/*
 * @(#)Permission.java	1.33 01/11/29
 *
 * Copyright 2002 Sun Microsystems, Inc. All rights reserved.
 * SUN PROPRIETARY/CONFIDENTIAL. Use is subject to license terms.
 */
 
package java.security;

/**
 * Abstract class for representing access to a system resource.
 * All permissions have a name (whose interpretation depends on the subclass),
 * as well as abstract functions for defining the semantics of the
 * particular Permission subclass. 
 * 
 * <p>Most Permission objects also include an "actions" list that tells the actions 
 * that are permitted for the object.  For example, 
 * for a <code>java.io.FilePermission</code> object, the permission name is
 * the pathname of a file (or directory), and the actions list
 * (such as "read, write") specifies which actions are granted for the
 * specified file (or for files in the specified directory).
 * The actions list is optional for Permission objects, such as 
 * <code>java.lang.RuntimePermission</code>,
 * that don't need such a list; you either have the named permission (such
 * as "system.exit") or you don't.
 * 
 * <p>An important method that must be implemented by each subclass is
 * the <code>implies</code> method to compare Permissions. Basically,
 * "permission p1 implies permission p2" means that
 * if one is granted permission p1, one is naturally granted permission p2.
 * Thus, this is not an equality test, but rather more of a
 * subset test.
 * 
 * <P> Permission objects are similar to String objects in that they
 * are immutable once they have been created. Subclasses should not
 * provide methods that can change the state of a permission
 * once it has been created.
 *
 * @see Permissions
 * @see PermissionCollection
 *
 * @version 1.33 01/11/29
 *
 * @author Marianne Mueller
 * @author Roland Schemers 
 */

public abstract class Permission implements Guard, java.io.Serializable {

    private String name;

    /**
     * Constructs a permission with the specified name.
     *
     * @param name name of the Permission object being created.
     *
     */

    public Permission(String name) {
	this.name = name;
    }

    /**
     * Implements the guard interface for a permission. The 
     * <code>SecurityManager.checkPermission</code> method is called, 
     * passing this permission object as the permission to check.
     * Returns silently if access is granted. Otherwise, throws
     * a SecurityException.
     * 
     * @param object the object being guarded (currently ignored).
     *
     * @throws SecurityException
     *        if a security manager exists and its 
     *        <code>checkPermission</code> method doesn't allow access.
     * 
     * @see Guard
     * @see GuardedObject
     * @see SecurityManager#checkPermission
     * 
     */
    public void checkGuard(Object object) throws SecurityException {
	SecurityManager sm = System.getSecurityManager();
	if (sm != null) sm.checkPermission(this);
    }

    /**
     * Checks if the specified permission's actions are "implied by" 
     * this object's actions.
     * <P>
     * This must be implemented by subclasses of Permission, as they are the 
     * only ones that can impose semantics on a Permission object.
     * 
     * <p>The <code>implies</code> method is used by the AccessController to determine
     * whether or not a requested permission is implied by another permission that
     * is known to be valid in the current execution context.
     *
     * @param permission the permission to check against.
     *
     * @return true if the specified permission is implied by this object,
     * false if not.
     */

    public abstract boolean implies(Permission permission);

    /**
     * Checks two Permission objects for equality.
     * <P>
     * Do not use the <code>equals</code> method for making access control
     * decisions; use the <code>implies</code> method.
     *  
     * @param obj the object we are testing for equality with this object.
     *
     * @return true if both Permission objects are equivalent.
     */

    public abstract boolean equals(Object obj);

    /**
     * Returns the hash code value for this Permission object.
     * <P>
     * The required <code>hashCode</code> behavior for Permission Objects is
     * the following: <p>
     * <ul>
     * <li>Whenever it is invoked on the same Permission object more than 
     *     once during an execution of a Java application, the 
     *     <code>hashCode</code> method
     *     must consistently return the same integer. This integer need not 
     *     remain consistent from one execution of an application to another 
     *     execution of the same application. <p>
     * <li>If two Permission objects are equal according to the 
     *     <code>equals</code> 
     *     method, then calling the <code>hashCode</code> method on each of the
     *     two Permission objects must produce the same integer result. 
     * </ul>
     *
     * @return a hash code value for this object.
     */

    public abstract int hashCode();

    /**
     * Returns the name of this Permission.
     * For example, in the case of a <code>java.io.FilePermission</code>,
     * the name will be a pathname.
     *
     * @return the name of this Permission.
     * 
     */

    public final String getName() {
	return name;
    }

    /**
     * Returns the actions as a String. This is abstract
     * so subclasses can defer creating a String representation until 
     * one is needed. Subclasses should always return actions in what they
     * consider to be their
     * canonical form. For example, two FilePermission objects created via
     * the following:
     * 
     * <pre>
     *   perm1 = new FilePermission(p1,"read,write");
     *   perm2 = new FilePermission(p2,"write,read"); 
     * </pre>
     * 
     * both return 
     * "read,write" when the <code>getActions</code> method is invoked.
     *
     * @return the actions of this Permission.
     *
     */

    public abstract String getActions();

    /**
     * Returns an empty PermissionCollection for a given Permission object, or null if
     * one is not defined. Subclasses of class Permission should 
     * override this if they need to store their permissions in a particular
     * PermissionCollection object in order to provide the correct semantics
     * when the <code>PermissionCollection.implies</code> method is called. 
     * If null is returned,
     * then the caller of this method is free to store permissions of this
     * type in any PermissionCollection they choose (one that uses a Hashtable,
     * one that uses a Vector, etc).
     *
     * @return a new PermissionCollection object for this type of Permission, or 
     * null if one is not defined.
     */

    public PermissionCollection newPermissionCollection() {
	return null;
    }

    /**
     * Returns a string describing this Permission.  The convention is to
     * specify the class name, the permission name, and the actions in
     * the following format: '("ClassName" "name" "actions")'.
     * 
     * @return information about this Permission.
     */

    public String toString() {
	return "(" + getClass().getName() + " " + 
	                  name + " " + getActions() + ")";
    }
}