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iPOJO in 10 minutes

This page presents how to use the iPOJO runtime and its associated service component model. The concepts of the service component model are introduced, followed by a simple example that demonstrates the features of iPOJO.

Introduction

IPOJO iPOJO aims to simplify service-oriented programming on OSGi frameworks; the name iPOJO is an abbreviation for injected POJO. IPOJO iPOJO provides a new way to develop OSGi service components with the main goal being to simplify service component implementation by transparently managing the dynamics of the environment as well as other non-functional requirements. The iPOJO framework allows developers to more clearly separate functional code (i.e., the POJO) from the non-functional code (i.e., dependency management, service provision, configuration, etc.). IPOJO iPOJO combines the functional and non-functional aspects at run time. To achieve this, iPOJO provides a simple and extensible service component model based on POJOs.

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POJO is an acronym for Plain Old Java Object, but it embodies a concept that the simpler and less intrusive the design of a given framework, the better. The name is used to emphasize that a given object is not somehow special, but is an ordinary Java Object. Martin Fowler, Rebecca Parsons and Josh Mackenzie coined the term POJO in September 2000: "We wondered why people were so against using regular objects in their systems and concluded that it was because simple objects lacked a fancy name. So we gave them one, and it's caught on very nicely." From the developer's perspective, the iPOJO framework strives to only require POJOs in as much as it is possible.

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iPOJO service component overview

A service component is able to provide and/or require services, where a service is an object that implements a given service interface embodied as a Java interface. In addition, iPOJO introduces a callback concept to notify a component about various state changes.

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Presentation of the Spell application

To illustrate IPOJO iPOJO features, we will implement a very simple application. This application is composed by three components:

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This class implements the SpellChecker service interface as it will provide it. Moreover, it has a special field "_m_dictionary_". This field represents the required service. IPOJO iPOJO will inject a Dictionary service when needed. So, the class can use it directly. Notice that this class as no OSGi specific code, both the service providing and the requiring are managed by iPOJO. If the used dictionary service leaves, iPOJO will try to find another provider. If no more providers are available, the instance is invalidated, and the provided service is withdrawn from the service registry.

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Look at the three last methods. The check methods collect the user input and use a Check speller service to check this input. This method is called when the button is pressed by the user. The start and stop methods are lifecycle callbacks. As we need to display the user interface when the instance is created and to delete it when the instance stops, we need a way to be notified when we need to execute these actions. IPOJO iPOJO provide an easy way to do this. The component provides two callback methods for its activation and deactivation, start and stop, respectively. Callbacks are used when the component needs to be informed about a component state change. In iPOJO, the component state is either INVALID (i.e., not all of the component's constraints are satisfied) or VALID (i.e., all of the component's constraints are satisfied). In this example, the start callback method set the GUI visibility to true; the stop callback method deletes the GUI. The component metadata will instruct iPOJO to invoke these methods when the component's state changes to VALID or INVALID respectively.

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-> ps
START LEVEL 1
   ID   State        Level  Name
[   0] [Active     ] [   0] System Bundle (1.0.3)
[   1] [Active     ] [   1] Apache Felix Shell Service (1.0.0)
[   2] [Active     ] [   1] Apache Felix Shell TUI (1.0.0)
[   3] [Active     ] [   1] Apache Felix Bundle Repository (1.0.2)
[   4] [Active     ] [   1] iPOJO (0.7.6.SNAPSHOT)
->

IPOJO iPOJO runtime is the bundle 4. Once started, install the four created bundles as above:

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• Publish OSGi services
• Require OSGi services
• Use lifecycle callbacks to activate and deactivate components

IPOJO provide iPOJO provides a lot of others features that you can try in the others available tutorials. Subscribe to the Felix users mailing list by sending a message to users-subscribe@felix.apache.org; after subscribing, email questions or feedback to users@felix.apache.org.