log4j中的DailyRollingFileAppender日志输出器工作原理
新年之际，碰到线上日志文件消失问题，借此机会解读了一下log4j日志的工作原理，以此记录一下
首先项目中的log4j.properties配置内容
log4j.rootLogger=info,logfile
log4j.appender.console=org.apache.log4j.ConsoleAppender
log4j.appender.console.layout=org.apache.log4j.PatternLayout
log4j.appender.console.layout.ConversionPattern=%d %l %-5p - %m%n
log4j.appender.logfile=org.apache.log4j.DailyRollingFileAppender
log4j.appender.logfile.File=/opt/appstack/apache-tomcat/logs/XX/XX.log
log4j.appender.logfile.maxBackupIndex=30
log4j.appender.logfile.layout=org.apache.log4j.PatternLayout
log4j.appender.logfile.layout.ConversionPattern====%d{yyyy-MM-dd HH:mm:ss.SSS} %-5level %logger Line:%-3L - %msg%n
DailyRollingFileAppender依赖关系
log4j日志工作原理
一、读取初始化日志配置信息
private final Logger log = LoggerFactory.getLogger(this.getClass());
log对象获取工作原理
org.slf4j.LoggerFactory#getLogger(java.lang.String)
org.slf4j.impl.Log4jLoggerFactory#getLogger
所以这里的log对象是Log4jLoggerAdapter类型
LogManager的静态有对log4j.properties读取解析
二、代码日志记录工作原理
打印记录
log.info("开始时间：{}，访问路径：/api/user/login，手机号：{}，fv:{},fsv:{},登录参数信息：{}", start, loginDTO.getPhone(),
loginDTO.getFv(), loginDTO.getFsv(), JSON.toJSONString(loginDTO));
所有的日志级别记录，最终都是进入到以下方法
org.apache.log4j.Category#log(java.lang.String, org.apache.log4j.Priority, java.lang.Object, java.lang.Throwable)
整个日志记录执行方法调用过程：
org.slf4j.impl.Log4jLoggerAdapter#info(java.lang.String, java.lang.Object...)
org.apache.log4j.Category#log(java.lang.String, org.apache.log4j.Priority, java.lang.Object, java.lang.Throwable)
org.apache.log4j.Category#forcedLog(java.lang.String, org.apache.log4j.Priority, java.lang.Object, java.lang.Throwable)
org.apache.log4j.Category#callAppenders(org.apache.log4j.spi.LoggingEvent)
org.apache.log4j.helpers.AppenderAttachableImpl#appendLoopOnAppenders(org.apache.log4j.spi.LoggingEvent)
org.apache.log4j.AppenderSkeleton#doAppend(org.apache.log4j.spi.LoggingEvent)
org.apache.log4j.WriterAppender#append(org.apache.log4j.spi.LoggingEvent)
org.apache.log4j.DailyRollingFileAppender#subAppend(org.apache.log4j.spi.LoggingEvent)
org.apache.log4j.WriterAppender#subAppend(org.apache.log4j.spi.LoggingEvent)
org.apache.log4j.helpers.QuietWriter#write(java.lang.String)
其中org.apache.log4j.DailyRollingFileAppender#subAppend(org.apache.log4j.spi.LoggingEvent)方法代码逻辑
protected void subAppend( LoggingEvent event ){
long n = System.currentTimeMillis();
if ( n >= nextCheck )
{
now.setTime( n );
nextCheck = rc.getNextCheckMillis( now );
try {
rollOver();
}catch ( IOException ioe ) {
if ( ioe instanceof InterruptedIOException ){
Thread.currentThread().interrupt();
}
LogLog.error( "rollOver() failed.", ioe );
}
}
super.subAppend( event );
}
如果是当前时间到了下次生成新的日志文件时间时，执行rollOver方法，该方法包含对之前归档的日志文件处理以及生成新的日志文件
