PreferNamespace.java

/*
 * This file or a portion of this file is licensed under the terms of
 * the Globus Toolkit Public License, found in file GTPL, or at
 * http://www.globus.org/toolkit/download/license.html. This notice must
 * appear in redistributions of this file, with or without modification.
 *
 * Redistributions of this Software, with or without modification, must
 * reproduce the GTPL in: (1) the Software, or (2) the Documentation or
 * some other similar material which is provided with the Software (if
 * any).
 *
 * Copyright 1999-2004 University of Chicago and The University of
 * Southern California. All rights reserved.
 */
package org.griphyn.vdl.router;

import java.util.*;
import org.griphyn.vdl.util.Logging;
import org.griphyn.vdl.classes.Definition;
import org.griphyn.vdl.classes.Derivation;

/**
 * This implementation of the arbitration interface prefers the first
 * derivation within the same current working namespace. If there is no
 * such derivation, returns the first one from the collection.
 *
 * @author Jens-S. Vöckler
 * @author Yong Zhao
 * @version $Revision $
 */
public class PreferNamespace implements Arbiter
{
  /**
   * Compares two strings, each of which may be null. If both are null,
   * they are considered equal by this function. This function relies on
   * the fact that equals() can deal with null arguments. 
   *
   * @param a a string which may be null
   * @param b a string which may be null
   * @return true, if the strings equal, or if both are null.
   */
  private static boolean matchWithNull( String a, String b )
  {
    return ( a == null ? b == null : a.equals(b) );
  }

  /**
   * The interface method of the arbitration determines the most likely
   * derivation to chose by chosing the first within the same namespace. 
   * The cwns is the only element from the environment used to determine
   * the likely candidate.
   *
   * @param dvlist is a set of candidates
   * @param environment is a map describing the environment.
   * @return the chosen candidate from the input set
   */
  public Derivation arbit( java.util.Collection dvlist, 
			   java.util.Map environment )
  {
    // sanity check
    if ( dvlist.size() == 0 ) return null;

    // linear search in list
    String cwns = (String) environment.get("cwns");
    Derivation result = null;
    for ( Iterator i = dvlist.iterator(); i.hasNext(); ) {
      Derivation dv = (Derivation) i.next();
      if ( matchWithNull( cwns, dv.getNamespace() ) ) {
	result = dv;
	break;
      }
    }
    
    // still not found? Use first element -- and complain!
    if ( result == null ) {
      Logging.instance().log( "route", 0, 
			      "Unable to match namespaces, using first element" );
      result = (Derivation) dvlist.iterator().next();
    }

    // done
    return result;
  }
}