How does the Class Loader work in Tomcat?

The Class Loader in Tomcat is responsible for loading and managing classes in a web application. Tomcat uses a hierarchical structure of ClassLoaders, each responsible for loading different classes.

When a web application is deployed to Tomcat, Tomcat will create a web application level ClassLoader for it. This ClassLoader is responsible for loading class files within the application, such as Servlet classes and Filter classes.

In Tomcat, class loaders cooperate through a delegation mechanism. When a class needs to be loaded, Tomcat first attempts to load it using the application-level ClassLoader. If the application-level ClassLoader cannot find the class, Tomcat will delegate the task to the parent ClassLoader and continue up the hierarchy until the system-level ClassLoader eventually loads it.

This delegation mechanism ensures that class files between different web applications do not interfere with each other, and also ensures that shared classes in the system are only loaded once. This way, Tomcat achieves isolation and management of class loading.