The @PostConstruct annotation
is used on a method that needs to be executed after dependency injection is
done to perform any initialization. This rule is sometimes "omitted"
by novices who need to perform initialization stuff based on the injected
artifacts. For example, let's suppose that we have the following session scoped
CDI bean:
@Named
@SessionScoped
public class
Buzz implements Serializable {
private static final long serialVersionUID =
1L;
private String buzz = "BUZZ!";
// getter and setter for buzz
}
An instance of this bean is further injected into another session bean,
and it will serve for some initialization stuff. Now, the wrong approach will
consist in placing the initialization stuff in the bean constructor, instead of
using a separate method annotated with @PostConstruct as below:
@Named
@SessionScoped
public class
Foo implements Serializable {
private static final Logger LOG =
Logger.getLogger(Foo.class.getName());
private static final long serialVersionUID =
1L;
@Inject
private Buzz buzz;
// WRONG - buzz is not available yet!
public Foo() {
LOG.log(Level.INFO, "Foo() - The
injected buzz is: {0}", buzz);
}
// CORRECT - dependency injection was done, so
buzz is available!
@PostConstruct
private void init() {
LOG.log(Level.INFO, "init() - The
injected buzz is: {0}", buzz);
}
// CORRECT - just some bean method action
public void fooAction() {
LOG.log(Level.INFO, "fooAction() - The
injected buzz is: {0}", buzz);
}
}
The output will reveal (check out the constructor null output):
Foo() - The
injected buzz is: null
init() - The
injected buzz is: beans.Buzz@5b9370d8
fooAction() -
The injected buzz is: beans.Buzz@5b9370d8
Since Buzz
is session scoped the init()
method is called only once per session. Using a request scoped bean will cause
the initialization to take place at each request.
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