nd 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 trusted package border guarding, class loaders must be written so they check the names of requested classes against a list of restricted and forbidden packages.
For a walk through of the process of writing a class loader, including sample code, see Chuck McManis's JavaWorld article, "The basics of Java class loaders."
In next month's article, I'll continue the discussion of the JVM's security model by describing the class verifier.
About the author
Bill Venners has been writing software professionally for 12 years. Based in Silicon Valley, he provides software consulting and training services under the name Artima Software Company. Over the years he has developed software for the consumer electronics, education, semiconductor, and life insurance industries. He has programmed in many languages on many platforms: assembly language on various microprocessors, C on Unix, C++ on Windows, Java on the Web. He is author of the book: Inside the Java Virtual Machine, published by McGraw-Hill. Reach Bill at email@example.com.
This article was first published under the name Security and the Class Loader Architecture in JavaWorld, a division of Web Publishing, Inc., August 1997.