Book Review: Modular Java

Modular Java by Craig Walls is a book on building modular Java applications on OSGi platforms.

Published in the Pragmatic Bookshelf series it keeps up to the standards of that great series by presenting content that matters in a format that makes you try it out immediately.

OSGi’s value proposition is to keep complexity in software products manageable. It keeps modules isolated from each other and encourages loose coupling through publishing and consuming services. You can think of OSGi as an incarnation of service oriented archietcure (SOA) within the Java Virtual Machine. OSGi has its roots in the embedded systems environment and became popular for desktop applications with Eclipse using it as their core infrastructure. From embedded to desktop OSGi is currently coming to the server side.

Craig’s book introduces the basics of OSGi, shows how it isolates modules by its unique approach to classpath handling and gives you an overview of the concept of OSGi services. One easy to follow example application is used consistently throughout the book to show the various aspects.
Most of the time the book uses Equinox as runtime but Felix and Knopflerfish are also mentioned. For building bundles Modular Java makes use of Pax especially Pax Construct. It does not mention Bundlor, Bnd or Tycho in depth. Especially the Maven based Tycho stack sounds really promising so it’s unfortunate that it isn’t covered. I guess however this is just an attribution to the current speed of development in the OSGi tooling space.

Spring Dynamic Modules are an attempt to bring the principles of Spring Framework to OSGi. In the spirit of Spring Framework Dynamic Modules (DM) build on proven solutions (OSGi in this case) and make them easier to use. They eliminate a lot of boilerplate code that is normally require to handle OSGi services that may appear and disappear at any time. Spring-DM also provides integration with Spring Application Contexts and has support for web applications through an extender. Spring-DM is covered really well by the book. An appendix describes the new OSGi Blueprint Services that are an attempt to standardize the ideas of Spring-DM. Spring’s new dm Server is not covered.

The book focuses on the core concepts and shows the benefits of using OSGi for application development. The target audience are experienced Java developers. It is very well written, easy and fun to read and serves as a great introduction. I recommend the book to Java developers who consider making use of OSGi in future projects.

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