Note that the class loader hierarchy is not a Java inheritance hierarchy, but a delegation hierarchy. In the delegation design, a class loader delegates classloading to its parent before attempting to load a class itself. A class loader parent can be either the System class loader or another custom class loader. If the parent class loader cannot load a class, the class loader attempts to load the class itself. In effect, a class loader is responsible for loading only the classes not available to the parent. Classes loaded by a class loader higher in the hierarchy cannot refer to classes available lower in the hierarchy.
The Java Servlet specification recommends that the Web class loader look in the local class loader before delegating to its parent. You can make the Web class loader follow the delegation inversion model in the Servlet specification by setting delegate="false" in the class-loader element of the sun-web.xml file. It is safe to do this only for a web module that does not interact with any other modules. For details, see class-loader in Sun Java System Application Server 9.1 Application Deployment Guide.
The default value is delegate="true", which causes the Web class loader to delegate in the same manner as the other class loaders. You must use delegate="true" for a web application that accesses EJB components or that acts as a web service client or endpoint. For details about sun-web.xml, see The sun-web.xml File in Sun Java System Application Server 9.1 Application Deployment Guide.