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Its an port to fix bugs in OSGi support. ... The Apache Commons Discovery 0.5 component is about discovering, or finding, implementations for pluggable interfaces.
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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.commons.discovery.tools;

import java.lang.reflect.Constructor;
import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.lang.reflect.Modifier;

import org.apache.commons.discovery.DiscoveryException;
import org.apache.commons.discovery.defaults.Defaults;
import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

/* 
 * Various utilities to interact with {@code Class} types.
 */
public class ClassUtils {

    private static Log log = LogFactory.getLog(ClassUtils.class);

    /* 
     * Sets the {@code Log} for this class.
     *
     * @param _log This class {@code Log}
     * @deprecated This method is not thread-safe
     */
    @Deprecated
    public static void setLog(Log _log) {
        log = _log;
    }

    /* 
     * Get package name.
     * Not all class loaders 'keep' package information,
     * in which case Class.getPackage() returns null.
     * This means that calling Class.getPackage().getName()
     * is unreliable at best.
     *
     * @param clazz The class from which the package has to be extracted
     * @return The string representation of the input class package
     */
    public static String getPackageName(Class clazz) {
        Package clazzPackage = clazz.getPackage();
        String packageName;
        if (clazzPackage != null) {
            packageName = clazzPackage.getName();
        } else {
            String clazzName = clazz.getName();
            packageName = clazzName.substring(0, clazzName.lastIndexOf('.'));
        }
        return packageName;
    }

    /* 
     * Looks for {@code public static returnType methodName(paramTypes)}.
     *
     * @param clazz The class where looking for the method
     * @param returnType The method return type
     * @param methodName The method name
     * @param paramTypes The method arguments types
     * @return Method {@code public static returnType methodName(paramTypes)},
     *         if found to be directly implemented by clazz.
     */
    public static Method findPublicStaticMethod(Class clazz,
                                                Class returnType,
                                                String methodName,
                                                Class[] paramTypes) {
        boolean problem = false;
        Method method = null;

        // verify '()' is directly in class.
        try {
            method = clazz.getDeclaredMethod(methodName, paramTypes);
        } catch(NoSuchMethodException e) {
            problem = true;
            log.debug("Class " + clazz.getName() + ": missing method '" + methodName + "(...)", e);
        }

        // verify 'public static '
        if (!problem  &&
            !(Modifier.isPublic(method.getModifiers()) &&
              Modifier.isStatic(method.getModifiers()) &&
              method.getReturnType() == returnType)) {
            if (log.isDebugEnabled()) {
                if (!Modifier.isPublic(method.getModifiers())) {
                    log.debug(methodName + "() is not public");
                }
                if (!Modifier.isStatic(method.getModifiers())) {
                    log.debug(methodName + "() is not static");
                }
                if (method.getReturnType() != returnType) {
                    log.debug("Method returns: "
                            + method.getReturnType().getName()
                            + "@@"
                            + method.getReturnType().getClassLoader());
                    log.debug("Should return:  "
                            + returnType.getName()
                            + "@@"
                            + returnType.getClassLoader());
                }
            }
            problem = true;
            method = null;
        }

        return method;
    }

    /* 
     * Creates a new instance of the input class using the following policy:
     *
     * 
     * if paramClasses or params is null,
     * the default constructor will be used;
     * the public constructor with paramClasses arguments type,
     * with params as arguments value, will be used.
     * 
     *
     * @param  The class type has to be instantiated
     * @param impl The class has to be instantiated
     * @param paramClasses The constructor arguments types (can be {@code null})
     * @param params The constructor arguments values (can be {@code null})
     * @return A new class instance
     * @throws DiscoveryException if the class implementing
     *            the SPI cannot be found, cannot be loaded and
     *            instantiated, or if the resulting class does not implement
     *            (or extend) the SPI
     * @throws InstantiationException see {@link Class#newInstance()}
     * @throws IllegalAccessException see {@link Class#newInstance()}
     * @throws NoSuchMethodException see {@link Class#newInstance()}
     * @throws InvocationTargetException see {@link Class#newInstance()}
     */
    public static  T newInstance(Class impl, Class paramClasses[], Object params[]) throws DiscoveryException,
               InstantiationException,
               IllegalAccessException,
               NoSuchMethodException,
               InvocationTargetException {
        if (paramClasses == null || params == null) {
            return impl.newInstance();
        }

        Constructor constructor = impl.getConstructor(paramClasses);
        return constructor.newInstance(params);
    }

    /* 
     * Throws exception if {@code impl} does not
     * implement or extend the SPI.
     *
     * @param spi The SPI type
     * @param impl The class has to be verified is a SPI implementation/extension
     * @throws DiscoveryException if the input implementation class is not an SPI implementation
     */
    public static void verifyAncestory(Class spi, Class impl) throws DiscoveryException {
        if (spi == null) {
            throw new DiscoveryException("No interface defined!");
        }

        if (impl == null)
        	impl = Defaults.instance().findClass(spi.getName());
        
        if (impl == null) {
            throw new DiscoveryException("No implementation defined for " + spi.getName());
        }

        if (!spi.isAssignableFrom(impl)) {
            throw new DiscoveryException("Class "
                    + impl.getName()
                    + " does not implement "
                    + spi.getName());
        }
    }

}