IBM User Guide for Java V7 on Windows > IBM SDK for Java > Class loading

The parent-delegation model

The delegation model requires that any request for a class loader to load a given class is first delegated to its parent class loader before the requested class loader tries to load the class itself. The parent class loader, in turn, goes through the same process of asking its parent. This chain of delegation continues through to the bootstrap class loader (also known as the primordial or system class loader). If a class loader's parent can load a given class, it returns that class. Otherwise, the class loader attempts to load the class itself.

The JVM has three class loaders, each possessing a different scope from which it can load classes. As you descend the hierarchy, the scope of available class repositories widens, and typically the repositories are less trusted:

Bootstrap
|
Extensions
|
Application

At the top of the hierarchy is the bootstrap class loader. This class loader is responsible for loading only the classes that are from the core Java™ API. These classes are the most trusted and are used to bootstrap the JVM.

The extensions class loader can load classes that are standard extensions packages in the extensions directory.

The application class loader can load classes from the local file system, and will load files from the CLASSPATH. The application class loader is the parent of any custom class loader or hierarchy of custom class loaders.

Because class loading is always delegated first to the parent of the class loading hierarchy, the most trusted repository (the core API) is checked first, followed by the standard extensions, then the local files that are on the class path. Finally, classes that are located in any repository that your own class loader can access, are accessible. This system prevents code from less-trusted sources from replacing trusted core API classes by assuming the same name as part of the core API.

Parent: Class loading

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