src/main/java/javax/xml/parsers/FactoryFinder.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.
 */

// $Id: FactoryFinder.java 670431 2008-06-23 01:40:03Z mrglavas $

package javax.xml.parsers;

import java.io.BufferedReader;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.util.Properties;

/**
 * This class is duplicated for each JAXP subpackage so keep it in
 * sync.  It is package private.
 *
 * This code is designed to implement the JAXP 1.1 spec pluggability
 * feature and is designed to run on JDK version 1.1 and later including
 * JVMs that perform early linking like the Microsoft JVM in IE 5.  Note
 * however that it must be compiled on a JDK version 1.2 or later system
 * since it calls Thread#getContextClassLoader().  The code also runs both
 * as part of an unbundled jar file and when bundled as part of the JDK.
 */
final class FactoryFinder {
    
    /**
     * <p>Debug flag to trace loading process.</p>
     */
    private static boolean debug = false;

    // Define system property "jaxp.debug" to get output
    static {
        // Use try/catch block to support applets, which throws
        // SecurityException out of this code.
        try {
            String val = System.getProperty("jaxp.debug");
            // Allow simply setting the prop to turn on debug
            debug = val != null && (! "false".equals(val));
        } catch (SecurityException se) {
            debug = false;
        }
    }
    
    private FactoryFinder() {}

    private static void dPrint(String msg) {
        if (debug) {
            System.err.println("JAXP: " + msg);
        }
    }
    
    /**
     * Create an instance of a class using the specified ClassLoader and
     * optionally fall back to the current ClassLoader if not found.
     *
     * @param className Name of the concrete class corresponding to the
     * service provider
     *
     * @param cl ClassLoader to use to load the class, null means to use
     * the bootstrap ClassLoader
     *
     * @param doFallback true if the current ClassLoader should be tried as
     * a fallback if the class is not found using cl
     */
    static Object newInstance(String className, ClassLoader cl,
                                      boolean doFallback)
        throws ConfigurationError
    {
        // assert(className != null);

        try {
            Class providerClass;
            if (cl == null) {
                // If classloader is null Use the bootstrap ClassLoader.  
                // Thus Class.forName(String) will use the current
                // ClassLoader which will be the bootstrap ClassLoader.
                providerClass = Class.forName(className);
            } else {
                try {
                    providerClass = cl.loadClass(className);
                } catch (ClassNotFoundException x) {
                    if (doFallback) {
                        // Fall back to current classloader
                        cl = FactoryFinder.class.getClassLoader();
                        if (cl != null) {
                            providerClass = cl.loadClass(className);
                        }
                        else {
                            providerClass = Class.forName(className);
                        }
                    } else {
                        throw x;
                    }
                }
            }
                        
            Object instance = providerClass.newInstance();
            if (debug) dPrint("created new instance of " + providerClass +
                   " using ClassLoader: " + cl);
            return instance;
        } catch (ClassNotFoundException x) {
            throw new ConfigurationError(
                "Provider " + className + " not found", x);
        } catch (Exception x) {
            throw new ConfigurationError(
                "Provider " + className + " could not be instantiated: " + x,
                x);
        }
    }
    
    /**
     * Finds the implementation Class object in the specified order.  Main
     * entry point.
     * @return Class object of factory, never null
     *
     * @param factoryId             Name of the factory to find, same as
     *                              a property name
     * @param fallbackClassName     Implementation class name, if nothing else
     *                              is found.  Use null to mean no fallback.
     *
     * Package private so this code can be shared.
     */
    static Object find(String factoryId, String fallbackClassName)
        throws ConfigurationError
    {        

        // Figure out which ClassLoader to use for loading the provider
        // class.  If there is a Context ClassLoader then use it.
        
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        
        if (classLoader == null) {
            // if we have no Context ClassLoader
            // so use the current ClassLoader
            classLoader = FactoryFinder.class.getClassLoader();
        }

        if (debug) dPrint("loaded from fallback value: " + fallbackClassName);
        return newInstance(fallbackClassName, classLoader, true);
    }

    static class ConfigurationError extends Error {
        private Exception exception;

        /**
         * Construct a new instance with the specified detail string and
         * exception.
         */
        ConfigurationError(String msg, Exception x) {
            super(msg);
            this.exception = x;
        }

        Exception getException() {
            return exception;
        }
    }

}