The Java™ Tutorials
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Understanding Extension Class Loading
Trail: The Extension Mechanism
Lesson: Creating and Using Extensions

Understanding Extension Class Loading

The extension framework makes use of the class-loading delegation mechanism. When the runtime environment needs to load a new class for an application, it looks for the class in the following locations, in order:

  1. Bootstrap classes: the runtime classes in rt.jar, internationalization classes in i18n.jar, and others.
  2. Installed extensions: classes in JAR files in the lib/ext directory of the JRE, and in the system-wide, platform-specific extension directory (such as /usr/jdk/packages/lib/ext on the Solaris™ Operating System, but note that use of this directory applies only to Java™ 6 and later).
  3. The class path: classes, including classes in JAR files, on paths specified by the system property java.class.path. If a JAR file on the class path has a manifest with the Class-Path attribute, JAR files specified by the Class-Path attribute will be searched also. By default, the java.class.path property's value is ., the current directory. You can change the value by using the -classpath or -cp command-line options, or setting the CLASSPATH environment variable. The command-line options override the setting of the CLASSPATH environment variable.

The precedence list tells you, for example, that the class path is searched only if a class to be loaded hasn't been found among the classes in rt.jar, i18n.jar or the installed extensions.

Unless your software instantiates its own class loaders for special purposes, you don't really need to know much more than to keep this precedence list in mind. In particular, you should be aware of any class name conflicts that might be present. For example, if you list a class on the class path, you'll get unexpected results if the runtime environment instead loads another class of the same name that it found in an installed extension.

The Java Class Loading Mechanism

The Java platform uses a delegation model for loading classes. The basic idea is that every class loader has a "parent" class loader. When loading a class, a class loader first "delegates" the search for the class to its parent class loader before attempting to find the class itself.

Here are some highlights of the class-loading API:

To see a sample application that uses some of the API as it relates to JAR files, see the Using JAR-related APIs lesson in this tutorial.

Class Loading and the java Command

The Java platform's class-loading mechanism is reflected in the java command.

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