I’ve had the idea for this blog post in my mind for a long time. It is based on all my personal experiences and observations over the past years and is in no way scientific. It is my personal opinion, and even though this is how I perceive the world, it might not be true for other people. With that said, let’s jump to the core of things.

A happy developer is a productive developer.

Now here’s something I didn’t know: XSLT 2 and XPath actually support date arithmetic! Took me a while to figure it out, but here’s how it works.

First, all your dates will have to be in ISO-8601 format. For dates only it looks like this: YYYY-MM-DD and for dates and times, like this: YYYY-MM-DDTHH:mm:SS.sssZ. There’s a few other formats, but these are the ones that probably cover all use cases.

In order to make use of all the functions regarding date and time, the values will have to be converted into the appropriate types. They data types are defined in the XMLSchema namespace http://www.w3.org/2001/XMLSchema and are:

Introduction

I’ve been trying to get Google Guice and Peaberry to work in my OSGI projects for a while. Google Guice is a great dependency injection framework, and Peaberry promises to bridge the gap between OSGI services and dependency injection.

However, getting Peaberry to work was not trivial, mostly because there aren’t many docs and the quality of the docs is somewhat lacking. The best piece is still this pdf Peaberry - blending services and extensions, but it’s a lot of information in very little space.

Yesterday I ran into a most strange error when compiling one of my OSGI bundles:

[ERROR] Failed to execute goal org.apache.felix:maven-scr-plugin:1.7.4:scr (generate-scr-scrdescriptor) on project XXX: A type incompatibility occured while executing org.apache.felix:maven-scr-plugin:1.7.4:scr: com.thoughtworks.qdox.model.Annotation cannot be cast to java.util.List

I saw several variations of that error and was initially clueless as how to fix it. However, I eventually found the problem:

@Service(value = { MyService.class, ManagedService.class })
@Component(immediate = true)
@Properties(@Property(name = "foo", value = "bar"))
public class NissanAutoDataUrlGenerator implements ManagedService { }

The mistake is that the @Properties annotation takes an array as it’s parameter. Eclipse does not mark the above as an error, however, the Maven plugin is not happy about it. Here’s the correct code:

I’ve been working on a small Java application I wrote a few years ago for some bug fixes and in the process of making it better, I introduced Google Guice, my favourite dependency injection framework. On of the great features of Guice is that it supports different scopes for injection. Per default, Guice will return a new object for every request. But sometimes you want to objects to be created a bit less liberally, for example, you want a certain object to be created only once. Guice has a @Singleton scope for that. Want an object to be created once for a request? Guice and guice-servlet offers @RequestScoped and SessionScoped. But there’s more, need JUnit per test scope? Guiceberry has exactly that: @TestScoped that will make sure every test gets exactly one object.

Recently I’ve been reading a lot about software development and philosophies on how to write better software. I really enjoyed the Rugged Manifesto as it emphasizes how software is crucial in our world and the responsibilities that come with it. Another great guide is Twelve-Factor App, a list of 12 principles on how to effectively implement and operate applications. And last but not least, Understanding JVM Internals, a really well written cross cut through the JVM, including class file format, JVM structure and stack memory layouts.

If you work with Apache Sling, you have probably encountered the ResourceResolver and its configuration rules. In short, the ResourceResolver is the part of Sling that resolves incoming requests to actual or virtual resources. For example, if a request for /foo/bar is coming in the resolver will resolve that to a corresponding node in the JCR. However, sometimes it is not desireable to expose the internal structure of the repository or the required external structure cannot be represented using the JCR. In that case the resolver can be customized by installing resolver rules. Resolver rules can be modified via the OSGI configuration editor or the OSGI ConfigurationAdmin.

While working on a small toy project using Apache Felix and commons-httpclient, I kept running into the issue that there is no official OSGI bundle for Apache commons-logging out there. While most Apache commons projects either provide simple bundles or full blown OSGI implementations with Activators and Services, commons-logging is an interesting exception. If you scan the Commons OSGI status page, you will notice that there is no OSGI version for commons-logging available, and a separate section to explain why. I haven’t dug into why and how it conflicts with the OSGI classloading scheme, but I can see that being a problem. Be it as it may, there is no official OSGI bundle for commons logging.

I’ve been working with OSGI based technology for a while now and it’s a great piece of technology. However, so far I’ve only worked with OSGI technology that runs in existing instances of the framework. A running framework is easy to deal with, but how do you get to this point? I spent some time today playing with that, and it took me a while to put it all together. So naturally, I’ll have to record my findings here.

I’ve been doing a lot of reading on how OSGI handles class loading. I’m starting to fully understand and leverage OSGI and I’m trying to keep my framework Object Mapper as compatible with OSGI as I can.

In OSGI classloading is very restricted, similar to what happens in enterprisy Java application servers, and completely unlike traditional Java applications where everything shares a single classloader. In OSGI each bundle has its own classloader and what is visible accross the bundle boundaries is subject to strict export rules. In order to use a class outside of a bundle you’ll have to explicitly declare its package as exported. There are many benefits to this strict architecture, clean architectures, reusable components and the ability to update bundles at runtime are probably the most important ones.