Jakarta Contexts and Dependency Injection defines a powerful set of complementary services that help to improve the structure of application code.
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A well-defined lifecycle for stateful objects bound to lifecycle contexts, where the set of contexts is extensible
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A sophisticated, typesafe dependency injection mechanism, including the ability to select dependencies at either development or deployment time, without verbose configuration
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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
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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
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The ability to decorate injected objects
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The ability to associate interceptors to objects via typesafe interceptor bindings
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An event notification model
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A web conversation context in addition to the three standard web contexts defined by the Jakarta Servlets specification
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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.