OCEJBCD -5. Singleton Session Bean

OCEJBCD (SCBCD) - 1Z0-895 - Enterprise JavaBeans Developer Certification

5.1. Understand the advantages and disadvantages of using a singleton session bean

A singleton is a stateless bean whose instance is accessed by all request in the application. Singletons have to be thread safe in order to manage the concurrently access.

Advantages:
- Only one instance. The bean will be created when the application is started and will exist during all application life.

Disadvantages:
- Could cause bottle-neck and performance, if it is not handle accurately.

5.2. Create a singleton session bean

javax.ejb.Singleton 

@Singleton
public class SingletonBean { ... }

Other annotations that can be used with singleton are:

@Startup (For eager initialization; it means the instance will be created at deployment time)

@DependsOn (For adding dependencies)

5.3. Describe the life cycle of a singleton session bean

5.4. Implement singleton session bean life cycle methods

For a full example using singleton see last section of this post.

5.5. Describe singleton concurrency access

Singleton beans are designed for concurrency access. There are two ways to manage concurrency.

• Container-managed concurrency (by default)
• Bean-managed concurrency

Example:

@ConcurrencyManagement(ConcurrencyManagementType.CONTAINER)
@Singleton
public class ExampleSingletonBean {...}

For managing concurrrency, there are the following annotations to lock blocks of code:

@Lock(LockType.READ)
@Lock(LockType.WRITE)

5.6. Implement a concurrency management strategy

The following example describes the use of a singleton bean called by a servlet in order to get a random number in the page after clicking a button.

The method that gets the random number can be accessed concurrently and the value will be consistent from different client accesses.

Bean class:

Servlet class:
JSP:
After running the application in the server, when the page is opened in a browser the first time:

If we open another browser (emulating another client), the request accesses the same value of the counter. The counter value is going to be shared by all the requests:

OCEJBCD (SCBCD) - 1Z0-895 - Enterprise JavaBeans Developer Certification