core/src/main/java/org/apache/commons/digester3/plugins/LogUtils.java

package org.apache.commons.digester3.plugins;

/*
 * 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.
 */

import org.apache.commons.digester3.Digester;
import org.apache.commons.logging.Log;

/**
 * Simple utility class to assist in logging.
 * <p>
 * This class is intended only for the use of the code in the plugins packages. No "user" code should use this package.
 * <p>
 * The Digester module has an interesting approach to logging: all logging should be done via the Log object stored on
 * the digester instance that the object *doing* the logging is associated with.
 * <p>
 * This is done because apparently some "container"-type applications such as Avalon and Tomcat need to be able to
 * configure different logging for different <i>instances</i> of the Digester class which have been loaded from the same
 * ClassLoader [info from Craig McClanahan]. Not only the logging of the Digester instance should be affected; all
 * objects associated with that Digester instance should obey the reconfiguration of their owning Digester instance's
 * logging. The current solution is to force all objects to output logging info via a single Log object stored on the
 * Digester instance they are associated with.
 * <p>
 * Of course this causes problems if logging is attempted before an object <i>has</i> a valid reference to its owning
 * Digester. The getLogging method provided here resolves this issue by returning a Log object which silently discards
 * all logging output in this situation.
 * <p>
 * And it also implies that logging filtering can no longer be applied to subcomponents of the Digester, because all
 * logging is done via a single Log object (a single Category). C'est la vie...
 *
 * @since 1.6
 */
class LogUtils
{

    /**
     * Get the Log object associated with the specified Digester instance, or a "no-op" logging object if the digester
     * reference is null.
     * <p>
     * You should use this method instead of digester.getLogger() in any situation where the digester might be null.
     */
    static Log getLogger( Digester digester )
    {
        if ( digester == null )
        {
            return new org.apache.commons.logging.impl.NoOpLog();
        }

        return digester.getLogger();
    }

}