/*
 * 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.xerces.xinclude;

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;

/**
 * This class is duplicated for each JAXP subpackage so keep it in sync.
 * It is package private and therefore is not exposed as part of the JAXP
 * API.
 * <p>
 * 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, and to compile on JDK 1.2 and onward.
 * The code also runs both as part of an unbundled jar file and
 * when bundled as part of the JDK.
 * <p>
 *
 * @version $Id$
 */
final class ObjectFactory {

    //
    // Constants
    //

    // name of default properties file to look for in JDK's jre/lib directory
    private static final String DEFAULT_PROPERTIES_FILENAME = "xerces.properties";

    /** Set to true for debugging */
    private static final boolean DEBUG = isDebugEnabled();

    /**
     * Default columns per line.
     */
    private static final int DEFAULT_LINE_LENGTH = 80;

    /** cache the contents of the xerces.properties file.
     *  Until an attempt has been made to read this file, this will
     * be null; if the file does not exist or we encounter some other error
     * during the read, this will be empty.
     */
    private static Properties fXercesProperties = null;

    /***
     * Cache the time stamp of the xerces.properties file so
     * that we know if it's been modified and can invalidate
     * the cache when necessary.
     */
    private static long fLastModified = -1;

    //
    // static methods
    //

    /**
     * Finds the implementation Class object in the specified order.  The
     * specified order is the following:
     * <ol>
     *  <li>query the system property using <code>System.getProperty</code>
     *  <li>read <code>META-INF/services/<i>factoryId</i></code> file
     *  <li>use fallback classname
     * </ol>
     *
     * @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.
     *
     * @exception ObjectFactory.ConfigurationError
     */
    static Object createObject(String factoryId, String fallbackClassName)
        throws ConfigurationError {
        return createObject(factoryId, null, fallbackClassName);
    } // createObject(String,String):Object

    /**
     * Finds the implementation Class object in the specified order.  The
     * specified order is the following:
     * <ol>
     *  <li>query the system property using <code>System.getProperty</code>
     *  <li>read <code>$java.home/lib/<i>propertiesFilename</i></code> file
     *  <li>read <code>META-INF/services/<i>factoryId</i></code> file
     *  <li>use fallback classname
     * </ol>
     *
     * @return Class object of factory, never null
     *
     * @param factoryId             Name of the factory to find, same as
     *                              a property name
     * @param propertiesFilename The filename in the $java.home/lib directory
     *                           of the properties file.  If none specified,
     *                           ${java.home}/lib/xerces.properties will be used.
     * @param fallbackClassName     Implementation class name, if nothing else
     *                              is found.  Use null to mean no fallback.
     *
     * @exception ObjectFactory.ConfigurationError
     */
    static Object createObject(String factoryId,
                                      String propertiesFilename,
                                      String fallbackClassName)
        throws ConfigurationError
    {
        try {
            return Class.forName(fallbackClassName).newInstance();
        } catch (Exception e) {
            throw new ConfigurationError(e.getMessage(), e);
        }
    } // createObject(String,String,String):Object

    //
    // Private static methods
    //
    
    /** Returns true if debug has been enabled. */
    private static boolean isDebugEnabled() {
        try {
            String val = SecuritySupport.getSystemProperty("xerces.debug");
            // Allow simply setting the prop to turn on debug
            return (val != null && (!"false".equals(val)));
        } 
        catch (SecurityException se) {}
        return false;
    } // isDebugEnabled()

    /** Prints a message to standard error if debugging is enabled. */
    private static void debugPrintln(String msg) {
        if (DEBUG) {
            System.err.println("XERCES: " + msg);
        }
    } // debugPrintln(String)

    /**
     * Figure out which ClassLoader to use.  For JDK 1.2 and later use
     * the context ClassLoader.
     */
    static ClassLoader findClassLoader()
        throws ConfigurationError
    {
        return ObjectFactory.class.getClassLoader();
    } // findClassLoader():ClassLoader

    /**
     * Create an instance of a class using the specified ClassLoader
     */
    static Object newInstance(String className, ClassLoader cl,
                                      boolean doFallback)
        throws ConfigurationError
    {
        // assert(className != null);
        try{
            Class providerClass = findProviderClass(className, cl, doFallback);
            Object instance = providerClass.newInstance();
            if (DEBUG) debugPrintln("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);
        }
    }

    /**
     * Find a Class using the specified ClassLoader
     */
    static Class findProviderClass(String className, ClassLoader cl,
                                      boolean doFallback)
        throws ClassNotFoundException, ConfigurationError
    {
        return Class.forName(className);
    }

    /*
     * Try to find provider using Jar Service Provider Mechanism
     *
     * @return instance of provider class if found or null
     */
    private static Object findJarServiceProvider(String factoryId)
        throws ConfigurationError
    {
        String serviceId = "META-INF/services/" + factoryId;
        InputStream is = null;

        // First try the Context ClassLoader
        ClassLoader cl = findClassLoader();

        is = SecuritySupport.getResourceAsStream(cl, serviceId);

        // If no provider found then try the current ClassLoader
        if (is == null) {
            ClassLoader current = ObjectFactory.class.getClassLoader();
            if (cl != current) {
                cl = current;
                is = SecuritySupport.getResourceAsStream(cl, serviceId);
            }
        }

        if (is == null) {
            // No provider found
            return null;
        }

        if (DEBUG) debugPrintln("found jar resource=" + serviceId +
               " using ClassLoader: " + cl);

        // Read the service provider name in UTF-8 as specified in
        // the jar spec.  Unfortunately this fails in Microsoft
        // VJ++, which does not implement the UTF-8
        // encoding. Theoretically, we should simply let it fail in
        // that case, since the JVM is obviously broken if it
        // doesn't support such a basic standard.  But since there
        // are still some users attempting to use VJ++ for
        // development, we have dropped in a fallback which makes a
        // second attempt using the platform's default encoding. In
        // VJ++ this is apparently ASCII, which is a subset of
        // UTF-8... and since the strings we'll be reading here are
        // also primarily limited to the 7-bit ASCII range (at
        // least, in English versions), this should work well
        // enough to keep us on the air until we're ready to
        // officially decommit from VJ++. [Edited comment from
        // jkesselm]
        BufferedReader rd;
        try {
            rd = new BufferedReader(new InputStreamReader(is, "UTF-8"), DEFAULT_LINE_LENGTH);
        } catch (java.io.UnsupportedEncodingException e) {
            rd = new BufferedReader(new InputStreamReader(is), DEFAULT_LINE_LENGTH);
        }

        String factoryClassName = null;
        try {
            // XXX Does not handle all possible input as specified by the
            // Jar Service Provider specification
            factoryClassName = rd.readLine();
        } catch (IOException x) {
            // No provider found
            return null;
        }
        finally {
            try {
                // try to close the reader.
                rd.close();
            }
            // Ignore the exception.
            catch (IOException exc) {}
        }

        if (factoryClassName != null &&
            ! "".equals(factoryClassName)) {
            if (DEBUG) debugPrintln("found in resource, value="
                   + factoryClassName);

            // Note: here we do not want to fall back to the current
            // ClassLoader because we want to avoid the case where the
            // resource file was found using one ClassLoader and the
            // provider class was instantiated using a different one.
            return newInstance(factoryClassName, cl, false);
        }

        // No provider found
        return null;
    }

    //
    // Classes
    //

    /**
     * A configuration error.
     */
    static final class ConfigurationError
        extends Error {

        /** Serialization version. */
        static final long serialVersionUID = 5061904944269807898L;
        
        //
        // Data
        //

        /** Exception. */
        private Exception exception;

        //
        // Constructors
        //

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

        //
        // methods
        //

        /** Returns the exception associated to this error. */
        Exception getException() {
            return exception;
        } // getException():Exception

    } // class ConfigurationError

} // class ObjectFactory
