Getting Started (original) (raw)

Contexts and Dependency Injection (CDI) enables your objects to have their dependencies provided to them automatically, instead of creating them or receiving them as parameters. CDI also manages the lifecycle of those dependencies for you.

For example, consider the following servlet:

@WebServlet("/cdiservlet")
public class NewServlet extends HttpServlet {
    private Message message;

    @Override
    public void init() {
        message = new MessageB();
    }

    @Override
    public void doGet(HttpServletRequest request, HttpServletResponse response)
                  throws IOException {
        response.getWriter().write(message.get());
    }
}

This servlet needs an instance of an object that implements theMessage interface:

public interface Message {
    public String get();
}

The servlet creates itself an instance of the following object:

public class MessageB implements Message {
    public MessageB() { }

    @Override
    public String get() {
        return "message B";
    }
}

Using CDI, this servlet can declare its dependency on a Messageinstance and have it injected automatically by the CDI runtime. The new servlet code is the following:

@WebServlet("/cdiservlet")
public class NewServlet extends HttpServlet {
    @Inject private Message message;

    @Override
    public void doGet(HttpServletRequest request, HttpServletResponse response)
                  throws IOException {
        response.getWriter().write(message.get());
    }
}

The CDI runtime looks for classes that implement the Messageinterface, finds the MessageB class, creates a new instance of it, and injects it into the servlet at runtime. To manage the lifecycle of the new instance, the CDI runtime needs to know what the scope of the instance should be. In this example, the servlet only needs the instance to process an HTTP request; the instance can then be garbage collected. This is specified using the javax.enterprise.context.RequestScopedannotation:

@RequestScoped
public class MessageB implements Message { ... }

For more information on scopes, see Using Scopes.