Chapter 6. Java Security

Java programs can dynamically load Java classes from a variety of sources, including untrusted sources, such as web sites reached across an insecure network. The ability to create and work with such mobile code is one of the great strengths and features of Java. To make it work successfully, however, Java puts great emphasis on a security architecture that allows untrusted code to run safely, without fear of damage to the host system.

The need for a security system in Java is most acutely demonstrated by appletsminiature Java applications designed to be embedded in web pages.^[1] When a user visits a web page (with a Java-enabled web browser) that contains an applet, the web browser downloads the Java class files that define that applet and runs them. In the absence of a security system, an applet could wreak havoc on the user's system by deleting files, installing a virus, stealing confidential information, and so on. Somewhat more subtly, an applet could take advantage of the user's system to forge email, generate spam, or launch hacking attempts on other systems.

^[1] Applets are documented in Java Foundation Classes in a Nutshell (O'Reilly) and are not covered in this book. Still, they serve as good examples here.

Java's main line of defense against such malicious code is access control: untrusted code is simply not given access to certain sensitive portions of the core Java API. For example, an untrusted applet is not typically allowed to read, write, or delete files on the host system or connect over the network to any computer other than the web server from which it was downloaded. This chapter describes the Java access control architecture and a few other facets of the Java security system.