XRebel for standalone apps with embedded Jetty

XRebel was designed to work specifically with Java web applications. Currently, it relies on Servlet API to serve its UI. It has been tested with a range of application servers (Apache Tomcat, WildFly, WebSphere, and others) and some full stack frameworks as well, including Spring Boot and JavaSpark.

Sometimes however, you might want to use XRebel with a standalone Java process that is not using Servlet API. Hence, XRebel cannot display its embedded UI to render the data it collected from the application. What can we do about this? Well, XRebel is tested on Jetty and it works quite well there. Jetty is also often used as an embedded HTTP server. So why don’t we use this trick and embed Jetty into our standalone app to serve XRebel UI.

Here’s our example application:

	package com.zt;
	import java.io.BufferedReader;
	import java.io.IOException;
	import java.io.InputStream;
	import java.io.InputStreamReader;
	import java.net.URL;
	import java.net.URLConnection;
	import java.util.concurrent.Executors;
	import java.util.concurrent.ScheduledExecutorService;
	import java.util.concurrent.TimeUnit;
	public class App {
	public static void main(String[] args) throws Exception {
	ScheduledExecutorService scheduler = Executors.newScheduledThreadPool(1);
	scheduler.scheduleAtFixedRate(App::get, 3, 5, TimeUnit.SECONDS);
	}
	private static String get() {
	StringBuilder sb = new StringBuilder();
	try {
	String spec = "http://www.ee";
	URL url = new URL(spec);
	URLConnection urlConnection = url.openConnection();
	InputStream inputStream = urlConnection.getInputStream();
	BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream));
	String line;
	while ((line = bufferedReader.readLine()) != null) {
	sb.append(line);
	}
	bufferedReader.close();
	} catch (IOException e) {
	e.printStackTrace();
	}
	return sb.toString();
	}
	}

view raw App.java hosted with ❤ by GitHub

Just for the demonstration purposes, it executes an HTTP request every 3 seconds and reads the response. Quick and dirty.

To embed Jetty server we need is to add a dependency to Jetty container and initialize the server when the application starts. The required dependencies are jetty-server and jetty-servlet:

	<dependencies>
	<dependency>
	<groupId>org.eclipse.jetty</groupId>
	<artifactId>jetty-server</artifactId>
	<version>9.4.6.v20170531</version>
	</dependency>
	<dependency>
	<groupId>org.eclipse.jetty</groupId>
	<artifactId>jetty-servlet</artifactId>
	<version>9.4.6.v20170531</version>
	</dependency>

view raw pom.xml hosted with ❤ by GitHub

Then we can start Jetty by adding the following code snippet into the static initializer of the class:

	public class App {
	static {
	new Thread(() -> {
	Server server = new Server(8080);
	ServletContextHandler context = new ServletContextHandler(ServletContextHandler.SESSIONS);
	context.setContextPath("/");
	server.setHandler(context);
	try {
	server.start();
	server.join();
	} catch (Exception e) {
	e.printStackTrace();
	}
	}).start();
	}
	//...
	}

view raw App.java hosted with ❤ by GitHub

To enable XRebel agent, you need to start the application with -javaagent VM argument pointing to the location of xrebel.jar. For instance:

-javaagent:/Users/anton/tools/xrebel/xrebel.jar -Dxrebel.traces.all=true

I also added -Dxrebel.traces.all=true VM property to enable tracing of non-HTTP activities by XRebel. Tracing of non-HTTP activities in XRebel is disabled by default, hence I need to add this parameter in order to see profiling data for the periodic tasks (if I wish).

Once I launch the application, it will boot up the embedded Jetty instance on port 8080. The standalone XRebel UI is deployed by the agent to /xrebel context. Hence, if we open http://localhost:8080/xrebel in the browser we will see the following:

As you can see, it is quite easy to use XRebel with the standalone apps with this little trick. Just start an embedded Jetty instance on some port and you will be able to see what is your application doing. Perhaps, you can spot a sneaky bug with it before it gets to production! :)

If you want to play with the example application, I have it at GitHub. Download XRebel and start the application with the appropriate VM arguments to enable the agent. It will be fun! :)