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Security and the class loader architecture

A look at the role played by class loaders in the JVM's overall security model

By Bill Venners, JavaWorld.com, 09/01/97

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Page 6 of 6

Conclusion

The class loader architecture contributes to the JVM's security model in two ways:

  1. by separating code into multiple name-spaces and placing a "shield" between code in different name-spaces
  2. by guarding the borders of trusted class libraries, such as the Java API


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."

Next month

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.
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Resources
  • Previous "Under The Hood" articles:
  • The Lean, Mean Virtual Machine -- Gives an introduction to the Java virtual machine.
  • The Java Class File Lifestyle -- Gives an overview of the Java class file, the file format into which all Java programs are compiled.
  • Java's Garbage- Collected Heap -- Gives an overview of garbage collection in general and the garbage-collected heap of the Java virtual machine in particular.
  • Bytecode Basics -- Introduces the bytecodes of the Java virtual machine, and discusses primitive types, conversion operations, and stack operations in particular.
  • Floating Point Arithmetic -- Describes the Java virtual machine's floating-point support and the bytecodes that perform floating point operations.
  • Logic and Arithmetic -- Describes the Java virtual machine's support for logical and integer arithmetic, and the related bytecodes.
  • Objects and Arrays -- Describes how the Java virtual machine deals with objects and arrays, and discusses the relevant bytecodes.
  • Exceptions -- Describes how the Java virtual machine deals with exceptions, and discusses the relevant bytecodes.
  • Try-Finally -- Describes how the Java virtual machine implements try-finally clauses, and discusses the relevant bytecodes.
  • Control Flow -- Describes how the Java virtual machine implements control flow and discusses the relevant bytecodes.
  • The Architecture of Aglets -- Describes the inner workings of aglets, IBM's autonomous Java-based software agent technology.
  • The Point of Aglets -- Analyzes the real-world utility of mobile agents such as aglets, IBM's autonomous Java- based software agent technology.
  • Method Invocation and Return -- Describes the four ways the Java virtual machine invokes methods, including the relevant bytecodes.
  • Thread Synchronization -- Shows how thread synchronization works in the Java virtual machine. Discusses the bytecodes for entering and exiting monitors.
  • Java's Security Architecture -- Gives an overview of the security model built into the JVM and looks at the JVM's built-in safety features.