Class loading broken for WebSocketClient used inside webapp

[sbordet]

Jetty version
9.4.x

Description
The fix for #5859 resolved the leaking of ClassLoaders, but now causes class loading issues in web applications, see for example: cometd/cometd#1037.

A web application using the WebSocket client may trigger the creation of a thread, which before #5859 was inheriting the webapp ClassLoader as context ClassLoader, but now the thread gets the server ClassLoader as the context ClassLoader.

If the task at hand needs to load a class from the webapp (for example due to JSON parsing using oej.util.Loader.loadClass(), it was succeeding before #5859, but fails now.

Obviously we want to keep the fix for #5859, but we need to explicitly set the context ClassLoader before calling webapp code (e.g. listeners, callbacks, etc.) in those components that are provided by the server, in particular WebSocketClient.

[Deepak-D]

Hello @sbordet / @lachlan-roberts, good day to you! I guess I'm facing this problem. More details here. Could this be confirmed?
Thanks in advance!!

[lachlan-roberts]

@Deepak-D from reading the description it doesn't seem to be the same issue. This issue is only relating to the use of WebSocketClient inside the webapp. Are you using the WebSocketClient and accessing the ContextClassLoader from within the websocket endpoint?

If not would it be possible to attach a simple reproducer where the getContextClassLoader() gives WebAppClassLoader in 9.4.36 and then startJarLoader if upgraded to 9.4.37. You should open a separate issue for this if this is the case.

[Deepak-D]

Hello @lachlan-roberts , thanks for your feedback..

Here is my case!

I have WebSocket ClientEndpoint in a jar (Say client.jar) which is deployed in JETTY_BASE/lib/ext (since this client is accessed by multiple web application deployed in JETTY_BASE/webapps and each webapp is differentiated by unique client id).

ClientEndPoint will look like below

@ClientEndpoint(decoders = {MessageDecoder.class}, encoders = {MessageEncoder.class})
public class WSClientEndPoint {
	WebSocketContainer container = ContainerProvider.getWebSocketContainer();
	
	public boolean connectToWSURL() {
		Session localSession = null;
		try {
			localSession = container.connectToServer(this, new URI(this.wsURL)); //wsURL has ServerEndPoint URL
		}
		catch (Exception e) {
		}
	}
	
	@OnOpen
	public void onOpen(Session session) {
	}
	
	@OnMessage
	public void onMessage(final Session session, Message msg) {
	}
	
	@OnError
	public void onError(Session session, Throwable error) {
	}
	
	@OnClose
	public void onClose(Session session, CloseReason closeReason) {
	}
}

WebSocket ServerEndpoint in another web application which is also deployed in the same JETTY_BASE/webapps. I use javax.websocket.WebSocketContainer to establish connection to the server endpoint with its endpoint url.

ServerEndPoint will look like below

@ServerEndpoint(value = "/server/{client-id}", decoders = {MessageDecoder.class}, encoders = {MessageEncoder.class})
public class ServerResource {
	@OnOpen
	public void onOpen(Session session, @PathParam("client-id") final String clientId) {
	}
	
	@OnClose
	public void onClose(Session session, CloseReason closeReason) {
	}
	
	@OnMessage
	public void message(final Session session, Message message) {
	}
	
	@OnError
	public void onError(Throwable t, Session session) {
	}
	
	@OnMessage
	public void onPong(PongMessage pongMessage, Session session) {
	}
}

This client and server is designed to work based on the publish/subscribe model using topics (Similar to mqtt).

I have a interface in client.jar and this interface will be implemented inside webapp which uses this client.jar to handle data received from server endpoint for the subscription of each topic.

interface in client.jar will look like below

public interface NotificationHandler {
	public boolean handle(String topic, String message);
}

Implementation inside webapp will look like below

package com.webappx.websocket.subscribehandlers
public class ConfigurationHandler implements NotificationHandler{
	@Override
	public boolean handle(String topic, String message) {
		InputStream inputStream;
		ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
		try {
			inputStream = classLoader.getResourceAsStream("x.properties");
		}catch (Exception ie) {
			ie.printStackTrace();
		}
	}
}

I have list of topics and its dedicated class full name implementing the NotificationHandler interface inside webapp. On webapp startup, I'm establishing websocket connection to the server endpoint using client.jar which has client endpoint using the webapp's unigue client id to differentiate client's session on server side. Then, I'm doing topic subscription to the server and I will create class instance dedicated for that specific topic and persist inside client.jar in a HashMap. Whenever I receive subscription message from the server for the topic, I will get the instance from HashMap and call handle method of the NotificationHandler interface passing topic and data received.

In this flow, when I try to get the class loader instance using ClassLoader classLoader = Thread.currentThread().getContextClassLoader() as shown above, I get startJarLoader from Jetty 9.4.37 which causes problem loading resource files from WEB-INF\classes but with Jetty 9.4.36 and lower versions, I get WebAppClassLoader.

Hope this helps! If not, I will try to prepare a reproducer.

[lachlan-roberts]

@Deepak-D this does sound like it is the same issue.

If you want to verify this, then you can build the branch for PR #6325 (jetty-9.4.x-6287-WebSocketClientClassLoading) and test with your code to see if this fixes it for you.