org.apache.wicket.application.AbstractClassResolver.java Source code

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Here is the source code for org.apache.wicket.application.AbstractClassResolver.java

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/*
 * 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.wicket.application;

import java.lang.ref.WeakReference;
import java.net.URL;
import java.util.Enumeration;
import java.util.Iterator;
import java.util.Set;
import java.util.TreeSet;
import java.util.concurrent.ConcurrentHashMap;
import java.util.concurrent.ConcurrentMap;

import org.apache.wicket.Application;
import org.apache.wicket.WicketRuntimeException;
import org.apache.wicket.util.collections.UrlExternalFormComparator;

/**
 * An abstract implementation of a {@link IClassResolver} which uses a {@link ClassLoader} for
 * resolving classes.
 * 
 * @see org.apache.wicket.settings.ApplicationSettings#getClassResolver()
 * 
 * @author Juergen Donnerstag
 * @author Jonathan Locke
 */
public abstract class AbstractClassResolver implements IClassResolver {
    /**
     * Usually class loaders implement more efficient caching strategies than we could possibly do,
     * but we experienced synchronization issue resulting in stack traces like:
     * java.lang.LinkageError: duplicate class definition:
     * 
     * <pre>
     *    wicket/examples/repeater/RepeatingPage at java.lang.ClassLoader.defineClass1(Native Method)
     * </pre>
     * 
     * This problem has gone since we synchronize the access.
     */
    private final ConcurrentMap<String, WeakReference<Class<?>>> classes = new ConcurrentHashMap<>();

    @Override
    public final Class<?> resolveClass(final String className) throws ClassNotFoundException {
        Class<?> clazz = null;
        WeakReference<Class<?>> ref = classes.get(className);

        // Might be garbage-collected between getting the WeakRef and retrieving
        // the Class from it.
        if (ref != null) {
            clazz = ref.get();
        }
        if (clazz == null) {
            switch (className) {
            case "byte":
                clazz = byte.class;
                break;
            case "short":
                clazz = short.class;
                break;
            case "int":
                clazz = int.class;
                break;
            case "long":
                clazz = long.class;
                break;
            case "float":
                clazz = float.class;
                break;
            case "double":
                clazz = double.class;
                break;
            case "boolean":
                clazz = boolean.class;
                break;
            case "char":
                clazz = char.class;
                break;
            default:
                // synchronize on the only class member to load only one class at a time and
                // prevent LinkageError. See above for more info
                synchronized (classes) {
                    clazz = Class.forName(className, false, getClassLoader());
                    if (clazz == null) {
                        throw new ClassNotFoundException(className);
                    }
                }
                classes.put(className, new WeakReference<Class<?>>(clazz));
                break;
            }
        }
        return clazz;
    }

    @Override
    public Iterator<URL> getResources(final String name) {
        Set<URL> resultSet = new TreeSet<>(new UrlExternalFormComparator());

        try {
            // Try the classloader for the wicket jar/bundle
            Enumeration<URL> resources = Application.class.getClassLoader().getResources(name);
            loadResources(resources, resultSet);

            // Try the classloader for the user's application jar/bundle
            resources = Application.get().getClass().getClassLoader().getResources(name);
            loadResources(resources, resultSet);

            // Try the context class loader
            resources = getClassLoader().getResources(name);
            loadResources(resources, resultSet);
        } catch (Exception e) {
            throw new WicketRuntimeException(e);
        }

        return resultSet.iterator();
    }

    /**
     * 
     * @param resources
     * @param loadedResources
     */
    private void loadResources(Enumeration<URL> resources, Set<URL> loadedResources) {
        if (resources != null) {
            while (resources.hasMoreElements()) {
                final URL url = resources.nextElement();
                loadedResources.add(url);
            }
        }
    }
}