LoadedClassCache.java

/*******************************************************************************
 * Copyright (c) 2007, 2014 Massimiliano Ziccardi
 *
 * Licensed 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 it.jnrpe.utils;

import java.util.Collection;
import java.util.HashMap;
import java.util.Iterator;
import java.util.Map;
import java.util.WeakHashMap;

/**
 * This class stores the already loaded plugin class so that no attempts will be
 * performed to load the same class twice.
 * 
 * @author Massimiliano Ziccardi
 * 
 * @version $Revision: 1.0 $
 */
@SuppressWarnings("rawtypes")
final class LoadedClassCache {

    /**
     * Private constructor to avoid instantiations.
     */
    private LoadedClassCache() {

    }

    /**
     * Stores data about each created class loader.
     */
    private static final Map<ClassLoader, ClassesData> LOADED_PLUGINS = new WeakHashMap<ClassLoader, ClassesData>();

    /**
     * This class stores data about all the classes loaded by a classloader.
     * 
     * @author Massimiliano Ziccardi
     * 
     */
    private static final class ClassesData {

        /**
         * Maps classname with corresponding Class object.
         */
        private final Map<String, Class> loadedClasses = new HashMap<String, Class>();

        /**
         * @param name
         *            the class name
        
         * @return the cached class object (if any) */
        public Class getClass(final String name) {
            return loadedClasses.get(name);
        }

        /**
         * Adds a class to the cache.
         * 
         * @param clazz
         *            the class to be added
         */
        public void addClass(final Class clazz) {
            loadedClasses.put(clazz.getName(), clazz);
        }

        /**
         * Method toString.
         * @return String
         */
        @Override
        public String toString() {
            final int maxLen = 10;
            return "ClassesData [loadedClasses=" + (loadedClasses != null ? toString(loadedClasses.entrySet(), maxLen) : null) + "]";
        }

        /**
         * Method toString.
         * @param collection Collection<?>
         * @param maxLen int
         * @return String
         */
        private String toString(Collection<?> collection, int maxLen) {
            StringBuilder builder = new StringBuilder();
            builder.append('[');
            int i = 0;
            for (Iterator<?> iterator = collection.iterator(); iterator.hasNext() && i < maxLen; i++) {
                if (i > 0) {
                    builder.append(", ");
                }
                builder.append(iterator.next());
            }
            builder.append(']');
            return builder.toString();
        }
    }

    /**
     * Stores a class in the cache.
     * 
     * @param cl
     *            The classloader
     * @param c
     *            the class to be stored
     */
    private static void saveClass(final ClassLoader cl, final Class c) {
        if (LOADED_PLUGINS.get(cl) == null) {
            LOADED_PLUGINS.put(cl, new ClassesData());
        }

        ClassesData cd = LOADED_PLUGINS.get(cl);
        cd.addClass(c);
    }

    /**
     * Returns a class object. If the class is new, a new Class object is
     * created, otherwise the cached object is returned.
     * 
     * @param cl
     *            the classloader
     * @param className
     *            the class name
    
    
     * @return the class object associated to the given class name * @throws ClassNotFoundException
     *             if the class can't be loaded */
    public static Class getClass(final ClassLoader cl, final String className) throws ClassNotFoundException {
        if (LOADED_PLUGINS.get(cl) == null) {
            LOADED_PLUGINS.put(cl, new ClassesData());
        }

        ClassesData cd = LOADED_PLUGINS.get(cl);
        Class clazz = cd.getClass(className);
        if (clazz == null) {
            clazz = cl.loadClass(className);
            saveClass(cl, clazz);
        }

        return clazz;
    }
}