CODEWITHSUNDEEP
javaosgiosgi-bundle
TryingToBeACivilEng
TryingToBeACivilEng

Reputation: 11

Osgi bundles and their relation with packages

For an application written using OSGi specifications

A service is a java object that represents a feature, recording a voice call for example Every service has a number of packages associated with it, for example, service interface, service implementation, and slick.

I can't get the relation between the above and OSGi bundles, as bundles contain services does that mean they contain the service packages or something else

Upvotes: 1

Views: 198

Answers (1)

Peter Kriens
Peter Kriens

Reputation: 15372

An OSGi service is a mechanism for bundles to communicate over a well defined Java interface known to all parties. OSGi acts as a broker where one party can register a service under the interface name, and other parties can get it under this interface name. I.e. the actual implementation class is decoupled from the users of the service. This allows the use of different implementation classes while the user of the service is unaware. I.e. for example, if you want to set up a voice call, there could be an PhoneExchange service that would provide you with the functions to setup the call and maybe record it. You can then write your code against the PhoneExchange interface. In runtime, you might be coupled to an Ericsson PBX or a Nokia one.

This decoupling is also present with Java factories. However, Java factories have many disadvantages, and tend to be very static. The OSGi broker is a dynamic broker. A service can be registered but also unregistered. The dynamism in OSGi is seen by many as hard to handle but the Declarative Services makes that more or less trivial. When you really get the dynamism of OSGi services you will find out that many complex dynamic scenarios, like for example communication channels, map extremely well.

The bundle is the module in OSGi. It contains all the Java code and resources. A bundle will export and import Java packages. In a well designed system, these are only the Java packages that hold the service interfaces. It is a bad pattern to export implementation packages. At startup, bundles are wired. This means that exporters get bound to importers. OSGi is unique in that it can handle the same package in multiple versions.

Bundles have their own life cycle. They get started and stopped. In both cases, they can run their own private code.

The aforementioned Declarative Services provide yet another layer. When a Java class declares it is a component with an annotation, it can also declare on what services it depends. When these services are registered, they are injected and the component is activated. Declarative Services allow you to eat your cake and have it to. You get the incredibly powerful dynamics without the associated complexity.

Upvotes: 2

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