ClassNotFoundException in Java Agent Instrumentation premain

Question

I read https://www.baeldung.com/java-instrumentation and I'm trying to instrument the doService method in org.springframework.web.servlet.DispatcherServlet , which is part of the Spring Boot framework, ie spring-webmvc.jar .

This is the way I start my spring boot application:

java -javaagent:agent.jar -jar myapp.jar

I understand my agent's pom.xml should package/include the javassist dependency but I don't expect the agent should package/include the jar spring-webmvc.jar containing the class org.springframework.web.servlet.DispatcherServlet , as it is already packaged and contained in myapp.jar

In the premain of the agent, when I try to instrument and load the class org.springframework.web.servlet.DispatcherServlet by using forName , it throws the ClassNotFoundException .

How should I instruct the agent to load it from the myapp.jar ?

Thank you!

        try {
            targetCls = Class.forName(className);
            targetClassLoader = targetCls.getClassLoader();
            LOGGER.log(Level.INFO, "Successfully obtain target class {0} and its classloader using 'forName' with class loader {1}", new Object[] {className, targetClassLoader.toString()});
        } catch (Exception ex) {
            LOGGER.log(Level.SEVERE, "Class {0} not found with Class.forName. {1}", new Object[] {className, ex.getClass().getCanonicalName()});
        }

Answer 1

If you are new to Java instrumentation and following this post , note that manually invoking forName is not necessary in the example. What you can do is to register the transformer in premain and then in transform method, check for your target class name and start the instrumentation as instructed in the link. Happy coding!