/*
 * Copyright 2005-2010 the original author or authors.
 * 
 * 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 org.wamblee.ioc;

import org.apache.commons.logging.Log;
import org.apache.commons.logging.LogFactory;

import org.wamblee.io.ClassPathResource;
import org.wamblee.io.InputResource;

import java.io.IOException;
import java.io.InputStream;

import java.util.Properties;

/**
 * The standard means to obtain the bean factory. This works by reading a
 * property {@value #BEAN_FACTORY_CLASS} from a property file named
 * {@value #BEAN_KERNEL_PROP_FILE} from the class path. This property identifies
 * the bean factory implementation to use. The configured bean factory must have
 * a no-arg constructor.
 */
public final class BeanKernel {
    private static final Log LOG = LogFactory.getLog(BeanKernel.class);

    /**
     * Bean factory kernel properties file.
     */
    private static final String BEAN_KERNEL_PROP_FILE = "org.wamblee.beanfactory.properties";

    /**
     * Name of the property to define the name of the bean factory class to use.
     * THis class must have a public default constructor.
     */
    private static final String BEAN_FACTORY_CLASS = "org.wamblee.beanfactory.class";

    /**
     * Cached bean factory.
     */
    private static BeanFactory BEAN_FACTORY;

    /**
     * Disabled constructor.
     * 
     */
    private BeanKernel() {
        // Empty
    }

    /**
     * Overrides the default mechanism for looking up the bean factory by
     * specifying it yourself.
     * 
     * @param aOverride
     *            Override bean factory.
     */
    public static void overrideBeanFactory(BeanFactory aOverride) {
        BEAN_FACTORY = aOverride;
    }

    /**
     * Gets the bean factory.
     * 
     * @return Bean factory.
     */
    public static BeanFactory getBeanFactory() {
        synchronized (BeanFactory.class) {
            if (BEAN_FACTORY == null) {
                BEAN_FACTORY = lookupBeanFactory(BEAN_KERNEL_PROP_FILE);
            }
        }

        return BEAN_FACTORY;
    }

    /**
     * Lookup the bean factory based on the properties file.
     * 
     * 
     * @return Bean factory.
     * 
     */
    static BeanFactory lookupBeanFactory(String aPropertyFilename) {
        InputResource resource = new ClassPathResource(aPropertyFilename);
        InputStream is;

        try {
            is = resource.getInputStream();
        } catch (IOException e) {
            throw new BeanFactoryException("Cannot open resource " + resource,
                e);
        }

        try {
            Properties props = new Properties();
            props.load(is);

            String className = props.getProperty(BEAN_FACTORY_CLASS);
            Class beanFactory = Class.forName(className);

            return (BeanFactory) beanFactory.newInstance();
        } catch (Exception e) {
            throw new BeanFactoryException("Cannot read from resource " +
                resource, e);
        } finally {
            try {
                is.close();
            } catch (IOException e) {
                // last resort cannot do much now.
                LOG.error("Error closing resource " + resource);
            }
        }
    }
}