Abstract Methods in EJB 2.0 CMP
One of the most visible changes from the EJB 1.1 model of container-managed persistence is the
introduction of abstract accessor methods.
An abstract accessor in the context of EJB 2.0 is an abstract get or set method used to
access part of the object's persistent state. An abstract accessor is part of the bean's
implementation class. Since all access to the bean's persistent state is through these
abstract accessors, it allows the container to manage this state intelligently. These
abstract accessors have nothing to do with the client view. They may or may not be
exposed in the bean's component interface.
For instance, an entity EJB representing a customer might have state to represent the customer's id, first
name, last name, and birth date. The corresponding abstract accessors for this business object might be
declared as follows (this code is not intended to be run, but just an example of accessor methods):
public abstract class CustomerBean implements EntityBean {
public abstract long getId();
public abstract void setId(long id);
public abstract String getFirstName();
public abstract void setFirstName(String firstName);
public abstract String getLastName();
public abstract void setLastName(String lastName);
public abstract Date getBirthDate();
public abstract void setBirthDate(Date birthDate);
}
The naming convention for abstract accessors corresponds to the standard naming convention for
JavaBean accessors. The first letter of the state variable name is capitalized, and either get or set is
prepended. Getter methods take no parameters and have a return type corresponding to the type of the
state variable. Setter methods take one parameter with a type corresponding to the type of the state
variable, and return void. Abstract getter and setter methods do not throw exceptions.
These abstract accessors must correspond to <cmp-field> elements in the deployment descriptor. This
is no different from the EJB 1.1 container-managed persistence model. The types of these container-
managed fields are determined from the parameters and return values of the abstract accessors. Legal
types are limited to:
-
Java primitive values (int, long, float, etc.)
-
Java serializable objects (java.util.Date, java.lang.String, etc.)
-
User-defined types that implement java.io.Serializable
-
EJB component home and remote interfaces
You cannot have a non-serializable Java class or a local interface class as a legal type for a cmp field.
The terms "cmp field" and "cmr field" are often used to refer to the corresponding
part of an entity's persistent state and persistent relationships, represented by
abstract accessor methods and declared in the deployment descriptor using the
<cmp-field> or <cmr-field> elements. A cmp field represents persistent state. A cmr
field represents an endpoint of a persistent relationship.
The bean developer uses these methods to access the persistent state of the bean. The implementation of
these methods is left to the EJB container. In other words, the EJB component developer never writes a
version of any of these accessor methods that has a body. The following is not legal for any abstract
accessor method:
public String getFirstName() {
return firstName;
}
public void setFirstName(String firstName) {
this.firstName = firstName;
}
Of course, there is no prohibition against writing getter and setter methods that do not correspond to
aspects of the EJB component's persistent state. For our customer entity, the following is perfectly legal:
public String getName() {
return getFirstName() + " " + getLastName();
}
The EJB component developer never declares any variables representing the persistent state of the
object. This, too, is left to the EJB container. All access to those state variables in the EJB component
must be through the abstract accessor methods. At deployment time, the EJB container will provide
some representation of the object's state and implementations of its abstract accessor methods. At
runtime, calls to an abstract accessor will become calls to the appropriate method implemented by the
EJB container, returning the appropriate state information.
Note: it is still legal to have state variables, as long as that state isn't part of the object's persistent
state. For instance, the EJB component might cache references to a resource that it needed.
Abstract accessor methods are used for both entity beans and their dependent objects. The resultant
Java bean-implementation classes can look a little unusual to experienced programmers, because they
often declare few if any object-scoped variables.
They must also be declared as abstract classes, because any class with an abstract
method must itself be abstract. A container-supplied class will provide a concrete
derivation of the component developer's abstract class.
New on the Java Boutique:
New Review:
Time Management Made Easy with the Quartz Enterprise Job Scheduler
Why not just use the Java timer API? This open source scheduling
API boasts simplicity, ease-of-integration, a well-rounded feature
set, and it's free!
New Applet:
Reverse Complement
Reverse Complement is a simple applet that converts DNA or RNA
sequences into three useful formats.
Elsewhere on internet.com:
WebDeveloper Java
Lots of Java information on webdeveloper.com
WDVL Java
Thorough Java resource at the Web Developer's Virtual Library.
ScriptSearch Java
Hundreds of free Java code files to download.
jGuru: Your View of the Java Universe
Customizable portal with online training, FAQs, regular news updates, and tutorials.
| 
