eqnode.java

一个完整的XACML工程,学习XACML技术的好例子!

/*
* Copyright (c) 2000-2005, University of Salford
* All rights reserved.
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*
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* list of conditions and the following disclaimer.
* 
* Redistributions in binary form must reproduce the above copyright notice, 
* this list of conditions and the following disclaimer in the documentation 
* and/or other materials provided with the distribution. 
*
* Neither the name of the University of Salford nor the names of its 
* contributors may be used to endorse or promote products derived from this 
* software without specific prior written permission. 
*
* THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS "AS IS" 
* AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT LIMITED TO, THE 
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package issrg.pba.rbac.xmlpolicy.ifstatement;

/**
 * This is the class for EQ node of the IF-statement. Its functionality is
 * very much determined by OperationNode and EqInterpreter, which are 
 * configured to deliver the following semantics:
 *
 * <p>This operation expects two and only two Terms of the same type, and 
 * returns a 
 * value of type Types.BOOLEAN_TYPE.
 * The evaluation result is "true", if and only if calling equals() method on
 * the first term returns boolean "true".
 *
 * <p>It is possible to register equality interpreters for specific 
 * combinations of Terms, which will be used in preference to the default 
 * EqInterpreter. Use registerInterpreter method to do that.
 *
 * @see #registerInterpreter
 *
 * @author A.Otenko
 */

public class EqNode extends OperationNode {
  public static final String EQ_NODE = "EQ";

  /**
   * Call this method to register the EqNode with the XMLPolicyParser. It also
   * registers the default EqInterpreter.
   */
  public static void register(){
    try{
      issrg.pba.rbac.xmlpolicy.XMLPolicyParser.registerXMLNode(EQ_NODE, EqNode.class);
    }catch (NoSuchMethodException nsme){
      nsme.printStackTrace();
    }

    registerInterpreter(new EqInterpreter());
  }

  /**
   * This method maintains a register of interpreters for specific combinations
   * of Terms. If no special interpreters are registered, the default one is 
   * used, as described above.
   *
   * @param i - the Interpreter instance that can evaluate equality expression
   *   for some combination of Terms
   */
  public static void registerInterpreter(Interpreter i){
    OperationNode.registerInterpreterForNode(EQ_NODE, i);
  }

  protected EqNode() {
  }

  /**
   * This constructor builds an EqNode given the XMLPolicyParser and the set of
   * attributes of this XML element. It expects that there are two and only two
   * child nodes.
   *
   * @param p - the XMLPolicyParser that builds this EqNode
   * @param attr - the attributes of this XML element
   */
  public EqNode(issrg.pba.rbac.xmlpolicy.XMLPolicyParser p, org.xml.sax.Attributes attrs){
    super(EQ_NODE, attrs, 2, 2);
  }
}


/**
 * This is a default equality interpreter. It can interpret any two terms of 
 * the same type
 * and will return the equality match as the corresponding Java object equals() 
 * method determines; that is, leftHandSideTerm.equals(rightHandSideTerm)
 *
 * @author A.Otenko
 */
class EqInterpreter implements Interpreter {
  public EqInterpreter(){}

  /**
   * This method returns null only if the types of the terms are different or 
   * there is more than
   * two terms.
   */
  public String canEvaluate(Term [] t){
    if (t.length==2){
      if (t[0].getType().intern()==t[1].getType().intern()){
        return Types.BOOLEAN_TYPE;
      }
    }

    return null;
  }

  public Object evaluate(Environment env, Term [] t) throws EvaluationException{
    if (canEvaluate(t)==null){
      throw new EvaluationException("Cannot evaluate expression");
    }

    return new Boolean(t[0].evaluate(env).equals(t[1].evaluate(env)));
  }
}