As mentioned above, the web application class loader diverges from the default Java delegation model (in accordance with the recommendations in the Servlet Specification, version 2.4, section 9.7.2 Web Application Classloader). When a request to load a class from the web application's WebappX class loader is processed, this class loader will look in the local repositories first, instead of delegating before looking. There are exceptions. Classes which are part of the JRE base classes cannot be overridden. For some classes (such as the XML parser components in J2SE 1.4+), the J2SE 1.4 endorsed feature can be used. Last, any JAR file that contains Servlet API classes will be explicitly ignored by the classloader — Do not include such JARs in your web application. All other class loaders in Tomcat follow the usual delegation pattern.
Therefore, from the perspective of a web application, class or resource loading looks in the following repositories, in this order:
- Bootstrap classes of your JVM
- /WEB-INF/classes of your web application
- /WEB-INF/lib/*.jar of your web application
- System class loader classes (described above)
- Common class loader classes (described above)
If the web application class loader is configured with <Loader delegate="true"/> then the order becomes:
- Bootstrap classes of your JVM
- System class loader classes (described above)
- Common class loader classes (described above)
- /WEB-INF/classes of your web application
- /WEB-INF/lib/*.jar of your web application