Log4j

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。

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【1】自动加载原理

JVM加载log4j的类(org.apache.log4j.LogManager)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);             }         }      ...// }

如下图所示配置了加载的两种配置文件：

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【2】非src路径下

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>

其配置参数说明如下图：