One way the Java platform uses a Security Manager to keep viruses from accessing your comuter through a java application. Currently JDK system code invokes security manager methods to perform resource access control checks.

Most browsers install a security manager, so applets usually run under the scrutiny of a security manager. No applet is allowed to access a resource unless the Security Manager finds that permission has been explicitly granted to access that system resource. That permission is granted by an entry in a policy file.

Here's the source code for an applet named WriteFile that tries to create and to write to a file named writetest in the current directory. This applet will not be able to create the file unless it has explicit permission in a policy file.

Type this command in your command window:

appletviewer http://java.sun.com/docs/books/tutorial/
security1.2/tour1/examples/WriteFile.html

Type this command on a single line, without spaces in the URL.

You should see a message about a security exception, as shown in the following figure. This is the expected behavior; the system caught the applet trying to access a resource it does not have permission to access.