Like many server applications, Tomcat installs a variety of class loaders (that is, classes that implementjava.lang.ClassLoader) to allow different portions of the container, and the web applications running on the container, to have access to different repositories of available classes and resources. This mechanism is used to provide the functionality defined in the Servlet Specification, version 2.4 — in particular, Sections 9.4 and 9.6.
In a Java environment, class loaders are arranged in a parent-child tree. Normally, when a class loader is asked to load a particular class or resource, it delegates the request to a parent class loader first, and then looks in its own repositories only if the parent class loader(s) cannot find the requested class or resource. Note, that the model for web application class loaders differs slightly from this, as discussed below, but the main principles are the same.
When Tomcat is started, it creates a set of class loaders that are organized into the following parent-child relationships, where the parent class loader is above the child class loader:
Bootstrap
|
System
|
Common
/ \
Webapp1 Webapp2 …
The characteristics of each of these class loaders, including the source of classes and resources that they make visible, are discussed in detail in the following section.
