org.apache.wicket.util.collections.ClassMetaCache.java Source code

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Here is the source code for org.apache.wicket.util.collections.ClassMetaCache.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.util.collections;

import java.util.Collections;
import java.util.Map;
import java.util.WeakHashMap;
import java.util.concurrent.ConcurrentHashMap;

/**
 * This class wraps a WeakHashMap that holds one ConcurrentHashMap per ClassLoader. In the rare
 * event of a previously unmapped ClassLoader, the WeakHashMap is replaced by a new one. This avoids
 * any synchronization overhead, much like a {@link java.util.concurrent.CopyOnWriteArrayList}
 * 
 * @param <T>
 *            type of objects stored in cache
 */
public class ClassMetaCache<T> {
    private volatile Map<ClassLoader, ConcurrentHashMap<String, T>> cache = Collections.emptyMap();

    /**
     * Puts value into cache
     * 
     * @param key
     *            the class that will be used as the value's key
     * @param value
     *            the value that should be stored in cache
     * @return value previously stored in cache for this key, or {@code null} if none
     */
    public T put(final Class<?> key, final T value) {
        ConcurrentHashMap<String, T> container = getClassLoaderCache(key.getClassLoader(), true);
        return container.put(key(key), value);
    }

    /**
     * Gets value from cache or returns {@code null} if not in cache
     * 
     * @param key
     *            the class that is the key for the value
     * @return value stored in cache or {@code null} if none
     */
    public T get(final Class<?> key) {
        ConcurrentHashMap<String, T> container = getClassLoaderCache(key.getClassLoader(), false);
        if (container == null) {
            return null;
        } else {
            return container.get(key(key));
        }
    }

    /**
     * @param classLoader
     * @param create
     * @return a {@link ConcurrentHashMap} mapping class names to injectable fields, never
     *         <code>null</code>
     */
    private ConcurrentHashMap<String, T> getClassLoaderCache(final ClassLoader classLoader, final boolean create) {
        ConcurrentHashMap<String, T> container = cache.get(classLoader);
        if (container == null) {
            if (!create) {
                return container;
            }

            // only lock in rare event of unknown ClassLoader
            synchronized (this) {
                // check again inside lock
                container = cache.get(classLoader);
                if (container == null) {
                    container = new ConcurrentHashMap<>();

                    /*
                     * don't write to current cache, copy instead
                     */
                    Map<ClassLoader, ConcurrentHashMap<String, T>> newCache = new WeakHashMap<>(cache);
                    newCache.put(classLoader, container);
                    cache = Collections.unmodifiableMap(newCache);
                }
            }
        }
        return container;
    }

    /**
     * converts class into a key used by the cache
     * 
     * @param clazz
     * 
     * @return string representation of the clazz
     */
    private static String key(final Class<?> clazz) {
        return clazz.getName();
    }
}