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Can anyone explain the full process of implementing a Singleton in a Java EE 6 app? I'm assuming that I shouldn't be creating a singleton in the typical way of declaring a static variable and should be using the @Singleton annotation? Do I have to do it this way?
Is it just a case of declaring it @Singleton and that's it? Do I have to do anymore to the class?
What do I then need to do to access the singleton in my other classes?
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In Java, Singleton is a design pattern that ensures that a class can only have one object. To create a singleton class, a class must implement the following properties: Create a private constructor of the class to restrict object creation outside of the class.
ejb. Singleton is a session bean with a guarantee that there is at most one instance in the application.
The primary purpose of a Singleton class is to restrict the limit of the number of object creation to only one. This often ensures that there is access control to resources, for example, socket or database connection.
Example. The Singleton pattern ensures that a class has only one instance and provides a global point of access to that instance. It is named after the singleton set, which is defined to be a set containing one element. The office of the President of the United States is a Singleton.
Singletons can be used while working with databases. They can be used to create a connection pool to access the database while reusing the same connection for all the clients. For example, When we run the program, the output will be: You are now connected to the database. We have created a singleton class Database.
Unlike other enterprise beans, once a singleton session bean instance is initialized, it is not destroyed if the singleton’s business or lifecycle methods cause system exceptions. This ensures that the same singleton instance is used throughout the application lifecycle.
That is, the EJB container could initialize the singletons in the following order: SecondaryBean, PrimaryBean, TertiaryBean. Singleton session beans are designed for concurrent access, situations in which many clients need to access a single instance of a session bean at the same time.
1 We have created a singleton class Database. 2 The dbObject is a class type field. ... 3 The private constructor Database () prevents object creation outside of the class. 4 The static class type method getInstance () returns the instance of the class to the outside world. 5 In the Main class, we have class type variable db1. ... More items...
Is it just a case of declaring it @Singleton and that's it?
Yes! That's it! Just design the class like any other Javabean.
Do however note that this is indeed not the same as GoF's Singleton design pattern. Instead, it's exactly the "just create one" pattern. Perhaps that's the source of your confusion. Admittedly, the annotation name is somewhat poorly chosen, in JSF and CDI the name @ApplicationScoped is been used.
What do I then need to do to access the singleton in my other classes?
Just the same way as every other EJB, by injecting it as @EJB:
@EJB
private YourEJB yourEJB;
The javax.ejb.Singleton annotation is used to specify that the enterprise bean implementation class is a singleton session bean.
This information is to tell the ejb container, not to create multiple instance of this bean and only create a singleton instance. Otherwise it is just a normal bean class. Read more here:
http://docs.oracle.com/javaee/6/tutorial/doc/gipvi.html
You don't have to create a static variable, and do all the related stuff to make it singleton. Just write a normal bean as mentioned here and container will take care of instantiating only object of it:
@Startup
@Singleton
public class StatusBean {
private String status;
@PostConstruct
void init {
status = "Ready";
}
...
}
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