OpenJPA is an open source implementation of the Java JPA (Java Persistence API) specification from Apache. JPA provides an agnostic Java-based API for storing and retrieving information to a backend database. It has a canonical query language named Java Persistence Query Language, or JPQL, that blends with the programming methods of Java and eliminates the need to tailor database queries for a particular database. However, JPA also supports native SQL which can be used for quick ports with a known backend database. This tutorial is designed to walk you through the steps of setting up a simple web application to use OpenJPA Geronimo and to transact the derby database that comes with Geronimo. The tutorial code uses a simple Java Server Page (JSP), backed up by some basic classes. It displays a table of inventory items and categories. In this tutorial, we will not dive into details regarding the JSP code. Its purpose is to be a window through which you can examine OpenJPA. The intended audience for this tutorial is those with some knowledge and understanding of the Java programming language and who are just beginning with OpenJPA. To start, you must download the following requirements and install them on your computer. For the purposes of this tutorial, we are using Eclipse as the IDE and Microsoft Windows as the operating system of choice.
Geronimo V2.2: You can get it here . Download this file and unzip it to a permanent location. There is no installer. The server will run from the command line.
Eclipse V3.2 or later: This version has annotation support included. Annotations play a large role in OpenJPA. Download Eclipse 3.2 or later.
Apache OpenJPA library: For the purpose of implementing this tutorial you can select OpenJPA v1.2 or greater. You can download Apache OpenJPA from the Apache site. Note that the Milestone (openjpa-all-2.0.0-M3.jar as of this writing) is an early release of OpenJPA 2.0 and may have some instabilities. No issues have been noted for the usage in this tutorial.
The tutorial code files: These files are provided with this tutorial. You will add them to your Eclipse project.
Setup and Running the Sample
Now, that you have all the prerequisites for this tutorial downloaded and installed, the following sections will walk you through the Eclipse project setup and the OpenJPA configuration. Make sure you read through and follow each part carefully.
Setting up Eclipse
After installing Eclipse, create a new project in a dedicated workspace for the tutorial. Complete the following setup instructions: First, make sure your Eclipse environment is updated and has the Geronimo plugin. If you do not know how to do that, follow the instructions found at the Geronimo website .
Running and Configuring Geronimo and Derby
Geronimo has no installer and runs from the command line. Here are some quick instructions to get you started.
Running and Deploying the Sample Code in Geronimo
The sample code provided with this tutorial is working code. It is a simple inventory database program that shows items and categories. But even this simple example requires the ability to add, edit and delete entries. It requires the ability to sort and filter database queries and it requires the identification of the relationship of the items to the categories. In this example, the relationship is one to many. Knowing that relationship is important to how the code is written. Before we analyze the code and OpenJPA, we will first deploy the sample and see it work. To deploy the sample code follow these instructions:
Examining the Sample Code
Now that everything is set up and you have seen it work, let's look more closely at the parts of the code that use OpenJPA. The JSP code is just a prop to explore OpenJPA and we will not examine it.The sample application source code is provided for this tutorial and you may run as-is with no customizations. However, you have the option of reproducing the code manually using the following explanations. Whichever method you choose, locate the code that corresponds to explanations as you follow along.