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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.
/**
 * 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.commons.crypto.utils;

import java.lang.ref.WeakReference;
import java.lang.reflect.Constructor;
import java.util.Collections;
import java.util.Map;
import java.util.WeakHashMap;

import org.apache.commons.crypto.cipher.CryptoCipher;

General utility methods for working with reflection.
/**
 * General utility methods for working with reflection.
 */
public final class ReflectionUtils {

    private static final Map<ClassLoader, Map<String, WeakReference<Class<?>>>> CACHE_CLASSES = new WeakHashMap<>();

    private static final ClassLoader CLASSLOADER;

    static {
        ClassLoader threadClassLoader = Thread.currentThread()
                .getContextClassLoader();
        CLASSLOADER = (threadClassLoader != null) ? threadClassLoader
                : CryptoCipher.class.getClassLoader();
    }

    
Sentinel value to store negative cache results in CACHE_CLASSES. /**
     * Sentinel value to store negative cache results in {@link #CACHE_CLASSES}.
     */
    private static final Class<?> NEGATIVE_CACHE_SENTINEL = NegativeCacheSentinel.class;

    
The private constructor of ReflectionUtils. /**
     * The private constructor of {@link ReflectionUtils}.
     */
    private ReflectionUtils() {
    }

    
A unique class which is used as a sentinel value in the caching for getClassByName. ReflectionUtils.getClassByNameOrNull(String). /**
     * A unique class which is used as a sentinel value in the caching for
     * getClassByName. {@link #getClassByNameOrNull(String)}.
     */
    private static abstract class NegativeCacheSentinel {
    }

    
Uses the constructor represented by this Constructor object to create and initialize a new instance of the constructor's declaring class, with the specified initialization parameters. 
Params:
klass – the Class object.
args – array of objects to be passed as arguments to the constructor
       call.
Type parameters:
<T> – type for the new instance
Returns:
a new object created by calling the constructor this object
        represents./**
     * Uses the constructor represented by this {@code Constructor} object to
     * create and initialize a new instance of the constructor's declaring
     * class, with the specified initialization parameters.
     *
     * @param <T> type for the new instance
     * @param klass the Class object.
     * @param args array of objects to be passed as arguments to the constructor
     *        call.
     * @return a new object created by calling the constructor this object
     *         represents.
     */
    public static <T> T newInstance(Class<T> klass, Object... args) {
        try {
            Constructor<T> ctor;

            if (args.length == 0) {
                ctor = klass.getDeclaredConstructor();
            } else {
                Class<?>[] argClses = new Class[args.length];
                for (int i = 0; i < args.length; i++) {
                    argClses[i] = args[i].getClass();
                }
                ctor = klass.getDeclaredConstructor(argClses);
            }
            ctor.setAccessible(true);
            return ctor.newInstance(args);
        } catch (Exception e) {
            throw new RuntimeException(e);
        }
    }

    
Loads a class by name.
Params:
name – the class name.
Throws:
ClassNotFoundException – if the class is not found.
Returns:
the class object./**
     * Loads a class by name.
     *
     * @param name the class name.
     * @return the class object.
     * @throws ClassNotFoundException if the class is not found.
     */
    public static Class<?> getClassByName(String name)
            throws ClassNotFoundException {
        Class<?> ret = getClassByNameOrNull(name);
        if (ret == null) {
            throw new ClassNotFoundException("Class " + name + " not found");
        }
        return ret;
    }

    
Loads a class by name, returning null rather than throwing an exception
if it couldn't be loaded. This is to avoid the overhead of creating an
exception.
Params:
name – the class name.
Returns:
the class object, or null if it could not be found./**
     * Loads a class by name, returning null rather than throwing an exception
     * if it couldn't be loaded. This is to avoid the overhead of creating an
     * exception.
     *
     * @param name the class name.
     * @return the class object, or null if it could not be found.
     */
    private static Class<?> getClassByNameOrNull(String name) {
        Map<String, WeakReference<Class<?>>> map;

        synchronized (CACHE_CLASSES) {
            map = CACHE_CLASSES.get(CLASSLOADER);
            if (map == null) {
                map = Collections
                        .synchronizedMap(new WeakHashMap<String, WeakReference<Class<?>>>());
                CACHE_CLASSES.put(CLASSLOADER, map);
            }
        }

        Class<?> clazz = null;
        WeakReference<Class<?>> ref = map.get(name);
        if (ref != null) {
            clazz = ref.get();
        }

        if (clazz == null) {
            try {
                clazz = Class.forName(name, true, CLASSLOADER);
            } catch (ClassNotFoundException e) {
                // Leave a marker that the class isn't found
                map.put(name, new WeakReference<Class<?>>(
                        NEGATIVE_CACHE_SENTINEL));
                return null;
            }
            // two putters can race here, but they'll put the same class
            map.put(name, new WeakReference<Class<?>>(clazz));
            return clazz;
        } else if (clazz == NEGATIVE_CACHE_SENTINEL) {
            return null; // not found
        } else {
            // cache hit
            return clazz;
        }
    }
}