The Servlet specification recommends that the Web class loader look in the local class loader before delegating to its parent. To make the Web class loader follow the delegation model in the Servlet specification, set delegate="false" in the class-loader element of the sun-web.xml file. It’s safe to do this only for a web module that does not interact with any other modules.
The default value is delegate="true", which causes the Web class loader to delegate in the same manner as the other class loaders. 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 Sun GlassFish Enterprise Server v2.1.1 Application Deployment Guide.
For general information about class loaders, see Chapter 2, Class Loaders.