So you’re keen to get started writing your first bean? Or perhaps you’re skeptical, wondering what kinds of hoops the CDI specification will make you jump through! The good news is that you’ve probably already written and used hundreds, perhaps thousands of beans. CDI just makes it easier to actually use them to build an application!
A bean is exactly what you think it is. Only now, it has a true identity in the container environment.
Prior to Java EE 6, there was no clear definition of the term "bean" in the Java EE platform. Of course, we’ve been calling Java classes used in web and enterprise applications "beans" for years. There were even a couple of different kinds of things called "beans" in EE specifications, including EJB beans and JSF managed beans. Meanwhile, other third-party frameworks such as Spring and Seam introduced their own ideas of what it meant to be a "bean". What we’ve been missing is a common definition.
Java EE 6 finally laid down that common definition in the Managed Beans specification. Managed Beans are defined as container-managed objects with minimal programming restrictions, otherwise known by the acronym POJO (Plain Old Java Object). They support a small set of basic services, such as resource injection, lifecycle callbacks and interceptors. Companion specifications, such as EJB and CDI, build on this basic model. But, at last, there’s a uniform concept of a bean and a lightweight component model that’s aligned across the Java EE platform.
With very few exceptions, almost every concrete Java class that has a
constructor with no parameters (or a constructor designated with the
annotation @Inject) is a bean. This includes every JavaBean and every
EJB session bean. If you’ve already got some JavaBeans or session beans
lying around, they’re already beans—you won’t need any additional
special metadata.
The JavaBeans and EJBs you’ve been writing every day, up until now, have not been able to take advantage of the new services defined by the CDI specification. But you’ll be able to use every one of them with CDI—allowing the container to create and destroy instances of your beans and associate them with a designated context, injecting them into other beans, using them in EL expressions, specializing them with qualifier annotations, even adding interceptors and decorators to them—without modifying your existing code. At most, you’ll need to add some annotations.
Now let’s see how to create your first bean that actually uses CDI.
Suppose that we have two existing Java classes that we’ve been using for years in various applications. The first class parses a string into a list of sentences:
public class SentenceParser {
public List<String> parse(String text) { ... }
}
The second existing class is a stateless session bean front-end for an external system that is able to translate sentences from one language to another:
@Stateless
public class SentenceTranslator implements Translator {
public String translate(String sentence) { ... }
}
Where Translator is the EJB local interface:
@Local
public interface Translator {
public String translate(String sentence);
}
Unfortunately, we don’t have a class that translates whole text documents. So let’s write a bean for this job:
public class TextTranslator {
private SentenceParser sentenceParser;
private Translator sentenceTranslator;
@Inject
TextTranslator(SentenceParser sentenceParser, Translator sentenceTranslator) {
this.sentenceParser = sentenceParser;
this.sentenceTranslator = sentenceTranslator;
}
public String translate(String text) {
StringBuilder sb = new StringBuilder();
for (String sentence: sentenceParser.parse(text)) {
sb.append(sentenceTranslator.translate(sentence));
}
return sb.toString();
}
}
But wait! TextTranslator does not have a constructor with no
parameters! Is it still a bean? If you remember, a class that does not
have a constructor with no parameters can still be a bean if it has a
constructor annotated @Inject.
As you’ve guessed, the @Inject annotation has something to do with
dependency injection! @Inject may be applied to a constructor or
method of a bean, and tells the container to call that constructor or
method when instantiating the bean. The container will inject other
beans into the parameters of the constructor or method.
We may obtain an instance of TextTranslator by injecting it into a
constructor, method or field of a bean, or a field or method of a Java
EE component class such as a servlet. The container chooses the object
to be injected based on the type of the injection point, not the name of
the field, method or parameter.
Let’s create a UI controller bean that uses field injection to obtain an
instance of the TextTranslator, translating the text entered by a
user:
@Named @RequestScoped
public class TranslateController {
@Inject TextTranslator textTranslator;
private String inputText;
private String translation;
// JSF action method, perhaps
public void translate() {
translation = textTranslator.translate(inputText);
}
public String getInputText() {
return inputText;
}
public void setInputText(String text) {
this.inputText = text;
}
public String getTranslation() {
return translation;
}
}
| | Field injection of |
Tip
Notice the controller bean is request-scoped and named. Since this
combination is so common in web applications, there’s a built-in
annotation for it in CDI that we could have used as a shorthand. When
the (stereotype) annotation @Model is declared on a class, it creates
a request-scoped and named bean.
Alternatively, we may obtain an instance of TextTranslator
programmatically from an injected instance of Instance, parameterized
with the bean type:
import javax.enterprise.inject.Instance;
import javax.inject.Inject;
....
@Inject Instance<TextTranslator> textTranslatorInstance;
...
public void translate() {
textTranslatorInstance.get().translate(inputText);
}
Notice that it isn’t necessary to create a getter or setter method to inject one bean into another. CDI can access an injected field directly (even if it’s private!), which sometimes helps eliminate some wasteful code. The name of the field is arbitrary. It’s the field’s type that determines what is injected.
At system initialization time, the container must validate that exactly
one bean exists which satisfies each injection point. In our example, if
no implementation of Translator is available—if the
SentenceTranslator EJB was not deployed—the container would inform us
of an unsatisfied dependency. If more than one implementation of
Translator were available, the container would inform us of the
ambiguous dependency.
Before we get too deep in the details, let’s pause and examine a bean’s anatomy. What aspects of the bean are significant, and what gives it its identity? Instead of just giving examples of beans, we’re going to define what makes something a bean.