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The Apache Commons Object Pooling Library.
There is a newer version: 2.12.0
/*
 * 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.pool2.impl;

import java.lang.reflect.ParameterizedType;
import java.lang.reflect.Type;
import java.lang.reflect.TypeVariable;

import org.apache.commons.pool2.PooledObjectFactory;

/**
 * Implementation specific utilities.
 *
 * @since 2.0
 */
class PoolImplUtils {

    /**
     * Identifies the concrete type of object that an object factory creates.
     *
     * @param factoryClass
     *            The factory to examine
     *
     * @return the type of object the factory creates
     */
    @SuppressWarnings("rawtypes")
    static Class getFactoryType(final Class factoryClass) {
        final Class type = PooledObjectFactory.class;
        final Object genericType = getGenericType(type, factoryClass);
        if (genericType instanceof Integer) {
            // POOL-324 org.apache.commons.pool2.impl.GenericObjectPool.getFactoryType() throws
            // java.lang.ClassCastException
            //
            // A bit hackish, but we must handle cases when getGenericType() does not return a concrete types.
            final ParameterizedType pi = getParameterizedType(type, factoryClass);
            if (pi != null) {
                final Type[] bounds = ((TypeVariable) pi.getActualTypeArguments()[(Integer) genericType]).getBounds();
                if (bounds != null && bounds.length > 0) {
                    final Type bound0 = bounds[0];
                    if (bound0 instanceof Class) {
                        return (Class) bound0;
                    }
                }
            }
            // last resort: Always return a Class
            return Object.class;
        }
        return (Class) genericType;
    }

    /**
     * Obtains the concrete type used by an implementation of an interface that uses a generic type.
     *
     * @param type
     *            The interface that defines a generic type
     * @param clazz
     *            The class that implements the interface with a concrete type
     * @param 
     *            The interface type
     *
     * @return concrete type used by the implementation
     */
    private static  Object getGenericType(final Class type, final Class clazz) {
        if (type == null || clazz == null) {
            // Error will be logged further up the call stack
            return null;
        }

        // Look to see if this class implements the generic interface
        final ParameterizedType pi = getParameterizedType(type, clazz);
        if (pi != null) {
            return getTypeParameter(clazz, pi.getActualTypeArguments()[0]);
        }

        // Interface not found on this class. Look at the superclass.
        @SuppressWarnings("unchecked")
        final Class superClass = (Class) clazz.getSuperclass();

        final Object result = getGenericType(type, superClass);
        if (result instanceof Class) {
            // Superclass implements interface and defines explicit type for generic
            return result;
        } else if (result instanceof Integer) {
            // Superclass implements interface and defines unknown type for generic
            // Map that unknown type to the generic types defined in this class
            final ParameterizedType superClassType = (ParameterizedType) clazz.getGenericSuperclass();
            return getTypeParameter(clazz, superClassType.getActualTypeArguments()[((Integer) result).intValue()]);
        } else {
            // Error will be logged further up the call stack
            return null;
        }
    }

    /**
     * Gets the matching parameterized type or null.
     * @param type
     *            The interface that defines a generic type
     * @param clazz
     *            The class that implements the interface with a concrete type
     * @param 
     *            The interface type
     */
    private static  ParameterizedType getParameterizedType(final Class type, final Class clazz) {
        for (final Type iface : clazz.getGenericInterfaces()) {
            // Only need to check interfaces that use generics
            if (iface instanceof ParameterizedType) {
                final ParameterizedType pi = (ParameterizedType) iface;
                // Look for the generic interface
                if (pi.getRawType() instanceof Class && type.isAssignableFrom((Class) pi.getRawType())) {
                    return pi;
                }
            }
        }
        return null;
    }

    /**
     * For a generic parameter, return either the Class used or if the type is unknown, the index for the type in
     * definition of the class
     *
     * @param clazz
     *            defining class
     * @param argType
     *            the type argument of interest
     *
     * @return An instance of {@link Class} representing the type used by the type parameter or an instance of
     *         {@link Integer} representing the index for the type in the definition of the defining class
     */
    private static Object getTypeParameter(final Class clazz, final Type argType) {
        if (argType instanceof Class) {
            return argType;
        }
        final TypeVariable[] tvs = clazz.getTypeParameters();
        for (int i = 0; i < tvs.length; i++) {
            if (tvs[i].equals(argType)) {
                return Integer.valueOf(i);
            }
        }
        return null;
    }
}