fmeschbe's blog

Bringing Back the Fun Apache Sling brings back the fun to Java developers and makes the life of a web developer much easier. It combines current state of the art technologies and methods like OSGi, REST, scripting, and JCR. The main focus of Sling deals with the important task of bringing your content into the web and providing a platform to manage/update the content in a REST style. Sling is built as OSGi bundles and therefore benefits from all advantages of OSGi. On the development side a scripting layer (leveraging the Java Scripting API) allows to use any scripting language with Sling (of course you can use plain old Java, too). And on top of this, Sling helps in developing an application in a RESTful way. As the first web framework dedicated to JSR-283 Java Content Repositories, Sling makes it very simple to implement simple applications, while providing an enterprise-level framework for more complex applications. Underneath the covers Apache Jackrabbit is used for the repository ...

At times you want (or need) to include third party libraries in your bundles. You basically have two options to do that: Embed the complete library Inline the required classes/packages (or everything) Until recently I was in the camp of embedding the complete libraries and setting the Bundle-ClassPath manifest header accordingly. The Embed-Dependency directive of the Apache Felix Maven Bundle Plugin makes this extremely easy to do. Lately, though, this has been questioned by Karl Paul's pojosr project. This project brings the OSGi Service Registry to regular applications. The nice thing here is, that it really is a stripped down OSGi Framework basically removing the modularity pillar. The drawback is that this causes the Bundle-ClassPath manifest header and embeddeded libraries to not be supported. Thus to run your regular bundle inside pojosr, you will have to inline all third party libraries instead of embedding them. The upside is that this really works. For example the Apac...

After subscribing to the OSGi Planet feed I felt like starting to read some old blog posts and stumbled upon a series of posts by BJ Hargave around the issues of the Eclipse ContextFinder caused by the Class.forName methods. For the full story please go and read Class.forName caches defined class in the initiating class loader (and folow the links !). So, these posts caused me to try and look how we behave in Apache Sling ... and of course hoped we would be clean. Well, hmm, turns out we are not ... I found nine classes using Class.forName . So we probably have to clean this up. Maybe or maybe not, these uses may be the cause for some strange failures we had over time. I cannot really tell. But I cannot exclude this possibility either. BTW, this is what I did to find the classes: $ find . -name "*.java" -exec fgrep -l Class.forName {} \;

I have been thinking about using version numbers lately while working on some API extension of the Sling Engine bundle. So here is what I think versions are all about and that we all should be very careful when changing code and assigning versions to it. On a high level versions have various aspects: Syntax There is no global agreement on the correct syntax of versions. I tend to like the OSGi syntax specification: The version has four parts separated by dots. The first three parts are numbers, called the major, minor and micro version. The fourth part is a plain (reduced character set) string which may be used to describe a particular version. Version numbers are compared as you would expect, except that the fourth part is employs case-sensitive string comparison comparing the actual Unicode codepoints of the characters. Semantic The semantics of a version define what it means to increment each place of a version. In the world of software development there is even less agreemen...

In the Apache Sling project we have an interesting problem : Knowing when the application has finished its startup. Coming from a background of a traditional application, you know when the system has finished its startup. For example, a servlet container knows it has finished the startup, when all web applications have been started. In Apache Sling, the situation is a bit different: Apache Sling is an extensible system, where extensions may simply be added by adding more bundles. "Easy", you say, "just wait for all bundles to have been started and you know when the application is ready". True, but there is a catch. To extend Apache Sling, you register services with the OSGi registry. "Still easy", you might say. Right, if the services are all started by bundle activators, we still can depend on having all bundles started for the system to be ready. Again, this is only part of the story: Some services depend on other services. So the dependent services may...

The OSGi framework and its compendium services provide a whole lot of fun to build applications. Defining bundles is a cool stuff to cut the big job into pieces and enjoy the coolness of separation of concerns just like the old Romans said: Divide et Impera ! One interesting compendium specification is the Declarative Services Specification. This specification tries and IMHO succeeds very well to bring some of the cool stuff of Spring, namely Dependency Injection, into the OSGi world. Just like the application descriptors in Spring you have component descriptors in Declarative Services. Using a component descriptor, you define the following properties of a component: The name of the component and whether it is activated immediately or not Whether the component is a service and the service interfaces to register the component with Which other services are used by the component. These services may be injected (bound in OSGi speak) or may be looked up. There is also the notion of mandator...