Maven has been around for a long time and is heavily used by a very large group of people. The greatest advantage of Maven is that it is very easy to use the outputs of Maven projects through their repository model. Especially Maven Central has grown to be a humongous collection of JARs and other files. In April 2016 there were almost 1.2 million JARs in the repository.
The Maven repository model is largely build around a naming scheme and a mapping from that naming scheme to file and URL paths. If you’re familiar with Maven then please don’t skip the next section where we define this scheme, we introduce some novel names to define this model a bit more formal and less ambiguous.
This naming scheme is based on:
groupId – A general grouping name of programs.artifactId – A general name for a program.version – A version (free form) that identifies a revision.extension – An extension identifies the packaging of a file. The pom extension is special, it identifies the Project Object Model (POM).classifier – The classifier is used to have multiple files in a revision. There is always the target of the revision and the pom file, but Maven does allow archives for sources, javadoc, tests, etc. The classifier identifies these additional archives.snapshots – Maven has different workflows for snapshots and final released versions. Snapshots are identified by a -SNAPSHOT extension of the version. Snapshots can be released multiple times while in principle an actual release can only be released once.Maven identifies archives with a combination of these identifiers, these are known as gavs or coordinates. There are several competing short form of the coordinates. In bnd, the following syntax is used to specify archives:
archive = groupId ':' artifactId ( ':' extension ( ':' classifier )? )? ':' version ( '-SNAPSHOT' )?
For example, com.google.guava:guava:jar:jdk5:19 is the coordinate for the released Guava library compiled for JDK 5.
If you’re familiar with Maven you will find that the concepts of program, revision, and archive are not defined by Maven. They are introduced here because they remove a lot of the ambiguity in the Maven documentation. (For example, what is an artifact?) They are defined as follows:
program – This is basically the combination of the groupId and artifactId. It defines a name but this name does not identify anything concrete. A program can have multiple revisions.revisions – A revision is named by a program and a version and has a collection of archives.archive – A file in a revision. The file with the pom extension contains the project information. Archives are identified by an extension and a classifier. Extensions are closely related to packaging.The bnd Maven Repository is a bnd repository that provides the following functions for bnd users:
The transitive dependency model of Maven (where you drag in all the dependencies of your dependencies) is highly popular among Maven users because it really solves a problem. Still, in bnd (and indirectly OSGi) there is no support for this oh so useful feature. A lot of Maven aficionados give up on bnd for this reason before they fully understand why OSGi does not full heartedly support this model. After all, it is conceptually a simpler model than the OSGi model.
The reason is identical to why Java got interfaces. The goal of OSGi is to create systems out of reusable components. The problem with the transitive dependency model is that it suffers from the same condition as pre-interface Object Oriented applications: [The Big Ball of Mud]. Without interfaces, your code does not only drag in the direct dependencies of the components you use, you also drag in any implementation dependencies.
In OSGi, instead of requiring another component, we require a capability. A capability is for example a specific service or package. Using the resolver we can then assemble applications. Therefore, the mindset in the OSGi world is to care deeply about your dependencies. The actual dependency graph is a primary citizen in the architecture design and not a consequence of what projects do.
This is quite a different philosophy that does generate some pain up-front. However, the resulting systems are significantly simpler.
So please bear with us even if it looks disgusting to you.