通過查看log4j源碼，版本1.2.16，log4j包 去默認的位置加載 配置文件；

文件名如下：

static public final String DEFAULT_CONFIGURATION_FILE = "log4j.properties";
  
  static final String DEFAULT_XML_CONFIGURATION_FILE = "log4j.xml";  

加載配置的過程：

LogManager 的 static 代碼段：

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."); 
    }  

Loader.getResource()代碼如下，其中getTCL()，獲取的是線程上下文類加載器，即{Thread.currentThread().getContextClassLoader()}，該類加載器是可設置的，默認為系統類加載器；

 static public URL getResource(String resource) {
    ClassLoader classLoader = null;
    URL url = null;
    
    try {
      if(!java1 && !ignoreTCL) {
        classLoader = getTCL();
        if(classLoader != null) {
          LogLog.debug("Trying to find ["+resource+"] using context classloader "
               +classLoader+".");
          url = classLoader.getResource(resource);      
          if(url != null) {
            return url;
          }
        }
      }

log4j默認加載的配置文件