operationnode.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.
*
* Redistribution and use in source and binary forms, with or without 
* modification, are permitted provided that the following conditions are met:
*
* Redistributions of source code must retain the above copyright notice, this 
* 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 
* IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR A PARTICULAR PURPOSE 
* ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT OWNER OR CONTRIBUTORS BE 
* LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL, SPECIAL, EXEMPLARY, OR 
* CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT LIMITED TO, PROCUREMENT OF 
* SUBSTITUTE GOODS OR SERVICES; LOSS OF USE, DATA, OR PROFITS; OR BUSINESS 
* INTERRUPTION) HOWEVER CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN 
* CONTRACT, STRICT LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) 
* ARISING IN ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE 
* POSSIBILITY OF SUCH DAMAGE.
*/

package issrg.pba.rbac.xmlpolicy.ifstatement;

/**
 * This is an abstract class which is a base to other operations. The 
 * sub-classes would only
 * need to have a valid constructor, so the XMLParser will be able to 
 * instantiate a working
 * copy of it, and perhaps a static registration method to update the collection 
 * of interpreters.
 *
 * <p>This class maintains a registry of Interpreters for each type of 
 * operation. During evaluation time an Interpreter will be found in the 
 * registry for the
 * required type of operation and it will be used to perform the evaluation.
 * If no Interpreter can be found for the specific set of Terms, 
 * EvaluationException occurs.
 *
 * @see Interpreter
 */
public class OperationNode extends TermNode {
  protected String type;
  protected Interpreter interpreter;

  protected OperationNode() {}

  /**
   * This constructor builds an OperationNode given the name and the attributes
   * of the XML element, and the acceptable numbers of operands.
   *
   * @param nodeName - the name of the XML element this OperationNode represents
   * @param attrs - the attributes of this XML element
   * @param maxOperandCount - the maximum acceptable number of operands; if 
   *   "-1", any number of operands is acceptable
   * @param minOperandCount - the minimum acceptable number of operands
   */
  public OperationNode(String nodeName, org.xml.sax.Attributes attrs, int maxOperandCount, int minOperandCount){
    super(nodeName, attrs, maxOperandCount, minOperandCount);
  }

  /**
   * The method builds a usual TermNode and then checks if an interpreter can 
   * be found that
   * would agree to interpret the relevant terms (children nodes) and deliver a 
   * definite result.
   * If no such interpreter is found, a PolicyParsingException is thrown; 
   * otherwise, the
   * interpreter will be used by the evaluate method, and the result type 
   * reported by the canEvaluate
   * method of the interpreter will be returned by the getType method.
   */
  public void construct() throws issrg.pba.rbac.PolicyParsingException {
    super.construct();

    Interpreter[] ints = TermNode.getInterpretersForNode(getName());
    if (ints!=null){
      for (int i=0; i<ints.length; i++){
        if ((type=ints[i].canEvaluate(terms))!=null){ // can evaluate; terms has been set by the super.construct(); this automatically sets the type of the result, too
          interpreter=ints[i];
          break;
        }
      }
    }

    if (interpreter==null){
      throw new issrg.pba.rbac.PolicyParsingException("No interpreter for such combination of operands has been found for node "+getName());
    }
  }

  public String getType(){
    return type;    // evaluated during the call to construct()
  }

  /**
   * This method evaluates the expression given the environment. It uses the
   * Interpreter found during the call to construct();
   *
   * @param env - the Environment with the variables in it
   *
   * @return the Interpreter-specific object, guaranteed to be of type returned
   *   by getType()
   *
   * @throws EvaluationException, if there was a problem evaluating the 
   *   expression
   */
  public Object evaluate(Environment env) throws EvaluationException{
    try{
      return interpreter.evaluate(env, terms);
    }catch (EvaluationException ee){
      throw ee;
    }catch (Throwable th){
      throw new EvaluationException("Run-time error", th);
    }
  }
}