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Applying @Alternative Beans and Lifecycle Annotations

Contributed by Andy Gibson

Content on this page applies to NetBeans IDE 6.9, 7.0 and 7.1

Contexts and Dependency Injection (CDI), specified by JSR-299, is an integral part of Java EE 6 and provides an architecture that allows Java EE components such as servlets, enterprise beans, and JavaBeans to exist within the lifecycle of an application with well-defined scopes. In addition, CDI services allow Java EE components such as EJB session beans and JavaServer Faces (JSF) managed beans to be injected and to interact in a loosely coupled way by firing and observing events.

This tutorial is based on the blog post by Andy Gibson, entitled Getting Started with CDI part 2 – Injection. It demonstrates how you can take advantage of the @Alternative annotation to configure your application for different deployments, and also shows how you can use managed bean lifecycle annotations, such as @PostConstruct and @PreDestroy, to combine CDI injection with functionality provided by the Java EE 6 Managed Bean Specification.

NetBeans IDE provides built-in support for Contexts and Dependency Injection, including the option of generating the beans.xml CDI configuration file upon project creation, editor and navigation support for annotations, as well as various wizards for creating commonly used CDI artifacts.


To complete this tutorial, you need the following software and resources.

Software or Resource Version Required
NetBeans IDE 6.9, 7.0, 7.1, Java EE version
Java Development Kit (JDK) version 6
GlassFish server Open Source Edition 3.1 or 3.0.1
cdiDemo2.zip n/a

Notes:

  • The NetBeans IDE Java bundle also includes the GlassFish Server Open Source Edition 3.x, which is a Java EE 6-compliant container.
  • Support for CDI is also available for NetBeans 6.8 via Patch 1.
  • The solution sample project for this tutorial can be downloaded: cdiDemo3.zip

Handling Multiple Deployments

CDI offers the use of the @Alternative annotation which lets you package multiple beans that match an injection point without ambiguity errors. In other words, you can apply the @Alternative annotation to two or more beans, then, based on your deployment, specify the bean you want to use in CDI's beans.xml configuration file.

To demonstrate this, consider the following scenario. We inject an ItemValidator into our main ItemProcessor class. The ItemValidator is implemented by both DefaultItemValidator and RelaxedItemValidator. Based on our deployment requirements, we'd like to use DefaultItemValidator for most cases, but also require RelaxedItemValidator for a specific deployment. To resolve this, we annotate both beans, then specify which bean to use for a given deployment by adding an entry to the application's beans.xml file.

CDI diagram showing objects created in this exercise
  1. Begin by extracting the sample start project from the cdiDemo2.zip file (See the table listing required resources above.) Open the project in the IDE by choosing File > Open Project (Ctrl-Shift-O; ⌘-Shift-O on Mac), then selecting the project from its location on your computer.
  2. Create an ItemValidator interface.

    Click the New File ( New File button ) button or press Ctrl-N (⌘-N on Mac) to open the File wizard.
  3. Select the Java category, then select Java Interface. Click Next.
  4. Type in ItemValidator as the class name, then enter exercise3 as the package.
  5. Click Finish. The new interface is generated and opens in the editor.
  6. Add a method called isValid() that takes an Item object and returns a boolean value. 
    public interface ItemValidator {
    boolean isValid(Item item);
}
    (Use the editor's hint to add the import statement for exercise2.Item.)
  7. Expand the ItemProcessor class to incorporate the new feature. Open ItemProcessor in the editor and make the following changes. 
    @Named
@RequestScoped
public class ItemProcessor {

    @Inject @Demo
    private ItemDao itemDao;

    @Inject
    private ItemValidator itemValidator;

    public void execute() {
      List<Item>  items = itemDao.fetchItems();
      for (Item item : items) {
          System.out.println("Item = " + item + " valid = " + itemValidator.isValid(item));
      }
    }
}

    Use the editor's hint to add the import statement for exercise3.ItemValidator.

  8. Create an implementation of ItemValidator named DefaultItemValidator that simply tests the limit against the value.

    In the Projects window, right-click the exercise3 package and choose New > Java Class. Name the class DefaultItemValidator and click Finish.

  9. Have DefaultItemValidator implement ItemValidator and override the isValid() method as follows. 
    public class DefaultItemValidator implements ItemValidator {

    @Override
    public boolean isValid(Item item) {
        return item.getValue() < item.getLimit();
    }
}

    (Use the editor's hint to add the import statement for exercise2.Item.)

  10. Click the Run Project ( Run Project button ) button in the IDE's main toolbar. The project is compiled and deployed to GlassFish, and the application's welcome page (process.xhtml) opens in the browser.
  11. Click the 'Execute' button that displays on the page. Switch back to the IDE and examine the GlassFish server log. The server log displays in the Output window (Ctrl-4; ⌘-4 on Mac) under the GlassFish tab. You can see that items are being validated, and the only valid item listed is the case where the value is less than the limit. 
    INFO: Item =  [Value=34, Limit=7] valid = false
INFO: Item =  [Value=4, Limit=37] valid = true
INFO: Item =  [Value=24, Limit=19] valid = false
INFO: Item =  [Value=89, Limit=32] valid = false
    Output window - GlassFish server log
  12. Now consider a scenario where you have to deploy to a different site that is more relaxed and considers an item invalid only if the value is more than twice the limit. You may want to have another bean that implements the ItemValidator interface for that logic.

    Create a new implementation of ItemValidator named RelaxedItemValidator. In the Projects window, right-click the exercise3 package and choose New > Java Class. Name the class RelaxedItemValidator and click Finish.

  13. Have RelaxedItemValidator implement ItemValidator and override the isValid() method as follows. 
    public class RelaxedItemValidator implements ItemValidator {

    @Override
    public boolean isValid(Item item) {
        return item.getValue() < (item.getLimit() * 2);
    }
}

    (Use the editor's hint to add the import statement for exercise2.Item.)

  14. Click the Run Project ( Run Project button ) button to run the project. Note that the project now fails to deploy.
  15. Examine the server log in the Output window (Ctrl-4; ⌘-4 on Mac). You see an error message reporting an 'ambiguous dependency' problem. This occurs because you now have two classes implementing the same interface. 
    org.glassfish.deployment.common.DeploymentException: Injection point has ambiguous dependencies.
Injection point: field exercise2.ItemProcessor.itemValidator;
Qualifiers: [@javax.enterprise.inject.Default()];
Possible dependencies: [exercise3.RelaxedItemValidator, exercise3.DefaultItemValidator]

    Weld, the implementation for CDI, cannot determine whether to use RelaxedItemValidator or DefaultItemValidator for the given injection point.

    As mentioned, the only difference is based on deployment. For most deployments, you want to use the default validator, but for one deployment you want to use the 'relaxed' implementation. CDI offers the use of the @Alternative annotation which lets you package multiple beans that match an injection point without ambiguity errors, and the bean to use is defined in the beans.xml. This allows you to deploy both implementations in the same module with the only difference being the beans.xml definition, which can change over different deployments.

  16. Add the @Alternative annotation and corresponding import statement to RelaxedItemValidator and DefaultItemValidator.

    Open RelaxedItemValidator in the editor and make the following change. 
    import javax.enterprise.inject.Alternative;
...

@Alternative
public class RelaxedItemValidator implements ItemValidator {

    public boolean isValid(Item item) {
        return item.getValue() < (item.getLimit() * 2);
    }
}

    Type '@Al' then press Ctrl-Space to invoke code completion. Because only one option is filtered, the @Alternative annotation is completed, and the corresponding import statement for javax.enterprise.inject.Alternative is automatically added to the top of the file. Typically, pressing Ctrl-Space on annotations also provides a Javadoc documentation popup.

    Javadoc documentation popup in editor

    Switch to DefaultItemValidator (press Ctrl-Tab) and make the following change.

    import javax.enterprise.inject.Alternative;
...

@Alternative
public class DefaultItemValidator implements ItemValidator {

    public boolean isValid(Item item) {
        return item.getValue() < item.getLimit();
    }
}

    If you deployed the application now you would get an 'unsatisfied dependency' error since you defined the two matching beans as alternative but you did not enable either of them in the beans.xml file.

  17. Use the IDE's Go to File dialog to quickly open the beans.xml file. Choose Navigate > Go to File from the IDE's main menu (Alt-Shift-O; Ctrl-Shift-O on Mac), then type 'beans'. Click OK. Go to File dialog
  18. Make the following change to the beans.xml file. 
    <beans xmlns="http://java.sun.com/xml/ns/javaee"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/beans_1_0.xsd">

    <alternatives>
        <class>exercise3.RelaxedItemValidator</class>
    </alternatives>

</beans>

    This tells CDI to use the RelaxedItemValidator for this deployment. You can think of the @Alternative annotation as effectively disabling the bean, making it unavailable for injection, but allowing the implementation to be packaged with the other beans. Adding it as an alternative in the beans.xml file effectively re-enables the bean, making it available for injection. By moving this type of metadata to the beans.xml file, we can bundle different versions of the file with different deployments.

  19. Click the Run Project ( Run Project button ) button to run the project (Alternatively, press F6; fn-F6 on Mac). In the browser, click the 'Execute' button that displays on the page. Switch back to the IDE and examine the GlassFish server log displayed in the Output window (Ctrl-4; ⌘-4 on Mac). 
    INFO: Item =  [Value=34, Limit=7] valid = false
INFO: Item =  [Value=4, Limit=37] valid = true
INFO: Item =  [Value=24, Limit=19] valid = true
INFO: Item =  [Value=89, Limit=32] valid = false

    You can see that the RelaxedItemValidator implementation is being used, as the third item displays as valid while the provided value (24) is greater than the given limit (19).


Applying Lifecycle Annotations to Managed Beans

In this exercise, you inject an ItemErrorHandler into the main ItemProcessor class. Because FileErrorReporter is the only implementation of the ItemErrorHandler interface, it is selected for the injection. To set up lifecycle-specific actions for the class, you use the @PostConstruct and @PreDestroy annotations from the Managed Bean specification (included in JSR 316: Java Platform, Enterprise Edition 6 Specification).

CDI diagram showing objects created in this exercise

Continuing with the example, create an ItemErrorHandler interface to handle invalid items when they are discovered.

  1. In the Projects window, right-click the exercise3 package and choose New > Java Interface.
  2. In the Java Interface wizard, type in ItemErrorHandler as the class name, then enter exercise3 as the package. Click Finish.

    The new interface is generated and opens in the editor.

  3. Add a method called handleItem() that takes an Item object as an argument. 
    public interface ItemErrorHandler {
    void handleItem(Item item);
}

    (Use the editor's hint to add the import statement for exercise2.Item.)

  4. Begin by implementing the ItemErrorHandler with a bogus handler named FileErrorReporter that saves item details to a file.

    In the Projects window, right-click the exercise3 package and choose New > Java Class. Name the class FileErrorReporter and click Finish.

  5. Have FileErrorReporter implement ItemErrorHandler and override the handleItem() method as follows. 
    public class FileErrorReporter implements ItemErrorHandler {

    @Override
    public void handleItem(Item item) {
        System.out.println("Saving " + item + " to file");
    }
}

    (Use the editor's hint to add the import statement for exercise2.Item.)

    You want to open the file before you start handling items, leave it open for the duration of the process as content is added to the file, and then close the file when we the processing is done. You could manually add initProcess() and finishProcess() methods to the error reporter bean, but then you could not code to the interface since the caller would need to know about those class specific methods. You could add those same methods to the ItemErrorReporter interface but then you would have to unnecessarily implement those methods in every class that implements that interface. Instead, you can use some of the lifecycle annotations from the Managed Bean specification (included in JSR 316: Java Platform, Enterprise Edition 6 Specification) to call methods on the bean at certain points in the bean lifecycle. A @PostConstruct annotated method is called when the bean has been constructed and any dependencies the bean has have been injected. Likewise, a @PreDestroy annotated method is called just before the bean is disposed of by the container.

  6. Add the following init() and release() methods with corresponding @PostConstruct and @PreDestroy annotations. 
    public class FileErrorReporter implements ItemErrorHandler {

    @PostConstruct
    public void init() {
        System.out.println("Creating file error reporter");
    }

    @PreDestroy
    public void release() {
        System.out.println("Closing file error reporter");
    }

    @Override
    public void handleItem(Item item) {
        System.out.println("Saving " + item + " to file");
    }
}
  7. Fix imports. Either right-click in the editor and choose Fix Imports, or press Ctrl-Shift-I (⌘-Shift-I on Mac). Import statements for javax.annotation.PostConstruct and javax.annotation.PreDestroy are added to the top of the file.
  8. Finally, add the new ItemErrorHandler bean to the ItemProcessor. 
    @Named
@RequestScoped
public class ItemProcessor {

    @Inject @Demo
    private ItemDao itemDao;

    @Inject
    private ItemValidator itemValidator;

    @Inject
    private ItemErrorHandler itemErrorHandler;

    public void execute() {
        List<Item>  items = itemDao.fetchItems();
        for (Item item : items) {
            if (!itemValidator.isValid(item)) {
                itemErrorHandler.handleItem(item);
            }
        }
    }
}

    (Use the editor's hint to add the import statement for exercise3.ItemErrorHandler.)

  9. Click the Run Project ( Run Project button ) button to run the project (Alternatively, press F6; fn-F6 on Mac). In the browser, click the 'Execute' button that displays on the page. Switch back to the IDE and examine the GlassFish server log displayed in the Output window (Ctrl-4; ⌘-4 on Mac). 
    INFO: Creating file error reporter
INFO: Saving  [Value=34, Limit=7] to file
INFO: Saving  [Value=89, Limit=32] to file
INFO: Closing file error reporter

See Also

Different application deployments might use different rules for handling invalid items, such as rejecting an item, sending notifications to individuals, flagging them, or just listing them in an output file. In addition, we may want to do a combination of these (e.g., reject an order, send an email to a sales representative, and list the order in a file). One great way to handle this kind of multi-faceted problem is by using events. CDI events are the subject of the final installment of this series:

For more information about CDI and Java EE, see the following resources.