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# Providing Your Own Security Manager

> Security becomes a major concern when writing and using programs that interact with the Internet. Will you download something that corrupts your file system? Will you be open for a virus attack? It's unlikely that computers on the Internet will ever be completely safe from attack from the few evil-doers out there. However, you can take steps to provide a significant level of protection and security for your computers and data. One of the ways that Java provides protection from attack is through the use of security managers. A security manager implements and imposes the security policy for an application.

> #### Introducing the Security Manager

> > The security manager is an application-wide object that determines whether potentially threatening operations should be allowed. The classes in the Java packages cooperate with the security manager by asking the application's security manager for permission to perform certain operations.

> #### Writing a Security Manager

> > This section walks you through a simple implementation of a security manager that requires the user to type in a password each time the application tries to open a file for reading or writing.

> #### Installing Your Security Manager

> > This page shows you how to put your security manager on duty for your Java application.

> > Note: The security manager for an application can be set only once. Typically, a browser sets its security manager during its startup procedure. So most of the time, applets cannot set the security manager because it's already been set. A SecurityException will result if your applet attempts to do so. See Understanding Applet Capabilities and Restrictions for information.

> #### Deciding What SecurityManager Methods to Override

> > And finally, this page looks at the SecurityManager class in greater detail, showing you which methods in the SecurityManager class affect which kinds of operations and helping you decide which methods your security manager needs to override.

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