Java Security, 2nd Edition

The Class Loader and Namespaces

Class loaders are used by the Java virtual machine to enforce certain rules about the namespaces used by Java classes. Recall that the full name of a Java class is qualified by the name of the package to which the class belongs; there is no standard class called String in the Java API, but there is the class java.lang.String. On the other hand, a class does not need to belong to a package, in which case its full name is just the name of the class. It’s often said that these classes are in the default package, but that’s slightly misleading: as it turns out, there is a different default package for each class loader in use by the virtual machine.

Consider what happens if you surf to a page at www.sun.com and load an applet that uses a class called Car (with no package name); after that, you surf to a page at www.ora.com and load a different applet that uses a class called Car (also with no package name). Clearly, these are two different classes, but they have the same fully qualified name -- how can the virtual machine distinguish between these two classes?

The answer to that question lies in the internal workings of the class loader. When a class is loaded by a class loader, it is stored in a reference internal to that class loader. A class loader in Java is simply an object whose type extends the ClassLoader class. When the virtual machine needs access to a particular class, it asks the appropriate class loader. For example, when the virtual machine ...