src/java.xml/share/classes/javax/xml/xpath/XPathFactoryFinder.java

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 * DO NOT ALTER OR REMOVE COPYRIGHT NOTICES OR THIS FILE HEADER.
 *
 * This code is free software; you can redistribute it and/or modify it
 * under the terms of the GNU General Public License version 2 only, as
 * published by the Free Software Foundation.  Oracle designates this
 * particular file as subject to the "Classpath" exception as provided
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 * This code is distributed 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.  See the GNU General Public License
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 * accompanied this code).
 *
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package javax.xml.xpath;

import com.sun.org.apache.xpath.internal.jaxp.XPathFactoryImpl;
import java.lang.reflect.InvocationTargetException;
import java.util.Properties;
import java.util.ServiceConfigurationError;
import java.util.ServiceLoader;
import java.util.function.Supplier;
import jdk.xml.internal.SecuritySupport;

/**
 * Implementation of {@link XPathFactory#newInstance(String)}.
 *
 * @author Kohsuke Kawaguchi
 * @since 1.5
 */
class XPathFactoryFinder  {
    private static final String DEFAULT_PACKAGE = "com.sun.org.apache.xpath.internal";

    /** debug support code. */
    private static boolean debug = false;
    static {
        debug = System.getProperty("jaxp.debug") != null;
    }

    /**
     * <p>Cache properties for performance.</p>
     */
    private static final Properties cacheProps = new Properties();

    /**
     * <p>First time requires initialization overhead.</p>
     */
    private volatile static boolean firstTime = true;

    /**
     * <p>Conditional debug printing.</p>
     *
     * @param msgGen Supplier function that returns debug message
     */
    private static void debugPrintln(Supplier<String> msgGen) {
        if (debug) {
            System.err.println("JAXP: " + msgGen.get());
        }
    }

    /**
     * <p><code>ClassLoader</code> to use to find <code>XPathFactory</code>.</p>
     */
    private final ClassLoader classLoader;

    /**
     * <p>Constructor that specifies <code>ClassLoader</code> to use
     * to find <code>XPathFactory</code>.</p>
     *
     * @param loader
     *      to be used to load resource and {@link XPathFactory}
     *      implementations during the resolution process.
     *      If this parameter is null, the default system class loader
     *      will be used.
     */
    public XPathFactoryFinder(ClassLoader loader) {
        this.classLoader = loader;
        if( debug ) {
            debugDisplayClassLoader();
        }
    }

    private void debugDisplayClassLoader() {
        try {
            if( classLoader == SecuritySupport.getContextClassLoader() ) {
                debugPrintln(() -> "using thread context class loader ("+classLoader+") for search");
                return;
            }
        } catch( Throwable unused ) {
             // getContextClassLoader() undefined in JDK1.1
        }

        if( classLoader==ClassLoader.getSystemClassLoader() ) {
            debugPrintln(() -> "using system class loader ("+classLoader+") for search");
            return;
        }

        debugPrintln(() -> "using class loader ("+classLoader+") for search");
    }

    /**
     * <p>Creates a new {@link XPathFactory} object for the specified
     * object model.</p>
     *
     * @param uri
     *       Identifies the underlying object model.
     *
     * @return <code>null</code> if the callee fails to create one.
     *
     * @throws NullPointerException
     *      If the parameter is null.
     */
    public XPathFactory newFactory(String uri) throws XPathFactoryConfigurationException {
        if (uri == null) {
            throw new NullPointerException();
        }
        XPathFactory f = _newFactory(uri);
        if (f != null) {
            debugPrintln(()->"factory '" + f.getClass().getName() + "' was found for " + uri);
        } else {
            debugPrintln(()->"unable to find a factory for " + uri);
        }
        return f;
    }

    /**
     * <p>Lookup a {@link XPathFactory} for the given object model.</p>
     *
     * @param uri identifies the object model.
     *
     * @return {@link XPathFactory} for the given object model.
     */
    private XPathFactory _newFactory(String uri) throws XPathFactoryConfigurationException {
        XPathFactory xpathFactory = null;

        String propertyName = SERVICE_CLASS.getName() + ":" + uri;

        // system property look up
        try {
            debugPrintln(()->"Looking up system property '"+propertyName+"'" );
            String r = System.getProperty(propertyName);
            if(r!=null) {
                debugPrintln(()->"The value is '"+r+"'");
                xpathFactory = createInstance(r);
                if (xpathFactory != null) {
                    return xpathFactory;
                }
            } else
                debugPrintln(()->"The property is undefined.");
        } catch( Throwable t ) {
            if( debug ) {
                debugPrintln(()->"failed to look up system property '"+propertyName+"'" );
                t.printStackTrace();
            }
        }

        String factoryClassName = SecuritySupport.readConfig(propertyName);
        if (factoryClassName != null) {
            xpathFactory = createInstance(factoryClassName);
            if(xpathFactory != null){
                return xpathFactory;
            }
        }

        // Try with ServiceLoader
        assert xpathFactory == null;
        xpathFactory = findServiceProvider(uri);

        // The following assertion should always be true.
        // Uncomment it, recompile, and run with -ea in case of doubts:
        // assert xpathFactory == null || xpathFactory.isObjectModelSupported(uri);

        if (xpathFactory != null) {
            return xpathFactory;
        }

        // platform default
        if(uri.equals(XPathFactory.DEFAULT_OBJECT_MODEL_URI)) {
            debugPrintln(()->"attempting to use the platform default W3C DOM XPath lib");
            return new XPathFactoryImpl();
        }

        debugPrintln(()->"all things were tried, but none was found. bailing out.");
        return null;
    }

    /** <p>Create class using appropriate ClassLoader.</p>
     *
     * @param className Name of class to create.
     * @return Created class or <code>null</code>.
     */
    private Class<?> createClass(String className) {
        Class<?> clazz;

        // use appropriate ClassLoader
        try {
            if (classLoader != null) {
                    clazz = Class.forName(className, false, classLoader);
            } else {
                    clazz = Class.forName(className);
            }
        } catch (Throwable t) {
            if(debug) {
                t.printStackTrace();
            }
            return null;
        }

        return clazz;
    }

    /**
     * <p>Creates an instance of the specified and returns it.</p>
     *
     * @param className
     *      fully qualified class name to be instantiated.
     *
     * @return null
     *      if it fails. Error messages will be printed by this method.
     */
    XPathFactory createInstance(String className)
            throws XPathFactoryConfigurationException
    {
        XPathFactory xPathFactory = null;

        debugPrintln(()->"createInstance(" + className + ")");

        // get Class from className
        Class<?> clazz = createClass(className);
        if (clazz == null) {
            debugPrintln(()->"failed to getClass(" + className + ")");
            return null;
        }
        debugPrintln(()->"loaded " + className + " from " + which(clazz));

        // instantiate Class as a XPathFactory
        try {
            xPathFactory = (XPathFactory) clazz.getConstructor().newInstance();
        } catch (ClassCastException | IllegalAccessException | IllegalArgumentException |
            InstantiationException | InvocationTargetException | NoSuchMethodException ex) {
            debugPrintln(()->"could not instantiate " + clazz.getName());
            if (debug) {
                    ex.printStackTrace();
            }
            return null;
        }

        return xPathFactory;
    }

    /**
     * Finds a service provider subclass of XPathFactory that supports the
     * given object model using the ServiceLoader.
     *
     * @param objectModel URI of object model to support.
     * @return An XPathFactory supporting the specified object model, or null
     *         if none is found.
     * @throws XPathFactoryConfigurationException if a configuration error is found.
     */
    private XPathFactory findServiceProvider(final String objectModel)
            throws XPathFactoryConfigurationException {

        assert objectModel != null;
        try {
            final ServiceLoader<XPathFactory> loader =
                    ServiceLoader.load(SERVICE_CLASS);
            for (XPathFactory factory : loader) {
                // factory.isObjectModelSupportedBy
                if (factory.isObjectModelSupported(objectModel)) {
                    return factory;
                }
            }
            return null; // no factory found.
        } catch (ServiceConfigurationError error) {
            throw new XPathFactoryConfigurationException(error);
        }
    }

    private static final Class<XPathFactory> SERVICE_CLASS = XPathFactory.class;

    // Used for debugging purposes
    private static String which( Class<?> clazz ) {
        return SecuritySupport.getClassSource(clazz);
    }

}