Application Lifecycle

In this section, we look into more technical details of application deployment, user sessions, and the UI instance lifecycle. These details are not generally needed for writing Vaadin applications, but may be useful for understanding how they actually work and, especially, in what circumstances their execution ends.

Deployment

Before a Vaadin application can be used, it has to be deployed to a Java web server. The deployment process reads the servlet classes annotated with the @WebServlet annotation or the web.xml deployment descriptor in the application to register servlets for specific URL paths. It then loads the classes. Deployment does not yet usually run any code in the application, although static blocks in classes are executed when they are loaded.

A servlet class should extend the VaadinServlet class. However, there is no need to define your own servlet class if you are using the Servlet 3.1 specification. It is only necessary to have at least one class that carries the @Route annotation. A VaadinServlet instance will be registered for you automatically and Vaadin will register all the required servlets automatically.

Automatic Servlet Registration

On start-up, Vaadin application tries to register the following servlets:

Vaadin application servlet, mapped to the path /*

This servlet is needed to serve the main application files.

The servlet will not be registered, if any VaadinServlet is already registered, or if there is no class annotated with the @Route annotation.

Additionally, a servlet will not be registered if

there is a servlet that has already been mapped to the same path, or
if the system property disable.automatic.servlet.registration is set to true.

Undeploying and Redeploying

Applications are undeployed when the server shuts down, during redeployment, and when they are explicitly undeployed. Undeploying a server-side Vaadin application ends its execution. All application classes are unloaded and the heap space allocated by the application is freed for garbage collection.

If any user sessions are open at this point, the client-side state of the UIs is left as it was. An Out of Sync error is displayed on the next server request.

Vaadin Servlet and Service

The VaadinServlet receives all server requests mapped to it by its URL, as defined in the deployment configuration, and associates them with sessions. The sessions further associate the requests with particular UIs.

When servicing requests, the Vaadin servlet handles all tasks common to servlets in a VaadinService. It manages sessions, gives access to the deployment configuration information, handles system messages, and does various other tasks. Any further servlet-specific tasks are handled in the corresponding VaadinServletService. The service acts as the primary low-level customization layer for processing requests.

Customizing Vaadin Servlet

Many common configuration tasks need to be performed in the Servlet class that you already have when using the @WebServlet annotation for Servlet 3.0 to deploy the application. You can handle most customization by overriding the servletInitialized() method. Here, the VaadinService object is available with getService(). It would not be available in a constructor. You should always call super.servletInitialized() at the beginning.

public class MyServlet extends VaadinServlet {
    @Override
    protected void servletInitialized() throws ServletException {
        super.servletInitialized();
        //...
    }
}

To add custom functionality around request handling, you can override the service() method.

Customizing Vaadin Service

To customize VaadinService, you first need to extend the VaadinServlet class and override createServletService() to create a custom service object.

User Session

A user session begins when a user first makes a request to a Vaadin servlet by opening the URL for a particular UI. All server requests belonging to a particular UI class are processed by the VaadinServlet class. When a new client connects, it creates a new user session, represented by an instance of VaadinSession. Sessions are tracked using cookies stored in the browser.

You can get the VaadinSession of a UI with getSession(), or globally with VaadinSession.getCurrent(). It also provides access to the lower-level session objects, HttpSession, through a WrappedSession. You can also access the deployment configuration through VaadinSession.

A session ends after the last UI instance expires or is closed, as described later.

Handling Session Initialization and Destruction

You can handle session initialization and destruction by implementing a SessionInitListener or SessionDestroyListener, respectively, to the VaadinService. You can do this best by extending VaadinServlet and overriding servletInitialized(), as outlined in Vaadin Servlet and Service.

public class MyServlet extends VaadinServlet
    implements SessionInitListener, SessionDestroyListener {

    @Override
    protected void servletInitialized() throws ServletException {
        super.servletInitialized();
        getService().addSessionInitListener(this);
        getService().addSessionDestroyListener(this);
    }

    @Override
    public void sessionInit(SessionInitEvent event)
            throws ServiceException {
        // Do session start stuff here
    }

    @Override
    public void sessionDestroy(SessionDestroyEvent event) {
        // Do session end stuff here
    }
}

Loading a UI

When a browser first accesses a URL mapped to the servlet of a particular UI class, the Vaadin servlet generates a loader page. The page loads the client-side engine (widget set), which in turn loads the UI in a separate request to the Vaadin servlet.

A UI instance is created when the client-side engine makes its first request.

After a new UI is created, its init() method is called. The method gets the request as a VaadinRequest.

Customizing the Loader Page

The HTML content of the loader page is generated as an HTML DOM object, which can be customized by implementing an IndexHtmlRequestListener that modifies the DOM object. To do this, you need to extend the VaadinServlet and add a SessionInitListener to the service object, as outlined in User Session. You can then add the bootstrap listener to a session with addIndexHtmlRequestListener() when the session is initialized.

Loading the widget set is handled in the loader page with functions defined in a separate BootstrapHandler.js script, whose content is included inline in the page.

UI Expiration

UI instances are cleaned up if no communication is received from them after a certain time. If no other server requests are made, the client-side sends "keep alive" heartbeat requests. A UI is kept alive for as long as requests or heartbeats are received from it. It expires if three consecutive heartbeats are missed.

The heartbeats occur at an interval of 5 minutes, which can be changed with the heartbeatInterval parameter of the servlet. You can configure the parameter in @VaadinServletConfiguration or in web.xml.

When the UI cleanup happens, a DetachEvent is sent to all DetachListener objects added to the UI. When the UI is detached from the session, detach() is called for it.

Closing UIs Explicitly

You can explicitly close a UI with close(). The method marks the UI to be detached from the session after processing the current request. Therefore, the method does not invalidate the UI instance immediately and the response is sent as usual.

Detaching a UI does not close the page or browser window in which the UI is running. Further server requests will cause an error. Typically, you should close the window, reload it, or redirect it to another URL. If the page is a regular browser window or tab, browsers do not usually allow them to be closed programmatically. However, redirection is possible. You can redirect the window to another URL via JavaScript.

If you close UIs other than the one associated with the current request, they will not be detached at the end of the current request. This will happen after the next request from the particular UI. You can make it happen more quickly by increasing the UI heartbeat frequency, or immediately by using server push.

Session Expiration

A session is kept alive by server requests caused by user interaction with the application, as well as by the heartbeat-monitoring mechanism of the UIs. Once all UIs have expired, the session still remains. It is cleaned up from the server when the session timeout configured in the web application elapses.

If there are active UIs in an application, their heartbeat keeps the session alive indefinitely. You may want to have the sessions time out if the user is inactive for a certain time. This is the original purpose of the session timeout setting.

If the closeIdleSessions deployment configuration parameter of the servlet is set to true, the closure mechanism works as follows. The session and all of its UIs are closed when the timeout specified by the session-timeout parameter of the servlet elapses after the last non-heartbeat request. After the session is gone, the browser will show an Out of sync error on the next server request.

See Configuration Properties for information on setting configuration parameters.

You can handle session expiration on the server side with a SessionDestroyListener, as described in User Session.

Closing a Session

You can close a session by calling close() on the VaadinSession. This is typically used when logging a user out, as the session and all the UIs belonging to the session should be closed. The session is closed immediately and any objects related to it are unavailable after calling the method.

@Route("")
public class MainLayout extends Div {

    protected void onAttach(AttachEvent attachEvent) {
        UI ui = getUI().get();
        Button button = new Button("Logout", event -> {
            // Redirect this page immediately
            ui.getPage().executeJs("window.location.href='logout.html'");

            // Close the session
            ui.getSession().close();
        });

        add(button);

        // Notice quickly if other UIs are closed
        ui.setPollInterval(3000);
    }
}

There is more to be done. When a session is closed from one UI, any other UIs attached to it are left hanging. When the client-side engine notices that a UI and the session are gone on the server-side, it displays a Session Expired message and, by default, reloads the UI when the message is clicked.

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