WebLogic: The Definitive Guide

WebLogic’s Classloading Framework

A classloader is used by the JVM to locate and load Java classes into memory at runtime. Java classloaders define a hierarchy, a tree-like collection of parent and child classloaders. The root of this classloader hierarchy is the bootstrap classloader, which is created by the JVM for loading its internal classes and the java.* packages included within the JVM. An extensions classloader, which is a child of the bootstrap classloader, is used to load any JARs placed in the extensions directory of the JDK. This means that any JAR in the JDK’s extensions directory may refer only to other classes in that directory, or to the JDK classes. Finally, the system classpath classloader, which is a child of the extensions classloader, is responsible for loading classes from the JVM’s classpath. Any custom classloader created by an application, including WebLogic’s classloaders, are all descendants of this system classpath classloader.

This classloader hierarchy plays an important role when locating and loading classes into memory. In general, classloaders try to reuse a preloaded version of a class. The hierarchy of classloaders intrinsically determines the scope from where the preloaded version of the class may be fetched. By default, child classloaders and classes loaded by these classloaders have direct access to their parent classloader and any classes loaded by it. This means that any classes loaded by a classloader are visible directly or indirectly, ...