axiom-api/src/main/java/org/apache/axiom/locator/ImplementationFactory.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.
 */
package org.apache.axiom.locator;

import java.io.IOException;
import java.io.InputStream;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;

import javax.xml.namespace.QName;
import javax.xml.parsers.DocumentBuilderFactory;
import javax.xml.parsers.ParserConfigurationException;

import org.apache.axiom.locator.loader.OMMetaFactoryLoader;
import org.apache.axiom.om.OMAbstractFactory;
import org.apache.axiom.om.OMMetaFactory;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;
import org.w3c.dom.Element;
import org.w3c.dom.Node;
import org.xml.sax.SAXException;

final class ImplementationFactory {
    static final String DESCRIPTOR_RESOURCE = "META-INF/axiom.xml";
    
    private static final String NS = "http://ws.apache.org/axiom/";
    private static final QName QNAME_IMPLEMENTATIONS = new QName(NS, "implementations");
    private static final QName QNAME_IMPLEMENTATION = new QName(NS, "implementation");
    private static final QName QNAME_FEATURE = new QName(NS, "feature");
    private static final QName QNAME_EXTENSION_INTERFACE = new QName(NS, "extensionInterface");
    
    private static final Log log = LogFactory.getLog(ImplementationFactory.class);
    
    private ImplementationFactory() {}
    
    static Implementation createDefaultImplementation(Loader loader, String className) {
        if (log.isDebugEnabled()) {
            log.debug("Creating default implementation for class " + className);
        }
        OMMetaFactory metaFactory = (OMMetaFactory)load(loader, className);
        return metaFactory == null ? null : new Implementation(null, metaFactory,
                new Feature[] { new Feature(OMAbstractFactory.FEATURE_DEFAULT, Integer.MAX_VALUE, new Class<?>[0]) });
    }
    
    private static Object load(Loader loader, String className) {
        Class<?> clazz;
        try {
            clazz = loader.load(className);
        } catch (ClassNotFoundException ex) {
            log.error("The class " + className + " could not be loaded", ex);
            return null;
        }
        try {
            return clazz.getDeclaredConstructor().newInstance();
        } catch (Exception ex) {
            log.error("The class " + className + " could not be instantiated", ex);
            return null;
        }
    }
    
    static List<Implementation> parseDescriptor(Loader loader, URL url) {
        if (log.isDebugEnabled()) {
            log.debug("Loading " + url);
        }
        List<Implementation> implementations = new ArrayList<Implementation>();
        try {
            // Since this code is used to discover Axiom implementations, we have to use DOM here.
            DocumentBuilderFactory dbf = DocumentBuilderFactory.newInstance();
            dbf.setNamespaceAware(true);
            // Use URL#openStream() (instead of converting the URL to a String and passing it to the
            // parser) to avoid situations where the parser can't reconstruct/interpret the URL.
            InputStream in = url.openStream();
            try {
                Element root = dbf.newDocumentBuilder().parse(in).getDocumentElement();
                QName rootQName = getQName(root);
                if (rootQName.equals(QNAME_IMPLEMENTATIONS)) {
                    Node child = root.getFirstChild();
                    while (child != null) {
                        if (child instanceof Element) {
                            QName childQName = getQName(child);
                            if (childQName.equals(QNAME_IMPLEMENTATION)) {
                                Implementation implementation = parseImplementation(loader, (Element)child);
                                if (implementation != null) {
                                    implementations.add(implementation);
                                }
                            } else {
                                log.warn("Skipping unexpected element " + childQName + "; only "
                                        + QNAME_IMPLEMENTATION + " is expected");
                            }
                        }
                        child = child.getNextSibling();
                    }
                } else {
                    log.error(url + " is not a valid implementation descriptor: unexpected root element "
                            + rootQName + "; expected " + QNAME_IMPLEMENTATIONS);
                }
            } finally {
                in.close();
            }
        } catch (ParserConfigurationException ex) {
            // If we get here, something went badly wrong
            throw new Error(ex);
        } catch (IOException ex) {
            log.error("Unable to read " + url, ex);
        } catch (SAXException ex) {
            log.error("Parser error while reading " + url, ex);
        }
        if (log.isDebugEnabled()) {
            log.debug("Discovered implementations: " + implementations);
        }
        return implementations;
    }
    
    private static Implementation parseImplementation(Loader loader, Element implementation) {
        String name = implementation.getAttributeNS(null, "name");
        if (name.length() == 0) {
            log.error("Encountered " + QNAME_IMPLEMENTATION + " element without name attribute");
            return null;
        }
        String loaderClassName = implementation.getAttributeNS(null, "loader");
        if (loaderClassName.length() == 0) {
            log.error("Encountered " + QNAME_IMPLEMENTATION + " element without loader attribute");
            return null;
        }
        OMMetaFactory metaFactory = ((OMMetaFactoryLoader)load(loader, loaderClassName)).load(null);
        if (metaFactory == null) {
            return null;
        }
        List<Feature> features = new ArrayList<Feature>();
        Node child = implementation.getFirstChild();
        while (child != null) {
            if (child instanceof Element) {
                QName childQName = getQName(child);
                if (childQName.equals(QNAME_FEATURE)) {
                    Feature feature = parseFeature(loader, (Element)child);
                    if (feature != null) {
                        features.add(feature);
                    }
                } else {
                    log.warn("Skipping unexpected element " + childQName + "; only "
                            + QNAME_FEATURE + " is expected");
                }
            }
            child = child.getNextSibling();
        }
        return new Implementation(name, metaFactory, features.toArray(new Feature[features.size()]));
    }

    private static Feature parseFeature(Loader loader, Element feature) {
        String name = feature.getAttributeNS(null, "name");
        if (name.length() == 0) {
            log.error("Encountered " + QNAME_FEATURE + " element without name attribute");
            return null;
        }
        String priority = feature.getAttributeNS(null, "priority");
        if (priority.length() == 0) {
            log.error("Encountered " + QNAME_FEATURE + " element without priority attribute");
            return null;
        }
        List<Class<?>> extensionInterfaces = new ArrayList<Class<?>>();
        Node child = feature.getFirstChild();
        while (child != null) {
            if (child instanceof Element) {
                QName childQName = getQName(child);
                if (childQName.equals(QNAME_EXTENSION_INTERFACE)) {
                    String className = ((Element)child).getTextContent();
                    try {
                        extensionInterfaces.add(loader.load(className));
                    } catch (ClassNotFoundException ex) {
                        log.error("The class " + className + " could not be loaded", ex);
                    }
                } else {
                    log.warn("Skipping unexpected element " + childQName + "; only "
                            + QNAME_EXTENSION_INTERFACE + " is expected");
                }
            }
            child = child.getNextSibling();
        }
        try {
            return new Feature(name, Integer.parseInt(priority), extensionInterfaces.toArray(new Class<?>[extensionInterfaces.size()]));
        } catch (NumberFormatException ex) {
            log.error("Invalid priority value '" + priority + "'; must be an integer");
            return null;
        }
    }
    
    private static QName getQName(Node node) {
        String namespaceURI = node.getNamespaceURI();
        return namespaceURI == null ? new QName(node.getLocalName()) : new QName(namespaceURI, node.getLocalName());
    }
}