Help needed,Doubt regarding AccessController.doPrivileged() usage

Question

Hi All,
I am trying understand the basics of java security and AccessController.doPrivileged() usage
i started with a sample program
import java.security.AccessController;
import java.security.PrivilegedAction;
public class AccessSystemProperty {
   public static void main(String[] args) {
         System.out.println(System.getSecurityManager());
        AccessController.doPrivileged(
          new PrivilegedAction<Boolean>(){
             public Boolean run(){
                  System.out.println(System.getProperty("java.home"));
                  return Boolean.TRUE;
}if i try to run above code using default security manage i am getting AccessControlException
My stacktrace is
C:\>java -Djava.security.manager AccessSystemProperty
java.lang.SecurityManager@923e30
Exception in thread "main" java.security.AccessControlException: access denied (
java.util.PropertyPermission java.home read)
        at java.security.AccessControlContext.checkPermission(Unknown Source)
        at java.security.AccessController.checkPermission(Unknown Source)
        at java.lang.SecurityManager.checkPermission(Unknown Source)
        at java.lang.SecurityManager.checkPropertyAccess(Unknown Source)
        at java.lang.System.getProperty(Unknown Source)
        at AccessSystemProperty$1.run(AccessSystemProperty.java:9)
        at AccessSystemProperty$1.run(AccessSystemProperty.java:8)
        at java.security.AccessController.doPrivileged(Native Method)
        at AccessSystemProperty.main(AccessSystemProperty.java:6)from one other forum , i saw one comment
"When you use AccessController.doPrivileged to invoke those operations, the operation is executed with all the rights(permissions) of your protection domain. Hence if your code has enough rights only then it could execute those operations."
http://stackoverflow.com/questions/2233761/when-should-accesscontroller-doprivileged-be-used
What i understood from above is ,if we try to access system property from AccessController.doPrivileged() it should work ,Please correct me .. i think i am wrong ..
Kindly help me to get a clear picture of
1)when we need to use AccessController.doPrivileged() ?.(if SecurityManager is present we use AccessController.doPrivileged why this is failing in above example )
2) what is the real advantage we are getting by using AccessController and PrivilegedAction?.
3) Do we need custom policy file for above example to work ?
Thanks,
Paul

Answer 1

Finally i got an answer.Pls find the Detailed explanation from Rob.
http://stackoverflow.com/questions/8703234/accesscontroller-usage