java class loaders

http://onjava.com/pub/a/onjava/2005/01/26/classloading.html

1. Bootstrap classes: the runtime classes in rt.jar, internationalization classes in i18n.jar, and others.
Whenever a new JVM is started by typing java MyMainClass, the "bootstrap class loader" is responsible for loading key Java classes like java.lang.Object and other runtime code into memory first. The runtime classes are packaged inside of theJRE\lib\rt.jar file.

1. 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). We can store extension libraries, those that provide features that go beyond the core Java runtime code, in the path given by the java.ext.dirs property. The ExtClassLoader is responsible for loading all .jar files kept in the java.ext.dirs path

1. 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 -cpcommand-line options, or setting the CLASSPATH environment variable. The command-line options override the setting of theCLASSPATH environment variable.
The third and most important class loader from the developer perspective is the AppClassLoader. The application class loader is responsible for loading all of the classes kept in the path corresponding to the java.class.path system property.