Build I/O framework-agnostic applications on the JVM

Asity is a lightweight abstraction layer for I/O frameworks which is designed to build applications that can run on any full-stack framework, any micro framework or any raw server on the JVM.

An Asity application can handle the following resources:

And run on the following platforms:

Getting started

Write your first Asity app

Asity is distributed through Maven Central. To write a web application running on any platform Asity supports, you need two artifacts: io.cettia.asity:asity-http:1.0.0-Beta1 and io.cettia.asity:asity-websocket:1.0.0-Beta1.


Once you’ve set up the build, all you need to do is to write actions receiving ServerHttpExchange and ServerWebSocket. As a simple example, let’s write echo actions sending any incoming messages such as HTTP chunk and WebSocket data frame back.

import io.cettia.asity.action.Action;
import io.cettia.asity.http.ServerHttpExchange;
import io.cettia.asity.websocket.ServerWebSocket;

public class EchoHandler {
  // Asity supports Java 7+
  // For better readability, public final fields and lambda expressions in Java 8 are used here
  public final Action<ServerHttpExchange> httpAction = (ServerHttpExchange http) -> {
    // Copies the content-type header of the request to the response
    http.setHeader("content-type", http.header("content-type"));
    // When a chunk is read from the request body, writes it to the response body
    http.onchunk((ByteBuffer bytes) -> http.write(bytes));
    // When the request is fully read, ends the response
    http.onend((Void v) -> http.end());
    // Reads the request body as binary to circumvent encoding issue
    // When the response is fully written and ends,
    http.onfinish((Void v) -> System.out.println("on finish"));
    // When some error happens in the request-response exchange,
    http.onerror((Throwable t) -> t.printStackTrace());
    // When the underlying connection is terminated,
    http.onclose((Void v) -> System.out.println("on close"));
  public final Action<ServerWebSocket> websocketAction = (ServerWebSocket ws) -> {
    // When a text frame is arrived, sends it back
    ws.ontext((String data) -> ws.send(data));
    // When a binary frame is arrived, sends it back
    ws.onbinary((ByteBuffer bytes) -> ws.send(bytes));
    // When some error happens in the connection,
    ws.onerror((Throwable t) -> t.printStackTrace());
    // When the connection is closed for any reason,
    ws.onclose((Void v) -> System.out.println("on close"));

Run your app

Now to run this application on the specific platform, we need to wrap HTTP resources and WebSocket resources provided by that specific platform into ServerHttpExchange and ServerWebSocket and feed them into an instance of EchoHandler. A module playing such roles is called bridge.

For example, to run EchoHandler on an implementation of Java Servlet 3 and Java WebSocket API 1 such as Jetty 9 and Tomcat 8, you need Java Servlet 3 bridge and Java WebSocket API 1 bridge. Let’s add the following bridge dependencies.


Then, you can feed the application with HTTP resources through AsityServlet and WebSocket resources through AsityServerEndpoint.

import io.cettia.asity.bridge.jwa1.AsityServerEndpoint;
import io.cettia.asity.bridge.servlet3.AsityServlet;

import javax.servlet.*;
import javax.servlet.annotation.WebListener;
import javax.websocket.DeploymentException;
import javax.websocket.server.*;

public class Bootstrap implements ServletContextListener {
  public void contextInitialized(ServletContextEvent event) {
    // An Asity application
    final EchoHandler handler = new EchoHandler();
    // Feeds the application with HTTP resources produced by Servlet 3
    ServletContext context = event.getServletContext();
    Servlet servlet = new AsityServlet().onhttp(handler.httpAction);
    ServletRegistration.Dynamic reg = context.addServlet(AsityServlet.class.getName(), servlet);
    // Feeds the application with WebSocket resources produced by Java WebSocket API 1
    ServerContainer container = (ServerContainer) context.getAttribute(ServerContainer.class.getName());
    ServerEndpointConfig config = ServerEndpointConfig.Builder.create(AsityServerEndpoint.class, "/echo")
    .configurator(new Configurator() {
      public <T> T getEndpointInstance(Class<T> endpointClass) throws InstantiationException {
        return endpointClass.cast(new AsityServerEndpoint().onwebsocket(handler.websocketAction));
    try {
    } catch (DeploymentException e) {
      throw new RuntimeException(e);

  public void contextDestroyed(ServletContextEvent sce) {}

Try out examples

The same pattern applies when bridging an application to other platforms. Here are working examples. They demonstrate how to run Cettia Java Server implementing the Cettia Protocol using Asity on each platform.

Build a custom bridge

Though your favorite platform is not supported? Take a look how Grizzly 2 bridge is written. Mostly, with more or less 200 lines, it’s enough to write a bridge.