> Markdown version of [Server Push](https://vaadin.com/docs/v23/advanced/server-push). Section index: [llms.txt](https://vaadin.com/docs/v23/advanced/llms.txt)

# <a id="push.configuration"></a>Server Push Configuration

Server push is based on a client-server connection which the client establishes and the server can then use to send updates to the client.

The server-client communication uses a WebSocket connection, if the browser and the server support it. If not, the connection falls back to a method supported by the browser. Vaadin uses the [Atmosphere framework](https://github.com/Atmosphere/atmosphere) internally.

## <a id="push.configuration.enabling"></a>Enabling Push in Your Application

To enable server push, you need to define the push mode either in the deployment descriptor or with the `@Push` annotation on your application shell class.

The application shell class is a plain Java class implementing the `AppShellConfigurator` interface. The class is detected and instantiated by Vaadin during bootstrap. Only one application shell class is allowed. If more than one class implements `AppShellConfigurator`, an exception is thrown.

For example, you can use the following `Application` class, which implements the `AppShellConfigurator` interface, to enable server push in your application by adding the `@Push` annotation to it.

```java
@Push
public class Application implements AppShellConfigurator {
  ...
}
```

### <a id="push.configuration.pushmode"></a>Push Modes and Transports

You can use server push in two modes: `automatic` and `manual`. The automatic mode pushes changes to the browser automatically after `access()` finishes. With the manual mode, you can do the push explicitly with `push()`, which allows more flexibility.

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Server push can use several transports: WebSockets, long polling, or combined WebSockets+XHR. WebSockets+XHR is the default transport.

<!-- vale Vaadin.Abbr = YES -->

### <a id="push.configuration.annotation"></a>The @Push Annotation

You can enable server push for the application annotating the application shell class with the `@Push` annotation, as follows. It defaults to automatic mode (`PushMode.AUTOMATIC`).

```java
@Push
public class Application implements AppShellConfigurator {
  ...
}
```

To enable manual mode, you need to pass the `PushMode.MANUAL` parameter, as follows:

```java
@Push(PushMode.MANUAL)
public class Application implements AppShellConfigurator {
  ...
}
```

To use the long polling transport, you need to set the transport parameter as `Transport.LONG_POLLING`, as follows:

```java
@Push(transport = Transport.LONG_POLLING)
public class Application implements AppShellConfigurator {
  ...
}
```

### <a id="push.configuration.servlet"></a>Servlet Configuration

If you are configuring your servlet manually, be sure to set the `async-supported` parameter.

You can enable server push and define the push mode for an entire application in the servlet configuration with the `pushMode` parameter for the servlet in the `web.xml` deployment descriptor or a corresponding `@WebServlet` annotation.

On the server side, the push endpoint is mapped to the `VAADIN/push` path. This mapping is added either under context root, context path, or the first URL mapping (sorted by natural order and ignoring `/VAADIN/*` and `/vaadinServlet/*`) of the Vaadin servlet, depending on application deployment configuration. In case of multiple servlet mappings, it’s possible to set the URL mapping for server push by configuring the `pushServletMapping` parameter to match the desired mapping.

## <a id="push.access"></a>Asynchronous Updates

Making changes to a UI from another thread and pushing them to the browser requires locking the user session. Otherwise, the UI update done from another thread could conflict with a regular event-driven update and cause either data corruption or deadlocks. Because of this, you may only access an UI using the `access()` method, which locks the session to prevent conflicts. It takes as parameter a `Command` to execute while the session is locked.

For example:

```java
ui.access(new Command() {
    @Override
    public void execute() {
        statusLabel.setText(statusText);
    }
});
```

You can also use a lambda expression to define your access command.

```java
ui.access(() -> statusLabel.setText(statusText));
```

If the push mode is `manual`, you need to push the pending UI changes to the browser explicitly with the `push()` method.

```java
ui.access(() -> {
    statusLabel.setText(statusText);
    ui.push();
});
```

The following is a complete example of making UI changes from another thread.

```java
@Route("push")
public class PushyView extends VerticalLayout {
    private FeederThread thread;

    @Override
    protected void onAttach(AttachEvent attachEvent) {
        add(new Span("Waiting for updates"));

        // Start the data feed thread
        thread = new FeederThread(attachEvent.getUI(), this);
        thread.start();
    }

    @Override
    protected void onDetach(DetachEvent detachEvent) {
        // Cleanup
        thread.interrupt();
        thread = null;
    }

    private static class FeederThread extends Thread {
        private final UI ui;
        private final PushyView view;

        private int count = 0;

        public FeederThread(UI ui, PushyView view) {
            this.ui = ui;
            this.view = view;
        }

        @Override
        public void run() {
            try {
                // Update the data for a while
                while (count < 10) {
                    // Sleep to emulate background work
                    Thread.sleep(500);
                    String message = "This is update " + count++;

                    ui.access(() -> view.add(new Span(message)));
                }

                // Inform that we're done
                ui.access(() -> {
                    view.add(new Span("Done updating"));
                });
            } catch (InterruptedException e) {
                e.printStackTrace();
            }
        }
    }
}
```

When sharing data between UIs or user sessions, you need to consider the message-passing mechanism, as explained in the next section.

## <a id="push.broadcaster"></a>Collaborative Views

Broadcasting messages to be pushed to UIs in other user sessions requires some sort of message-passing mechanism that sends the messages to all UIs that are registered as recipients. As processing server requests for different UIs happens concurrently in different threads of the application server, locking the data structures is important to avoid deadlock situations.

### <a id="push.broadcaster.broadcaster"></a>The Broadcaster

The standard pattern for sending messages to other users is to use a *broadcaster* singleton that registers recipients and broadcasts messages to them. To avoid deadlocks, it’s recommended that the messages be sent through a message queue in a separate thread. Using a Java `ExecutorService` running a single thread is one of the easiest and safest ways. The methods in the class are defined as `synchronized` to prevent race conditions.

```java
public class Broadcaster {
    static Executor executor = Executors.newSingleThreadExecutor();

    static LinkedList<Consumer<String>> listeners = new LinkedList<>();

    public static synchronized Registration register(
            Consumer<String> listener) {
        listeners.add(listener);

        return () -> {
            synchronized (Broadcaster.class) {
                listeners.remove(listener);
            }
        };
    }

    public static synchronized void broadcast(String message) {
        for (Consumer<String> listener : listeners) {
            executor.execute(() -> listener.accept(message));
        }
    }
}
```

### <a id="push.broadcaster.receiving"></a>Receiving Broadcasts

The receivers need to register a consumer to the broadcaster to receive the broadcasts. The registration should be removed when the component is no longer attached. When updating the UI in a receiver, you should do this safely by executing the update through the `access()` method of the `UI`, as described in the previous section, [Asynchronous Updates](#push.access).

```java
@Route("broadcaster")
public class BroadcasterView extends Div {
    VerticalLayout messages = new VerticalLayout();
    Registration broadcasterRegistration;

    // Creating the UI shown separately

    @Override
    protected void onAttach(AttachEvent attachEvent) {
        UI ui = attachEvent.getUI();
        broadcasterRegistration = Broadcaster.register(newMessage -> {
            ui.access(() -> messages.add(new Span(newMessage)));
        });
    }

    @Override
    protected void onDetach(DetachEvent detachEvent) {
        broadcasterRegistration.remove();
        broadcasterRegistration = null;
    }
}
```

### <a id="push.broadcaster.sending"></a>Sending Broadcasts

To send broadcasts with a broadcaster singleton, such as the one described previously, you would only need to call the `broadcast()` method, as follows.

```java
@Route("broadcaster")
public BroadcasterView() {
    TextField message = new TextField();
    Button send = new Button("Send", e -> {
        Broadcaster.broadcast(message.getValue());
        message.setValue("");
    });

    HorizontalLayout sendBar = new HorizontalLayout(message, send);

    add(sendBar, messages);
}
```

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