/** 
 * Copyright 2005 Hiram Chirino
 * 
 * Licensed under the Apache License, Version 2.0 (the "License"); 
 * you may not use this file except in compliance with the License. 
 * You may obtain a copy of the License at 
 * 
 * http://www.apache.org/licenses/LICENSE-2.0
 * 
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS, 
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied. 
 * See the License for the specific language governing permissions and 
 * limitations under the License. 
 * 
 **/
package org.activemq.filter;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;

import javax.jms.JMSException;
import javax.jms.Message;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/**
 * Used to evaluate an XPath Expression in a JMS selector.
 */
public final class XPathExpression implements BooleanExpression {

    private static final Log log = LogFactory.getLog(XPathExpression.class);
    private static final String EVALUATOR_SYSTEM_PROPERTY = "org.activemq.XPathEvaluatorClassName";
    private static final String DEFAULT_EVALUATOR_CLASS_NAME=XalanXPathEvaluator.class.getName();
    
    private static final String EVALUATOR_CLASS_NAME;
    private static final Constructor EVALUATOR_CONSTRUCTOR;
    
    static {
        String cn = System.getProperty(EVALUATOR_SYSTEM_PROPERTY, DEFAULT_EVALUATOR_CLASS_NAME);        
        Constructor m = null;
        try {
            try {
                m = getXPathEvaluatorConstructor(cn);
            } catch (Throwable e) {
                log.warn("Invalid "+XPathEvaluator.class.getName()+" implementation: "+cn+", reason: "+e,e);
                cn = DEFAULT_EVALUATOR_CLASS_NAME;
                try {
                    m = getXPathEvaluatorConstructor(cn);
                } catch (Throwable e2) {
                    log.error("Default XPath evaluator could not be loaded",e);
                }
            }
        } finally {
            EVALUATOR_CLASS_NAME = cn;
            EVALUATOR_CONSTRUCTOR = m;
        }
    }

    private static Constructor getXPathEvaluatorConstructor(String cn) throws ClassNotFoundException, SecurityException, NoSuchMethodException {
        Class c = XPathExpression.class.getClassLoader().loadClass(cn);
        if( !XPathEvaluator.class.isAssignableFrom(c) ) {
            throw new ClassCastException(""+c+" is not an instance of "+XPathEvaluator.class);
        }
        return c.getConstructor(new Class[]{String.class});
    }
    
    private final String xpath;
    private final XPathEvaluator evaluator;
    
    static public interface XPathEvaluator {
        public boolean evaluate(Message message) throws JMSException;
    }    
    
    XPathExpression(String xpath) {
        this.xpath = xpath;
        this.evaluator = createEvaluator(xpath);
    }

    private XPathEvaluator createEvaluator(String xpath2) {
        try {
            return (XPathEvaluator)EVALUATOR_CONSTRUCTOR.newInstance(new Object[]{xpath});
        } catch (InvocationTargetException e) {
            Throwable cause = e.getCause();
            if( cause instanceof RuntimeException ) {
                throw (RuntimeException)cause;
            }
            throw new RuntimeException("Invalid XPath Expression: "+xpath+" reason: "+e.getMessage(), e);
        } catch (Throwable e) {
            throw new RuntimeException("Invalid XPath Expression: "+xpath+" reason: "+e.getMessage(), e);
        }
    }

    public Object evaluate(Message message) throws JMSException {
        return evaluator.evaluate(message) ? Boolean.TRUE : Boolean.FALSE;
    }

    public String toString() {
        return "XPATH "+ConstantExpression.encodeString(xpath);
    }
}