--- title: Server Push order: 16 layout: page --- [[advanced.push]] = Server Push When you need to update a UI from another UI, possibly of another user, or from a background thread running in the server, you usually want to have the update show immediately, not when the browser happens to make the next server request. For this purpose, you can use __server push__ that sends the data to the browser immediately. Push is based on a client-server connection, usually a WebSocket connection, that the client establishes and the server can then use to send updates to the client. The server-client communication is done by default with a WebSocket connection if the browser and the server support it. If not, Vaadin will fall back to a method supported by the browser. Vaadin Push uses a custom build of the link:https://github.com/Atmosphere/atmosphere[Atmosphere framework] for client-server communication. [[advanced.push.installation]] == Installing the Push Support The server push support in Vaadin requires the separate Vaadin Push library. It is included in the installation package as [filename]#vaadin-push.jar#. [[advanced.push.installation.ivy]] === Retrieving with Ivy With Ivy, you can get it with the following declaration in the [filename]#ivy.xml#: [source, xml] ---- ---- In some servers, you may need to exlude a [literal]#++sl4j++# dependency as follows: [source, xml] ---- ---- Pay note that the Atmosphere library is a bundle, so if you retrieve the libraries with Ant, for example, you need to retrieve [literal]#++type="jar,bundle"++#. [[advanced.push.installation.maven]] === Retrieving with Maven In Maven, you can get the push library with the following dependency in the POM: [source, xml] ---- com.vaadin vaadin-push ${vaadin.version} ---- [[advanced.push.enabling]] == Enabling Push for a UI To enable server push, you need to define the push mode either in the deployment descriptor or with the [classname]#@Push# annotation for the UI. [[advanced.push.enabling.pushmode]] === Push Modes and Transports You can use server push in two modes: [literal]#++automatic++# and [literal]#++manual++#. The automatic mode pushes changes to the browser automatically after access() finishes. With the manual mode, you can do the push explicitly with [methodname]#push()#, which allows more flexibility. Sever push can use several transports: WebSockets, long polling, or combined WebSockets+XHR. [literal]#++WebSockets++# is the default transport. [[advanced.push.enabling.pushmode]] === The [classname]#@Push# annotation You can enable server push for a UI with the [classname]#@Push# annotation as follows. It defaults to automatic mode ( [parameter]#PushMode.AUTOMATIC#). [source, java] ---- @Push public class PushyUI extends UI { ---- To enable manual mode, you need to give the [parameter]#PushMode.MANUAL# parameter as follows: [source, java] ---- @Push(PushMode.MANUAL) public class PushyUI extends UI { ---- To use the long polling transport, you need to set the transport parameter as [parameter]#Transport.LONG_POLLING# as follows: [source, java] ---- @Push(transport=Transport.LONG_POLLING) public class PushyUI extends UI { ---- [[advanced.push.enabling.servlet]] === Servlet Configuration You can enable the server push and define the push mode also in the servlet configuration with the [parameter]#pushmode# parameter for the servlet in the [filename]#web.xml# deployment descriptor. If you use a Servlet 3.0 compatible server, you also want to enable asynchronous processing with the [literal]#++async-supported++# parameter. Note the use of Servlet 3.0 schema in the deployment descriptor. [subs="normal"] ---- <?xml version="1.0" encoding="UTF-8"?> <web-app id="WebApp_ID" version="**3.0**" xmlns="**http://java.sun.com/xml/ns/javaee**" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="**http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd**"> <servlet> <servlet-name>Pushy UI</servlet-name> <servlet-class> com.vaadin.server.VaadinServlet</servlet-class> <init-param> <param-name>UI</param-name> <param-value>**com.example.my.PushyUI**</param-value> </init-param> <!-- Enable server push --> <init-param> <param-name>pushmode</param-name> <param-value>**automatic**</param-value> </init-param> <async-supported>**true**</async-supported> </servlet> </web-app> ---- [[advanced.push.running]] == Accessing UI from Another Thread Making changes to a [classname]#UI# object from another thread and pushing them to the browser requires locking the user session when accessing the UI. 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 [methodname]#access()# method, which locks the session to prevent conflicts. It takes a [interfacename]#Runnable# which it executes as its parameter. For example: [source, java] ---- ui.access(new Runnable() { @Override public void run() { series.add(new DataSeriesItem(x, y)); } }); ---- In Java 8, where a parameterless lambda expression creates a runnable, you could simply write: [source, java] ---- ui.access(() -> series.add(new DataSeriesItem(x, y))); ---- If the push mode is [literal]#++manual++#, you need to push the pending UI changes to the browser explicitly with the [methodname]#push()# method. [source, java] ---- ui.access(new Runnable() { @Override public void run() { series.add(new DataSeriesItem(x, y)); ui.push(); } }); ---- Below is a complete example of a case where we make UI changes from another thread. [source, java] ---- public class PushyUI extends UI { Chart chart = new Chart(ChartType.AREASPLINE); DataSeries series = new DataSeries(); @Override protected void init(VaadinRequest request) { chart.setSizeFull(); setContent(chart); // Prepare the data display Configuration conf = chart.getConfiguration(); conf.setTitle("Hot New Data"); conf.setSeries(series); // Start the data feed thread new FeederThread().start(); } class FeederThread extends Thread { int count = 0; @Override public void run() { try { // Update the data for a while while (count < 100) { Thread.sleep(1000); access(new Runnable() { @Override public void run() { double y = Math.random(); series.add( new DataSeriesItem(count++, y), true, count > 10); } }); } // Inform that we have stopped running access(new Runnable() { @Override public void run() { setContent(new Label("Done!")); } }); } catch (InterruptedException e) { e.printStackTrace(); } } } } ---- When sharing data between UIs or user sessions, you need to consider the message-passing mechanism more carefully, as explained next. [[advanced.push.pusharound]] == Broadcasting to Other Users Broadcasting messages to be pushed to UIs in other user sessions requires having some sort of message-passing mechanism that sends the messages to all UIs that register as recipients. As processing server requests for different UIs is done concurrently in different threads of the application server, locking the threads properly is very important to avoid deadlock situations. [[advanced.push.pusharound.broadcaster]] === The Broadcaster The standard pattern for sending messages to other users is to use a __broadcaster__ singleton that registers the UIs and broadcasts messages to them safely. To avoid deadlocks, it is recommended that the messages should be sent through a message queue in a separate thread. Using a Java [classname]#ExecutorService# running in a single thread is usually the easiest and safest way. [source, java] ---- public class Broadcaster implements Serializable { static ExecutorService executorService = Executors.newSingleThreadExecutor(); public interface BroadcastListener { void receiveBroadcast(String message); } private static LinkedList listeners = new LinkedList(); public static synchronized void register( BroadcastListener listener) { listeners.add(listener); } public static synchronized void unregister( BroadcastListener listener) { listeners.remove(listener); } public static synchronized void broadcast( final String message) { for (final BroadcastListener listener: listeners) executorService.execute(new Runnable() { @Override public void run() { listener.receiveBroadcast(message); } }); } } ---- In Java 8, you could use lambda expressions for the listeners instead of the interface, and a parameterless expression to create the runnable: [source, java] ---- for (final Consumer listener: listeners) executorService.execute(() -> listener.accept(message)); ---- [[advanced.push.pusharound.receiving]] === Receiving Broadcasts The receivers need to implement the receiver interface and register to the broadcaster to receive the broadcasts. A listener should be unregistered when the UI expires. When updating the UI in a receiver, it should be done safely as described earlier, by executing the update through the [methodname]#access()# method of the UI. [source, java] ---- @Push public class PushAroundUI extends UI implements Broadcaster.BroadcastListener { VerticalLayout messages = new VerticalLayout(); @Override protected void init(VaadinRequest request) { ... build the UI ... // Register to receive broadcasts Broadcaster.register(this); } // Must also unregister when the UI expires @Override public void detach() { Broadcaster.unregister(this); super.detach(); } @Override public void receiveBroadcast(final String message) { // Must lock the session to execute logic safely access(new Runnable() { @Override public void run() { // Show it somehow messages.addComponent(new Label(message)); } }); } } ---- [[advanced.push.pusharound.sending]] === Sending Broadcasts To send broadcasts with a broadcaster singleton, such as the one described above, you would only need to call the [methodname]#broadcast()# method as follows. [source, java] ---- final TextField input = new TextField(); sendBar.addComponent(input); Button send = new Button("Send"); send.addClickListener(new ClickListener() { @Override public void buttonClick(ClickEvent event) { // Broadcast the message Broadcaster.broadcast(input.getValue()); input.setValue(""); } }); ----