed page of HTML is displayed, albeit missing a little human-readable white-space.

The output produced is created by an OpenLaszlo application, written in a declarative, XML-based programming language called LZX. Here's the source code to hello.lzx, which is pretty much run-of-the-mill XML:


<canvas> 
    <text>Hello to Linux Journal from Laszlo!</text>
</canvas>

This simple example illustrates an important point about OpenLaszlo. Openlaszlo's programming language is declarative in nature, not procedural. What this means is that you specify what you want OpenLaszlo to do as opposed to specifying how OpenLaszlo is to go about performing what you want done. OpenLaszlo then works out the series of steps that need to be performed and performs them for you. (In a way, this is exactly like how regular expressions work, in that you specify the pattern you are looking for, not how to find it.) So, when you program OpenLaszlo, you declare the behaviour you require in LZX, and you write LZX in XML. Hard-core programming types might think that writing code in XML is far too unwieldy. But, it's not code per se; it's a declaration of the desired behaviour. Once you get your head around this idea, LZX and OpenLaszlo make quite a bit of sense.

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