Log4j配置为何失效

昨天上线折腾了半天，发现日志始终不能正确打印到指定文件，反而Tomcat的catalina.out打印了业务日志，仔细检查线上log4j配置，未发现任何问题，console日志输入并没有打开。于是迅速使用上次上线包进行替换重新发布，发现问题解决。

考虑到本次上线新依赖了其他系统的jar包，检查该jar后发现，jar包中包含了Log4j.xml！！

log4j为何选择了jar中的log4j.xml而不是自己的log4j.properties文件呢，原因就在log4j的LogManager类中

看段代码:

/**
   * @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";

以上是LogManager的成员变量。

然后看static模块：

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+"].");
      }
    } else {
        LogLog.debug("Default initialization of overridden by " +
            DEFAULT_INIT_OVERRIDE_KEY + "property.");
    }
  } 

关键点就在

if(configurationOptionStr == null) {
	url = Loader.getResource(DEFAULT_XML_CONFIGURATION_FILE);
	if(url == null) {
	  url = Loader.getResource(DEFAULT_CONFIGURATION_FILE);
	}
      }

说明log4j优先使用程序制定的配置路径，然后使用类路径下的xml配置文件，然后才选择properties文件。这就是原因，希望对大家有所帮助。

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