Java Server Events Client

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The Java ServerEventClient is an idiomatic port of ServiceStack's C# Server Events Client to Java providing a productive client to consume ServiceStack's real-time Server Events that can be used in any Java/JVM (JRE 7+) Client/Server Applications or Java/Kotlin Android applications.


The AndroidServerEventsClient for Android is available in the net.servicestack:android package which can be installed in your build.gradle with:

dependencies {
    implementation 'net.servicestack:android:1.0.48'

Or in Maven with:


Other Java/JVM languages running on the JVM (JRE 7+) can use the ServerEventsClient in the net.servicestack:client package which can be installed using Gradle:

compile 'net.servicestack:client:1.0.48'

Or Maven:


The AndroidServerEventsClient class for Android inherits ServerEventsClient to provide enhanced functionality like alternative non-blocking APIs for all Sync APIs using Android Async Tasks. It also requires the use of an external OkHttp Client dependency since the HttpURLConnection implementation in Android doesn't support cancellable non-blocking requests on HTTP Streams. Otherwise both implementations provide the same functionality and are interchangeable, for the purposes of demonstration we'll be using AndroidServerEventsClient.

To configure Server Sent Events on the client create a new instance of AndroidServerEventsClient with the baseUrl and the channels you want to connect to, e.g:

ServerEventsClient client = new AndroidServerEventsClient(baseUrl, "home")
    .setOnConnect(sub -> {                        // Successful SSE connection
        Log.d("You've connected! welcome " + sub.getDisplayName());
    .setOnJoin(e -> {                             // User has joined subscribed channel
        Log.d("Welcome, " + e.getDisplayName());
    .setOnLeave(e -> {                            // User has left subscribed channel
        Log.d(e.getDisplayName() + " has left the building");
    .setOnUpdate(e -> {                           // User channel subscription was changed
        Log.d(e.getDisplayName() + " has left the building");
    .setOnMessage(msg -> { })                     // Invoked for each other message
    //... Register custom handlers
    .registerHandler("chat", (client, e) -> {     // Invoked for adhoc messages
        ChatMessage chatMsg = JsonUtils.fromJson(e.getJson(), ChatMessage.class);
    .registerReceiver(MyReceiver.class)           // Register Global 'cmd.' default receiver
    .registerNamedReceiver("tv",TvReceiver.class) // Register named 'tv.' receiver
    .addListener("theEvent", msg -> {})           // Add listener for pub/sub event trigger
    .setOnException(e -> { })                     // Invoked on each Error
    .setOnReconnect(() -> { })                    // Invoked after each auto-reconnect
    .start();                                     // Start listening for Server Events!

//Global Receiver Class
public class MyReceiver extends ServerEventReceiver {
    public void announce(String message){}       // Handle messages with simple argument
    public void chat(ChatMessage message){}      // Handle messages with complex type argument
    public void customType(CustomType message){} // Handle complex types with default selector      
    @Override                                    // Handle other unknown messages
    public void noSuchMethod(String selector, Object message){}

//Named Receiver Class
public class TvReciever extends ServerEventReceiver {
    public void watch(String videoUrl){}         // Handle ' {url}' messages 
    public void off(){}                          // Handle '' messages 

Message Events

ServiceStack Server Events has 4 built-in events sent during a subscriptions life-cycle:

  • onConnect - sent when successfully connected, includes the subscriptions private subscriptionId as well as heartbeat and unregister urls that's used to automatically setup periodic heartbeats
  • onJoin - sent when a new user joins the channel
  • onLeave - sent when a user leaves the channel
  • onUpdate - sent when a user's channels subscription was updated

The onJoin/onLeave/onUpdate events can be turned off with ServerEventsFeature.NotifyChannelOfSubscriptions=false.

All other messages can be handled with the catch-all:

  • onMessage - fired when any other message is sent

Server Event Client Events

Other top-level events the ServerEventClient fires that can be handled include:

  • onException - Invoked on each error the client receives
  • onReconnect - Invoked after each time the client had to auto-reconnect


A selector is a string that identifies what should handle the message, it's used by the client to route the message to different handlers. The client bindings supports 4 different handlers out of the box:

Global Event Handlers

The easiest way to handle a custom event is to define a handler, e.g:

ServerEventsClient client = new AndroidServerEventsClient(baseUrl, "home")
    .registerHandler("paint", (client, e) -> {
        String color = JsonUtils.fromJson(e.getJson(), String.class);
        Log.d("Painting the " + e.getCssSelector() + " " + color);
    .registerHandler("chat", (client, e) -> {
        ChatMessage chatMsg = JsonUtils.fromJson(e.getJson(), ChatMessage.class);
        Log.d("Received " + chatMsg.getMessage() + " from " + chatMsg.getFromName());

The selector to invoke a global event handler is:

cmd.{handler} {message}

Which can be sent in ServiceStack with:

ServerEvents.NotifyChannel("home", "cmd.paint$#town", "red");
ServerEvents.NotifyChannel("home", "", new ChatMessage { ... });

Where {handler} is the name of the handler you want to invoke, e.g cmd.paint. The first argument is the ServerEventsClient instance whilst the 2nd argument a structured ServerEventMessage which for the above Server Event is populated with:

.registerHandler("paint", (client, e) -> {
    e.getChannel()     //= home
    e.getData()        //= home@cmd.paint$#town "red"
    e.getSelector()    //= cmd.paint
    e.getJson()        //= "red"
    e.getOp()          //= cmd
    e.getTarget()      //= paint
    e.getCssSelector() //= #town

The message body is serialized as JSON and accessible from e.getJson() and can be extracted using JsonUtils,e.g:

String color = JsonUtils.fromJson(e.getJson(), String.class); //Simple string message body
ChatMessage chatMsg = JsonUtils.fromJson(e.getJson(), ChatMessage.class); //Complex Type body

Postfix CSS selector

All server event handler options also support a postfix CSS selector for specifying what each handler should be bound to with a $ followed by the CSS selector, e.g:

cmd.{handler}${cssSelector} {value}

A concrete example for calling the above API would be:

cmd.paint$#town red


Spaces in CSS selectors need to be encoded with %20

Handling Messages with the Default Selector

All IServerEvents Notify API's includes overloads for sending messages without a selector that by convention will take the format cmd.{TypeName}.

As they're prefixed with cmd.* these events can be handled with either a handler (as above) or a global receiver based on Message type name, e.g:

ServerEventsClient client = new AndroidServerEventsClient(baseUrl, "home")

public class TestGlobalReceiver extends ServerEventReceiver {
    public void setterType(SetterType value) {
    public void customType(CustomType request) {

Which will be called when messages are sent without a selector, e.g:

public class MyServices : Service
    public IServerEvents ServerEvents { get; set; }

    public void Any(Request request)
        ServerEvents.NotifyChannel("home", new CustomType { ... });
        ServerEvents.NotifyChannel("home", new SetterType { ... });

Whilst Named Receivers are used to handle messages sent to a specific namespaced selector, registering a Global Receiver allows you to handle messages sent with the cmd.* selector which is also the default selector used when sending messages with no selector.


In programming languages based on message-passing like Smalltalk and Objective-C invoking a method is done by sending a message to a receiver. This is conceptually equivalent to invoking a method on an instance in C# where both these statements are roughly equivalent:

// Objective-C
[receiver method:argument]
// C#

Support for receivers is available in the following format:

{receiver}.{target} {msg}

Registering Receivers

Registering a receiver can be done with a map of the object instance and the name you want it to be exported as. E.g. we can add a "css" receiver to handle with:

ServerEventsClient client = new AndroidServerEventsClient(baseUrl, "home")
    .registerNamedReceiver("css", CssReceiver.class)
    .setResolver(new MyResolver(mainActivity))

public class CssReceiver extends ServerEventReceiver {
    private MainActivity parentActivity;
    public CssReceiver(MainActivity parentActivity) {
        this.parentActivity = parentActivity;

    public void backgroundImage(String message){
        String url = message.startsWith("url(")
            ? message.substring(4, message.length() - 1)
            : message;

        App.get().readBitmap(url, bitmap -> {
            ImageView chatBackground = (ImageView)parentActivity.findViewById(;
            parentActivity.runOnUiThread(() -> chatBackground.setImageBitmap(bitmap));

    public void background(String message){
        String color = message.replace("#", "#AA");
        String cssSelector = super.getRequest().getCssSelector();
        parentActivity.runOnUiThread(() -> {
            if (Objects.equals(cssSelector, "#top")){
                    new ColorDrawable(colorVal)

public class MyResolver implements IResolver {
    private MainActivity parentActivity;
    public MyResolver(MainActivity parentActivity) {
        this.parentActivity = parentActivity;

    public Object TryResolve(Class cls){
        if (cls == CssReceiver.class){
            return new CssReceiver(this.parentActivity);
        return cls.newInstance();

Which will invoke backgroundImage method off a new instance of the CssReceiver class that's triggered with:

css.background-image url(

and can be sent to all subscriptions on the home channel in ServiceStack with:

ServerEvents.NotifyChannel("home", "css.background-image", "url(");

This works the same with Global Receivers.

Inheriting ServerEventReceiver

By inheriting ServerEventReceiver:

class ServerEventReceiver implements IReceiver {
    public client: ServerEventsClient;
    public request: ServerEventMessage;
    noSuchMethod(selector: string, message:any) {}

Receivers can access additional built-in functionality where it will allow receivers to access the ServerEventsClient client dependency, the ServerEventMessage that was received, it also lets you handle any unhandled messages sent by implementing noSuchMethod(), e.g:

class JavaScriptReceiver extends ServerEventReceiver {

    public void chat(ChatMessage chatMessage){
        LogMessage request = new LogMessage()

    public void announce(String message){
        Toast.makeText(this.parentActivity, message, Toast.LENGTH_LONG);

    public void toggle(){
        if ("#sidebar".equals(super.request.getCssSelector())){
            LinearLayout sidebarLayout=(LinearLayout)this.findViewById(;
            sidebarLayout.setVisibility(sidebarLayout.getVisibility() == LinearLayout.INVISIBLE
                ? LinearLayout.VISIBLE
                : LinearLayout.INVISIBLE

    public void noSuchMethod(String selector, Object message){
        ServerEventMessage msg = (ServerEventMessage)message;
        Log.d("Unhandled " + selector + " was sent message: " + msg.getJson());

client.registerReceiver(JavaScriptReceiver.class); //register Global Receiver

These can triggered with:

ServerEvents.NotifyChannel(channel, new ChatMessage { ... });
ServerEvents.NotifyChannel(channel, "cmd.announce", "Hello, World!");
ServerEvents.NotifyChannel(channel, "cmd.toggle$#sidebar");
ServerEvents.NotifyChannel(channel, "cmd.UnknownSelector", new Message { ... });

Dependency Resolvers

You can control the lifetime of the receivers by injecting a custom resolver which will let you reuse the same Receiver instance by using a SingletonInstanceResolver, e.g:

ServerEventsClient client = new AndroidServerEventsClient(baseUrl, "home")
    .setResolver(new SingletonInstanceResolver())

Which is implemented with:

public class SingletonInstanceResolver implements IResolver {
    ConcurrentMap<Class, Object> cache = new ConcurrentHashMap<>();
    public Object TryResolve(Class cls) {
        Object instance = cache.get(cls);
        if (instance == null){
            try {
                Object newInstance = cls.newInstance();
                instance = (instance = cache.putIfAbsent(cls, newInstance)) == null
                        ? newInstance
                        : instance;
            } catch (Exception e) {
                throw new RuntimeException(e);
        return instance;

Custom Resolvers are also useful configuring the dependencies in your Receiver classes, e.g. we use this above to inject our MainActivity into our CssReceiver class.

ServerEventsClient client = new AndroidServerEventsClient(baseUrl, "home")
    .setResolver(new MyResolver())

public class MyResolver implements IResolver {
    private MainActivity parentActivity;
    public MyResolver(MainActivity parentActivity) {
        this.parentActivity = parentActivity;

    public Object TryResolve(Class cls){
        if (cls == CssReceiver.class){
            return new CssReceiver(this.parentActivity);
        return cls.newInstance();

Event Triggers

Triggers enable a pub/sub event model where multiple listeners can subscribe and be notified of an event.

Registering an event handler can be done at anytime using the addListener() API, e.g:

Action<ServerEventMessage> handler = e -> {
    Log.d("received event " + e.getTarget() + " with arg: " + e.getJson());

ServerEventsClient client = new AndroidServerEventsClient(baseUrl, "home")
    .addListener("customEvent", handler)

//Register another listener to 'customEvent' event
List<ServerEventMessage> msgs1 = new ArrayList<>();
client.addListener("customEvent", msgs1::add);

The selector to trigger this custom event is:

trigger.customEvent arg
trigger.customEvent {json}

Which can be sent in ServiceStack with a simple or complex type argument, e.g:

ServerEvents.NotifyChannel(channel, "trigger.customEvent", "arg");
ServerEvents.NotifyChannel(channel, "trigger.customEvent", new ChatMessage { ... });

Removing Listeners

Use removeListener() to stop listening for an event, e.g:

//Remove first event listener
client.removeListener("customEvent", handler);

Channel Subscriber APIs

You can use any of the APIs below to update an active Subscriptions Channels:


//Alternatively subscribe/unsubscribe to channels in the same request with:
UpdateEventSubscriber request = new UpdateEventSubscriber()


All Service Client APIs in AndroidServiceClient also have non-blocking versions with an an Async suffix that utilize Android's Async Task and optional callbacks for performing non-blocking Service Client requests, e.g:

client.updateSubscriberAsync(request, () -> {
}, e -> {
    Log.d("FAILED: " + e.toString());

Get Channel Subscribers

Once connected, you can get a list of channel subscribers the ServerEventsClient is currently connected to with:

List<ServerEventUser> channelUsers = client.getChannelSubscribers();
Func.each(channelUsers, user -> {
    Log.d(user.getUserId() + " @" + user.getDisplayName() + " " + user.getProfileUrl());

Accessing ServiceClient

Alternatively you can access the channel subscribers using the built-in JsonServiceClient, e.g:

client.getServiceClient().get(new GetEventSubscribers()

Which you can also use to call your own Services:

client.getServiceClient().post(new MyRequest());

Integration Test Examples

More examples of ServerEventClient usage can be found in the Test Suites below:

Java ServerEvents Examples

Android Java Chat

Java Chat client utilizing Server Events for real-time notifications and enabling seamless OAuth Sign In's using Facebook, Twitter and Google's native SDKs: