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.

This project is part of Jakarta EE 8.