Hello, World!

To kick off our introduction to server-side Echo3 development, we'll start with the simplest of applications: "Hello, World!".

To create a web application with Echo, you will create two Java classes: an ApplicationInstance and a WebContainerServlet. The ApplicationInstance will represent the state of a single user-instance of the application. The WebContainerServlet is a Servlet-derivative that will process HTTP connections from web clients and instantiate new ApplicationInstances for new users.

The ApplicationInstance

Purpose: An ApplicationInstance represents the state of the user interface of a single user. Every user who visits an Echo application will have his/her own unique instance. Echo will store the ApplicationInstance in the servlet container's user session, so the instance will be available until the user exits the application or the session times out due to inactivity.

Implementation: ApplicationInstance is an abstract class, requiring a single method to be defined by the developer: init(). The init() method is invoked to initialize the state of the user interface for a new user. It must return an Echo Window object representing the state of the initial window of an application. An application's implementation of the init() method should create a new Window and configure it for a new user by assembling a hierarchy of Echo components within it. The following code example shows an ApplicationInstance implementation for a simple "Hello, world!" application:

public class HelloWorldApp extends ApplicationInstance {
    public Window init() {
        Window window = new Window();

        ContentPane contentPane = new ContentPane();

        Label label = new Label("Hello, world!");
        return window;

Properties: Because an ApplicationInstance represents a single user of an application, it is safe to add properties to this class in your implementation with the expectation that the properties will be unique on a per-user basis. For example, if an application required authentication, it might be practical to store the login name of the user in a property of the ApplicationInstance object.

The WebContainerServlet

The WebContainerServlet class is an Echo-specific derivative of the Java Servlet Specification's HttpServlet class. The WebContainerServlet is responsible for processing all requests from the client-side Echo engine, including rendering the initial HTML page, handling XML synchronization services, and sending graphic images to the client. All such client interaction work is done behind the scenes. As an application developer, your only required interaction with the WebContainerServlet class is to create a derivative implementation that returns new ApplicationInstances. The following example shows a WebContainerServlet that returns new ApplicationInstances of the previous "Hello, world!" application:

public class HelloWorldServlet extends WebContainerServlet {
    public ApplicationInstance newApplicationInstance() {
        return new HelloWorldApp();