log4j Tricks (log4j 1.2)

1. 开启 log4j 框架内部的日志输出到控制台

# 在 log4j.properties 中添加
log4j.debug=true # 配置 log4j 框架内部的日志通过 System.out 输出

通过开启 log4j 内部日志，可以方便查看配置问题

n. 类作用

PropertyConfigurator : log4j.properties 对应的配置类

Q&A:

1. log4j.properties 配置了日志输出到控制台和文件，结果控制台的没有输出

log4j.rootLogger=STDOUT,INFO,ERROR

rootLogger 配置有问题，rootLogger 值对应的第一个串代表 rootLogger 的日志 level，应该改为：

log4j.rootLogger=INFO,STDOUT,INFO,ERROR

对应的源码：PropertyConfigurator#parseCategory(...)

 void parseCategory(Properties props, Logger logger, String optionKey, String loggerName, String value) {

     LogLog.debug("Parsing for [" + loggerName + "] with value=[" + value + "].");
     // We must skip over ',' but not white space
     StringTokenizer st = new StringTokenizer(value, ",");

     // If value is not in the form ", appender.." or "", then we should set
     // the level of the loggeregory.
     if (!(value.startsWith(",") || value.equals(""))) {

         // just to be on the safe side...
         if (!st.hasMoreTokens())
             return;

         // rootLogger 配置的第一个串为 levelStr
         String levelStr = st.nextToken();
         LogLog.debug("Level token is [" + levelStr + "].");

         // If the level value is inherited, set category level value to
         // null. We also check that the user has not specified inherited for the
         // root category.
         if (INHERITED.equalsIgnoreCase(levelStr) || NULL.equalsIgnoreCase(levelStr)) {
             if (loggerName.equals(INTERNAL_ROOT_NAME)) {
                 LogLog.warn("The root logger cannot be set to null.");
             } else {
                 logger.setLevel(null);
             }
         } else {
             logger.setLevel(OptionConverter.toLevel(levelStr, (Level) Level.DEBUG));
         }
         LogLog.debug("Category " + loggerName + " set to " + logger.getLevel());
     }

     // Begin by removing all existing appenders.
     logger.removeAllAppenders();

     Appender appender;
     String appenderName;
     // rootLogger 配置的第二个串往后的串为 appenderName
     while (st.hasMoreTokens()) {
         appenderName = st.nextToken().trim();
         if (appenderName == null || appenderName.equals(","))
             continue;
         LogLog.debug("Parsing appender named \"" + appenderName + "\".");
         appender = parseAppender(props, appenderName);
         if (appender != null) {
             logger.addAppender(appender);
         }
     }
 }