/* * Copyright 2001-2004 The Apache Software Foundation. *  * 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.apache.commons.logging;import java.io.BufferedReader;import java.io.IOException;import java.io.InputStream;import java.io.InputStreamReader;import java.lang.reflect.InvocationTargetException;import java.lang.reflect.Method;import java.security.AccessController;import java.security.PrivilegedAction;import java.util.Enumeration;import java.util.Hashtable;import java.util.Properties;/** * <p>Factory for creating {@link Log} instances, with discovery and * configuration features similar to that employed by standard Java APIs * such as JAXP.</p> * * <p><strong>IMPLEMENTATION NOTE</strong> - This implementation is heavily * based on the SAXParserFactory and DocumentBuilderFactory implementations * (corresponding to the JAXP pluggability APIs) found in Apache Xerces.</p> * * @author Craig R. McClanahan * @author Costin Manolache * @author Richard A. Sitze * @version $Revision: 1.27 $ $Date: 2004/06/06 21:15:12 $ */public abstract class LogFactory {    // ----------------------------------------------------- Manifest Constants    /**     * The name of the property used to identify the LogFactory implementation     * class name.     */    public static final String FACTORY_PROPERTY =        "org.apache.commons.logging.LogFactory";    /**     * The fully qualified class name of the fallback <code>LogFactory</code>     * implementation class to use, if no other can be found.     */    public static final String FACTORY_DEFAULT =        "org.apache.commons.logging.impl.LogFactoryImpl";    /**     * The name of the properties file to search for.     */    public static final String FACTORY_PROPERTIES =        "commons-logging.properties";    /**     * JDK1.3+ <a href="http://java.sun.com/j2se/1.3/docs/guide/jar/jar.html#Service%20Provider">     * 'Service Provider' specification</a>.     *      */    protected static final String SERVICE_ID =        "META-INF/services/org.apache.commons.logging.LogFactory";    // ----------------------------------------------------------- Constructors    /**     * Protected constructor that is not available for public use.     */    protected LogFactory() { }    // --------------------------------------------------------- Public Methods    /**     * Return the configuration attribute with the specified name (if any),     * or <code>null</code> if there is no such attribute.     *     * @param name Name of the attribute to return     */    public abstract Object getAttribute(String name);    /**     * Return an array containing the names of all currently defined     * configuration attributes.  If there are no such attributes, a zero     * length array is returned.     */    public abstract String[] getAttributeNames();    /**     * Convenience method to derive a name from the specified class and     * call <code>getInstance(String)</code> with it.     *     * @param clazz Class for which a suitable Log name will be derived     *     * @exception LogConfigurationException if a suitable <code>Log</code>     *  instance cannot be returned     */    public abstract Log getInstance(Class clazz)        throws LogConfigurationException;    /**     * <p>Construct (if necessary) and return a <code>Log</code> instance,     * using the factory's current set of configuration attributes.</p>     *     * <p><strong>NOTE</strong> - Depending upon the implementation of     * the <code>LogFactory</code> you are using, the <code>Log</code>     * instance you are returned may or may not be local to the current     * application, and may or may not be returned again on a subsequent     * call with the same name argument.</p>     *     * @param name Logical name of the <code>Log</code> instance to be     *  returned (the meaning of this name is only known to the underlying     *  logging implementation that is being wrapped)     *     * @exception LogConfigurationException if a suitable <code>Log</code>     *  instance cannot be returned     */    public abstract Log getInstance(String name)        throws LogConfigurationException;    /**     * Release any internal references to previously created {@link Log}     * instances returned by this factory.  This is useful in environments     * like servlet containers, which implement application reloading by     * throwing away a ClassLoader.  Dangling references to objects in that     * class loader would prevent garbage collection.     */    public abstract void release();    /**     * Remove any configuration attribute associated with the specified name.     * If there is no such attribute, no action is taken.     *     * @param name Name of the attribute to remove     */    public abstract void removeAttribute(String name);    /**     * Set the configuration attribute with the specified name.  Calling     * this with a <code>null</code> value is equivalent to calling     * <code>removeAttribute(name)</code>.     *     * @param name Name of the attribute to set     * @param value Value of the attribute to set, or <code>null</code>     *  to remove any setting for this attribute     */    public abstract void setAttribute(String name, Object value);    // ------------------------------------------------------- Static Variables    /**     * The previously constructed <code>LogFactory</code> instances, keyed by     * the <code>ClassLoader</code> with which it was created.     */    protected static Hashtable factories = new Hashtable();    // --------------------------------------------------------- Static Methods    /**     * <p>Construct (if necessary) and return a <code>LogFactory</code>     * instance, using the following ordered lookup procedure to determine     * the name of the implementation class to be loaded.</p>     * <ul>     * <li>The <code>org.apache.commons.logging.LogFactory</code> system     *     property.</li>     * <li>The JDK 1.3 Service Discovery mechanism</li>     * <li>Use the properties file <code>commons-logging.properties</code>     *     file, if found in the class path of this class.  The configuration     *     file is in standard <code>java.util.Properties</code> format and     *     contains the fully qualified name of the implementation class     *     with the key being the system property defined above.</li>     * <li>Fall back to a default implementation class     *     (<code>org.apache.commons.logging.impl.LogFactoryImpl</code>).</li>     * </ul>     *     * <p><em>NOTE</em> - If the properties file method of identifying the     * <code>LogFactory</code> implementation class is utilized, all of the     * properties defined in this file will be set as configuration attributes     * on the corresponding <code>LogFactory</code> instance.</p>     *     * @exception LogConfigurationException if the implementation class is not     *  available or cannot be instantiated.     */    public static LogFactory getFactory() throws LogConfigurationException {        // Identify the class loader we will be using        ClassLoader contextClassLoader =            (ClassLoader)AccessController.doPrivileged(                new PrivilegedAction() {                    public Object run() {                        return getContextClassLoader();                    }                });        // Return any previously registered factory for this class loader        LogFactory factory = getCachedFactory(contextClassLoader);        if (factory != null)            return factory;        // Load properties file.        // Will be used one way or another in the end.        Properties props=null;        try {            InputStream stream = getResourceAsStream(contextClassLoader,                                                     FACTORY_PROPERTIES);            if (stream != null) {                props = new Properties();                props.load(stream);                stream.close();            }        } catch (IOException e) {        } catch (SecurityException e) {        }        // First, try the system property        try {            String factoryClass = System.getProperty(FACTORY_PROPERTY);            if (factoryClass != null) {                factory = newFactory(factoryClass, contextClassLoader);            }        } catch (SecurityException e) {            ;  // ignore        }        // Second, try to find a service by using the JDK1.3 jar        // discovery mechanism. This will allow users to plug a logger        // by just placing it in the lib/ directory of the webapp ( or in        // CLASSPATH or equivalent ). This is similar to the second        // step, except that it uses the (standard?) jdk1.3 location in the jar.        if (factory == null) {            try {                InputStream is = getResourceAsStream(contextClassLoader,                                                     SERVICE_ID);                if( is != null ) {                    // This code is needed by EBCDIC and other strange systems.                    // It's a fix for bugs reported in xerces                    BufferedReader rd;                    try {                        rd = new BufferedReader(new InputStreamReader(is, "UTF-8"));                    } catch (java.io.UnsupportedEncodingException e) {                        rd = new BufferedReader(new InputStreamReader(is));                    }                    String factoryClassName = rd.readLine();                    rd.close();                    if (factoryClassName != null &&                        ! "".equals(factoryClassName)) {                        factory= newFactory( factoryClassName, contextClassLoader );                    }                }            } catch( Exception ex ) {                ;            }        }        // Third try a properties file.        // If the properties file exists, it'll be read and the properties        // used. IMHO ( costin ) System property and JDK1.3 jar service        // should be enough for detecting the class name. The properties        // should be used to set the attributes ( which may be specific to        // the webapp, even if a default logger is set at JVM level by a        // system property )