https://tomcat.apache.org/: Tomcat Core Logging Package 
/*
 * 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.juli.logging;

import java.lang.reflect.Constructor;
import java.nio.file.FileSystems;
import java.util.ServiceLoader;
import java.util.logging.LogManager;


This is a modified LogFactory that uses a simple ServiceLoader based discovery mechanism with a default of using JDK based logging. An implementation that uses the full Commons Logging discovery mechanism is available as part of the Tomcat extras download. Why? It is an attempt to strike a balance between simpler code (no discovery) and providing flexibility - particularly for those projects that embed Tomcat or some of Tomcat's components - is an alternative logging implementation is desired. Note that this implementation is not just a wrapper around JDK logging (like the original commons-logging impl). It adds 2 features - a simpler configuration (which is in fact a subset of log4j.properties) and a formatter that is less ugly. The removal of 'abstract' preserves binary backward compatibility. It is possible to preserve the abstract - and introduce another (hardcoded) factory - but I see no benefit. Since this class is not intended to be extended - all protected methods are removed. This can be changed - but again, there is little value in keeping dead code. Just take a quick look at the removed code ( and it's complexity). -------------- Original comment: 
Factory for creating Log instances, with discovery and configuration features similar to that employed by standard Java APIs such as JAXP.


IMPLEMENTATION NOTE - This implementation is heavily
based on the SAXParserFactory and DocumentBuilderFactory implementations
(corresponding to the JAXP pluggability APIs) found in Apache Xerces.


Author:Craig R. McClanahan, Costin Manolache, Richard A. Sitze
/**
 * This is a modified LogFactory that uses a simple {@link ServiceLoader} based
 * discovery mechanism with a default of using JDK based logging. An
 * implementation that uses the full Commons Logging discovery mechanism is
 * available as part of the Tomcat extras download.
 *
 * Why? It is an attempt to strike a balance between simpler code (no discovery)
 * and providing flexibility - particularly for those projects that embed Tomcat
 * or some of Tomcat's components - is an alternative logging
 * implementation is desired.
 *
 * Note that this implementation is not just a wrapper around JDK logging (like
 * the original commons-logging impl). It adds 2 features - a simpler
 * configuration  (which is in fact a subset of log4j.properties) and a
 * formatter that is less ugly.
 *
 * The removal of 'abstract' preserves binary backward compatibility. It is
 * possible to preserve the abstract - and introduce another (hardcoded) factory
 * - but I see no benefit.
 *
 * Since this class is not intended to be extended - all protected methods are
 * removed. This can be changed - but again, there is little value in keeping
 * dead code. Just take a quick look at the removed code ( and it's complexity).
 *
 * --------------
 *
 * Original comment:
 * <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
 */
public class LogFactory {

    private static final LogFactory singleton = new LogFactory();

    private final Constructor<? extends Log> discoveredLogConstructor;

    
Private constructor that is not available for public use.

/**
     * Private constructor that is not available for public use.
     */
    private LogFactory() {
        /*
         * Work-around known a JRE bug.
         * https://bugs.openjdk.java.net/browse/JDK-8194653
         *
         * Pre-load the default file system. No performance impact as we need to
         * load the default file system anyway. Just do it earlier to avoid the
         * potential deadlock.
         *
         * This can be removed once the oldest JRE supported by Tomcat includes
         * a fix.
         */
        FileSystems.getDefault();

        // Look via a ServiceLoader for a Log implementation that has a
        // constructor taking the String name.
        ServiceLoader<Log> logLoader = ServiceLoader.load(Log.class);
        Constructor<? extends Log> m=null;
        for (Log log: logLoader) {
            Class<? extends Log> c=log.getClass();
            try {
                m=c.getConstructor(String.class);
                break;
            }
            catch (NoSuchMethodException | SecurityException e) {
                throw new Error(e);
            }
        }
        discoveredLogConstructor=m;
    }


    // --------------------------------------------------------- Public Methods

    // only those 2 methods need to change to use a different direct logger.

    
Construct (if necessary) and return a Log instance,
using the factory's current set of configuration attributes.


NOTE - Depending upon the implementation of
the LogFactory you are using, the Log
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.


Params:
  • name – Logical name of the Log instance to be
 returned (the meaning of this name is only known to the underlying
 logging implementation that is being wrapped)
Throws:
Returns:A log instance with the requested name
/**
     * <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)
     *
     * @return A log instance with the requested name
     *
     * @exception LogConfigurationException if a suitable <code>Log</code>
     *  instance cannot be returned
     */
    public Log getInstance(String name) throws LogConfigurationException {
        if (discoveredLogConstructor == null) {
            return DirectJDKLog.getInstance(name);
        }

        try {
            return discoveredLogConstructor.newInstance(name);
        } catch (ReflectiveOperationException | IllegalArgumentException e) {
            throw new LogConfigurationException(e);
        }
    }


    
Convenience method to derive a name from the specified class and
call getInstance(String) with it.

Params:
  • clazz – Class for which a suitable Log name will be derived
Throws:
Returns:A log instance with a name of clazz.getName()
/**
     * 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
     *
     * @return A log instance with a name of clazz.getName()
     *
     * @exception LogConfigurationException if a suitable <code>Log</code>
     *  instance cannot be returned
     */
    public Log getInstance(Class<?> clazz) throws LogConfigurationException {
        return getInstance( clazz.getName());
    }


    // ------------------------------------------------------- Static Variables


    // --------------------------------------------------------- Static Methods


    
Construct (if necessary) and return a LogFactory
instance, using the following ordered lookup procedure to determine
the name of the implementation class to be loaded.


    

  • The org.apache.commons.logging.LogFactory system
    property.
    • 

  • The JDK 1.3 Service Discovery mechanism
    • 

  • Use the properties file commons-logging.properties
    file, if found in the class path of this class.  The configuration
    file is in standard java.util.Properties format and
    contains the fully qualified name of the implementation class
    with the key being the system property defined above.
    • 

  • Fall back to a default implementation class
    (org.apache.commons.logging.impl.LogFactoryImpl).
    • 



NOTE - If the properties file method of identifying the
LogFactory implementation class is utilized, all of the
properties defined in this file will be set as configuration attributes
on the corresponding LogFactory instance.


Throws:
Returns:The singleton LogFactory instance
/**
     * <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>
     *
     * @return The singleton LogFactory instance
     *
     * @exception LogConfigurationException if the implementation class is not
     *  available or cannot be instantiated.
     */
    public static LogFactory getFactory() throws LogConfigurationException {
        return singleton;
    }


    
Convenience method to return a named logger, without the application
having to care about factories.

Params:
  • clazz – Class from which a log name will be derived
Throws:
Returns:A log instance with a name of clazz.getName()
/**
     * Convenience method to return a named logger, without the application
     * having to care about factories.
     *
     * @param clazz Class from which a log name will be derived
     *
     * @return A log instance with a name of clazz.getName()
     *
     * @exception LogConfigurationException if a suitable <code>Log</code>
     *  instance cannot be returned
     */
    public static Log getLog(Class<?> clazz)
        throws LogConfigurationException {
        return getFactory().getInstance(clazz);
    }


    
Convenience method to return a named logger, without the application
having to care about factories.

Params:
  • name – Logical name of the Log instance to be
 returned (the meaning of this name is only known to the underlying
 logging implementation that is being wrapped)
Throws:
Returns:A log instance with the requested name
/**
     * Convenience method to return a named logger, without the application
     * having to care about factories.
     *
     * @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)
     *
     * @return A log instance with the requested name
     *
     * @exception LogConfigurationException if a suitable <code>Log</code>
     *  instance cannot be returned
     */
    public static Log getLog(String name)
        throws LogConfigurationException {
        return getFactory().getInstance(name);
    }


    
Release any internal references to previously created LogFactory instances that have been associated with the specified class loader (if any), after calling the instance method release() on
each of them.

Params:
  • classLoader – ClassLoader for which to release the LogFactory
/**
     * Release any internal references to previously created {@link LogFactory}
     * instances that have been associated with the specified class loader
     * (if any), after calling the instance method <code>release()</code> on
     * each of them.
     *
     * @param classLoader ClassLoader for which to release the LogFactory
     */
    public static void release(ClassLoader classLoader) {
        // JULI's log manager looks at the current classLoader so there is no
        // need to use the passed in classLoader, the default implementation
        // does not so calling reset in that case will break things
        if (!LogManager.getLogManager().getClass().getName().equals(
                "java.util.logging.LogManager")) {
            LogManager.getLogManager().reset();
        }
    }
}