Singleton Session Beans - Tutorial

Introduction

Singleton session beans are an important component in Enterprise JavaBeans (EJB) for building enterprise applications that require globally accessible and stateful components. In this tutorial, you will learn about the characteristics of singleton session beans, how to create and use them, and their advantages in enterprise application development.

Characteristics of Singleton Session Beans

Singleton session beans possess the following key characteristics:

• Global Access: Singleton session beans are globally accessible within an application. They provide a single instance shared by multiple clients or components.
• Stateful: Singleton session beans can maintain conversational state with clients or maintain application-level state.
• Non-Concurrent: By default, singleton session beans are not designed to handle concurrent requests. However, concurrency can be managed by the EJB container using locking mechanisms.
• Long-Lived: Singleton session beans live for the duration of the application and are created when the application starts and destroyed when it stops.

Creating and Using Singleton Session Beans

To create and use singleton session beans, follow these steps:

1. Create the Singleton Session Bean Class

   Start by creating a Java class that represents the singleton session bean. Annotate the class with the @Singleton annotation to mark it as a singleton session bean. Implement the business methods that provide the desired functionality.

   @Singleton
   public class MySingletonBean {
     private int counter = 0;
   
   public void incrementCounter() {
   counter++;
   }
   
   public int getCounter() {
   return counter;
   }
   }

2. Deploy the Singleton Session Bean

   Package the singleton session bean class into a Java Archive (JAR) file along with any required dependencies. Deploy the JAR file to an EJB container, such as a Java EE application server. The container will instantiate and manage the singleton session bean instance.

3. Access the Singleton Session Bean

   Clients can access the singleton session bean by obtaining a reference to it using JNDI (Java Naming and Directory Interface). The client can then invoke the business methods exposed by the singleton session bean.

   Context context = new InitialContext();
   MySingletonBean bean = (MySingletonBean) context.lookup("java:global/myApp/MySingletonBean");
   bean.incrementCounter();
   int counter = bean.getCounter();

Common Mistakes

• Not properly managing the concurrency of singleton session beans, leading to potential data inconsistency or performance issues.
• Overusing singleton session beans when other EJB components or design patterns could be more suitable.
• Not handling the destruction of singleton session beans properly, leading to resource leaks or improper cleanup.
• Depending on mutable shared state within singleton session beans, leading to potential concurrency issues.
• Not considering the impact of singleton session beans on application scalability and performance.

FAQs

Q1: Can multiple clients access a singleton session bean simultaneously?

By default, singleton session beans are not designed to handle concurrent requests. However, the EJB container can manage concurrency by using locking mechanisms. Developers can also implement concurrency control manually within the singleton session bean to handle concurrent access.

Q2: Can singleton session beans be accessed from different applications?

Singleton session beans are scoped within an application. They can be accessed by components or clients within the same application. If multiple applications need access to a singleton session bean, each application will have its own instance of the singleton session bean.

Q3: Can a singleton session bean maintain state across method invocations?

Yes, a singleton session bean can maintain conversational state across multiple method invocations. The state is specific to the client or application and can be accessed and modified by different clients or components.

Q4: When should I use a singleton session bean?

Singleton session beans are suitable when you need to create globally accessible components that maintain state or provide shared functionality within an application. Examples include caching mechanisms, application configuration managers, and stateful application-level services.

Q5: How is the lifecycle of singleton session beans managed?

The lifecycle of singleton session beans is managed by the EJB container. The container creates the singleton instance when the application starts and destroys it when the application stops. The container ensures that the singleton session bean is available for access throughout the application's lifetime.

Summary

Singleton session beans provide a way to create globally accessible and stateful components within an enterprise application. They allow the sharing of resources and functionality across multiple clients or components. Understanding the characteristics and proper usage of singleton session beans is essential for building scalable and efficient enterprise applications in Java.