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);
        }
    }
}