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
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.
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)).
The default @Lock value for a method 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. The important thing to keep in mind is that when an @Lock(WRITE) method is invoked the container will block all access to the bean, even to @Lock(READ) methods, until the method completes. This is very good and is an advanced form of synchronization that allows for safe multi-threaded reading and single-threaded writing.
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.
------------------------------------------------------- T E S T S ------------------------------------------------------- Running org.superbiz.registry.ComponentRegistryBeanTest Apache OpenEJB 3.1-SNAPSHOT build: 20080820-09:53 http://openejb.apache.org/ INFO - openejb.home = /Users/dblevins/work/openejb3/examples/simple-singleton INFO - openejb.base = /Users/dblevins/work/openejb3/examples/simple-singleton INFO - Configuring Service(id=Default Security Service, type=SecurityService, provider-id=Default Security Service) INFO - Configuring Service(id=Default Transaction Manager, type=TransactionManager, provider-id=Default Transaction Manager) INFO - Found EjbModule in classpath: /Users/dblevins/work/openejb3/examples/simple-singleton/target/classes INFO - Beginning load: /Users/dblevins/work/openejb3/examples/simple-singleton/target/classes INFO - Configuring enterprise application: classpath.ear INFO - Configuring Service(id=Default Singleton Container, type=Container, provider-id=Default Singleton Container) INFO - Auto-creating a container for bean ComponentRegistryBean: Container(type=SINGLETON, id=Default Singleton Container) INFO - Enterprise application "classpath.ear" loaded. INFO - Assembling app: classpath.ear INFO - Jndi(name=ComponentRegistryBeanLocal) --> Ejb(deployment-id=ComponentRegistryBean) INFO - Jndi(name=PropertyRegistryBeanLocal) --> Ejb(deployment-id=PropertyRegistryBean) INFO - Created Ejb(deployment-id=ComponentRegistryBean, ejb-name=ComponentRegistryBean, container=Default Singleton Container) INFO - Created Ejb(deployment-id=PropertyRegistryBean, ejb-name=PropertyRegistryBean, container=Default Singleton Container) INFO - Deployed Application(path=classpath.ear) Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.879 sec Running org.superbiz.registry.PropertiesRegistryBeanTest Tests run: 1, Failures: 0, Errors: 0, Skipped: 0, Time elapsed: 0.009 sec Results : Tests run: 2, Failures: 0, Errors: 0, Skipped: 0