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One Server-Sent Events lib for Java to rule them all
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Jeasse Asity

Jeasse Asity is a jEaSSE connector for Asity, which allows to add the Server Sent Events feature to your web application no matter what Java web framework you use. Jeasse Asity is based on two libraries:

  • jEaSSE: A Server Sent Events (SSE) implementation for Java.
  • Asity: An HTTP/WebSocket abstraction layer for Java.

Getting Started

Adding Dependencies

Add the following dependencies to your application. These modules allow to create a SSE application which is framework-agnostic.


Add an Asity bridge according to your web framework. This module allows to plug your SSE application into the framework of your choice. Asity supports the following web frameworks as of 3Q 2019.

  • asity-bridge-vertx3: Vert.x 3
  • asity-bridge-vertx2: Vert.x 2
  • asity-bridge-spring-webmvc4: Spring MVC 4
  • asity-bridge-spring-webflux5: Spring WebFlux 5
  • asity-bridge-servlet3: Java Servlet 3
  • asity-bridge-play2: Play Framework 2
  • asity-bridge-netty4: Netty 4
  • asity-bridge-grizzly2: Grizzly 2
  • asity-bridge-atmosphere2: Atmosphere 2

Writing a Web Fragment

A web fragment is a labmda that takes a ServerHttpExchange instance which an abstraction of framework-specific HTTP request-response exchanges.

public class MyJeasseAsityAction implements Action<ServerHttpExchange> {
  public void on(ServerHttpExchange http) {
    switch (http.method()) {
      case GET:
        // Handle GET requests
      case POST:
        // Handle POST requests

The above web fragment handles only GET and POST requests and responds with 405, Method Not Allowed, for other requests. You'll learn how to plug it into your framework later.

Working with a SSE Connection

To open a SSE connection, when handling GET requests, create an AsityEventTarget with a given ServerHttpExchange which implements jEaSSE's EventTarget interface and call its ok() and open() methods.

EventTarget eventTarget = new AsityEventTarget((ServerHttpExchange) http);

To send an event through a SSE connection, use send(MessageEvent messageEvent) or send(String event, String data) of EventTarget interface.

MessageEvent event = new MessageEvent.Builder().setData("Hello World").build();

Working with SSE Connections

jEaSSE provides the EventBroadcast class that streamlines handling of multiple SSE connections. Assume that it's created as a field of the web fragment and shared by each different requests.

EventBroadcast broadcaster = new EventBroadcast();

To open a SSE connection through EventBroadcast and add it as a subscriber, call its addSubscriber(EventTarget) method. You don't need to call EventTarget's ok() and open() methods in this case.

EventTarget eventTarget = new AsityEventTarget(http);

To send some events to all the subscribers, call its broadcast(MessageEvent messageEvent) or broadcast(String event, String data) method.

broadcaster.broadcast("message", "Hello World");

Plugging the Web Fragment to Your Web Application

The last step is to plug the web fragment, MyJeasseAsityAction, into your web application using an Asity bridge for the web framework you are using. Here's an example of Servlet 3.

public class ChatServerInitializer implements ServletContextListener {
  public void contextInitialized(ServletContextEvent event) {
    MyJeasseAsityAction httpAction = new MyJeasseAsityAction();

    ServletContext context = event.getServletContext();
    Servlet servlet = new AsityServlet().onhttp(httpAction);
    ServletRegistration.Dynamic reg = context.addServlet("chat", servlet);

asity-bridge-servlet3 module provides the AsityServlet class which extends Servlet's HttpServlet and abstracts Servlet's HttpServletRequest and HttpServletResponse into Asity's ServerHttpExchange. With the AsityServlet, you can feed the web fragment with HTTP request-response exchanges in a Servlet application as above.

Likewise, other Asity bridges provide such adapters per framework so that you can set up web fragments in your webapp with ease. For the details, see the Asity website's Run Anywhere section.


Play around with the following examples to better understand jEaSSE and Asity. Examples require Java 8+ and Maven 3+. Clone or download the repository.

git clone
cd jeasse-asity

To run a Servlet 3 example,

cd example-servlet3
mvn jetty:run

To run a Grizzly 2 example,

cd example-girzzly2
mvn package exec:java

To run a Play Framework 2 example,

cd example-play2
sbt run

All examples behave the same. Run one of examples and open a browser to http://localhost:8080/index.html. You'll see a screen similar to the following.


The web fragment of the example, MyJeasseAsityAction, is located at here.

For your information, as you may know, examples borrowed most code from the original example in jEaSSE.


If you have any questions or issues with Jeasse Asity, create an issue in the issue tracker in Github or DM me on Twitter (@flowersits).

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