Compared to EJB 2.1, EJB 3.0 simplifies the process of creating
Enterprise JavaBean applications.
The underlying concept of the EJB 3.0 specification centers on a plain
old Java™ object
(POJO) programming model that uses Java annotations to capture information
that deployment descriptors used to contain. Deployment descriptors are now
optional in most cases. Using default values liberally also means that you
need to write and maintain less supporting code. This greatly simplifies the
programming experience in creating and using EJB 3.0 components.
While EJB 2.1 added support for Web services, changes in the implementation
of session beans, changes in how enterprise beans are invoked and a new XML
schema to replace the DTD that defined ejb-jar.xml deployment descriptor,
EJB 3.0 has taken this one step further. EJB 3.0 has introduced a lightweight
entity bean persistence mechanism through the Java Persistence API. These entities are
POJO based and can then be run outside of an EJB container and do not require
any interfaces or EJB code in them. Similarly, session beans also no longer
require EJB-specific component interfaces either.
Comparison of EJB 2.1 class plus Deployment Descriptor file with equivalent
EJB 3.0 class
The examples in Table 1 are functionally equivalent:
Table 1. Comparison of EJB 2.1 and EJB 3.0
| EJB 2.1 |
EJB 3.0 |
|
Java Class
public class AccountBean
implements javax.ejb.SessionBean {
SessionContext ctx;
DataSource accountDB;
public void setSessionContext(SessionContext ctx) {
this.ctx = ctx;
}
public void ejbCreate() {
accountDB = (DataSource)ctx.lookup(
"jdbc/accountDB");
}
public void ejbActivate() { }
public void ejbPassivate() { }
public void ejbRemove() { }
public void setAccountDeposit(int empId,
double deposit) {
...
Connection conn = accountDB.getConnection();
...
}
...
}
|
Java Class
@Stateless
public class AccountBean implements Account
{
@Resource private DataSource accountDB;
public void setAccountDeposit(int customerId,
double deposit) {
...
Connection conn = accountDB.getConnection();
...
}
...
}
|
|
Deployment Descriptor
<session>
<ejb-name>AccountBean</ejb-name>
<local-home>AccountHome</local-home>
<local>Account</local>
<ejb-class>com.example.AccountBean</ejb-class>
<session-type>Stateless</session-type>
<transaction-type>Container</transaction-type>
<resource-ref>
<res-ref-name>jdbc/accountDB</res-ref-name>
<res-ref-type>javax.sql.DataSource</res-ref-type>
<res-auth>Container</res-auth>
</resource-ref>
</session>
...
<assembly-descriptor>...</assembly-descriptor>
|
|
The following four facets of Java EE 5 contributed the most to the changes
in EJB 3.0 from EJB 2.1:
- Plain old Java objects
- Annotations
- Implied values
- Persistence
