/*
 * 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.util.Hashtable;

import org.apache.commons.logging.impl.SLF4JLogFactory;

/**
 * <p>
 * Factory for creating {@link Log} instances, which always delegates to an
 * instance of {@link SLF4JLogFactory}.
 * 
 * </p>
 * 
 * @author Craig R. McClanahan
 * @author Costin Manolache
 * @author Richard A. Sitze
 * @author Ceki G&uuml;lc&uuml;
 */

public abstract class LogFactory {

  static String UNSUPPORTED_OPERATION_IN_JCL_OVER_SLF4J = "http://www.slf4j.org/codes.html#unsupported_operation_in_jcl_over_slf4j";

  static LogFactory logFactory = new SLF4JLogFactory();

  /**
   * The name (<code>priority</code>) of the key in the config file used to
   * specify the priority of that particular config file. The associated value
   * is a floating-point number; higher values take priority over lower values.
   * 
   * <p>
   * This property is not used but preserved here for compatibility.
   */
  public static final String PRIORITY_KEY = "priority";

  /**
   * The name (<code>use_tccl</code>) of the key in the config file used to
   * specify whether logging classes should be loaded via the thread context
   * class loader (TCCL), or not. By default, the TCCL is used.
   * 
   * <p>
   * This property is not used but preserved here for compatibility.
   */
  public static final String TCCL_KEY = "use_tccl";

  /**
   * The name of the property used to identify the LogFactory implementation
   * class name.
   * <p>
   * This property is not used but preserved here for compatibility.
   */
  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.
   * 
   * <p>
   * This property is not used but preserved here for compatibility.
   */
  public static final String FACTORY_DEFAULT = "org.apache.commons.logging.impl.SLF4JLogFactory";

  /**
   * The name of the properties file to search for.
   * <p>
   * This property is not used but preserved here for compatibility.
   */
  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>.
   * <p>
   * This property is not used but preserved here for compatibility.
   */
  protected static final String SERVICE_ID =
      "META-INF/services/org.apache.commons.logging.LogFactory";
  
  /**
   * The name (<code>org.apache.commons.logging.diagnostics.dest</code>) of
   * the property used to enable internal commons-logging diagnostic output, in
   * order to get information on what logging implementations are being
   * discovered, what classloaders they are loaded through, etc.
   * 
   * <p>
   * This property is not used but preserved here for compatibility.
   */
  public static final String DIAGNOSTICS_DEST_PROPERTY = "org.apache.commons.logging.diagnostics.dest";

  /**
   * <p>
   * Setting this system property value allows the <code>Hashtable</code> used
   * to store classloaders to be substituted by an alternative implementation.
   * <p>
   * This property is not used but preserved here for compatibility.
   */
  public static final String HASHTABLE_IMPLEMENTATION_PROPERTY = "org.apache.commons.logging.LogFactory.HashtableImpl";
  
  /**
   * The previously constructed <code>LogFactory</code> instances, keyed by
   * the <code>ClassLoader</code> with which it was created.
   * 
   * <p>
   * This property is not used but preserved here for compatibility.
   */
  protected static Hashtable factories = null;
  
  /**
   * <p>
   * This property is not used but preserved here for compatibility.
   */
  protected static LogFactory nullClassLoaderFactory = null;

  /**
   * 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
   * @return configuration attribute
   */
  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.
   * 
   * @return names of all currently defined configuration attributes
   */
  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 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.SLF4FLogFactory</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 {
    return logFactory;
  }

  /**
   * 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
   * 
   * @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.
   * 
   * @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 static Log getLog(String name) throws LogConfigurationException {
    return (getFactory().getInstance(name));
  }

  /**
   * 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) {
    // since SLF4J based JCL does not make use of classloaders, there is nothing
    // to do here
  }

  /**
   * Release any internal references to previously created {@link LogFactory}
   * instances, after calling the instance method <code>release()</code> on
   * each of them. 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 static void releaseAll() {
    // since SLF4J based JCL does not make use of classloaders, there is nothing
    // to do here
  }

  /**
   * Returns a string that uniquely identifies the specified object, including
   * its class.
   * <p>
   * The returned string is of form "classname@hashcode", ie is the same as the
   * return value of the Object.toString() method, but works even when the
   * specified object's class has overidden the toString method.
   * 
   * @param o
   *                may be null.
   * @return a string of form classname@hashcode, or "null" if param o is null.
   * @since 1.1
   */
  public static String objectId(Object o) {
    if (o == null) {
      return "null";
    } else {
      return o.getClass().getName() + "@" + System.identityHashCode(o);
    }
  }

  // protected methods which were added in JCL 1.1. These are not used
  // by SLF4JLogFactory

  /**
   * This method exists to ensure signature compatibility.
   */
  protected static Object createFactory(String factoryClass, ClassLoader classLoader) {
    throw new UnsupportedOperationException(
        "Operation [factoryClass] is not supported in jcl-over-slf4j. See also "
            + UNSUPPORTED_OPERATION_IN_JCL_OVER_SLF4J);
  }

  /**
   * This method exists to ensure signature compatibility.
   */
  protected static ClassLoader directGetContextClassLoader() {
    throw new UnsupportedOperationException(
        "Operation [directGetContextClassLoader] is not supported in jcl-over-slf4j. See also "
            + UNSUPPORTED_OPERATION_IN_JCL_OVER_SLF4J);
  }

  /**
   * This method exists to ensure signature compatibility.
   */
  protected static ClassLoader getContextClassLoader()
      throws LogConfigurationException {
    throw new UnsupportedOperationException(
        "Operation [getContextClassLoader] is not supported in jcl-over-slf4j. See also "
            + UNSUPPORTED_OPERATION_IN_JCL_OVER_SLF4J);
  }
  
  /**
   * This method exists to ensure signature compatibility.
   */
  protected static ClassLoader getClassLoader(Class clazz) {
    throw new UnsupportedOperationException(
        "Operation [getClassLoader] is not supported in jcl-over-slf4j. See also "
            + UNSUPPORTED_OPERATION_IN_JCL_OVER_SLF4J);
  }

  /**
   * This method exists to ensure signature compatibility.
   */
  protected static boolean isDiagnosticsEnabled() {
    throw new UnsupportedOperationException(
        "Operation [isDiagnosticsEnabled] is not supported in jcl-over-slf4j. See also "
            + UNSUPPORTED_OPERATION_IN_JCL_OVER_SLF4J);
  }
  
  /**
   * This method exists to ensure signature compatibility.
   */
  protected static void logRawDiagnostic(String msg) {
    throw new UnsupportedOperationException(
        "Operation [logRawDiagnostic] is not supported in jcl-over-slf4j. See also "
            + UNSUPPORTED_OPERATION_IN_JCL_OVER_SLF4J);
  }
  
  /**
   * This method exists to ensure signature compatibility.
   */
  protected static LogFactory newFactory(final String factoryClass,
      final ClassLoader classLoader, final ClassLoader contextClassLoader) {
    throw new UnsupportedOperationException(
        "Operation [logRawDiagnostic] is not supported in jcl-over-slf4j. See also "
            + UNSUPPORTED_OPERATION_IN_JCL_OVER_SLF4J);
  }
 
  /**
   * This method exists to ensure signature compatibility.
   */
  protected static LogFactory newFactory(final String factoryClass,
                                         final ClassLoader classLoader) {
    throw new UnsupportedOperationException(
        "Operation [newFactory] is not supported in jcl-over-slf4j. See also "
            + UNSUPPORTED_OPERATION_IN_JCL_OVER_SLF4J);
  }


}