Access Control Contexts The AccessController Class

Chapter 6. Java Class Loaders

In this chapter, were going to look at the third major component that determines the security policy of a Java program: the Java class loader. Class loaders are the mechanism by which files or other sources containing Java bytecodes are read into the Java virtual machine and converted into class definitions. There are three areas in which the class loader operates with the security model. First, the class loader cooperates with the virtual machine to define namespaces, which protect the integrity of the security features built into the Java language. Second, the class loader calls the security manager when appropriate, ensuring that code has the appropriate permissions in order to access or define classes. And third, the class loader sets up the mapping of permissions to class objects the protection domain of each class so that the access controller knows which classes have which permissions. The last of these areas is the one which is of most use to developers: if you want to establish a different security policy in your application, its easier to do it by writing a custom class loader and establishing the permissions of classes within that class loader than by writing a new implementation of the Policy class. In this chapter, well address all of these points. Well also look into the class loader classes that come with Java and how to write your own class loader. As with the other elements of the Java sandbox, the ability to create and use a class loader is limited. If the default security model is in place, then you must explicitly grant code permission to create a class loader.

6.1 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. Its often said that these classes are in the default package, but thats 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 is executing the code from sun.com and needs access to the Car class, it asks the class loader that loaded the applet r1 in Figure 6−1 to provide that class. Figure 6−1. Different instances of the class loaders help to disambiguate class names