在提供静态方法中，创建单例，并提供静态方法访问单例中的方法

本文介绍了Apache Log4j框架的配置与初始化过程，详细解释了如何通过系统属性指定配置文件位置，以及如何使用默认配置文件进行日志记录。此外还介绍了如何设置RepositorySelector和获取Logger实例。

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/*
* 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.log4j;

import org.apache.log4j.spi.LoggerRepository;
import org.apache.log4j.spi.LoggerFactory;
import org.apache.log4j.spi.RepositorySelector;
import org.apache.log4j.spi.DefaultRepositorySelector;
import org.apache.log4j.spi.RootLogger;
import org.apache.log4j.spi.NOPLoggerRepository;
import org.apache.log4j.helpers.Loader;
import org.apache.log4j.helpers.OptionConverter;
import org.apache.log4j.helpers.LogLog;

import java.net.URL;
import java.net.MalformedURLException;

import java.util.Enumeration;

/**
* Use the <code>LogManager</code> class to retreive {@link Logger}
* instances or to operate on the current {@link
* LoggerRepository}. When the <code>LogManager</code> class is loaded
* into memory the default initalzation procedure is inititated. The
* default intialization procedure</a> is described in the <a
* href="../../../../manual.html#defaultInit">short log4j manual</a>.
*
* @author Ceki Gülcü */
public class LogManager {

/**
   * @deprecated This variable is for internal use only. It will
   * become package protected in future versions.
   * */
static public final String DEFAULT_CONFIGURATION_FILE = "log4j.properties";

static final String DEFAULT_XML_CONFIGURATION_FILE = "log4j.xml";

/**
   * @deprecated This variable is for internal use only. It will
   * become private in future versions.
   * */
static final public String DEFAULT_CONFIGURATION_KEY="log4j.configuration";

/**
   * @deprecated This variable is for internal use only. It will
   * become private in future versions.
   * */
static final public String CONFIGURATOR_CLASS_KEY="log4j.configuratorClass";

/**
* @deprecated This variable is for internal use only. It will
* become private in future versions.
*/
public static final String DEFAULT_INIT_OVERRIDE_KEY =
"log4j.defaultInitOverride";

static private Object guard = null;
static private RepositorySelector repositorySelector;

static {
    // By default we use a DefaultRepositorySelector which always returns 'h'.
    Hierarchy h = new Hierarchy(new RootLogger((Level) Level.DEBUG));
    repositorySelector = new DefaultRepositorySelector(h);

    /** Search for the properties file log4j.properties in the CLASSPATH. */
    String override =OptionConverter.getSystemProperty(DEFAULT_INIT_OVERRIDE_KEY,
             null);

    // if there is no default init override, then get the resource
    // specified by the user or the default config file.
    if(override == null || "false".equalsIgnoreCase(override)) {

      String configurationOptionStr = OptionConverter.getSystemProperty(
         DEFAULT_CONFIGURATION_KEY,
         null);

      String configuratorClassName = OptionConverter.getSystemProperty(
                                                   CONFIGURATOR_CLASS_KEY,
         null);

URL url = null;

      // if the user has not specified the log4j.configuration
      // property, we search first for the file "log4j.xml" and then
      // "log4j.properties"
      if(configurationOptionStr == null) {
url = Loader.getResource(DEFAULT_XML_CONFIGURATION_FILE);
if(url == null) {
   url = Loader.getResource(DEFAULT_CONFIGURATION_FILE);
}
      } else {
try {
   url = new URL(configurationOptionStr);
} catch (MalformedURLException ex) {
   // so, resource is not a URL:
   // attempt to get the resource from the class path
   url = Loader.getResource(configurationOptionStr);
}
      }

      // If we have a non-null url, then delegate the rest of the
      // configuration to the OptionConverter.selectAndConfigure
      // method.
      if(url != null) {
     LogLog.debug("Using URL ["+url+"] for automatic log4j configuration.");
        try {
            OptionConverter.selectAndConfigure(url, configuratorClassName,
        LogManager.getLoggerRepository());
        } catch (NoClassDefFoundError e) {
            LogLog.warn("Error during default initialization", e);
        }
      } else {
     LogLog.debug("Could not find resource: ["+configurationOptionStr+"].");
      }
    }
}

/**
 Sets <code>LoggerFactory</code> but only if the correct
 guard is passed as parameter.

 Initally the guard is null. If the guard is
 <code>null</code>, then invoking this method sets the logger
 factory and the guard. Following invocations will throw a {@link
 IllegalArgumentException}, unless the previously set
 <code>guard</code> is passed as the second parameter.

This allows a high-level component to set the {@link
 RepositorySelector} used by the <code>LogManager</code>.

 For example, when tomcat starts it will be able to install its
 own repository selector. However, if and when Tomcat is embedded
 within JBoss, then JBoss will install its own repository selector
 and Tomcat will use the repository selector set by its container,
 JBoss. */
static
public
void setRepositorySelector(RepositorySelector selector, Object guard)
 throws IllegalArgumentException {
 if((LogManager.guard != null) && (LogManager.guard != guard)) {
 throw new IllegalArgumentException(
 "Attempted to reset the LoggerFactory without possessing the guard.");
 }

    if(selector == null) {
      throw new IllegalArgumentException("RepositorySelector must be non-null.");
    }

LogManager.guard = guard;
LogManager.repositorySelector = selector;
}

static
public
LoggerRepository getLoggerRepository() {
    if (repositorySelector == null) {
        repositorySelector = new DefaultRepositorySelector(new NOPLoggerRepository());
        guard = null;
        LogLog.error("LogMananger.repositorySelector was null likely due to error in class reloading, using NOPLoggerRepository.");
    }
    return repositorySelector.getLoggerRepository();
}

/**
     Retrieve the appropriate root logger.
   */
public
static
Logger getRootLogger() {
     // Delegate the actual manufacturing of the logger to the logger repository.
    return getLoggerRepository().getRootLogger();
}

/**
     Retrieve the appropriate {@link Logger} instance.
*/
public
static
Logger getLogger(final String name) {
     // Delegate the actual manufacturing of the logger to the logger repository.
    return getLoggerRepository().getLogger(name);
}

/**
     Retrieve the appropriate {@link Logger} instance.
*/
public
static
Logger getLogger(final Class clazz) {
     // Delegate the actual manufacturing of the logger to the logger repository.
    return getLoggerRepository().getLogger(clazz.getName());
}

/**
     Retrieve the appropriate {@link Logger} instance.
*/
public
static
Logger getLogger(final String name, final LoggerFactory factory) {
     // Delegate the actual manufacturing of the logger to the logger repository.
    return getLoggerRepository().getLogger(name, factory);
}

public
static
Logger exists(final String name) {
return getLoggerRepository().exists(name);
}

public
static
Enumeration getCurrentLoggers() {
return getLoggerRepository().getCurrentLoggers();
}

public
static
void shutdown() {
getLoggerRepository().shutdown();
}

public
static
void resetConfiguration() {
getLoggerRepository().resetConfiguration();
}
}