5. The Middle Tier

5.1 Introduction

GreenPages’ middle-tier provides implementations of the Directory and Listing interfaces that can be used by the Web bundle. The implementation will use EclipseLink JPA to access a database via a DataSource published in the OSGi service registry.

The database

The GreenPages application uses a very simple database that contains a single table. The table, named LISTING, consists of four columns:


Scripts are provided with the sample source code (in $GREENPAGES_HOME/db) to start, create, and populate the database. These will be used during the creation of the middle tier.

Using JPA

The middle tier will provide JPA-based implementations of the Directory and Listing interfaces with the four attributes of a Listing (first name, last name, email address, and id) being mapped to the corresponding columns in the LISTING. JPA will be used to implement the queries that search the database and return Listings.


The middle tier consists of two bundles, greenpages.jpa that publishes a Directory implementation for consumption by the Web bundle, and greenpages.db to configure and publish the DataSource used to access the database.