Log4j.properties文件的加载

org.apache.log4j.LogManager类有一个静态块,首先是找 log4j.xml，找不到的情况下才找 log4j.properties

Java code

    
    
    
    
            
     
     
     
     //
     
     
     
      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);
                }
            }
    
    
    
    

2，然后是怎么找呢：如下代码，是委托给classloader(加载Loader类的classloader)去找了，

Java code

    
    
    
    
      
     
     
     
     //
     
     
     
      We could not find resource. Ler us now try with the 
     
     
     
     //
     
     
     
      classloader that loaded this class.
     
     
     
            classLoader = Loader.
     
     
     
     class.getClassLoader(); 
      
     
     
     
     if(classLoader != 
     
     
     
     null) {
        LogLog.debug("Trying to find ["+resource+"] using "+classLoader
                 +" class loader.");
        url = classLoader.getResource(resource);
        
     
     
     
     if(url != 
     
     
     
     null) {
          
     
     
     
     return url;
        }
      }
    
    
    
    

3，classloader的getResource(...)又是怎么找呢：总是先从父classloader里去找，找不到才自己去找

Java code

    
    
    
    
    
     
     
     
     public URL getResource(String name) {
    URL url;
    
     
     
     
     if (parent != 
     
     
     
     null) {
        url = parent.getResource(name);
    } 
     
     
     
     else {
        url = getBootstrapResource(name);
    }
    
     
     
     
     if (url == 
     
     
     
     null) {
        url = findResource(name);
    }
    
     
     
     
     return url;
    }
    
    
    
    

总结：对于不同的应用服务器(或者web服务器)来说，classloader的层次不尽相同。这里以最简单的tomcat来说，如果你的应用是部署到tomcat下的，使用log4j配置文件的顺序就是$TOMCAT_HOME/lib/log4j.xml或者log4j.properties==>你自己web应用/WEB-INF/classes(或者lib)/log4j.xml或者log4j.properties.
对于WEB-INF下是classes优先还是lib优先 你可以自己测试一下。



以上是个人意见，希望你结合自己的测试结果看看。