kernel/src/java/org/gbean/kernel/KernelFactory.java

/**
 *
 * Copyright 2005 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.gbean.kernel;

import java.io.BufferedReader;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;

import org.gbean.kernel.simple.SimpleKernelFactory;

/**
 * @version $Rev$ $Date$
 */
public abstract class KernelFactory {
    public static final String KERNEL_FACTORY_KEY = KernelFactory.class.getName();

    public static KernelFactory newInstance() {
        ClassLoader classLoader = Thread.currentThread().getContextClassLoader();
        if (classLoader == null) {
            classLoader = KernelFactory.class.getClassLoader();
        }

        // System property
        try {
            String kernelFactoryName = System.getProperty(KERNEL_FACTORY_KEY);
            if (kernelFactoryName != null) {
                return createKernelFactory(kernelFactoryName, classLoader);
            }
        } catch (SecurityException se) {
        }

        // Jar Service Specification - http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html
        String serviceId = "META-INF/services/" + KERNEL_FACTORY_KEY;
        InputStream inputStream = null;
        try {
            inputStream = classLoader.getResourceAsStream(serviceId);
            if (inputStream != null) {
                BufferedReader reader = new BufferedReader(new InputStreamReader(inputStream, "UTF-8"));
                String kernelFactoryName = reader.readLine();
                reader.close();

                if (kernelFactoryName != null && kernelFactoryName.length() > 0) {
                    return createKernelFactory(kernelFactoryName, classLoader);
                }
            }
        } catch (Exception ignored) {
        } finally {
            if (inputStream != null) {
                try {
                    inputStream.close();
                } catch (IOException ignored) {
                }
                inputStream = null;
            }
        }

        // Default is the basic kernel
        return new SimpleKernelFactory();
    }

    private static KernelFactory createKernelFactory(String className, ClassLoader classLoader) {
        try {
            return (KernelFactory) classLoader.loadClass(className).newInstance();
        } catch (ClassCastException e) {
            throw new KernelFactoryError("Kernel factory class does not implement KernelFactory: " + className);
        } catch (ClassNotFoundException e) {
            throw new KernelFactoryError("Kernel factory class not found: " + className);
        } catch (Exception e) {
            throw new KernelFactoryError("Unable to instantiate kernel factory class: " + className, e);
        }
    }

    public abstract Kernel createKernel(String kernelName);
}