Permission.java

/* Permission.java -- The superclass for all permission objects
   Copyright (C) 1998, 2001, 2002 Free Software Foundation, Inc.

This file is part of GNU Classpath.

GNU Classpath is free software; you can redistribute it and/or modify
it under the terms of the GNU General Public License as published by
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MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU
General Public License for more details.

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02111-1307 USA.

Linking this library statically or dynamically with other modules is
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As a special exception, the copyright holders of this library give you
permission to link this library with independent modules to produce an
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terms of your choice, provided that you also meet, for each linked
independent module, the terms and conditions of the license of that
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this exception to your version of the library, but you are not
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package java.security;

import java.io.Serializable;

/**
 * This class is the abstract superclass of all classes that implement
 * the concept of a permission.  A permission consists of a permission name
 * and optionally a list of actions that relate to the permission.  The
 * actual meaning of the name of the permission is defined only in the
 * context of a subclass.  It may name a resource to which access permissions
 * are granted (for example, the name of a file) or it might represent
 * something else entirely.  Similarly, the action list only has meaning
 * within the context of a subclass.  Some permission names may have no
 * actions associated with them.  That is, you either have the permission
 * or you don't.
 *
 * <p>The most important method in this class is <code>implies</code>.  This
 * checks whether if one has this permission, then the specified
 * permission is also implied.  As a conceptual example, consider the
 * permissions "Read All Files" and "Read File foo".  The permission
 * "Read All Files" implies that the caller has permission to read the
 * file foo.
 *
 * <p><code>Permission</code>'s must be immutable - do not change their
 * state after creation.
 *
 * @author Aaron M. Renn <arenn@urbanophile.com>
 * @see Permissions
 * @see PermissionCollection
 * @since 1.1
 * @status updated to 1.4
 */
public abstract class Permission implements Guard, Serializable
{
  /**
   * Compatible with JDK 1.1+.
   */
  private static final long serialVersionUID = -5636570222231596674L;

  /**
   * This is the name assigned to this permission object.
   *
   * @serial the name of the permission
   */
  private String name;

  /**
   * Create an instance with the specified name.
   *
   * @param name the permission name
   */
  public Permission(String name)
  {
    this.name = name;
  }

  /**
   * This method implements the <code>Guard</code> interface for this class.
   * It calls the <code>checkPermission</code> method in
   * <code>SecurityManager</code> with this <code>Permission</code> as its
   * argument.  This method returns silently if the security check succeeds
   * or throws an exception if it fails.
   *
   * @param obj the <code>Object</code> being guarded - ignored by this class
   * @throws SecurityException if the security check fails
   * @see GuardedObject
   * @see SecurityManager#checkPermission(Permission)
   */
  public void checkGuard(Object obj)
  {
    SecurityManager sm = System.getSecurityManager();
    if (sm != null)
      sm.checkPermission(this);
  }

  /**
   * This method tests whether this <code>Permission</code> implies that the
   * specified <code>Permission</code> is also granted.
   *
   * @param perm the <code>Permission</code> to test against
   * @return true if perm is implied by this
   */
  public abstract boolean implies(Permission perm);

  /**
   * Check to see if this object equals obj. Use <code>implies</code>, rather
   * than <code>equals</code>, when making access control decisions.
   *
   * @param obj the object to compare to
   */
  public abstract boolean equals(Object obj);

  /**
   * This method returns a hash code for this <code>Permission</code>. It
   * must satisfy the contract of <code>Object.hashCode</code>: it must be
   * the same for all objects that equals considers to be the same.
   *
   * @return a hash value
   */
  public abstract int hashCode();

  /**
   * Get the name of this <code>Permission</code>.
   *
   * @return the name
   */
  public final String getName()
  {
    return name;
  }

  /**
   * This method returns the list of actions for this <code>Permission</code>
   * as a <code>String</code>. The string should be in canonical order, for
   * example, both <code>new FilePermission(f, "write,read")</code> and
   * <code>new FilePermission(f, "read,write")</code> have the action list
   * "read,write".
   *
   * @return the action list for this <code>Permission</code>
   */
  public abstract String getActions();

  /**
   * This method returns an empty <code>PermissionCollection</code> object
   * that can store permissions of this type, or <code>null</code> if no
   * such collection is defined. Subclasses must override this to provide
   * an appropriate collection when one is needed to accurately calculate
   * <code>implies</code>.
   *
   * @return a new <code>PermissionCollection</code>
   */
  public PermissionCollection newPermissionCollection()
  {
    return null;
  }

  /**
   * This method returns a <code>String</code> representation of this
   * <code>Permission</code> object. This is in the format:
   * <code>'(' + getClass().getName() + ' ' + getName() + ' ' + getActions
   * + ')'</code>.
   *
   * @return this object as a <code>String</code>
   */
  public String toString()
  {
    return '(' + getClass().getName() + ' ' + getName() + ' '
      + getActions() + ')';
  }
} // class Permission