Tip
	This is an abbreviated introduction to OSGi bundles. Please refer to the Spring Dynamic Modules for OSGi documentation for full details.

An OSGi bundle is simply a jar file with metadata that describe additional characteristics such as version and imported and exported packages.

A bundle exports types and services to be used by other bundles:

A bundle may import types and services exported by other bundles:

Let's see an example from the hotel booking sample application. The following listing shows the MANIFEST.MF file for the org.springframework.booking.service.internal bundle.

Manifest-Version: 1.0
Bundle-ManifestVersion: 2
Bundle-Name: Hotel Booking Service Implementation
Bundle-SymbolicName: org.springframework.booking.service.internal
Bundle-Vendor: SpringSource Inc.
Import-Package: org.springframework.booking.domain,
 org.springframework.booking.service,
 org.springframework.booking.repository.user,
 org.springframework.booking.repository.hotel
Export-Package: org.springframework.booking.service.internal
Import-Library: org.springframework.spring;version="2.5.4",
 org.hibernate;version="3.2.6.ga"
		

The org.springframework.booking.service.internal bundle expresses its dependencies on the org.springframework.booking.domain, org.springframework.booking.service, org.springframework.booking.repository.user, and org.springframework.booking.repository.hotel packages. It also exposes the org.springframework.booking.service.internal package. The bundle also imports version 2.5.4 (or later) of the org.springframework.spring library since it uses Spring types in its implementation (we will examine the details of the library artifact in Section 5.3, “Simplifying dependencies through libraries” section). It also imports version 3.2.6.ga (or later) of the org.hibernate library for reasons described in Section 8.1, “Working with Hibernate”).

Note that you do not specify the bundle that will provide the imported packages. The SpringSource Application Platform will examine the available bundles and satisfy the required dependencies.

The following osgi-context.xml file for the same bundle declares services exported by the bundle and references to services exported by other bundles.

		
<?xml version="1.0" encoding="UTF-8"?>
<beans:beans
   xmlns="http://www.springframework.org/schema/osgi"
   xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
   xmlns:beans="http://www.springframework.org/schema/beans"
   xsi:schemaLocation="http://www.springframework.org/schema/osgi
       http://www.springframework.org/schema/osgi/spring-osgi.xsd
       http://www.springframework.org/schema/beans
       http://www.springframework.org/schema/beans/spring-beans.xsd">

    <service ref="bookingService"
             interface="org.springframework.booking.service.BookingService" />

    <reference id="transactionManager"
               interface="org.springframework.transaction.PlatformTransactionManager" />

    <reference id="userRepository"
               interface="org.springframework.booking.repository.user.UserRepository" />

    <reference id="hotelRepository"
               interface="org.springframework.booking.repository.hotel.HotelRepository" />

</beans:beans>

		

The service element exports the bookingService bean (a regular Spring bean declared in the module-context.xml file) and specifies org.springframework.booking.service.BookingService as the type of the exported service. The reference elements define various beans as references to services exported by other bundles.