12.2.2 Application and Web module class loaders

J2EE applications consist of five primary elements: Web modules, EJB modules, application client modules, resource adapters (RAR files), and utility JARs. Utility JARs contain code used by both EJBs and servlets. Utility frameworks such as log4j are good examples of a utility JAR.

EJB modules, utility JARs, resource adapter files, and shared libraries associated with an application are always grouped together into the same class loader. This class loader is called the application class loader. Depending on the class loader policy, this class loader can be shared by multiple applications (EARs), or be unique for each application, which is the default.

By default, Web modules receive their own class loader, a WAR class loader, to load the contents of the WEB-INF/classes and WEB-INF/lib directories. You can modify the default behavior by changing the application's WAR class loader policy. The default is to Class loader for each WAR file in the application (this setting was called Module in previous releases). If the WAR class loader policy is set to Single class loader for application (called Application in previous releases), the Web module contents are loaded by the application class loader in addition to the EJBs, RARs, utility JARs, and shared libraries. The application class loader is the parent of the WAR class loader.

The application and the WAR class loaders are reloadable class loaders. They monitor changes in the application code to automatically reload modified classes. You can modify this behavior at deployment time.