Log4j - 自动加载原理

有时候会发现在项目中直接在src(即，编译后的WEN-INF/classes)下放置log4j.properties or log4j.xml，项目会自动加载并打印日志。

原理如下：

JVM加载log4j的类(LogManager.class)后，执行静态代码块，会尝试加载默认路径下(WEN-INF/classes)的log4j.properties or log4j.xml:

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";
...//

部分静态代码块：

 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);

-- 注意下面的代码 -- 注意下面的代码 -- 注意下面的代码 -- 注意下面的代码

      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); 
    }   
      }
      ...//
 }

另外，如果log4j.properties or log4j.xml 放到了另外路径下，比如src/config ，那么可以在spring.xml 或者 web.xml加载该配置文件，如下：

spring方式加载，配置于web.xml中：

Spring加载log4j.properties，它提供了一个Log4jConfigListener，本身就能通过web.xml配置从指定位置加载log4j配置文件和log4j的输出路径，要注意的是Log4jConfigListener必须要在Spring的Listener之前。

<!-- 设置由Sprng载入的Log4j配置文件位置 -->

<context-param>  

    <param-name>log4jConfigLocation</param-name>  

    <param-value>WEB-INF/classes/config/log4j.properties</param-value>  

</context-param>  

  <!-- Spring刷新Log4j配置文件变动的间隔,单位为毫秒 -->

<context-param>  

    <param-name>log4jRefreshInterval</param-name>  

    <param-value>10000</param-value>  

</context-param>  

<listener>  

    <listener-class>org.springframework.web.util.Log4jConfigListener</listener-class>  

</listener>