Now that we have some hands-on experience with OSGi and the first curiosity is satisfied, let’s lean back and have a closer look at what OSGi is all about. This is more difficult than might be expected, because OSGi addresses several, mostly independent topics.
The big challenge when doing a modular design is to find the right modules. The big challenge when implementing a modular design is to prevent developers from using some other module’s classes that are not part of the module’s public API. There is only a limited provision for this in Java. You can hide classes in a package by declaring them as “protected”1. This mechanism supports very simple modularity at the package level: a public class serves as API and protected classes in the same package implement the required functionality. The approach falls apart when your module becomes too complex to be implemented in a single package. If you use different packages for structuring your implementation, you have to change the visibility of the classes in sub-packages to public, else you won’t be able to use them within your module2.
One means to prevent programmers from using classes that are intended for a module’s internal use only is to do regular dependency analysis using source code analysis tools3. Of course, this requires that the information about the modules’ APIs (the permitted use of interfaces and classes of a module) is maintained properly and that the compliance with the rules is enforced organizationally4.
Formally supported modularity is therefore a highly desirable feature in any
non-trivial project5. As mentioned already, OSGi provides such
a support: Everything that a module
comprises of is assembled in a jar (the “bundle”), and the Java packages that
are provided or required are explicitly declared in that jar’s manifest by
Import-Package headers. The OSGi framework uses this
information to configure class loaders in such a way that the restrictions
are enforced at runtime.
An alternative approach is JBoss Modules. However, it seems that this approach hasn’t caught on outside its initial usage in JBoss Application Server 7.
Coming with Java 9, we now have support for modularity from the “Java Platform Module System” (JPMS), as defined in JSR 376. JPMS has its roots in project Jigsaw, which aimed at providing a modular JDK. Contrary to OSGi, JPMS checks and enforces the rules defined for modularity at compile-time. It does, however, not define a service layer and supports neither sophisticated versioning nor dynamic modules (see below). Only time will show if the approach developed with a focus on providing a modularized platform is equally well suited to build modularized applications. An article on the topic distinguishes those two areas and claims that OSGi has advantages over JPMS as an application module system6.
A nasty property of modules is that they tend to evolve over time. The changes can be bug-fixes, additional features offered by a package or more fundamental changes of the API. In order to be able to track the changes, it is necessary to assign a version to every published package and to clearly state the version (or version range) that a modules is prepared to accept for imported packages.
In the OSGi context, version numbers follow the convention of
The basic idea is that changes in the third part of the version
number indicate bug fixes, i.e. the API is unchanged. An increment
of the second part of the version number indicates a backward compatible
change of the API (backward compatible from the point of view of the
user of the API). A change of the first part indicates some major
change of the API. Usually, modules programmed against version
will not be able to use an implementation with version
This is the reason why Bndtools generated a version range
when we added the import
of the framework.
The OSGi specification defines a life cycle for bundles. This includes that bundles can be installed into and removed from a running framework at any time. While this feature is certainly nice to play with, it causes developers a lot of trouble, especially the latter possibility to remove a bundle.
Handling added modules is easy. It is common in an application that modules are started one after the other. Often, you are in control of the startup, because you have to invoke some kind of initialization method before being able to use the module. The required startup code can also be hidden in the module’s implementation using a static object that triggers the execution of arbitrarily complex code as part of its initialization. The classloader logic ensures that any required classes are initialized before the triggering module. And after the initial startup, you can extend your program further by setting up additional class loaders and use them to load more classes.
The use case for removing classes from a running system is less common.
It’s not easy to do this because it requires that there are no more
instances of the class that you want to remove. Enforcing this is
effectively impossible. Our simple bundle is an example of the
omplexity of the problem7. We cannot simply establish a
reference to the log service at startup and then use it for the
rest of the program. Rather, we have to provide a significant
amount of code8 to handle the case that the log service
goes away, thus invalidating the reference. Of course, we could simple
keep the reference despite the notification about the removal of the
log service, which would prevent the log service object and its class
definition from being garbage collected. But this attempt to adhere
to the service doesn’t guarantee that it continues to work. Chances
are high the we get some
RuntimeException if we call a method after
the module has been removed. So we have to take all the trouble to
either work around the unavailability of the other module or
“invalidate” (stop) our own module if another required module
There are two reasons why you would want to remove a module from a running system. The first is resource consumption. When OSGi started in 1999, hardware wasn’t as cheap as it is today. I found a post by Peter Kriens9 from 2005 where he wanted to evaluate OSGi on a device with 32-64MB RAM and 2-8MB Flash, USB and Ethernet costing less than $100. Compare these requirements with what you get nowadays for $30 (Raspberry Pi B+, 900MHz quad-core, 1GB RAM for $25; flash depending on the microSD, e.g. 8GB for $5). But of course, depending on the requirements, resources may still be too limited to put everything at work at once. Today’s cheap availability of the resources, however, oftem leads to solving the problem on a different level. Take Android as an example. The apps aren’t installed as modules into a single virtual machine. Rather, you have the “old fashioned pattern” of an operating systems that runs the apps as independent process.
The other reason why you’d want to remove a module is the desire to replace it with an improved (often bug-fixed) version without shutting down the complete system. Obviously, this doesn’t apply to the common desktop application or a server application with a small group of users. It may be a point for an embedded system that is part of your infrastructure. And it may be a point for 24x7 enterprise systems – though those usually have a fail-over concept that e.g. allows you to provide the new version on a secondary system and switch over. Consider the (additional) testing that has to be performed for bringing a new module into a running productive system. Keep in mind that “No matter how clean the framework, administrators will always prefer a freshly booted machine.” (Caucho Blog) And, of course, consider that being able to replace a module does not mean that you have an uninterrupted service. Other modules may depend on the module being replaced and may go down until the substitute becomes available, just like our simple bundle will, when you replace the log service10.
Dynamic modules are something that most scenarios don’t need. Nevertheless, when using OSGi you buy into them and and this can cause of lot of additional effort if you don’t make them easy to use (or even better: transparent) for the developers, using one of the techniques shown in the next parts of this introduction.
It has been mentioned before, that
loose coupling between the components is an important aspect of a modularized design. The functionalities made available (called “services” by OSGi) are defined by Java interfaces. The challenge is to provide a mechanism that allows the user of a service to find a provider for the service, i.e. an instance of a class that implements the interface. JavaSE provides such a mechanism (since JavaSE 6) in the form of the ServiceLoader. The service loader implements a rather static approach. It searches all jars in the classpath for files “
META-INF/services/<interface name>”. The file with a given interface name contains the name of an implementation class for that interface.
The OSGi core framework defines a different, more dynamic approach as its “Service Layer”. This layer works almost independent of the module related features described above. It depends on bundles only in so far as a relationship between services and the bundles that provide them is maintained11. This relationship is used by the framework to e.g. automatically remove all services provided by a bundle if this bundle is stopped.
At the core of the service layer is the service registry. It can be
accessed using methods from the
Bundles that provide services register them with the registry, usually
in the activator’s start method. Bundles that need a service look it
getServiceObjects, which returns all registered services
of the requested kind. The service objects are instances of classes
that implement the required interface. After the initial lookup,
there is no overhead in using an object returned by the service registry.
Changes of the service registry can be tracked by registering service listeners. This allows components to react to the appearance or disappearance of a service. (This is what we did in our simple bundle with the log service.) The service registry and its operations should be considered implementation primitives. They are powerful enough to build anything from them, but you’ll want something more sophisticated for every day’s work. Or, to quote Peter Kriens: “The original model of handling your service dependency manually is and was broken from the start”. The service tracker was introduced as a minor amendment in release 2 (October 2001)12. But as we have seen, it fails to really simplify the implementation of dependent services.
The only way to use the OSGi service layer efficiently is to wrap it in some “higher level API”. Actually, the best way to manage services is to externalize the task of registering and unregistering them by providing a component that takes care of the registration process based on information supplied by the bundles and also uses this information for dependency resolutions. There are several approaches to this (see next part). The good thing is that you don’t have to make a final choice at once, as most solutions are based on the OSGi service layer and can therefore interoperate. The bad thing is that your application is rarely ever based on OSGi alone. Usually you’ll have some OSGi based stack – which implies a steeper learning curve.
The OSGi alliance didn’t stop at defining a framework for modules and service in the “OSGi Core” specification. Rather, they have also defined various services based on and to be used in that framework. A complete list of these services can be found in the “OSGi Compendium” specification.
The Services can roughly be grouped in three categories:
Don’t try to read the compendium and understand the purpose of all services. The descriptions usually lack an introductory illustrative use case. Rather, keep the titles in mind and come back to the service descriptions when you face a use case that might be related with the service.
You should also not assume that the services are best-of-breed in all cases. I admit, that – after working my way through the log service specification – I was a bit disappointed to find a remark by Peter Kriens that the OSGi alliance decided to not upgrade the “14 years old” service because there are “already so (too) many logging systems around”13.
It would be ridiculous to claim that I could summarize the more than 400 pages of the framework specification (not to speak of the compendium) in a single part of this introduction. I think I have covered the topics that you need to get started. As I have already recommended with regard to the services from the companion specification, have a look at the titles of the framework specification’s chapters and return to them when something “rings a bell”.
Or declaring no visibility at all, as “protected” is the default visibility. ↩
A “fine grained” access control to the content of Java packages using “normal” Java syntax was proposed in the (now withdrawn) JSR 294. A lot of information that went along with it seems to have vanished from the web together with the withdrawal. I found some explanations in a blog and in an article about new features in Java 7 (written too early, obviously). The cause for the withdrawal is the advent of the “successor” JSR 376 (see below). And politics. Java and modules seems to be a mined area. ↩
When using Java, those are usually source code/byte code analysis tools, i.e. they can analyze dependencies even if only the byte code is available. ↩
This usually falls apart when a developer claims that the only way to meet the deadline is to let him use a particular class, “no matter what the rules say”. ↩
If you became curious about OSGi because it is used in Eclipse, you should note that the modularity is the main usage of OSGi in Eclipse (together with the dynamic modules – though considering the constant prompt for a restart after installing a plugin, Eclipse doesn’t seem to believe in this feature). ↩
I admit that I am mixing up module dependencies and service dependencies here. Take it as an illustration of the underlying problem. I don’t want to complicate things by introducing a “pure module” example. ↩
Let me emphasize again that there are means to greatly simplify this for the programmer (see below). But we should have a clear idea about what kind of functionality fully dynamic modules actually provide and at what cost. ↩
One of the drivers behind OSGi. ↩
If there is a dependency between the modules on the service
level only (and you have put the interface in a bundle of its own),
you can alternatively add the new version in parallel before removing the old version. Properly programmed consumers (such as our simple bundle) will switch to the new version on-the-fly by updating the reference. However, modules that take possession of a unique resource (e.g. a TCP server port) cannot coexist in two versions, so there are limits to this approach. ↩
Which makes it impossible to use the service layer in your code without providing the code in bundles. My impression is that some people looking for a service layer stumble upon the OSGi platform, and then complain about its complexity when they notice that they have to build modularized applications in order to use the OSGi service layer. ↩
I think it’s unfortunate that an explicit reference to the very basic character of the service layer hasn’t found its way into the specification of the service layer, not even into the latest release 6. ↩