The Security Manager

The security manager is an object that is used to check whether the operation we want to perform is not violating security policy of the operation system.
Sometime we want to perform any operation and got a exception

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.<init>(FileOutputStream.java:169)
        at java.io.FileOutputStream.<init>(FileOutputStream.java:70)
        at java.io.FileWriter.<init>(FileWriter.java:46)

...

To Prevent from this exception we can use SecurityManager class to check for the permission before performing operation.

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.

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 owncheckXXX() 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.