Table of Contents
1. OSGi Revealed
2. Mastering Modularity
3. Learning Lifecycle
4. Studying Services
5. Delving Deeper into Modularity
6. Moving Toward Bundles
7. Managing Bundles and Applications
8. Testing Applications
9. Debugging Applications
10. Component Models
11. Launching and Embedding an OSGi
Framework
12. Security
13. Web Applications and Services
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1
OSGi Revealed
The Java™ platform is an unqualified success story. It is used to develop applications for
small mobile devices to massive enterprise endeavors. This is a testament to its well thought
out design and continued evolution. However, this success has come in spite of the fact that
Java does not have explicit support for building modular systems beyond ordinary object-
oriented data encapsulation.
So, what does this mean to you? If Java is a success despite its lack of advanced
modularization support, then you might wonder if its absence is a problem. Most well
managed projects have to build up a repertoire of comparable, but project specific,
techniques to compensate for the lack of modularization in Java. These include:
Programming practices,
Tricks with multiple class loaders, and
Serialization between in-process components.
However these techniques are inherently brittle and error prone since they are not
enforceable via any specific compile-time or run-time checks. The end result has detrimental
impacts on multiple stages of an application's lifecycle:
Development – you are unable to clearly and explicitly partition development into
independent pieces.
Deployment – you are unable to easily analyze, understand, and resolve requirements
imposed by the collection of independently developed pieces that make up the
system.
Execution – you are unable to manage and evolve the constituent pieces of a running
system, nor minimize the impact of doing so.
It is definitely possible to manage these issues in Java, and lots of projects do so using
the custom techniques mentioned above, but it is much more difficult than it should be.
We're tying ourselves in knots to work around the lack of a fundamental feature. If Java had
explicit support for modularity, then you would be freed from such issues and could
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concentrate on what you really want to do, which is developing the functionality of your
application.
Welcome to the OSGi™ Service Platform. The OSGi Service Platform is an industry
standard defined by the OSGi Alliance to specifically address the lack of support for
modularity in the Java platform. Additionally, it also introduces a new service-oriented
programming model, referred to by some as “SOA in a VM.” This chapter will give you an
overview of the OSGi Service Platform and the issues it is intended to address. Once we have
finished this chapter we will have enough background knowledge to start digging into the
details in chapter 2.
1.1 The what and why of OSGi
The sixty-four-thousand dollar question is, “What is OSGi?” The simplest answer to this
question is it is a modularity layer for the Java platform. Of course, the next question that
might spring to mind is, “What do you mean by modularity?” Here we use modularity more
or less in the traditional computer science sense, where the code of your software application
is divided into logical parts representing separate concerns as in Figure 1.1.1. If your
software is modular, then you can simplify development and improve maintainability by
enforcing the logical module boundaries; we will discuss more modularity details in Chapter
2.
The notion of modularity is not new. The concept actually became fashionable back in the
1970s. So, why is OSGi all the rage right now? To better understand what OSGi can do for
you, it is worthwhile to understand what Java is not doing for you with respect to modularity.
Once you understand that, then you can see how OSGi can help.
1.1.1 Java's modularity limitations
Java was never intended to support modular programming, so we admit that criticizing its
inability to do so is a little unfair. Java has been promoted as a platform for building all sorts
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Figure 1.1.1 Modularity refers to the logical
decomposition of a large system into smaller
collaborating pieces
Module
A
Module
B
Module
C
uses
uses
uses
uses
