Sizing Up Open Source Java Persistence : Page 2

The Contenders
What is a Java persistence framework (also referred to as a Java object-relational mapping mechanism)? It's the part of a program that moves data in Java objects to and from a relational data store. There are several persistence frameworks. Of course, you can also write your own using the JDBC API.

The goal of these frameworks is to reduce the amount of Java code (in other words your own JDBC code) needed to get the Java object data in and out of the database. This article compares the following frameworks:

• Hibernate, Version 3: The very popular open source persistence framework
• JPOX JDO, Version 1.2: The reference implementation of Sun's persistence Java Data Objects 2.0 specification.
• Castor JDO, Version 1.0.5: An open source framework that provides Java-to-XML binding and Java-to-SQL persistence. (Side note: Even though JDO is in its name, Castor JDO is NOT a JDO implementation. To avoid confusion, I’ll refer to it simply as Castor in this article.)
• iBatis for Java, Version 2.2: Another open source, lightweight data persistence framework.
• JDBC: The JDBC API is provided with any standard Java Standard Edition download. Homegrown JDBC solutions provide a baseline for comparison for all frameworks.

A student of the Java persistence problem will recognize that this is not a complete list of options. There are several commercial persistence frameworks such as Oracle's TopLink, Thought Inc.'s CocoBase, and ATG's SQL repository mechanisms. There are also other lesser-known open source frameworks. Many of these can be found at www.java-source.net. The Spring framework also offers JDBC abstraction although it "integrates out of the box" with a number of the frameworks examined here. And there is, of course, Enterprise JavaBeans 3.0 and the relatively new Java Persistence API that originated as part of the work of the JSR 220 (EJB 3.0). So, while there are other options, this article concentrates on those frameworks that are free (open source), relatively popular and do not require a Java Enterprise Edition (JEE) container.

Framework Acquisition and Setup
Each of the persistence frameworks is very simple to download and get setup. In fact, all of the frameworks here simply require adding the appropriate libraries to the build and execution paths. Most of these, as open source frameworks, require additional open source libraries for tasks such as XML parsing and logging. Except for Castor, the persistence framework download obtained from the project Web site will contain both the framework JAR as well as all the required libraries (except for the database driver). With Castor, you will have to make an extra visit to some additional projects' Web sites for required libraries. Table 1 provides an overview of what is downloaded and required for using each of the frameworks in your Java application.

Persistence Framework	Version	Download included libraries	Additional libraries needs
Hibernate	3.0	antlr.jar cglib.jar asm.jar asm-attrs.jars commons-collections.jar commons-logging.jar hibernate3.jar jta.jar dom4j.jar ehcache.jar c3po.jar	database driver
Castor	1.0.5	castor-1.0.5.jar	commons-logging.jar xercesImpl.jar database driver
JPOX JDO	1.2	jdo.jar jpox.jar log4j.jar	database driver
IBatis	2.2.0	ibatis-common-2.jar ibatis-sqlmap-2.jar	database driver
JDBC	Java 1.5	JDBC API with standard Java	database driver

Table 1. What's required? An overview of what is downloaded and required for using each of the frameworks in your Java application.

IDE Considerations
One development consideration when building an application that uses any of the frameworks is to look at the support your favorite IDE gives in XML editing. It's common practice to configure applications now by XML and these persistence frameworks make heavy use of XML in object-relational mapping. Most of these IDEs allow you to create/edit an XML file, but the good ones also provide code assist based on the persistence framework's Document Type Definitions. Each of the frameworks configuration files are structured via DTD.

JDO is a little unusual in that it also requires an additional step before you can deploy/execute an application—a special pre-deployment operation. After compiling your persistence-capable Java objects into the standard class byte codes (.class files), JDO requires you run these classes through a special JDO compilation called byte code enhancement. Below is an example of the JDO compile command that is completed on the persistent classes that result in modified byte code files:

java -cp C:\NetBeansProjects\Persistence\build\web\WEB-INF\classes;
c:\java\jpox\jpox-enhancer-1.1.4.jar;c:\java\jpox\jpox.jar;
c:\java\jpox\jdo.jar;c:\java\jpox\log4j.jar;c:\java\bcel-5.2\bcel-5.2.jar 
-Dlog4j.configuration=file:log4j.properties org.jpox.enhancer.JPOXEnhancer 
-v com\intertech\domain\package.jdo

Therefore, in addition to the libraries listed above, JDO also requires you (and your development environment) to use a JDO tool provided with the JDO libraries. In many cases, you'll want to create an Ant task to perform this activity.

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