The Class Loader Hierarchy

Bootstrap

The Bootstrap class loader loads the basic runtime classes provided by the JVM software. 

Extension

The Extension class loader loads classes from JAR files present in the system extensions directory, domain-dir/lib/ext. It is parent to the Public API class loader. See Using the Java Optional Package Mechanism.

Public API

The Public API class loader makes available all classes specifically exported by the Enterprise Server runtime for use by deployed applications. This includes, but is not limited to, Java EE APIs and other Sun GlassFish APIs. It is parent to the Common class loader. 

Common

The Common class loader loads JAR files in the as-install/lib directory, then classes in the domain-dir/lib/classes directory, followed by JAR files in the domain-dir/lib directory. Using domain-dir/lib/classes or domain-dir/lib is recommended whenever possible, and required for custom login modules and realms. It is parent to the Connector class loader. See Using the Common Class Loader.

Connector

The Connector class loader is a single class loader instance that loads individually deployed connector modules, which are shared across all applications. It is parent to the Applib class loader. 

Applib

The Applib class loader loads the library classes, specified during deployment, for a specific enabled module; see Application-Specific Class Loading. One instance of this class loader is present in each class loader universe; see Class Loader Universes. It is parent to the Archive class loader.

When multiple deployed applications use the same library, they share the same instance of the library. One library cannot reference classes from another library. 

Archive

The Archive class loader loads classes from the WAR and JAR files or directories (for directory deployment) of modules deployed to the Enterprise Server. This class loader also loads any application-specific classes generated by the Enterprise Server runtime, such as stub classes or servlets generated by JSP pages.