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EJB 3.0
Enterprise JavaBeans (EJB) was introduced for building distributed components. When it arrived it came with a promise to solve all issues and complexities of CORBA. EJB being the heart of J2EE went through several major revisions and got fattened with many features. Early on, most of the developers fell in love with EJB and used EJB in their application even it did not make any sense. “Blame it on EJB” has been the attitude for many developers when their projects did not scale well and they used EJB.
Development of EJB was never easier and it became more complex with every release of EJB specification. EJB has been compared with an elephant due to its complexity and heavy weight nature. Many developers feel EJB is like an extra layer of sugar syrup on a doughnut. In an age where low carb and Atkins diet is craze, the EJB expert committee has no option but to produce a low carb incarnation of EJB thus simplifying the development of EJB. EJB 3.0 expert committee released a sample picture of the lightweight model
What's new in ejb 3.0?
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Using Annotations Instead of DeploymentDescriptors
EJB 3.0 uses metadata annotations as an alternative to deployment descriptors. Annotations were introduced in J2SE 5.0 and are a key element in the EJB 3.0 simplification. In EJB 3.0, all Enterprise JavaBeans are Plain Old Java Objects (POJO), with proper annotations. So a developer marks up his/her Java code with annotations and the annotation processor creates the deployment descriptors at runtime. This mechanism allows the deployer to override the default configs so they can replace data sources, etc. Another enhancement here is that the code and annotations are in one file--the developer doesn't have to maintain multiple files for one bean.
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Callback Methods and Listener Classes
The EJB 2.1 spec required you to implement either the interface javax.ejb.SessionBean or javax.ejb.EntityBean. Methods like ejbCreate(), ejbPassivate(), and ejbActivate() were never used in your application and just cluttered up your code. Fortunately, they're not required in EJB 3.0.
In EJB 3.0, bean developers do not have to implement unnecessary callback methods and can instead designate any arbitrary method as a callback method to receive notifications for lifecycle events for a SessionBean or MessageDrivenBean (MDB). Callback methods can be indicated using callback annotations.
Also, we can design a callback listener class instead of writing callback methods in the bean class itself. The annotations used for callback methods are the same in both cases--only the method signatures are different. A callback method defined in a listener class must take a Object as a parameter, which is not needed when the callback is in the bean itself.
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Interceptors
The runtime services like transaction and security are applied to the bean objects at the method's invocation time. These services are often implemented as the interceptor methods managed by the container. However, EJB 3.0 allows developers to write the custom interceptor methods that are called before and after the bean method. It is useful to give the control to the developer for the actions like commit transaction, security check, etc. You can develop, reuse, and execute your own services. Or, you can re-implement the transaction and security services to override the container's default behaviors.
Interceptors offer fine-grained control over method invocation flow. They can be used on SessionBeans (stateful and stateless) and MessageDrivenBeans. They can be defined in the same bean class or in an external class. The interceptor's methods will be called before the actual bean class methods are called.
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Dependency Injection
Dependency injection is a term used to describe a separation between the implementation of an object and the construction of an object it depends upon. Instead of complicated XML ejb-refs or resource refs, you can use the @Inject annotation to set the value of a field or to call a setter method within your session bean with anything registered within JNDI. EJB 3.0 facilitates this feature by providing annotations to inject the dependencies into the bean class itself. Dependency annotation may be attached to the bean class, instance variables, or methods. The main reason for introducing @Inject is to avoid JNDI lookup to get the resources set the JNDI tree. Also another great effect of using @Inject is to allow a bean to be tested outside of the container.
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EntityBeans Made Easy
To create an EntityBean, a developer only needs to code a bean class and annotate it with appropriate metadata annotations. The bean class is a POJO.
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Security Annotations
EJB 3.0 provides annotations to specify security options. The following are the security-related annotations defined in EJB 3.0:
- @SecurityRoles
- @MethodPermissions
- @Unchecked
- @Exclude
- @RunAs
Annotations applied for package-level elements are called package-level annotations. These annotations are placed in the file package-info.java. The security roles are applied to the entire EJB module. The @SecurityRoles annotation must be placed in the package-info.java file with the package information. When the compiler parses package-info.java, it will create a synthetic interface. It does not have any source code, because it is created by the compiler. This interface makes package-level annotations available at runtime. The file package-info.java is created and stored inside of every package. For example, if your bean class is inside of the package ejb3.login, then you must put your package-info.java file inside the ejb3.login package with the user role details.
The package-info.java file is new in J2SE 5.0. It contain package declaration, annotations, package tags and Javadoc tags. It is preferred over the package.html file used in the previous versions, because package.html can contain only package comments and Javadocs, not annotations. A package may contain either package-info.java or package.html, but not both. Place either file in the package directory in the source tree along with your .java files.
EJB 3.0 Articles
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