Overview

As the name implies a javax.ejb.Singleton is a session bean with a guarantee that there is at most one instance in the application.

What it gives you that is completely missing in EJB 3.0 and prior versions is the ability to have an EJB that is notified when the application starts and notified when the application stops. So you can do all sorts of things that you previously could only do with a load-on-startup servlet. It also gives you a place to hold data that pertains to the entire application and all users using it, without the need for a static. Additionally, Singleton beans can be invoked by several threads at one time similar to a Servlet.

See the Singleton Beans page for a full description of the javax.ejb.Singleton api.

The Code

PropertyRegistryBean

Here we see a bean that uses the Bean-Managed Concurrency option as well as the @Startup annotation which causes the bean to be instantiated by the container when the application starts. Singleton beans with @ConcurrencyManagement(BEAN) are responsible for their own thread-safety. The bean shown is a simple properties "registry" and provides a place where options could be set and retrieved by all beans in the application.

ComponentRegistryBean

Here we see a bean that uses the Container-Managed Concurrency option, the default. With @ConcurrencyManagement(CONTAINER) the container controls whether multi-threaded access should be allowed to the bean (@Lock(READ)) or if single-threaded access should be enforced (@Lock(WRITE)).

Unless specified explicitly on the bean class or a method, the default @Lock value is @Lock(WRITE). The code above uses the @Lock(READ) annotation on bean class to change the default so that multi-threaded access is granted by default. We then only need to apply the @Lock(WRITE) annotation to the methods that modify the state of the bean.

Essentially @Lock(READ) allows multithreaded access to the Singleton bean instance unless someone is invoking an @Lock(WRITE) method. With @Lock(WRITE), the thread invoking the bean will be guaranteed to have exclusive access to the Singleton bean instance for the duration of its invocation. This combination allows the bean instance to use data types that are not normally thread safe. Great care must still be used, though.

In the example we see ComponentRegistryBean using a java.util.HashMap which is not synchronized. To make this ok we do three things:

1. Encapsulation. We don't expose the HashMap instance directly; including its iterators, key set, value set or entry set.
2. We use @Lock(WRITE) on the methods that mutate the map such as the put() and remove() methods.
3. We use @Lock(READ) on the get() and values() methods as they do not change the map state and are guaranteed not to be called at the same as any of the @Lock(WRITE) methods, so we know the state of the HashMap is no being mutated and therefore safe for reading.

The end result is that the threading model for this bean will switch from multi-threaded access to single-threaded access dynamically as needed depending on the which methods are being invoked. This gives Singletons a bit of an advantage over Servlets for processing multi-threaded requests.

See the Singleton Beans page for more advanced details on Container-Managed Concurrency.

Test Case

Running

Running the example is fairly simple. In the "simple-singleton" directory run:

$ mvn clean install

Which should create output like the following.