Entity Beans

Last month, we began to look at Enterprise JavaBeans (EJB), the centerpiece of Sun's J2EE (Java 2 Enterprise Edition) standard for server-side web applications. While neither the Java language nor the J2EE specification are open standards, increasing numbers of Linux advocates have begun to use them to write server-side web applications. The fact that J2EE appears to be the only mainstream alternative to Microsoft's .NET framework makes it even more appealing to many.

Enterprise JavaBeans come in two basic flavors, known as session beans and entity beans. Session beans typically model processes and lack any state, allowing us to place our business logic in EJB, rather than in our servlets or JavaServer Pages (JSPs). The calculator object we designed last month, which allowed us to multiply two numbers, was a simple example of a session bean with a single method.

Entity beans are meant to contain state, possibly even complex state. This state normally reflects the contents of a row in a relational database, with the bean managing its own object-relational mapping (bean-managed persistence or BMP) or allowing EJB to handle this task instead (container-managed persistence or CMP). The EJB container also provides transactions, giving us all-or-nothing operations in our objects as well as the database.

This month, we write a simple entity bean, connect it to a database and access it via a session bean from a Java application. We will use the open-source JBoss EJB container (released under the GNU Lesser General Public License, aka LGPL), but the code should work with little modification on any J2EE server that supports EJB.

As we saw last month, writing a session bean really means writing three Java classes:

We need to implement all three of these classes for an entity bean. In addition, we often need to implement a fourth “primary key” class. While this month's example does not need to define a primary key class, we will do so in the interest of completeness.

Most EJB applications will end up using at least one entity bean (to model the data) and at least one session bean (to implement the business logic). Given that a core idea of object-oriented programming is to put data and code in a single package, it seems a bit strange to split entity and session beans in this way. But this strategy does seem to work overall and makes it relatively easy to split work among multiple people, once the specification has been agreed upon.

J2EE is a specification; the actual implementation of that specification depends on whoever has written the server. One of the most important parts of a J2EE application server is the object-relational mapper, which transparently turns Java classes into rows of a relational database table (and vice versa). An object-relational mapper should remain as invisible as possible, allowing us to change our back-end storage from Oracle to MySQL without modifying our Java code.

The JBoss object-relational mapping system is known as JAWS and normally requires very little configuration. However, it can be instructive to look at the JAWS configuration file (standardjaws.xml, in the JBoss conf/default directory) to see what's happening behind the scenes.

The definitions at the top of standardjaws.xml set parameters for the entire JBoss server. In this way, we indicate which database we want to use; the HyperSonic database is included with JBoss, and we will use it for this month's examples.

The core of standardjaws.xml is the multiple <type-mapping> (singular) sections, which connect each <java-type> to a <jdbc-type> and an <sql-type> for each database. Since our EJBs do not create tables or write SQL explicitly, it's important that these values be accurate. You may be able to increase the efficiency or flexibility of your application by modifying these values. However, remember that modifying JAWS after you already have inserted data into a database may lead to confusion, corruption or errors.

If you simply want to get started with EJB, then you won't need to modify standardjaws.xml at all. Rather, you'll need to modify jboss.jcml, an XML file that defines the different managed beans (MBeans) that JBoss uses for system configuration and control.

The file jboss.jcml includes support for HyperSonic and InstantDB; in order for it to work with HyperSonic, I had to remove any reference to InstantDB from jboss.jcml. I did this by editing the “JDBC” section of jboss.jcml, removing the mention of org.enhydra.instantdb.jdbc.idbDriver from the “Drivers” attribute for the JdbcProvider MBean and the entire XADataSourceLoader <mbean>, whose service is XADataSource and whose service name is InstantDB.

Once you have removed all mentions of InstantDB from jboss.jcml, start up JBoss:

cd $JBOSS_DIST/bin
sh run.sh