xml/datatype/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 670432 2008-06-23 02:02:08Z mrglavas $

package javax.xml.datatype;

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.net.URL;
import java.util.Properties;
import libcore.io.IoUtils;

/**
 * <p>Implement pluggable data types.</p>
 *
 * <p>This class is duplicated for each JAXP subpackage so keep it in
 * sync.  It is package private for secure class loading.</p>
 *
 * @author <a href="mailto:Jeff.Suttor (at) Sun.com">Jeff Suttor</a>
 * @version $Revision: 670432 $, $Date: 2008-06-22 19:02:08 -0700 (Sun, 22 Jun 2008) $
 * @since 1.5
 */
final class FactoryFinder {

    /** <p>Name of class to display in output messages.</p> */
    private static final String CLASS_NAME = "javax.xml.datatype.FactoryFinder";

    /** <p>Debug flag to trace loading process.</p> */
    private static boolean debug = false;

    /**
     * <p>Cache properties for performance. Use a static class to avoid double-checked
     * locking.</p>
     */
    private static class CacheHolder {

        private static Properties cacheProps = new Properties();

        static {
            String javah = System.getProperty("java.home");
            String configFile = javah + File.separator + "lib" + File.separator + "jaxp.properties";
            File f = new File(configFile);
            if (f.exists()) {
                if (debug) debugPrintln("Read properties file " + f);
                try (FileInputStream inputStream = new FileInputStream(f)) {
                    cacheProps.load(inputStream);
                } catch (Exception ex) {
                    if (debug) {
                        ex.printStackTrace();
                    }
                }
            }
        }
    }

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

    /**
     * <p>Check to see if debugging enabled by property.</p>
     *
     * <p>Use try/catch block to support applets, which throws
     * SecurityException out of this code.</p>
     */
    static {
        String val = System.getProperty("jaxp.debug");
        // Allow simply setting the prop to turn on debug
        debug = val != null && (! "false".equals(val));
    }

    private FactoryFinder() {}

    /**
     * <p>Output debugging messages.</p>
     *
     * @param msg <code>String</code> to print to <code>stderr</code>.
     */
    private static void debugPrintln(String msg) {
        if (debug) {
            System.err.println(
                CLASS_NAME
                + ":"
                + msg);
        }
    }

    /**
     * <p>Find the appropriate <code>ClassLoader</code> to use.</p>
     *
     * <p>The context ClassLoader is preferred.</p>
     *
     * @return <code>ClassLoader</code> to use.
     *
     * @throws ConfigurationError If a valid <code>ClassLoader</code> cannot be identified.
     */
    private static ClassLoader findClassLoader() 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 (debug) debugPrintln(
            "Using context class loader: "
            + classLoader);

        if (classLoader == null) {
            // if we have no Context ClassLoader
            // so use the current ClassLoader
            classLoader = FactoryFinder.class.getClassLoader();
            if (debug) debugPrintln(
                "Using the class loader of FactoryFinder: "
                + classLoader);
        }

        return classLoader;
    }

    /**
     * <p>Create an instance of a class using the specified ClassLoader.</p>
     *
     * @param className Name of class to create.
     * @param classLoader ClassLoader to use to create named class.
     *
     * @return New instance of specified class created using the specified ClassLoader.
     *
     * @throws ConfigurationError If class could not be created.
     */
    static Object newInstance(
        String className,
        ClassLoader classLoader)
        throws ConfigurationError {

        try {
            Class spiClass;
            if (classLoader == null) {
                spiClass = Class.forName(className);
            } else {
                spiClass = classLoader.loadClass(className);
            }

            if (debug) {
                debugPrintln("Loaded " + className + " from " + which(spiClass));
            }

            return spiClass.newInstance();
        } 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.
     * Package private so this code can be shared.
     *
     * @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.
     *
     * @return Class Object of factory, never null
     *
     * @throws ConfigurationError If Class cannot be found.
     */
    static Object find(String factoryId, String fallbackClassName) throws ConfigurationError {

        ClassLoader classLoader = findClassLoader();

        // Use the system property first
        String systemProp = System.getProperty(factoryId);
        if (systemProp != null && systemProp.length() > 0) {
            if (debug) debugPrintln("found " + systemProp + " in the system property " + factoryId);
            return newInstance(systemProp, classLoader);
        }

        // try to read from $java.home/lib/jaxp.properties
        try {
            String factoryClassName = CacheHolder.cacheProps.getProperty(factoryId);
            if (debug) debugPrintln("found " + factoryClassName + " in $java.home/jaxp.properties");

            if (factoryClassName != null) {
                return newInstance(factoryClassName, classLoader);
            }
        } catch (Exception ex) {
            if (debug) {
                ex.printStackTrace();
            }
        }

        // Try Jar Service Provider Mechanism
        Object provider = findJarServiceProvider(factoryId);
        if (provider != null) {
            return provider;
        }

        if (fallbackClassName == null) {
            throw new ConfigurationError(
                "Provider for " + factoryId + " cannot be found", null);
        }

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

    /*
     * 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 = Thread.currentThread().getContextClassLoader();
        if (cl != null) {
            is = cl.getResourceAsStream(serviceId);
        }

        if (is == null) {
            cl = FactoryFinder.class.getClassLoader();
            is = cl.getResourceAsStream(serviceId);
        }

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

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

        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 {
            IoUtils.closeQuietly(rd);
        }

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

            return newInstance(factoryClassName, cl);
        }

        // No provider found
        return null;
    }

    /**
     * <p>Configuration Error.</p>
     */
    static class ConfigurationError extends Error {

        private static final long serialVersionUID = -3644413026244211347L;

        /**
         * <p>Exception that caused the error.</p>
         */
        private Exception exception;

        /**
         * <p>Construct a new instance with the specified detail string and
         * exception.</p>
         *
         * @param msg Detail message for this error.
         * @param x Exception that caused the error.
         */
        ConfigurationError(String msg, Exception x) {
            super(msg);
            this.exception = x;
        }

        /**
         * <p>Get the Exception that caused the error.</p>
         *
         * @return Exception that caused the error.
         */
        Exception getException() {
            return exception;
        }
    }

    /**
     * Returns the location where the given Class is loaded from.
     */
    private static String which(Class clazz) {
        try {
            String classnameAsResource = clazz.getName().replace('.', '/') + ".class";

            ClassLoader loader = clazz.getClassLoader();

            URL it;

            if (loader != null) {
                it = loader.getResource(classnameAsResource);
            } else {
                it = ClassLoader.getSystemResource(classnameAsResource);
            }

            if (it != null) {
                return it.toString();
            }
        }
        // The VM ran out of memory or there was some other serious problem. Re-throw.
        catch (VirtualMachineError vme) {
            throw vme;
        }
        // ThreadDeath should always be re-thrown
        catch (ThreadDeath td) {
            throw td;
        }
        catch (Throwable t) {
            // work defensively.
            if (debug) {
                t.printStackTrace();
            }
        }
        return "unknown location";
    }
}