XPathFactory.java

package org.tigris.juxy.xpath;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.tigris.juxy.JuxyRuntimeException;
import org.tigris.juxy.util.ArgumentAssert;

/**
 * Factory for XPath expressions.
 *
 * @author Pavel Sher
 */
public class XPathFactory {
  private final static Log logger = LogFactory.getLog(XPathFactory.class);
  private static Boolean JAVAX_XPATH_NOT_AVAILABLE = null;
  private static Boolean JAXEN_XPATH_NOT_AVAILABLE = null;

  /**
   * Constructs new XPath expression.
   *
   * @param expression an XPath expression
   * @return newly created XPathExpr object
   */
  public static XPathExpr newXPath(String expression) {
    ArgumentAssert.notEmpty(expression, "XPath expression must not be empty");

    XPathExpr xpath = createJavaxXPath(expression);
    if (xpath != null) return xpath;

    xpath = createJaxenXPath(expression);
    if (xpath != null) return xpath;

    throw new JuxyRuntimeException("Unable to locate XPath implementation");
  }

  protected static XPathExpr createJavaxXPath(String expression) {
    if (Boolean.TRUE.equals(JAVAX_XPATH_NOT_AVAILABLE)) return null;

    try {
      Class.forName("javax.xml.xpath.XPathFactory");
      return new JavaxXPathExpr(expression);
    }
    catch (Throwable t) {
      logger.debug("Failed to obtain instance of the javax.xml.xpath.XPathFactory");
      JAVAX_XPATH_NOT_AVAILABLE = Boolean.TRUE;
    }

    return null;
  }

  protected static XPathExpr createJaxenXPath(String expression) {
    if (Boolean.TRUE.equals(JAXEN_XPATH_NOT_AVAILABLE)) return null;

    try {
      Class.forName("org.jaxen.dom.DOMXPath");
      return new JaxenXPathExpr(expression);
    }
    catch (Throwable t) {
      logger.debug("Failed to obtain instance of the org.jaxen.dom.DOMXPath");
      JAXEN_XPATH_NOT_AVAILABLE = Boolean.TRUE;
    }

    return null;
  }

  protected static void reset() {
    JAVAX_XPATH_NOT_AVAILABLE = null;
    JAXEN_XPATH_NOT_AVAILABLE = null;
  }
}