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An implementation of Group that allows that acts as a stack of Principals with a single Principal Group member active at any time. When one adds a Principal to a NestablePrincipal the Principal is pushed onto the active Princpal stack and any of the Group methods operate as though the Group contains only the Principal. When removing the Principal that corresponds to the active Principal, the active Principal is popped from the stack and the new active Principal is effectively set to the new top of the stack. The typical usage of this class is when doing a JAAS LoginContext login to runAs a new Principal with a new CallerPrincipal identity without destroying the current CallerPrincipal identity and roles.

 public class NestablePrincipal extends SimplePrincipal implements GroupCloneable
The serialVersionUID
    private static final long serialVersionUID = 4628473920470890923L;
The stack of the Principals. Elements are pushed/poped by inserting/removing element 0.
     private LinkedList<PrincipalprincipalStack;

Creates new NestablePrincipal with the given name
     public NestablePrincipal(String name)
          = new LinkedList<Principal>();
 // --- Begin Group interface methods
Returns an enumeration that contains the single active Principal.

an Enumeration of the single active Principal.
     public Enumeration<Principalmembers()
         return new IndexEnumeration<Principal>();

Removes the first occurence of user from the Principal stack.

user the principal to remove from this group.
true if the principal was removed, or false if the principal was not a member.
     public boolean removeMember(Principal user)
         return .remove(user);

Pushes the user onto the Principal stack and makes it the active Principal.

true always.
     public boolean addMember(Principal user)
         return true;

Returns true if the passed principal is a member of the group. This method does a recursive search, so if a principal belongs to a group which is a member of this group, true is returned.

member the principal whose membership is to be checked.
true if the principal is a member of this group, false otherwise.
    public boolean isMember(Principal member)
        if.size() == 0 )
            return false;
        Object activePrincipal = .getFirst();
        return member.equals(activePrincipal);
    public synchronized Object clone() throws CloneNotSupportedException    
       NestablePrincipal clone = (NestablePrincipalsuper.clone(); 
       if(clone != null
         clone.principalStack = (LinkedList)this..clone();  
       return clone
// --- End Group interface methods
    private class IndexEnumeration<T extends Principalimplements Enumeration<Principal>
        private boolean hasMoreElements;
             = .size() > 0;
        public boolean hasMoreElements()
            return ;
        public Principal nextElement()
            Principal next = .getFirst();
             = false;
            return next;
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