src/main/java/org/apache/commons/jxpath/JXPathContextFactory.java

/*
 * Licensed to the Apache Software Foundation (ASF) under one or more
 * contributor license agreements.  See the NOTICE file distributed with
 * this work for additional information regarding copyright ownership.
 * The ASF licenses this file to You 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.apache.commons.jxpath;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.Properties;
import org.apache.commons.jxpath.util.ClassLoaderUtil;

/**
 * Defines a factory API that enables applications to obtain a
 * {@link JXPathContext} instance.  To acquire a JXPathContext, first call the
 * static {@link #newInstance} method of JXPathContextFactory.
 * This method returns a concrete JXPathContextFactory.
 * Then call {@link #newContext} on that instance.  You will rarely
 * need to perform these steps explicitly: usually you can call one of the
 * <code>JXPathContex.newContext</code> methods, which will perform these steps
 * for you.
 *
 * @see JXPathContext#newContext(Object)
 * @see JXPathContext#newContext(JXPathContext,Object)
 *
 * @author Dmitri Plotnikov
 * @version $Revision$ $Date$
 */
public abstract class JXPathContextFactory {

    /** The default property */
    public static final String FACTORY_NAME_PROPERTY =
        "org.apache.commons.jxpath.JXPathContextFactory";

    /** The default factory class */
    private static final String DEFAULT_FACTORY_CLASS =
        "org.apache.commons.jxpath.ri.JXPathContextFactoryReferenceImpl";

    /** Avoid reading all the files when the findFactory
        method is called the second time ( cache the result of
        finding the default impl )
    */
    private static final String FACTORY_IMPL_NAME = findFactory( FACTORY_NAME_PROPERTY, DEFAULT_FACTORY_CLASS );

    /**
     * Create a new JXPathContextFactory.
     */
    protected JXPathContextFactory () {

    }

    /**
     * Obtain a new instance of a <code>JXPathContextFactory</code>.
     * This static method creates a new factory instance.
     * This method uses the following ordered lookup procedure to determine
     * the <code>JXPathContextFactory</code> implementation class to load:
     * <ul>
     * <li>
     * Use  the <code>org.apache.commons.jxpath.JXPathContextFactory</code>
     * system property.
     * </li>
     * <li>
     * Alternatively, use the JAVA_HOME (the parent directory where jdk is
     * installed)/lib/jxpath.properties for a property file that contains the
     * name of the implementation class keyed on
     * <code>org.apache.commons.jxpath.JXPathContextFactory</code>.
     * </li>
     * <li>
     * Use the Services API (as detailed in the JAR specification), if
     * available, to determine the classname. The Services API will look
     * for a classname in the file
     * <code>META- INF/services/<i>org.apache.commons.jxpath.
     * JXPathContextFactory</i></code> in jars available to the runtime.
     * </li>
     * <li>
     * Platform default <code>JXPathContextFactory</code> instance.
     * </li>
     * </ul>
     *
     * Once an application has obtained a reference to a
     * <code>JXPathContextFactory</code> it can use the factory to
     * obtain JXPathContext instances.
     *
     * @return JXPathContextFactory
     * @throws JXPathContextFactoryConfigurationError if the implementation
     *            is not available or cannot be instantiated.
     */
    public static JXPathContextFactory newInstance() {
        JXPathContextFactory factoryImpl;
        try {
            Class clazz = ClassLoaderUtil.getClass( FACTORY_IMPL_NAME, true );
            factoryImpl = (JXPathContextFactory) clazz.newInstance();
        }
        catch (ClassNotFoundException cnfe) {
            throw new JXPathContextFactoryConfigurationError(cnfe);
        }
        catch (IllegalAccessException iae) {
            throw new JXPathContextFactoryConfigurationError(iae);
        }
        catch (InstantiationException ie) {
            throw new JXPathContextFactoryConfigurationError(ie);
        }
        return factoryImpl;
    }

    /**
     * Creates a new instance of a JXPathContext using the
     * currently configured parameters.
     * @param parentContext parent context
     * @param contextBean Object bean
     * @return JXPathContext
     * @throws JXPathContextFactoryConfigurationError if a JXPathContext
     *            cannot be created which satisfies the configuration requested
     */

    public abstract JXPathContext newContext(
        JXPathContext parentContext,
        Object contextBean);

    // -------------------- private methods --------------------
    // This code is duplicated in all factories.
    // Keep it in sync or move it to a common place
    // Because it's small probably it's easier to keep it here

    /** Temp debug code - this will be removed after we test everything
     */
    private static boolean debug = false;
    static {
        try {
            debug = System.getProperty("jxpath.debug") != null;
        }
        catch (SecurityException se) { //NOPMD
            // This is ok
        }
    }

    /**
     * Private implementation method - will find the implementation
     * class in the specified order.
     * @param property    Property name
     * @param defaultFactory Default implementation, if nothing else is found
     *
     * @return class name of the JXPathContextFactory
     */
    private static String findFactory(String property, String defaultFactory) {
        // Use the factory ID system property first
        try {
            String systemProp = System.getProperty(property);
            if (systemProp != null) {
                if (debug) {
                    System.err.println(
                        "JXPath: found system property" + systemProp);
                }
                return systemProp;
            }

        }
        catch (SecurityException se) { //NOPMD
            // Ignore
       }

        // try to read from $java.home/lib/xml.properties
        try {
            String javah = System.getProperty("java.home");
            String configFile =
                javah
                    + File.separator
                    + "lib"
                    + File.separator
                    + "jxpath.properties";
            File f = new File(configFile);
            if (f.exists()) {
                Properties props = new Properties();
                FileInputStream fis = new FileInputStream(f);
                try {
                    props.load(fis);
                }
                finally {
                    if (fis != null) {
                        try {
                            fis.close();
                        }
                        catch (IOException e) { //NOPMD
                            //swallow
                        }
                    }
                }
                String factory = props.getProperty(property);
                if (factory != null) {
                    if (debug) {
                        System.err.println(
                            "JXPath: found java.home property " + factory);
                    }
                    return factory;
                }
            }
        }
        catch (IOException ex) {
            if (debug) {
                ex.printStackTrace();
            }
        }

        String serviceId = "META-INF/services/" + property;
        // try to find services in CLASSPATH
        try {
            ClassLoader cl = JXPathContextFactory.class.getClassLoader();
            InputStream is;
            if (cl == null) {
                is = ClassLoader.getSystemResourceAsStream(serviceId);
            }
            else {
                is = cl.getResourceAsStream(serviceId);
            }

            if (is != null) {
                if (debug) {
                    System.err.println("JXPath: found  " + serviceId);
                }
                BufferedReader rd = new BufferedReader(new InputStreamReader(is, "UTF-8"));

                String factory = null;
                try {
                    factory = rd.readLine();
                }
                finally {
                    try {
                        rd.close();
                    }
                    catch (IOException e) { //NOPMD
                        //swallow
                    }
                }

                if (factory != null && !"".equals(factory)) {
                    if (debug) {
                        System.err.println(
                            "JXPath: loaded from services: " + factory);
                    }
                    return factory;
                }
            }
        }
        catch (Exception ex) {
            if (debug) {
                ex.printStackTrace();
            }
        }
        return defaultFactory;
    }
}