If you are facing the afore-mentioned issue while trying to open a JMX file on JMeter, you are in the right place. Let’s investigate and solve the issue together. If you are familiar with Java language, you may know that “CannotResolveClassException” errors usually indicate that something in your code/project is missing and your application doesn’t know what to do about that. “ CannotResolveClassException: kg.apc.jmeter.threads.UltimateThreadGroup ” error occurs when your script contains an Ultimate Thread Group element, but your JMeter setup doesn’t have a proper Ultimate Thread Group element installed.

You may think that installing the relevant plugin from JMeter will immediately solve the problem. But unfortunately, it is not that easy. Some versions of JMeter still support these thread group elements but some of them don’t. So you may have to follow some sort of step by step troubleshooting process.

1. Try to Perform a Fresh Install for the Plugin

Install Plugin Manager for JMeter from the link here. Then restart JMeter and click on Plugin Manager icon. Type “Custom Thread Groups” for available plugins and install the relevant result.

If your problem still persists, it indicates that your JMeter version may not be compatible with Custom Thread Groups plugins. Because of this, you should try another version of JMeter.

2- Install an Old Version of JMeter

If you are using one of the latest versions of JMeter, try to downgrade. If you are using a version that is higher than 4.0 for example, you can start with JMeter 4.0. After downloading JMeter, install the Plugin Manager and Custom Thread Groups again. Then check the result.

3- Try to Download the Plugin from JMeter’s Website

Navigate to this link and download the latest version for Custom Thread Groups (the latest version for now is 2.10). After your downloading is finished, you will have two different jar files; one for the plugin and one for the Plugin Manager. Then copy/paste these jar files to the “lib/ext” path in your JMeter installation folder.

Hopefully, after following these steps, your problem will be solved. Happy load testing!