What’s so wrong with EJB’s?

It’s not that they are heavy. It’s not that they are difficult to write. So, what’s wrong with EJB’s?

EJB’s can be either Stateless, Stateful or Singleton. However, when you are working on a web environment for example, these lifecycles will not suffice. Request, session and application lifecycles would be much better suited in that case. Another example is a business process application, where you could have a task or a process lifecycle; again, EJB’s will not match those lifecycles. This is what I call the Lifecycle Mismatch.

Through the years, EJB’s have become more POJO-like, through the use of annotations and without requirements such as XML descriptors and Java interfaces. However, it doesn’t matter how simple they get, until they don’t remove the lifecycle restriction, you will end up creating additional layers in your application and different component models to adapt your required lifecycles.

Note: The lifecycles I’m referring are also known as contexts or scopes in other frameworks/specifications.

For example, take a look at JSF. They introduced the JSF Managed Beans to handle this problem. JSF Managed Beans can be bound to request, session or application contexts. From there, you are on your own on how you connect them to the EJB’s different lifecycle.

CDI (Contexts and Dependency Injection) is a specification, part of the JEE stack, that is trying to fill that gap. It provides dependency injection and lifecycle management to POJO’s and EJB’s. But providing lifecycle management to EJB’s doesn’t feels right, because they already provide their own. CDI does its best effort to accommodate this mismatch.

On the other hand, CDI could provide to POJO’s, all the services EJB’s currently have, through extensions. So, do we really need EJB’s? Well, not really! You can check my post EJB’s - Time to let them go? for more information.

Conclusion

EJB’s have become light weighted and easy to write. But the lifecycle mismatch is still there. POJO’s, on the other hand, don’t have this restriction and, with CDI, they could have all the services EJB’s currently provide.

From JBoss Seam to CDI (JEE6)

Seam is a popular open source application framework that has led to the creation of CDI (Contexts and Dependency Injection), a specification that is now part of the JEE 6 ecosystem. In this post I’ll show you the architectural similarities and differences between both and how Seam has influenced CDI. So lets start!

Contexts

Contexts are definitely one of the most exciting features of Seam (you can read more about them in this post) and remain in the core of CDI.  Seam defines six stateful contexts: request (event), session, application, page, conversation and business process context. CDI only define 4, leaving page and business process contexts out. However, CDI allows you to define new contexts which was not possible in Seam. You can see an example of creating a context (the equivalent of the page context in Seam) by Steven Verborgh here.

Additionally, CDI now defines a new “dependent” context which allows a bean to have the same context as the bean that declares it. They will share the same lifecycle meaning that “it [the dependent bean] is instantiated when the object it belongs to is created and destroyed when the object it belongs is destroyed”, as stated in the Weld documentation.

Dependency Injection

Even though Seam and CDI both make heavy use of the Dependecy Injection pattern, they take different approaches to implement it. The most notable differences are the following (I’ll explain each one in more detail below):

• Seam uses a string-based approach to search for components which is not type safe while CDI brings type safety to the limit.
• Seam uses dynamic injection while CDI uses static injection.

Type safety

In Seam, every component has a name defined by the @Name annotation. When an injection occurs, Seam searches for the name of the component which can lead to runtime exceptions. For example, suppose you have the following component:

@Name("myBean")
public class MyBean {
 ...
}

Now, suppose you mistakenly inject MyBean into a String like this:

@In
private String myBean;

This will compile just fine, but at runtime you will get an exception. In large applications, this is a problem as you will potentially have a lot of bugs at runtime, something you really want to avoid.

CDI implements dependency injection based on the type of the bean like in Google Guice. There is no need to name a bean unless you want to access it from EL, in which case, you will need to annotate your bean with the @Named annotation. Lets look at a simple case of injection in CDI:

public class Bean1 {
 ...
}

public class Bean2 {
 @Inject
 private Bean1 bean1;
 ...
}

When Bean2 gets instantiated, Bean1 will be injected on Bean2. As you can see, there is no need to name the beans.

Static vs. Dynamic Injection

In Seam, injection occurs on each method call. After the method completes, all injected values are desinjected (set to null). This is known as dynamic injection and it has its benefits and drawbacks. The most outstanding benefit is that you can inject components from different contexts (i.e. a request component injected in a conversation component)  and you will always have the “correct” instance on each method call. However, dynamic injection has some problems including performance and null values on concurrency.

CDI uses static injection, meaning that all the values are injected on the creation of the bean and they are not desinjected between method calls. To handle injection of beans from different contexts, it uses proxies that reference the “correct” instance of each bean. A much more elegant solution in my opinion.

Factories

Remember the @Factory annotation in Seam? It allows you to obtain objects that are not Seam components; or it could also be used when you need to perform some custom initialization on a component before returning it. In CDI, there is an equivalent functionality called “producer method”:

@Produces
public User getUser {
      ...
}

Every time a User object is injected, this method will be called to obtain a reference. CDI provides a simpler alternative to a producer method called a producer field:

@Produces
private User user;

Events

In Seam, you can decouple your components using events. Each event must have a name. You can raise an event using the @Raise(“<name_of_event>”) annotation or programmatically using the Events.instance().raiseEvent(“<name_of_event>”) method. To observe for an event, you mark the method you want to be called on the event with the @Observer(“<name_of_event>”) annotation.

CDI grabs this concept from Seam and makes it type safe. So, for raising an event we can obtain an Event object by injection:

@Inject
private Event<User> userEvent;

And then call the fire method on the Event object:

userEvent.fire(user);

Now, for observing an event you use the @Observes annotation:

public void observeUserEvent(@Observes User user) {
 ...
}

Interceptors

Seam relies heavily on interceptors for injection, security, persistence and much more. CDI also provides an interception facility called interceptors bindings, based on the Java Interceptors specification. Furthermore, CDI provides the concept of “decorator”, which is a kind of interceptor for implementing the decorator pattern.

Wrapping up

Let’s take a look at the services provided by CDI and see if they were inspired by Seam:

• Contexts: heavily inspired by Seam.
• Dependency Injection: mostly inspired by Google Guice but we have an equivalent in Seam (which is not type safe).
• EL integration (the #{} notation): completely inspired by Seam.
• Interceptors: completely inspired by Seam.
• Events: heavily inspired by Seam but type safe.

Wait a minute! Are these all the services provided by CDI? What about all the out-of-the-box functionality I used to have in Seam (page flows, business process integration, security, scheduling, caching, remoting, etc.)? Don’t worry! Portable Extensions, the only service we are missing in this list, will solve this question, so just keep reading!

Portable Extensions and Seam 3

Besides the services described above, CDI provides a powerful SPI (Service Provider Interface) you can use to create extensions (called Portable Extensions in CDI). This is where Seam 3 enters the game!

Seam 3 will be a set of portable extensions that will provide almost all of the out-of-the-box functionality you actually have in Seam 2. At the time of this writing, Seam 3 is on its first steps and there is no official release yet. However, you can start developing your own portable extensions on top of CDI. Take a look at this post by Matt Corey to start writing your own first portable extension!

Conclusion

CDI has been heavily influenced by Seam with two major architectural changes:

• Instead of providing everything out-of-the-box as Seam, CDI provides a set of core technologies and a powerful SPI that will enable any one write its own portable extensions.
• Type safety. This was included in every aspect of CDI (Contexts, DI and events), except for the EL integration.

Remember that Seam is a framework and CDI is a specification (Weld is the Reference Implementation). For more information on CDI, check the Weld documentation and the CDI specification.

Seam and JSF

In my last post, I talked about Seam contexts and how to use them from your application. Now, I’ll show you how you can access them from JSF, especially how to use the event context correctly. At the end, I’ll give you some tips you may find useful when developing the web tier of your application with Seam and JSF.

Accessing contexts from your JSF pages

In Seam, you can access anything saved in the contexts from a JSF page using the #{} notation. For example, consider the following line of code:

<h:outputText value="#{employee.name}" />

This will search all the stateful contexts for the “employee” key using the Contexts.lookupInStatefulContexts() method as I showed you in my last post. Remember that the context search priority is: event, page, conversation, session, business process and application. If the key is not found in any of the contexts, it will return null. Otherwise, it will try to call the getName() method on the object.

JSF and the event context

The event context is the narrowest of the stateful contexts; it only lasts during a single http request. It is very useful when working with data iteration because Seam binds the iterated variable into the event context. Consider the following code fragment:

<rich:dataTable value="#{employees}" var="employee">
 <rich:column>
  <h:outputText value="#{employee.name}" />
 </rich:column>
 <rich:column>
  <h:outputText value="#{employeeSalaryResolver.netSalary}" />
 </rich:column>
</rich:dataTable>

In line 6 we are calling another component called employeeSalaryResolver to retrieve the net salary of the employee. Seam binds the actual employee object into the event context, so, we can just inject it in the EmployeeSalaryResolver component:

@Name("employeeSalaryResolver")
public class EmployeeSalaryResolver {
 
 @In
 private Employee employee;

 public double getNetSalary() {
  double netSalary = employee.getBaseSalary();
  
  // do other complex calculations (extra hours, bonus, etc.)  
  
  return netSalary;
 }
}

The employee attribute is being injected from the iteration variable (the event context) and used to retrieve the net salary of the employee. This component can be then reused in other use cases.

You can also use the event context to pass information between pages. For example, imagine a component that searches for an employee using the social security number and shows the result in another page.

@Name("searchEmployee")
public class SearchEmployee {
 
 private String ssNumber;
 
 @Out(scope=ScopeType.EVENT,required=false)
 private Employee employee;

 public String search() {
  Employee employee = ...; // search for the employee
  if (employee != null) {
   return "success";
  }
  
  return null; 
 }
}

When the search method is called (from a page I’m not showing) and an employee is found, the searchEmployee component will outject the Employee object to the event context and render the result in another page. The navigation in pages.xml would be something like this:

<page view-id="searchEmployee.xhtml">
 <navigation>
  <rule if-outcome="success">
   <render view-id="searchResult.xhtml"/>
  </rule>
 </navigation>
</page>

If you notice, we are using render tag instead of redirect tag. This will forward the request to the searchResult.xhtml page instead of redirecting, so the event context will not be lost.

Seam and JBoss EL

Standard EL (the #{} notation you use in your JSF pages) does not allow you to call methods with predefined parameters. However, Seam overcomes this limitation by using JBoss EL, which is an extension of the standard EL. For example, in our employees application, suppose we have a list of employees and we want to add an option on each row to fire employees. So, we create a fireEmployee component with a fire method that receives an Employee as an argument.

@Name("fireEmployee")
public class FireEmployee {

 public void fireEmployee(Employee employee) {
  // fire employee
 }
}

Now, we just need to add the action to the employees' page.

<rich:dataTable value="#{employees}" var="employee">
 <rich:column>
  <h:outputText value="#{employee.name}" />
 </rich:column>
 <rich:column>
  <h:outputText value="#{employeeSalaryResolver.netSalary}" />
 </rich:column>
 <rich:column>
  <s:link value="Fire Employee" action="#{fireEmployee.fire(employee)}" />
 </rich:column>
</rich:dataTable>

Really cool feature! Look at the <s:link> tag. Seam introduces some JSF tags to ease development. I won’t describe them here, but if you want, you can check them here.

Tips using Seam and JSF

1. Backing beans

In Seam, you can use EJB’s as your backing beans. However, by default, use JavaBeans (POJO’s). The reason is that you usually bind the backing bean to the page scope, but, if you use EJB Session Beans, you will not be able to do this.

@Name("someBean")
@Scope(ScopeType.PAGE)
@Synchronized
public class SomeBean implements Serializable {

 private static final long serialVersionUID = -2524956479693778917L;

 // your code ...
}

You can also bind your backing beans to the conversation or session scopes when needed. The @Synchronized annotation tells Seam to serialize requests from the backing beans as concurrency may cause some problems occasionally, especially when you are using AJAX calls. A better approach, however, is to use the eventQueue attribute in your richfaces components. See this for more information.

2. Use a naming convention for your components

You have to choose carefully the name you assign to each of your components because as the application starts growing, you may repeat component names and having some conflicts. So, you can use the package as a prefix for the name of the component.

However, if the project is small, you also can use a simpler prefix if you want. For example, you can use the “controller” prefix for your backing beans, “logic” for your business components and “model” for your entities and DAO’s.

3. You can use more than one Backing Bean per page

A lot of people use one backing bean per page. However, I really discourage this at it does not promotes code reusing and increments coupling. For example, if you have something you show in every page, use a separate backing bean and bind it to the session scope instead of retrieving the same information in every page.

Conclusion

Seam allows you to access any component in any context from JSF, this gives you the opportunity to choose how you want to architect your solution with some cool features that help you speed development time.

Understanding Seam Contexts

One of the most exciting features of JBoss Seam is the contextual component model that radically changes the way we design applications. It has become so important, it is now part of the JEE6 specification through the JSR-299. In this post, I will concentrate on explaining how Seam contexts work and how to use them from your code.

You may already be familiar with the request, session and application scopes of the Servlet specification. They are very useful to save and retrieve data but they have a huge limitation: you can only access them from a Servlet/JSP.

JBoss Seam overcomes this limitation by making the scopes (called contexts in Seam) available to the whole application instead of only to Servlets/JSP’s. So, you’ll find request(event), session and application contexts and 3 more they’ve created: page, conversation and business process contexts.

The request(event), session and application contexts work exactly as they do in Servlets/JSP’s so I’ll just skip their explanation. I will, however, explain briefly the page, conversation and business process contexts.

Page Context

The page context is broader than the request context as it spans multiple requests over the same page. This is very useful if you need to make roundtrips to the server to perform validation, bring information, etc. while maintaining the same page state.

Conversation Context

The conversation context is broader than the page context as it can expand to multiple request and pages but narrower than the session context. It is very useful to implement wizards-like interfaces in which the user has to fill data across multiple pages while maintaining the state over the whole process. You control the creation and destruction of each conversation.

Business Process Context

As it name implies, it is used to maintain state information during the execution of a business process instance in jBPM.

Using contexts from your code

You can use the Contexts class to gain access to one of the contexts, defined by the Context interface, and then use the set or get methods to save or retrieve information respectively.

For example, to save the current user information in the session context you use the following code:

User user = new User();
Contexts.getSessionContext().set("user", user);

Then, when you need to retrieve the user information, you use:

User user = (User) Contexts.getSessionContext().get("user");

Really simple, ah? You can also use the @Out and @In annotations in class attributes to save and retrieve information:

@Out(value="user",scope=ScopeType.SESSION)
private User user;

What the @Out annotation is telling Seam is to save the User object in the Session context with the key “user”. Then, you can retrieve the user with the following code:

@In(value="user",scope=ScopeType.SESSION)
private User user;

Again, the @In attribute is telling Seam to inject the User object from the Session context saved with the “user” key. You don’t have to write any getters or setters in your class, Seam will just outject or inject the information whenever an @Out or @In annotation is present. If no object is found under the “user” key, null will be injected.

As you can see, the value attribute of the @In and @Out annotations are used as a key for searching or saving data in the contexts. The scope attribute is used to specify the context in which the value should be injected or outjected. This attributes are optional; so, what happens if we don’t specify them as in the following example?

@In
private String name;

@Out
private User user;

When you don’t specify the value attribute, Seam uses the attribute’s name as key by default. In the preceding code fragment, Seam will use “name” as the key for the @In annotation and “user” for the @Out annotation.

If you don’t specify the scope of an @In annotation, Seam will scan all the contexts in the following order: event, page, conversation, session, business process and application. If the key is not found, it will inject null. You can use this feature programmatically by calling the Contexts.lookupInStatefulContexts() method.

If you don’t specify the scope of an @Out annotation … well, as the @Out JavaDoc states: “If no scope is explicitly specified, the default scope depends upon whether the value is an instance of a Seam component. If it is, the component scope is used. Otherwise, the scope of the component with the @Out attribute is used.” I know, I haven’t talked anything about Seam components as it is not the topic of this article. If you want to know about Seam components read this.

Conclusion

I hope this introduction has given you the basics of Seam contexts. It is a very simple, yet powerful, concept that has to be taken seriously when designing your next applications.