Protection and Policy Domain in Detail
		Remember, Policy maps the code sources to their permissions, and the Policy object is, essentially, a Singleton.  The methods we commonly use on Policy are getPermissions(), getPolicy(), refresh(), and setPolicy().  Now we'll look at getPermissions().   getPermissions() returns to us a PermissionCollection object.  This is fairly self-explanatory; a PermissionCollection is simply a collection of allowed permissions. Additionally, each class has a ProtectionDomain that encapsulates the code source and corresponding PermissionCollection.   You can get this by calling the getProtectionDomain of the Class object.   The Class object must represent the class in which you are interested in finding permissions. Use the forName() static method of class Class to get this.   Sound confusing?  How about this: ProtectionDomain pd = Class.forName("Car").getProtectionDomain() ProtectionDomain has four methods: CodeSource getCodeSource() - returns a CodeSource object that encapsulates both the location (URL) of the code and any certificates.  PermissionCollection getPermissions() - returns the PermissionCollection, discussed earlier. boolean implies(Permission permission) - returns true if the ProtectionDomain implies the permission represented in the Permission object. String toString() - overrides the default toString() method of class Object. The ProtectionDomain is set when the class is loaded. The SecureClassLoader uses the policy object to find permissions for the code source.  Then it creates a ProtectionDomain with the permissions and code sources. The SecurityManager checks requested permissions against granted permissions. It gets the ProtectionDomains of all of the classes on the stack, and asks if they permit the operation being requested.  If so, the operation can continue.  If not, a SecurityException is thrown. By checking all classes, the SecurityManager ensures that the most restrictive policy of all classes on the stack is enforced.  Security Policy Files