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/*

 * Copyright (c) 2004 World Wide Web Consortium,

 *

 * (Massachusetts Institute of Technology, European Research Consortium for

 * Informatics and Mathematics, Keio University). All Rights Reserved. This

 * work is distributed under the W3C(r) Software License [1] in the hope that

 * it will be useful, but WITHOUT ANY WARRANTY; without even the implied

 * warranty of MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

 *

 * [1] http://www.w3.org/Consortium/Legal/2002/copyright-software-20021231

 */

package org.w3c.dom.bootstrap;

import java.util.StringTokenizer;

import java.util.Vector;

import org.w3c.dom.DOMImplementationSource;

import org.w3c.dom.DOMImplementationList;

import org.w3c.dom.DOMImplementation;

import java.io.InputStream;

import java.io.BufferedReader;

import java.io.InputStreamReader;

import java.security.AccessController;

import java.security.PrivilegedAction;

/**

 * A factory that enables applications to obtain instances of

 * <code>DOMImplementation</code>.

 *

 * <p>

 * Example:

 * </p>

 *

 * <pre class='example'>

 *  // get an instance of the DOMImplementation registry

 *  DOMImplementationRegistry registry =

 *       DOMImplementationRegistry.newInstance();

 *  // get a DOM implementation the Level 3 XML module

 *  DOMImplementation domImpl =

 *       registry.getDOMImplementation("XML 3.0");

 * </pre>

 *

 * <p>

 * This provides an application with an implementation-independent starting

 * point. DOM implementations may modify this class to meet new security

 * standards or to provide *additional* fallbacks for the list of

 * DOMImplementationSources.

 * </p>

 *

 * @see DOMImplementation

 * @see DOMImplementationSource

 * @since DOM Level 3

 */

public final class DOMImplementationRegistry {

    /**

     * The system property to specify the

     * DOMImplementationSource class names.

     */

    public static final String PROPERTY =

        "org.w3c.dom.DOMImplementationSourceList";

    /**

     * Default columns per line.

     */

    private static final int DEFAULT_LINE_LENGTH = 80;

    /**

     * The list of DOMImplementationSources.

     */

    private Vector sources;

    /**

     * Private constructor.

     * @param srcs Vector List of DOMImplementationSources

     */

    private DOMImplementationRegistry(final Vector srcs) {

        sources = srcs;

    }

    /**

     * Obtain a new instance of a <code>DOMImplementationRegistry</code>.

     *

     * The <code>DOMImplementationRegistry</code> is initialized by the

     * application or the implementation, depending on the context, by

     * first checking the value of the Java system property

     * <code>org.w3c.dom.DOMImplementationSourceList</code> and

     * the the service provider whose contents are at

     * "<code>META_INF/services/org.w3c.dom.DOMImplementationSourceList</code>"

     * The value of this property is a white-space separated list of

     * names of availables classes implementing the

     * <code>DOMImplementationSource</code> interface. Each class listed

     * in the class name list is instantiated and any exceptions

     * encountered are thrown to the application.

     *

     * @return an initialized instance of DOMImplementationRegistry

     * @throws ClassNotFoundException

     *     If any specified class can not be found

     * @throws InstantiationException

     *     If any specified class is an interface or abstract class

     * @throws IllegalAccessException

     *     If the default constructor of a specified class is not accessible

     * @throws ClassCastException

     *     If any specified class does not implement

     * <code>DOMImplementationSource</code>

     */

    public static DOMImplementationRegistry newInstance()

        throws

        ClassNotFoundException,

        InstantiationException,

        IllegalAccessException,

        ClassCastException {

        Vector sources = new Vector();

        ClassLoader classLoader = getClassLoader();

        // fetch system property:

        String p = getSystemProperty(PROPERTY);

        //

        // if property is not specified then use contents of

        // META_INF/org.w3c.dom.DOMImplementationSourceList from classpath

        if (p == null) {

            p = getServiceValue(classLoader);

        }

        if (p == null) {

            //

            // DOM Implementations can modify here to add *additional* fallback

            // mechanisms to access a list of default DOMImplementationSources.

            p = "gnu.xml.dom.ImplementationSource";

        }

        if (p != null) {

            StringTokenizer st = new StringTokenizer(p);

            while (st.hasMoreTokens()) {

                String sourceName = st.nextToken();

                // Use context class loader, falling back to Class.forName

                // if and only if this fails...

                Class sourceClass = null;

                if (classLoader != null) {

                    sourceClass = classLoader.loadClass(sourceName);

                } else {

                    sourceClass = Class.forName(sourceName);

                }

                DOMImplementationSource source =

                    (DOMImplementationSource) sourceClass.newInstance();

                sources.addElement(source);

            }

        }

        return new DOMImplementationRegistry(sources);

    }

    /**

     * Return the first implementation that has the desired

     * features, or <code>null</code> if none is found.

     *

     * @param features

     *            A string that specifies which features are required. This is

     *            a space separated list in which each feature is specified by

     *            its name optionally followed by a space and a version number.

     *            This is something like: "XML 1.0 Traversal +Events 2.0"

     * @return An implementation that has the desired features,

     *         or <code>null</code> if none found.

     */

    public DOMImplementation getDOMImplementation(final String features) {

        int size = sources.size();

        String name = null;

        for (int i = 0; i < size; i++) {

            DOMImplementationSource source =

                (DOMImplementationSource) sources.elementAt(i);

            DOMImplementation impl = source.getDOMImplementation(features);

            if (impl != null) {

                return impl;

            }

        }

        return null;

    }

    /**

     * Return a list of implementations that support the

     * desired features.

     *

     * @param features

     *            A string that specifies which features are required. This is

     *            a space separated list in which each feature is specified by

     *            its name optionally followed by a space and a version number.

     *            This is something like: "XML 1.0 Traversal +Events 2.0"

     * @return A list of DOMImplementations that support the desired features.

     */

    public DOMImplementationList getDOMImplementationList(final String features) {

        final Vector implementations = new Vector();

        int size = sources.size();

        for (int i = 0; i < size; i++) {

            DOMImplementationSource source =

                (DOMImplementationSource) sources.elementAt(i);

            DOMImplementationList impls =

                source.getDOMImplementationList(features);

            for (int j = 0; j < impls.getLength(); j++) {

                DOMImplementation impl = impls.item(j);

                implementations.addElement(impl);

            }

        }

        return new DOMImplementationList() {

                public DOMImplementation item(final int index) {

                    if (index >= 0 && index < implementations.size()) {

                        try {

                            return (DOMImplementation)

                                implementations.elementAt(index);

                        } catch (ArrayIndexOutOfBoundsException e) {

                            return null;

                        }

                    }

                    return null;

                }

                public int getLength() {

                    return implementations.size();

                }

            };

    }

    /**

     * Register an implementation.

     *

     * @param s The source to be registered, may not be <code>null</code>

     */

    public void addSource(final DOMImplementationSource s) {

        if (s == null) {

            throw new NullPointerException();

        }

        if (!sources.contains(s)) {

            sources.addElement(s);

        }

    }

    /**

     *

     * Gets a class loader.

     *

     * @return A class loader, possibly <code>null</code>

     */

    private static ClassLoader getClassLoader() {

        try {

            ClassLoader contextClassLoader = getContextClassLoader();

            if (contextClassLoader != null) {

                return contextClassLoader;

            }

        } catch (Exception e) {

            // Assume that the DOM application is in a JRE 1.1, use the

            // current ClassLoader

            return DOMImplementationRegistry.class.getClassLoader();

        }

        return DOMImplementationRegistry.class.getClassLoader();

    }

    /**

     * This method attempts to return the first line of the resource

     * META_INF/services/org.w3c.dom.DOMImplementationSourceList

     * from the provided ClassLoader.

     *

     * @param classLoader classLoader, may not be <code>null</code>.

     * @return first line of resource, or <code>null</code>

     */

    private static String getServiceValue(final ClassLoader classLoader) {

        String serviceId = "META-INF/services/" + PROPERTY;

        // try to find services in CLASSPATH

        try {

            InputStream is = getResourceAsStream(classLoader, serviceId);

            if (is != null) {

                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 serviceValue = rd.readLine();

                rd.close();

                if (serviceValue != null && serviceValue.length() > 0) {

                    return serviceValue;

                }

            }

        } catch (Exception ex) {

            return null;

        }

        return null;

    }

    /**

     * A simple JRE (Java Runtime Environment) 1.1 test

     *

     * @return <code>true</code> if JRE 1.1

     */

    private static boolean isJRE11() {

        try {

            Class c = Class.forName("java.security.AccessController");

            // java.security.AccessController existed since 1.2 so, if no

            // exception was thrown, the DOM application is running in a JRE

            // 1.2 or higher

            return false;

        } catch (Exception ex) {

            // ignore

        }

        return true;

    }

    /**

     * This method returns the ContextClassLoader or <code>null</code> if

     * running in a JRE 1.1

     *

     * @return The Context Classloader

     */

    private static ClassLoader getContextClassLoader() {

        return isJRE11()

            ? null

            : (ClassLoader)

              AccessController.doPrivileged(new PrivilegedAction() {

                    public Object run() {

                        ClassLoader classLoader = null;

                        try {

                            classLoader =

                                Thread.currentThread().getContextClassLoader();

                        } catch (SecurityException ex) {

                        }

                        return classLoader;

                    }

                });

    }

    /**

     * This method returns the system property indicated by the specified name

     * after checking access control privileges. For a JRE 1.1, this check is

     * not done.

     *

     * @param name the name of the system property

     * @return the system property

     */

    private static String getSystemProperty(final String name) {

        return isJRE11()

            ? (String) System.getProperty(name)

            : (String) AccessController.doPrivileged(new PrivilegedAction() {

                    public Object run() {

                        return System.getProperty(name);

                    }

                });

    }

    /**

     * This method returns an Inputstream for the reading resource

     * META_INF/services/org.w3c.dom.DOMImplementationSourceList after checking

     * access control privileges. For a JRE 1.1, this check is not done.

     *

     * @param classLoader classLoader

     * @param name the resource

     * @return an Inputstream for the resource specified

     */

    private static InputStream getResourceAsStream(final ClassLoader classLoader,

                                                   final String name) {

        if (isJRE11()) {

            InputStream ris;

            if (classLoader == null) {

                ris = ClassLoader.getSystemResourceAsStream(name);

            } else {

                ris = classLoader.getResourceAsStream(name);

            }

            return ris;

        } else {

            return (InputStream)

                AccessController.doPrivileged(new PrivilegedAction() {

                        public Object run() {

                            InputStream ris;

                            if (classLoader == null) {

                                ris =

                                    ClassLoader.getSystemResourceAsStream(name);

                            } else {

                                ris = classLoader.getResourceAsStream(name);

                            }

                            return ris;

                        }

                    });

        }

    }

}