The first popular use of Java was to permit web surfers to download applets into their web browsers. This greatly extended the capabilities of web browsers, but some people were concerned that the capabilities had been extended too far, allowing malicious applet writers to wreak havoc on the computer of anybody using the applets. They pointed out that Java is a full-featured language, which would allow applet writers to do just about anything. It is extremely difficult to prove things about computer programs written in conventional languages, and it is usually easy to hide malicious code in an innocent-looking program.
The Java platform provides a solution to these security fears. The platform contains checks designed to prevent unfriendly behavior by limiting the capabilities of applets, placing potentially harmful operations off-limits. To ensure that these checks cannot be circumvented, the Java virtual machine contains a verification algorithm that eliminates the possibility of behavior that could be used to bypass the security checks.
This chapter examines the relationship between the Java platform's security architecture and the verification algorithm, and it shows how the verification algorithm helps ensure the promises of the Java platform's security policy.