log4j,如何“自动加载”?

今天看代码,发现log4j.properties。没有相应的加载代码,但它却生效了,这多神奇!

 

看进去,org.apache.log4j, LogManager.java,其有一个static方法块:

  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.");      
    OptionConverter.selectAndConfigure(url, configuratorClassName, 
                       LogManager.getLoggerRepository());
      } else {
    LogLog.debug("Could not find resource: ["+configurationOptionStr+"].");
      }
    }  
  } 

换句话说,是什么呢?也就是说:

1. 获取系统属性,看是否用户设置了override。默认是不设置的。

2. 如果确实没有设置,那么尝试找一下,有没有log4j.xml,有则加载。

3. 如果还没有,那么尝试找一下,有没有log4j.properites,有则加载。

其中,2、3里提到的“尝试找一下”,可能是去哪个目录里面找呢?翻译了一下,效果不好,还是上原文清晰 :

  1. Search for resource using the thread context class loader under Java2. If that fails, search for resource using the class loader that loaded this class (Loader). Under JDK 1.1, only the the class loader that loaded this class (Loader) is used.
  2. Try one last time with ClassLoader.getSystemResource(resource), that is is using the system class loader in JDK 1.2 and virtual machine's built-in class loader in JDK 1.1. 

 

所以,你把log4j.xml或log4j.properties放在这些目录下,那么log4j会“自动去加载”到,不用程序里手工写加载代码了。

但我个人,还是倾向于自己写加载。因为这种“悄悄被人做掉”,一是代码很难理解,二是假如A同学放了一个log4j,B同学又写了一个放在其他目录,这种默认加载机制,不一定哪个生效及生效顺序。这种不确定性,还是自己写两行代码,消灭在摇篮里吧。

 

posted @ 2013-04-18 13:34  技术草根女  Views(...)  Comments(...Edit  收藏