Singleton EJB tutorial

If you need to make sure you are exposing an Enterprise Java  Bean as a Singleton, there is a simple approach which requires adding the @Singleton annotation on your EJB.

 

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. Until now if you wanted to expose a Service as Singleton you had to use either Service POJOS (See an article here JBoss MBeans POJO ) or tweak the EJB pool maximum size.

All of these approach worked, however they were not portable because they used JBoss annotations/configuration to do the trick.

Making an EJB as Singleton is pretty easy: just add the @java.ejb.Singleton annotation and that’s all.

Here’s an example:

@Singleton
@Startup
public class UserRegistry {

        public ArrayList<String> listUsers;
        @PostConstruct
        public void init() {
                listUsers = new ArrayList<String>();
                listUsers.add("administrator");

        }

    	public void addUser(String username) {
    		listUsers.add(username);
    	}
    	public void removeUser(String username) {
    		listUsers.remove(username);
    	}

        public ArrayList<String> getListUsers() {
            return listUsers;
    }
}

In this example, the EJB exposes some methods to manage an in-memory cache based on an ArrayList.

Notice also the @Startup annotation which is not mandatory but can be used to signal to the container to invoke the @PostConstruct method just after the Bean has been created. So, if you need a proper Bean initialization, just add a @Startup annotation at Class level and a @PostConstruct at method level.

Testing the Singleton EJB is pretty simple, here’s a Servlet client:

@WebServlet("/servlet")
public class ServletClient extends HttpServlet {

    @EJB  UserRegistry ejb;

    @Override
    protected void doGet(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        resp.setContentType("text/html");
        PrintWriter out = resp.getWriter();
        ejb.addUser("frank");
        out.println("List of users:")
        ArrayList<String> list = ejb.getListUsers();       
        for (String s:list) {
            out.println(s);
        }    	

    }

    protected void doPost(HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException {
        doGet(req, resp);
    }
}

Please note that you won’t be able to look up remotely a Singleton EJB. If you need to access the Singleton EJB remotely you should include a wrapper Stateless Bean with a remote interface.

Managing concurrency in EJB singleton

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.

There are two ways to manage concurrent access to Singleton EJBs. Let’s see them both.

Singleton Container-Managed Concurrency

If a singleton uses container-managed concurrency, the EJB container will manage access to the business methods of the singleton. This is the default if you don’t use any @ConcurrencyManagement annotation.

You can use the javax.ejb.Lock and javax.ejb.LockType annotation to specify the access level of the singleton’s business methods.

• Annotate a method with @Lock(LockType.READ) if the method can be concurrently accessed.
• Annotate a method with @Lock(LockType.WRITE) if the singleton session bean should be locked to other clients while a client is calling that method.

Typically, the @Lock(LockType.WRITE) annotation is used when clients are modifying the state of the singleton

@Singleton
@ConcurrencyManagement(ConcurrencyManagementType.CONTAINER)
@Startup
public class UserRegistry {

        public ArrayList<String> listUsers;
        @PostConstruct
        public void init() {
                listUsers = new ArrayList<String>();
                listUsers.add("administrator");

        }

        @Lock(LockType.WRITE)
        @AccessTimeout(value=60, timeUnit=SECONDS)
    	public void addUser(String username) {
    		listUsers.add(username);
    	}
        @Lock(LockType.WRITE)
        @AccessTimeout(value=60, timeUnit=SECONDS)
    	public void removeUser(String username) {
    		listUsers.remove(username);
    	}
        @Lock(LockType.READ)
        public ArrayList<String> getListUsers() {
            return listUsers;
    }
}

@Lock(LockType.WRITE)
@AccessTimeout(value=60, timeUnit=SECONDS)
public void addUser(String username) {
    listUsers.add(username);
}
@Lock(LockType.WRITE)
@AccessTimeout(value=60, timeUnit=SECONDS)
public void removeUser(String username) {
    listUsers.remove(username);
}

@ConcurrencyManagement(BEAN)
@Singleton
public class UserRegistry { ... }