Learning Java, 4th Edition

Policy Files

One of the truly novel things about Java is that security is built into the language. As described in Chapter 1, the Java VM can verify class files and Java’s security manager can impose limits on what classes do. In early versions of Java, it was necessary to implement security policies programmatically by writing a Java security manager class and using it in your application. Later, a declarative security system was added. This system allows you to write policy files—text-based descriptions of permissions—which are much simpler and don’t require code changes. These policy files tell the security manager what to allow and disallow and for whom.

In early versions of Java, much of the buzz had to do with the security of applets. Applets that were downloaded from untrusted locations could be run with security restrictions that prevented them from doing questionable things such as reading from or writing to the disk or contacting arbitrary computers on the network. With security policy files, it’s easy to apply applet-style security to any application without modifying it. Furthermore, it’s easy to fine-tune the access you grant. For example, you can allow an application to access only a specific directory on the disk, or you can allow network access to certain addresses.

Understanding security and security policies can be important, so we’ll cover it here. However, in practice, you probably won’t use this facility yourself, unless you are writing a framework for running ...