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## Chapter 4. Developing Applications

Applications that take advantage of the OSGi capabilities of Virgo are typically comprised of multiple bundles. Each bundle may have dependencies on other bundles. Furthermore, each bundle exposes only certain packages and services. In this chapter, we look at how to create bundles, import and export appropriate functionality, and create artifacts to deploy web applications on the Virgo Server for Apache Tomcat.

## Anatomy of a Bundle

### 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 publishes services to be used by other bundles:

- Types: via the OSGi `Export-Package` directive,

- Services: via Spring-DM’s `<service ... />` XML namespace element.

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

- Types: via the OSGi `Import-Package` directive,

- Services: via Spring-DM’s `<reference ... />` XML namespace element.

Let’s see an example from the PetClinic sample application. The following listing shows the `MANIFEST.MF` file for the `org.springframework.petclinic.infrastructure.hsqldb` bundle.

    Manifest-Version: 1.0
    Bundle-ManifestVersion: 2
    Bundle-Name: PetClinic HSQL Database Infrastructure
    Bundle-SymbolicName: org.springframework.petclinic.infrastructure.hsqldb
    Bundle-Version: 1.0
    Import-Library: org.springframework.spring;version="[2.5,2.6]"
    Import-Bundle: com.springsource.org.apache.commons.dbcp;version="[1.2.2.osgi,1.2.2.osgi]",
     com.springsource.org.hsqldb;version="[1.8.0.9,1.8.0.9]"
    Import-Package: javax.sql
    Export-Package: org.springframework.petclinic.infrastructure

The `org.springframework.petclinic.infrastructure.hsqldb` bundle expresses its dependencies on the `javax.sql` package, the Commons DBCP and HSQLDB bundles, and the Spring library (we will examine the details of the library artifact in Defining Libraries). The Commons DBCP bundle is imported at a version of exactly 1.2.2.osgi and the HSQLDB bundle is imported at a version of exactly 1.8.0.9. The Spring library is imported at a version between 2.5 inclusive and 2.6 exclusive.

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

The following `osgi-context.xml` file from the PetClinic sample’s `org.springframework.petclinic.repository.jdbc` bundle declares a service published by the bundle and references a service published by another bundle.

    <?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 id="osgiClinic" ref="clinic" interface="org.springframework.petclinic.repository.Clinic" />
        <reference id="dataSource" interface="javax.sql.DataSource"/>

    </beans:beans>

The `service` element publishes the `clinic` bean (a regular Spring bean declared in the `module-context.xml` file) and specifies `org.springframework.petclinic.repository.Clinic` as the type of the published service.

The `reference` elements define a `dataSource` bean that references a service published by another bundle with a an interface type of `javax.sql.DataSource`.

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