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Trail: Essential Classes
Lesson: The Platform Environment
Section: System Utilities
The Platform Environment
Configuration Utilities
Properties
Command-Line Arguments
Environment Variables
Other Configuration Utilities
System Utilities
Command-Line I/O Objects
System Properties
The Security Manager
Miscellaneous Methods in System
PATH and CLASSPATH
Questions and Exercises
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The Security Manager

> A security manager is an object that defines a security policy for an application. This policy specifies actions that are unsafe or sensitive. Any actions not allowed by the security policy cause a `SecurityException` to be thrown. An application can also query its security manager to discover which actions are allowed.

> Typically, a web applet runs with a security manager provided by the browser or Java Web Start plugin. Other kinds of applications normally run without a security manager, unless the application itself defines one. If no security manager is present, the application has no security policy and acts without restrictions.

> This section explains how an application interacts with an existing security manager. For more detailed information, including information on how to design a security manager, refer to the Security Guide.

> ### Interacting with the Security Manager

> The security manager is an object of type `SecurityManager`; to obtain a reference to this object, invoke `System.getSecurityManager`.

> >     SecurityManager appsm = System.getSecurityManager();

> If there is no security manager, this method returns `null`.

> Once an application has a reference to the security manager object, it can request permission to do specific things. Many classes in the standard libraries do this. For example, `System.exit`, which terminates the Java virtual machine with an exit status, invokes `SecurityManager.checkExit` to ensure that the current thread has permission to shut down the application.

> The SecurityManager class defines many other methods used to verify other kinds of operations. For example, `SecurityManager.checkAccess` verifies thread accesses, and `SecurityManager.checkPropertyAccess` verifies access to the specified property. Each operation or group of operations has its own `checkXXX()` method.

> In addition, the set of `checkXXX()` methods represents the set of operations that are already subject to the protection of the security manager. Typically, an application does not have to directly invoke any `checkXXX()` methods.

> ### Recognizing a Security Violation

> Many actions that are routine without a security manager can throw a `SecurityException` when run with a security manager. This is true even when invoking a method that isn't documented as throwing `SecurityException`. For example, consider the following code used to read a file:

> >     reader = new FileReader("xanadu.txt");

> In the absence of a security manager, this statement executes without error, provided `xanadu.txt` exists and is readable. But suppose this statement is inserted in a web applet, which typically runs under a security manager that does not allow file input. The following error messages might result:

> >     appletviewer fileApplet.html
> >     Exception in thread "AWT-EventQueue-1" java.security.AccessControlException: access denied (java.io.FilePermission characteroutput.txt write)
> >     	at java.security.AccessControlContext.checkPermission(AccessControlContext.java:323)
> >     	at java.security.AccessController.checkPermission(AccessController.java:546)
> >     	at java.lang.SecurityManager.checkPermission(SecurityManager.java:532)
> >     	at java.lang.SecurityManager.checkWrite(SecurityManager.java:962)
> >     	at java.io.FileOutputStream.(FileOutputStream.java:169)
> >     	at java.io.FileOutputStream.(FileOutputStream.java:70)
> >     	at java.io.FileWriter.(FileWriter.java:46)
> >     ...

> Note that the specific exception thrown in this case, `java.security.AccessControlException`, is a subclass of `SecurityException`.

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