src/main/java/org/codehaus/groovy/tools/RootLoader.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.codehaus.groovy.tools;

import java.io.File;
import java.net.MalformedURLException;
import java.net.URL;
import java.net.URLClassLoader;
import java.util.HashMap;
import java.util.List;
import java.util.Map;

/**
 * This ClassLoader should be used as root of class loaders. Any
 * RootLoader does have it's own classpath. When searching for a
 * class or resource this classpath will be used. Parent
 * Classloaders are ignored first. If a class or resource
 * can't be found in the classpath of the RootLoader, then parent is
 * checked.
 * <p>
 * <b>Note:</b> this is very against the normal behavior of
 * classloaders. Normal is to first check parent and then look in
 * the resources you gave this classloader.
 * <p>
 * It's possible to add urls to the classpath at runtime through {@link #addURL(URL)}.
 * <p>
 * <b>Why using RootLoader?</b>
 * If you have to load classes with multiple classloaders and a
 * classloader does know a class which depends on a class only
 * a child of this loader does know, then you won't be able to
 * load the class. To load the class the child is not allowed
 * to redirect it's search for the class to the parent first.
 * That way the child can load the class. If the child does not
 * have all classes to do this, this fails of course.
 * <p>
 * For example:
 * <p>
 * <pre>
 *       parentLoader   (has classpath: a.jar;c.jar)
 *           |
 *           |
 *       childLoader    (has classpath: a.jar;b.jar;c.jar)
 * </pre>
 *
 * class C (from c.jar) extends B (from b.jar)
 *
 * childLoader.find("C")
 * <pre>
 * --&gt; parentLoader does know C.class, try to load it
 * --&gt; to load C.class it has to load B.class
 * --&gt; parentLoader is unable to find B.class in a.jar or c.jar
 * --&gt; NoClassDefFoundException!
 * </pre>
 *
 * if childLoader had tried to load the class by itself, there
 * would be no problem. Changing childLoader to be a RootLoader
 * instance will solve that problem.
 */
public class RootLoader extends URLClassLoader {
    private static final URL[] EMPTY_URL_ARRAY = new URL[0];
    private final Map<String, Class> customClasses = new HashMap<String, Class>();
    private static final String ORG_W3C_DOM_NODE = "org.w3c.dom.Node";

    /**
     * constructs a new RootLoader without classpath
     *
     * @param parent the parent Loader
     */
    private RootLoader(ClassLoader parent) {
        this(EMPTY_URL_ARRAY, parent);
    }

    /**
     * constructs a new RootLoader with a parent loader and an
     * array of URLs as classpath
     */
    public RootLoader(URL[] urls, ClassLoader parent) {
        super(urls, parent);
        // major hack here...!
        try {
            customClasses.put(ORG_W3C_DOM_NODE, super.loadClass(ORG_W3C_DOM_NODE, false));
        } catch (Exception e) { /* ignore */ }
    }

    private static ClassLoader chooseParent() {
        ClassLoader cl = RootLoader.class.getClassLoader();
        if (cl != null) return cl;
        return ClassLoader.getSystemClassLoader();
    }

    /**
     * constructs a new RootLoader with a {@link LoaderConfiguration}
     * object which holds the classpath
     */
    public RootLoader(LoaderConfiguration lc) {
        this(chooseParent());
        Thread.currentThread().setContextClassLoader(this);
        URL[] urls = lc.getClassPathUrls();
        for (URL url : urls) {
            addURL(url);
        }
        // TODO M12N eventually defer this until later when we have a full Groovy
        // environment and use normal Grape.grab()
        String groovyHome = System.getProperty("groovy.home");
        List<String> grabUrls = lc.getGrabUrls();
        for (String grabUrl : grabUrls) {
            Map<String, Object> grabParts = GrapeUtil.getIvyParts(grabUrl);
            String group = grabParts.get("group").toString();
            String module = grabParts.get("module").toString();
            String name = grabParts.get("module").toString() + "-" + grabParts.get("version") + ".jar";
            File jar = new File(groovyHome + "/repo/" + group + "/" + module + "/jars/" + name);
            try {
                addURL(jar.toURI().toURL());
            } catch (MalformedURLException e) {
                // ignore
            }
        }
    }

    /**
     * loads a class using the name of the class
     */
    protected synchronized Class loadClass(final String name, boolean resolve) throws ClassNotFoundException {
        Class c = this.findLoadedClass(name);
        if (c != null) return c;
        c = customClasses.get(name);
        if (c != null) return c;

        try {
            c = oldFindClass(name);
        } catch (ClassNotFoundException cnfe) {
            // IGNORE
        }
        if (c == null) c = super.loadClass(name, resolve);

        if (resolve) resolveClass(c);

        return c;
    }

    /**
     * returns the URL of a resource, or null if it is not found
     */
    public URL getResource(String name) {
        URL url = findResource(name);
        if (url == null) url = super.getResource(name);
        return url;
    }

    /**
     * adds an url to the classpath of this classloader
     */
    public void addURL(URL url) {
        super.addURL(url);
    }

    private Class oldFindClass(String name) throws ClassNotFoundException {
        return super.findClass(name);
    }

    protected Class findClass(String name) throws ClassNotFoundException {
        throw new ClassNotFoundException(name);
    }
}