Jakarta Contexts and Dependency Injection

Jakarta Contexts and Dependency Injection defines a powerful set of complementary services that help to improve the structure of application code.

• A well-defined lifecycle for stateful objects bound to lifecycle contexts, where the set of contexts is extensible

• A sophisticated, typesafe dependency injection mechanism, including the ability to select dependencies at either development or deployment time, without verbose configuration

• Support for Jakarta EE modularity and the Jakarta EE component architecture - the modular structure of a Jakarta EE application is taken into account when resolving dependencies between Jakarta EE components

• Integration with the Unified Expression Language (EL), allowing any contextual object to be used directly within a Jakarta Server Faces or Jakarta Server Pages page

• The ability to decorate injected objects

• The ability to associate interceptors to objects via typesafe interceptor bindings

• An event notification model

• A web conversation context in addition to the three standard web contexts defined by the Jakarta Servlets specification

• An SPI allowing portable extensions to integrate cleanly with the container

For instance the following class defines a CDI bean and inject user credentials and Jakarta Persistence entity manager. It produces another bean of type User with qualifier @LoggedIn

@SessionScoped @Model
public class Login implements Serializable {

@Inject Credentials credentials;
@Inject @Users EntityManager userDatabase;

private CriteriaQuery<User> query;
private Parameter<String> usernameParam;
private Parameter<String> passwordParam;

private User user;

@Inject
void initQuery(@Users EntityManagerFactory emf) {
CriteriaBuilder cb = emf.getCriteriaBuilder();
usernameParam = cb.parameter(String.class);
passwordParam = cb.parameter(String.class);
query = cb.createQuery(User.class);
Root<User> u = query.from(User.class);
query.select(u);
query.where( cb.equal(u.get(User_.username), usernameParam),
cb.equal(u.get(User_.password), passwordParam) );
}

public void login() {

List<User> results = userDatabase.createQuery(query)
.setParameter(usernameParam, credentials.getUsername())
.setParameter(passwordParam, credentials.getPassword())
.getResultList();

if ( !results.isEmpty() ) {
user = results.get(0);
}

}

public void logout() {
user = null;
}

public boolean isLoggedIn() {
return user!=null;
}

@Produces @LoggedIn User getCurrentUser() {
if (user==null) {
throw new NotLoggedInException();
}
else {
return user;
}
}
}

CDI provides far more features. More example can be found in the existing specification document.