Java Platform, Enterprise Edition (Java EE) 8 The Java EE Tutorial

## A Singleton Session Bean Example: counter

The `counter` example demonstrates how to create a singleton session bean.

The following topics are addressed here:

- Creating a Singleton Session Bean

- The Architecture of the counter Example

- Running the counter Example

### Creating a Singleton Session Bean

The `javax.ejb.Singleton` annotation is used to specify that the enterprise bean implementation class is a singleton session bean:

    @Singleton
    public class SingletonBean { ... }

#### Initializing Singleton Session Beans

The EJB container is responsible for determining when to initialize a singleton session bean instance unless the singleton session bean implementation class is annotated with the `javax.ejb.Startup` annotation. In this case, sometimes called eager initialization, the EJB container must initialize the singleton session bean upon application startup. The singleton session bean is initialized before the EJB container delivers client requests to any enterprise beans in the application. This allows the singleton session bean to perform, for example, application startup tasks.

The following singleton session bean stores the status of an application and is eagerly initialized:

    @Startup
    @Singleton
    public class StatusBean {
      private String status;

      @PostConstruct
      void init {
        status = "Ready";
      }
      ...
    }

Sometimes multiple singleton session beans are used to initialize data for an application and therefore must be initialized in a specific order. In these cases, use the `javax.ejb.DependsOn` annotation to declare the startup dependencies of the singleton session bean. The `@DependsOn` annotation’s `value` attribute is one or more strings that specify the name of the target singleton session bean. If more than one dependent singleton bean is specified in `@DependsOn`, the order in which they are listed is not necessarily the order in which the EJB container will initialize the target singleton session beans.

The following singleton session bean, `PrimaryBean`, should be started up first:

    @Singleton
    public class PrimaryBean { ... }

`SecondaryBean` depends on `PrimaryBean`:

    @Singleton
    @DependsOn("PrimaryBean")
    public class SecondaryBean { ... }

This guarantees that the EJB container will initialize `PrimaryBean` before `SecondaryBean`.

The following singleton session bean, `TertiaryBean`, depends on `PrimaryBean` and `SecondaryBean`:

    @Singleton
    @DependsOn({"PrimaryBean", "SecondaryBean"})
    public class TertiaryBean { ... }

`SecondaryBean` explicitly requires `PrimaryBean` to be initialized before it is initialized, through its own `@DependsOn` annotation. In this case, the EJB container will first initialize `PrimaryBean`, then `SecondaryBean`, and finally `TertiaryBean`.

If, however, `SecondaryBean` did not explicitly depend on `PrimaryBean`, the EJB container may initialize either `PrimaryBean` or `SecondaryBean` first. That is, the EJB container could initialize the singletons in the following order: `SecondaryBean`, `PrimaryBean`, `TertiaryBean`.

#### Managing Concurrent Access in a Singleton Session Bean

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. A singleton’s client needs only a reference to a singleton in order to invoke any business methods exposed by the singleton and doesn’t need to worry about any other clients that may be simultaneously invoking business methods on the same singleton.

When creating a singleton session bean, concurrent access to the singleton’s business methods can be controlled in two ways: container-managed concurrency and bean-managed concurrency.

The `javax.ejb.ConcurrencyManagement` annotation is used to specify container-managed or bean-managed concurrency for the singleton. With `@ConcurrencyManagement`, a type attribute must be set to either `javax.ejb.ConcurrencyManagementType.CONTAINER` or `javax.ejb.ConcurrencyManagementType.BEAN`. If no `@ConcurrencyManagement` annotation is present on the singleton implementation class, the EJB container default of container-managed concurrency is used.

Container-Managed Concurrency

If a singleton uses container-managed concurrency, the EJB container controls client access to the business methods of the singleton. The `javax.ejb.Lock` annotation and a `javax.ejb.LockType` type are used to specify the access level of the singleton’s business methods or `@Timeout` methods. The `LockType` enumerated types are `READ` and `WRITE`.

Annotate a singleton’s business or timeout method with `@Lock(LockType.READ)` if the method can be concurrently accessed, or shared, with many clients. Annotate the business or timeout 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.

Annotating a singleton class with `@Lock` specifies that all the business methods and any timeout methods of the singleton will use the specified lock type unless they explicitly set the lock type with a method-level `@Lock` annotation. If no `@Lock` annotation is present on the singleton class, the default lock type, `@Lock(LockType.WRITE)`, is applied to all business and timeout methods.

The following example shows how to use the `@ConcurrencyManagement`, `@Lock(LockType.READ)`, and `@Lock(LockType.WRITE)` annotations for a singleton that uses container-managed concurrency.

Although by default singletons use container-managed concurrency, the `@ConcurrencyManagement(CONTAINER)` annotation may be added at the class level of the singleton to explicitly set the concurrency management type:

    @ConcurrencyManagement(ConcurrencyManagementType.CONTAINER)
    @Singleton
    public class ExampleSingletonBean {
      private String state;

      @Lock(LockType.READ)
      public String getState() {
        return state;
      }

      @Lock(LockType.WRITE)
      public void setState(String newState) {
        state = newState;
      }
    }

The `getState` method can be accessed by many clients at the same time because it is annotated with `@Lock(LockType.READ)`. When the `setState` method is called, however, all the methods in `ExampleSingletonBean` will be locked to other clients because `setState` is annotated with `@Lock(LockType.WRITE)`. This prevents two clients from attempting to simultaneously change the `state` variable of `ExampleSingletonBean`.

The `getData` and `getStatus` methods in the following singleton are of type `READ`, and the `setStatus` method is of type `WRITE`:

    @Singleton
    @Lock(LockType.READ)
    public class SharedSingletonBean {
      private String data;
      private String status;

      public String getData() {
        return data;
      }

      public String getStatus() {
        return status;
      }

      @Lock(LockType.WRITE)
      public void setStatus(String newStatus) {
        status = newStatus;
      }
    }

If a method is of locking type `WRITE`, client access to all the singleton’s methods is blocked until the current client finishes its method call or an access timeout occurs. When an access timeout occurs, the EJB container throws a `javax.ejb.ConcurrentAccessTimeoutException`. The `javax.ejb.AccessTimeout` annotation is used to specify the number of milliseconds before an access timeout occurs. If added at the class level of a singleton, `@AccessTimeout` specifies the access timeout value for all methods in the singleton unless a method explicitly overrides the default with its own `@AccessTimeout` annotation.

The `@AccessTimeout` annotation can be applied to both `@Lock(LockType.READ)` and `@Lock(LockType.WRITE)` methods. The `@AccessTimeout` annotation has one required element, `value`, and one optional element, `unit`. By default, the `value` is specified in milliseconds. To change the `value` unit, set `unit` to one of the `java.util.concurrent.TimeUnit` constants: `NANOSECONDS`, `MICROSECONDS`, `MILLISECONDS`, or `SECONDS`.

The following singleton has a default access timeout value of 120,000 milliseconds, or 2 minutes. The `doTediousOperation` method overrides the default access timeout and sets the value to 360,000 milliseconds, or 6 minutes:

    @Singleton
    @AccessTimeout(value=120000)
    public class StatusSingletonBean {
      private String status;

      @Lock(LockType.WRITE)
      public void setStatus(String new Status) {
        status = newStatus;
      }

      @Lock(LockType.WRITE)
      @AccessTimeout(value=360000)
      public void doTediousOperation {
        ...
      }
    }

The following singleton has a default access timeout value of 60 seconds, specified using the `TimeUnit.SECONDS` constant:

    @Singleton
    @AccessTimeout(value=60, unit=TimeUnit.SECONDS)
    public class StatusSingletonBean { ... }

Bean-Managed Concurrency

Singletons that use bean-managed concurrency allow full concurrent access to all the business and timeout methods in the singleton. The developer of the singleton is responsible for ensuring that the state of the singleton is synchronized across all clients. Developers who create singletons with bean-managed concurrency are allowed to use the Java programming language synchronization primitives, such as `synchronization` and `volatile`, to prevent errors during concurrent access.

Add a `@ConcurrencyManagement` annotation with the type set to `ConcurrencyManagementType.BEAN` at the class level of the singleton to specify bean-managed concurrency:

    @ConcurrencyManagement(ConcurrencyManagementType.BEAN)
    @Singleton
    public class AnotherSingletonBean { ... }

#### Handling Errors in a Singleton Session Bean

If a singleton session bean encounters an error when initialized by the EJB container, that singleton instance will be destroyed.

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.

### The Architecture of the counter Example

The `counter` example consists of a singleton session bean, `CounterBean`, and a JavaServer Faces Facelets web front end.

`CounterBean` is a simple singleton with one method, `getHits`, that returns an integer representing the number of times a web page has been accessed. Here is the code of `CounterBean`:

    package javaeetutorial.counter.ejb;

    import javax.ejb.Singleton;

    /**
     * CounterBean is a simple singleton session bean that records the number
     * of hits to a web page.
     */
    @Singleton
    public class CounterBean {
        private int hits = 1;

        // Increment and return the number of hits
        public int getHits() {
            return hits++;
        }
    }

The `@Singleton` annotation marks `CounterBean` as a singleton session bean. `CounterBean` uses a local, no-interface view.

`CounterBean` uses the EJB container’s default metadata values for singletons to simplify the coding of the singleton implementation class. There is no `@ConcurrencyManagement` annotation on the class, so the default of container-managed concurrency access is applied. There is no `@Lock` annotation on the class or business method, so the default of `@Lock(WRITE)` is applied to the only business method, `getHits`.

The following version of `CounterBean` is functionally equivalent to the preceding version:

    package javaeetutorial.counter.ejb;

    import javax.ejb.Singleton;
    import javax.ejb.ConcurrencyManagement;
    import static javax.ejb.ConcurrencyManagementType.CONTAINER;
    import javax.ejb.Lock;
    import javax.ejb.LockType.WRITE;

    /**
     * CounterBean is a simple singleton session bean that records the number
     * of hits to a web page.
     */
    @Singleton
    @ConcurrencyManagement(CONTAINER)
    public class CounterBean {
        private int hits = 1;

        // Increment and return the number of hits
        @Lock(WRITE)
        public int getHits() {
            return hits++;
        }
    }

The web front end of `counter` consists of a JavaServer Faces managed bean, `Count.java`, that is used by the Facelets XHTML files `template.xhtml` and `index.xhtml`. The `Count` JavaServer Faces managed bean obtains a reference to `CounterBean` through dependency injection. `Count` defines a `hitCount` JavaBeans property. When the `getHitCount` getter method is called from the XHTML files, `CounterBean`'s `getHits` method is called to return the current number of page hits.

Here’s the `Count` managed bean class:

    @Named
    @ConversationScoped
    public class Count implements Serializable {
        @EJB
        private CounterBean counterBean;

        private int hitCount;

        public Count() {
            this.hitCount = 0;
        }

        public int getHitCount() {
            hitCount = counterBean.getHits();
            return hitCount;
        }

        public void setHitCount(int newHits) {
            this.hitCount = newHits;
        }
    }

The `template.xhtml` and `index.xhtml` files are used to render a Facelets view that displays the number of hits to that view. The `index.xhtml` file uses an expression language statement, `#{count.hitCount}`, to access the `hitCount` property of the `Count` managed bean. Here is the content of `index.xhtml`:

    <html lang="en"
          xmlns="http://www.w3.org/1999/xhtml"
          xmlns:ui="http://xmlns.jcp.org/jsf/facelets"
          xmlns:h="http://xmlns.jcp.org/jsf/html">
        <ui:composition template="/template.xhtml">
            <ui:define name="title">
                This page has been accessed #{count.hitCount} time(s).
            </ui:define>
            <ui:define name="body">
                Hooray!
            </ui:define>
        </ui:composition>
    </html>

### Running the counter Example

You can use either NetBeans IDE or Maven to build, package, deploy, and run the `counter` example.

The following topics are addressed here:

The following topics are addressed here:

- To Run the counter Example Using NetBeans IDE

- To Run the counter Example Using Maven

#### To Run the counter Example Using NetBeans IDE

1. Make sure that GlassFish Server has been started (see Starting and Stopping GlassFish Server).

2. From the File menu, choose Open Project.

3. In the Open Project dialog box, navigate to:

       tut-install/examples/ejb

4. Select the `counter` folder.

5. Click Open Project.

6. In the Projects tab, right-click the `counter` project and select Run.

   A web browser will open the URL `http://localhost:8080/counter`, which displays the number of hits.

7. Reload the page to see the hit count increment.

#### To Run the counter Example Using Maven

1. Make sure that GlassFish Server has been started (see Starting and Stopping GlassFish Server).

2. In a terminal window, go to:

       tut-install/examples/ejb/counter/

3. Enter the following command:

       mvn install

   This will build and deploy `counter` to your GlassFish Server instance.

4. In a web browser, enter the following URL:

       http://localhost:8080/counter

5. Reload the page to see the hit count increment.

Copyright © 2017, Oracle and/or its affiliates. All rights reserved.