Jitsi is an application that allows people to make video and voice calls, share their desktops, and exchange files and messages. More importantly it allows people to do this over a number of different protocols, ranging from the standardized XMPP (Extensible Messaging and Presence Protocol) and SIP (Session Initiation Protocol) to proprietary ones like Yahoo! and Windows Live Messenger (MSN). It runs on Microsoft Windows, Apple Mac OS X, Linux, and FreeBSD. It is written mostly in Java but it also contains parts written in native code. In this chapter, we'll look at Jitsi's OSGi-based architecture, see how it implements and manages protocols, and look back on what we've learned from building it.

The three most important constraints that we had to keep in mind when designing Jitsi (at the time called SIP Communicator) were multi-protocol support, cross-platform operation, and developer-friendliness.

From a developer's perspective, being multi-protocol comes down to having a common interface for all protocols. In other words, when a user sends a message, our graphical user interface needs to always call the same sendMessage method regardless of whether the currently selected protocol actually uses a method called sendXmppMessage or sendSipMsg.

The fact that most of our code is written in Java satisfies, to a large degree, our second constraint: cross-platform operation. Still, there are things that the Java Runtime Environment (JRE) does not support or does not do the way we'd like it to, such as capturing video from your webcam. Therefore, we need to use DirectShow on Windows, QTKit on Mac OS X, and Video for Linux 2 on Linux. Just as with protocols, the parts of the code that control video calls cannot be bothered with these details (they are complicated enough as it is).

Finally, being developer-friendly means that it should be easy for people to add new features. There are millions of people using VoIP today in thousands of different ways; various service providers and server vendors come up with different use cases and ideas about new features. We have to make sure that it is easy for them to use Jitsi the way they want. Someone who needs to add something new should have to read and understand only those parts of the project they are modifying or extending. Similarly, one person's changes should have as little impact as possible on everyone else's work.

To sum up, we needed an environment where different parts of the code are relatively independent from each other. It had to be possible to easily replace some parts depending on the operating system; have others, like protocols, run in parallel and yet act the same; and it had to be possible to completely rewrite any one of those parts and have the rest of the code work without any changes. Finally, we wanted the ability to easily switch parts on and off, as well as the ability to download plugins over the Internet to our list.

We briefly considered writing our own framework, but soon dropped the idea. We were itching to start writing VoIP and IM code as soon as possible, and spending a couple of months on a plugin framework didn't seem that exciting. Someone suggested OSGi, and it seemed to be the perfect fit.

People have written entire books about OSGi, so we're not going to go over everything the framework stands for. Instead we will only explain what it gives us and the way we use it in Jitsi.

Above everything else, OSGi is about modules. Features in OSGi applications are separated into bundles. An OSGi bundle is little more than a regular JAR file like the ones used to distribute Java libraries and applications. Jitsi is a collection of such bundles. There is one responsible for connecting to Windows Live Messenger, another one that does XMPP, yet another one that handles the GUI, and so on. All these bundles run together in an environment provided, in our case, by Apache Felix, an open source OSGi implementation.

All these modules need to work together. The GUI bundle needs to send messages via the protocol bundles, which in turn need to store them via the bundles handling message history. This is what OSGi services are for: they represent the part of a bundle that is visible to everyone else. An OSGi service is most often a group of Java interfaces that allow use of a specific functionality like logging, sending messages over the network, or retrieving the list of recent calls. The classes that actually implement the functionality are known as a service implementation. Most of them carry the name of the service interface they implement, with an "Impl" suffix at the end (e.g., ConfigurationServiceImpl). The OSGi framework allows developers to hide service implementations and make sure that they are never visible outside the bundle they are in. This way, other bundles can only use them through the service interfaces.

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The Architecture of Open Source Applications: Jitsi