To get the security benefits offered by name-spaces, you need to make sure you load classes from different sources through different class loaders. This is the scheme, described above, used by Java-enabled Web browsers. The Java application fired off by a Web browser usually creates a different applet class loader object for each source of classes it downloads across the network. For example, a browser would use one class loader object to download classes from http://www.niceapplets.com, and another class loader object to download classes from http://www.meanapplets.com.
Java allows classes in the same package to grant each other special access privileges that aren't granted to classes outside the package. So, if your class loader receives a request to load a class that by its name brazenly declares itself to be part of the Java API (for example, a class named
java.lang.Virus), your class loader should proceed
cautiously. If loaded, such a class could gain special access to
the trusted classes of
java.lang and could possibly
use that special access for devious purposes.
Consequently, you would normally write a class loader so that it simply refuses to load any class that claims to be part of the Java API (or any other trusted runtime library) but that doesn't exist in the local trusted repository. In other words, after your class loader passes a request to the primordial class loader, and the primordial class loader indicates it can't load the class, your class loader should check to make sure the class doesn't declare itself to be a member of a trusted package. If it does, your class loader, instead of trying to download the class across the network, should throw a security exception.
In addition, you may have installed some packages in the trusted repository that contain classes you want your application to be able to load through the primordial class loader, but that you don't want to be accessible to classes loaded through your class loader. For example, assume you have created a package named
absolutepower and installed it on the local
repository accessible by the primordial class loader. Assume also
that you don't want classes loaded by your class loader to be
able to load any class from the
package. In this case, you would write your class loader such
that the very first thing it does is to make sure the requested
class doesn't declare itself as a member of the
absolutepower package. If such a class is requested,
your class loader, rather than passing the class name to the
primordial class loader, should throw a security exception.
The only way a class loader can know whether or not a class is
from a restricted package, such as
java.lang, or a
forbidden package, such as
absolutepower, is by the
name of the class. Thus, a class loader must be given a list of
the names of restricted and forbidden packages. Because the name
java.lang.Virus indicates it is from the
java.lang package, and
java.lang is on
the list of restricted packages, your class loader should throw a
security exception if the primordial class loader can't load it.
Likewise, because the name of class
absolutepower.FancyClassLoader indicates it is part
absolutepower package, and the
absolutepower package is on the list of forbidden
packages, your class loader should throw a security
A security-minded class
A common way to write a security-minded class loader is to use the following four steps:
By performing steps one and three as outlined above, the class loader guards the borders of the trusted packages. With step one, it prevents a class from a forbidden package to be loaded at all. With step three, it doesn't allow an untrusted class to insert itself into a trusted package.
The class loader architecture contributes to the JVM's security model in two ways:
Both of these capabilities of Java's class loader architecture must be used properly by programmers so as to reap the security benefit they offer. To take advantage of the name-space shield, code from different sources should be loaded through different class loader objects. To take advantage of tr