Joshua Engel - Programming for the Java¿ Virtual Machine

The core of Java technology, the Java virtual machine is an abstract computing machine that enables the Java platform to host applications on any computer or operating system without rewriting or recompiling. Anyone interested in designing a language or writing a compiler for the Java virtual machine must have an in-depth understanding of its binary class format and instruction set. If you are programming with the Java programming language, knowledge of the Java virtual machine will give you valuable insight into the Java platform's security capabilities and cross-platform portability. It will increase your understanding of the Java programming language, enabling you to improve the security and performance of your programs.

The author employs a tutorial approach that provides a detailed look into the central workings of the technology and teaches the reader how to write real programs for the Java virtual machine. He describes methods for becoming a better programmer through an advanced understanding of the Java virtual machine and Java technology. Programming for the Java Virtual Machine offers comprehensive coverage of all the major elements of the Java virtual machine--classes and objects, control instructions, debugging, class loaders, compiling the Java programming language, performance issues, security, and threads and synchronization. The book provides an introduction to the Java Virtual Machine Specification (JVMS), with a collection of topics that help programmers understand the Java virtual machine and the JVMS better. In addition, the book features implementations of Prolog and Scheme, a language that runs on top of the Java virtual machine, generating Java virtual machine code as it runs and using a Java virtual machine class loader to load the generated code into the system.

You will find detailed information on such topics as:

• The Java virtual machine verification algorithm

• How Java virtual machine security works, and what it can and can't do

• Using class loaders to incorporate code and dynamically generated code from the Internet, the Java Foundation Classes, database queries, and other languages

• The mechanics of compiling the Java programming language for the Java virtual machine

• Implementing other languages using the Java virtual machine, including Scheme, Prolog, Sather, Eiffel, and regular expressions

The Java platform builds a security architecture on top of the protections promised by the JVM. A security architecture is a way of organizing the software that makes up the Java platform so that potentially harmful operations are isolated from unprivileged code but available to privileged code. Most code is unprivileged; only carefully selected pieces of code are privileged to perform potentially dangerous operations. The security architecture is responsible for making sure that unprivileged code does not masquerade as privileged code.

The core of the Java platform security architecture under Java platforms 1.0 and 1.1 is the SecurityManager class. This class decides which pieces of code can perform certain operations and which cannot. Collectively, these decisions are called the security policy . The security policy is enforced by the Java platform classes, which check the SecurityManager before proceeding with any operations under the control of the SecurityManager.

[1] On the Java 2 platform, the core of the security architecture is shifted to a class called AccessController, which falls outside the scope of this book. See http//java.sun.com for more information.

Only one instance of the SecurityManager can be installed, and once it is installed it cannot be removed. It is called the security manager . By default, there is no security manager, and all operations are permitted. The class java.lang.System is responsible for ensuring that there is only one security manager. It provides the static methods getSecurityManager and setSecurityManager to get and set the security manager.

The SecurityManager class has a set of methods that are called by the Java platform code before proceeding with certain potentially harmful operations. These methods throw a SecurityException if the operation is forbidden. If no exception is thrown, then the caller may assume that the operation is permitted, and it can proceed with the operation. describes the operations that are checked by the SecurityManager in the Java 1.02 platform.

The security manager uses a variety of factors to determine whether an operation is permitted or not. These factors include the source of the code attempting the operation and the preferences of the user (discussed further in ). First we present an example of how the security architecture and the security policy interact.