Entity Beans

Entity beings? No, beans. Learn to write 'em, connect 'em to a database and access 'em via their cousin, the session bean.

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.

Creating Entity Beans

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

  • The bean class performs the actual work.

  • The remote interface has methods that match those on the bean class and is our proxy to the bean.

  • The home interface helps us to create new instances of the bean class, as well as search for beans matching particular criteria.

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.

Working with JAWS

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
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ejb

jagruti's picture

can anyone help to write code for jsp entering bookname and displaying authorname from schema using ejb strut
how to connect jsp to session bean and further to entity bean and all configuration of jbosstomcat4.1

Re: At the Forge: Entity Beans

Anonymous's picture

Experienced database programmers know that primary keys should be hidden from view,

I don't agree with this. How would we identify the entities then?
The primary key is the set of attributes that identifies the entity, and this means that it has to be visibale.

and most databases have a way to automate this; PostgreSQL's SERIAL type, MySQL's AUTO INCREMENT and Oracle's sequences are common solutions to this problem.

Those are only artificial primary keys. You can run into few problems when you wnat to use them.
For example you can't be sure what key belongs to the entity you have currently written into the table, because if the primary key is given by the database. If the written row has no primary key except the assigned one you can't even select by the attributes to get the ID, because you can get multiple rows as the response and then the bet is only way to decide. If you have multiple concurent writes it gets even worse.

Unfortunately, there is no easy way to use such automatically generated primary keys within EJB.

UID object from java.rmi.server can give you primary key.
It is a bit strange (out of sequence) but unique, and that is what counts.

Re: At the Forge: Entity Beans

Anonymous's picture

Where to get the database schema for this example ? thanks

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