> Markdown version of [Application Lifecycle](https://vaadin.com/docs/v14/flow/advanced/tutorial-application-lifecycle). Section index: [llms.txt](https://vaadin.com/docs/v14/flow/llms.txt)

# <a id="application.lifecycle"></a>Application Lifecycle

In this section, we look into more technical details of application deployment, user sessions, and 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.

## <a id="application.lifecycle.deployment"></a>Deployment

Before a Vaadin application can be used, it has to be deployed to a Java web server. Deploying 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 and loads the classes. Deployment does not yet normally run any code in the application, although static blocks in classes are executed when they are loaded.

There is no need to define your own servlet class (which should extend the `VaadinServlet` class) if you are using Servlet 3.0 specification. You just need to have at least one class annotated with `@Route` annotation and a `VaadinServlet` instance will be registered for you automatically and Vaadin will register all servlets required automatically.

### <a id="automatic-servlet-registration"></a>Automatic servlet registration

When starting, Vaadin application tries to registed the following servlets:

- Vaadin application servlet, mapped to `/*` path

This servlet is needed to serve the main application files.

The servlet won’t be registered, if any `VaadinServlet` is registered already or if there is no class annotated with `Route` annotation.

- Frontend files servlet, mapped to `/frontend/*` path

This servlet is required in the development mode to serve the WebJar contents and is only registered when the application is started in the development mode.

In addition to the rules mentioned above, a servlet won’t be registered, if \* there is a servlet that had been mapped to the same path already \* or if `disable.automatic.servlet.registration` system property is set to `true`

### <a id="application.lifecycle.deployment.redeployment"></a>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 hanging and an Out of Sync error is displayed on the next server request.

## <a id="application.lifecycle.servlet-service"></a>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.

### <a id="application.lifecycle.servlet-service.servletcustomization"></a>Customizing Vaadin Servlet

Many common configuration tasks need to be done in the servlet class, which you already have if you are using the @WebServlet annotation for Servlet 3.0 to deploy the application. You can handle most customization by overriding the `servletInitialized()` method, where the `VaadinService` object is available with `getService()` (it would not be available in a constructor). You should always call `super.servletInitialized()` in the beginning.

```java
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.

### <a id="application.lifecycle.servlet-service.servicecustomization"></a>Customizing Vaadin Service

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

## <a id="application.lifecycle.session"></a>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 obtain 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.

### <a id="application.lifecycle.session.init"></a>Handling Session Initialization and Destruction

You can handle session initialization and destruction by implementing a \[interfacename]\`SessionInitListener or `SessionDestroyListener`, respectively, to the `VaadinService`. You can do that best by extending \[classname]\`VaadinServlet and overriding the `servletInitialized()` method, as outlined in [Vaadin Servlet and Service](#application.lifecycle.servlet-service).

```java
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
    }
}
```

## <a id="application.lifecycle.ui"></a>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.

Once a new UI is created, its \[methodname]\`init() method is called. The method gets the request as a `VaadinRequest`.

### <a id="application.lifecycle.ui.loaderpage"></a>Customizing the Loader Page

The HTML content of the loader page is generated as an HTML DOM object, which can be customized by implementing a `BootstrapListener` that modifies the DOM object. To do so, you need to extend the `VaadinServlet` and add a `SessionInitListener` to the service object, as outlined in [User Session](#application.lifecycle.session). You can then add the bootstrap listener to a session with `addBootstrapListener()` 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 inlined into the page.

## <a id="application.lifecycle.ui-expiration"></a>UI Expiration

`UI` instances are cleaned up if no communication is received from them after some 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##s added to the UI. When the [classname]`UI `` is detached from the session, `detach()` is called for it.

## <a id="application.lifecycle.ui-closing"></a>Closing UIs Explicitly

You can explicitly close a UI with \[methodname]\`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 and further server request will cause error. Typically, you either want to close the window, reload it, or redirect it to another URL. If the page is a regular browser window or tab, browsers generally do not allow closing them programmatically, but redirection is possible. You can redirect the window to another URL via JS execution.

If you close other UI than the one associated with the current request, they will not be detached at the end of the current request, but after next request from the particular UI. You can make that occur quicker by making the UI heartbeat faster or immediately by using server push.

## <a id="application.lifecycle.session-expiration"></a>Session Expiration

A session is kept alive by server requests caused by user interaction with the application as well as the heartbeat monitoring 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 expires.

If there are active UIs in an application, their heartbeat keeps the session alive indefinitely. You may want to have the sessions timeout if the user is inactive long enough, which is the original purpose of the session timeout setting. If the closeIdleSessions deployment configuration parameter of the servlet is set to true the session and all of its UIs are closed when the timeout specified by the session-timeout parameter of the servlet expires after the last non-heartbeat request. Once the session is gone, the browser will show an Out Of Sync error on the next server request.

See [Runtime Configuration](https://vaadin.com/docs/v14/flow/configuration/overview.md) about setting configuration parameters.

You can handle session expiration on the server-side with a \[interfacename]\`SessionDestroyListener, as described in [User Session](#application.lifecycle.session).

## <a id="application.lifecycle.session-closing"></a>Closing a Session

You can close a session by calling \[methodname]\`close() on the `VaadinSession`. It is typically used when logging a user out and 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 not available after calling the method.

```java
@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);
    }
}
```

This is not enough. 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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