log4j 是怎么取到行号的

log4j 是怎么取到行号的

我使用的是 log4j_1.2.15，在其源代码上进行分析。

修改 examples 目录下的 MyPatternLayout.java

		//Layout layout = new MyPatternLayout("[counter=%.10#] - %m%n");
		Layout layout = new MyPatternLayout("[counter=%.10#]%L - %m%n");

让其输出行号。


修改 org/apache/log4j/spi/LocationInfo.java

加入两个输出：

		this.fullInfo = s.substring(ibegin, iend);//150 行左右
		System.out.println("151 s:" + s);
		System.out.println("152 fullInfo:" + fullInfo);

分析一下运行结果，很明显，发起一个异常，然后得到行号。

LocationInfo.java 里，主要有如下几个方法：
getClassName
getFileName
getLineNumber
getMethodName

除了行号，其它几个属性的获得，也应该是通过异常来得到的。这样，将极其耗费资源。


结论：要让 log4j 输出行号以及一些附属信息，要耗费很多资源的。一定要慎重使用。


附
运行结果：

151 s:java.lang.Throwable
	at org.apache.log4j.spi.LoggingEvent.getLocationInformation(LoggingEvent.java:247)
	at org.apache.log4j.helpers.PatternParser$LocationPatternConverter.convert(PatternParser.java:483)
	at org.apache.log4j.helpers.PatternConverter.format(PatternConverter.java:65)
	at org.apache.log4j.PatternLayout.format(PatternLayout.java:502)
	at org.apache.log4j.WriterAppender.subAppend(WriterAppender.java:302)
	at org.apache.log4j.WriterAppender.append(WriterAppender.java:160)
	at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
	at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
	at org.apache.log4j.Category.callAppenders(Category.java:208)
	at org.apache.log4j.Category.forcedLog(Category.java:396)
	at org.apache.log4j.Category.debug(Category.java:263)
	at examples.MyPatternLayout.main(MyPatternLayout.java:59)

152 fullInfo:examples.MyPatternLayout.main(MyPatternLayout.java:59)
[counter=1]59 - Hello, log
151 s:java.lang.Throwable
	at org.apache.log4j.spi.LoggingEvent.getLocationInformation(LoggingEvent.java:247)
	at org.apache.log4j.helpers.PatternParser$LocationPatternConverter.convert(PatternParser.java:483)
	at org.apache.log4j.helpers.PatternConverter.format(PatternConverter.java:65)
	at org.apache.log4j.PatternLayout.format(PatternLayout.java:502)
	at org.apache.log4j.WriterAppender.subAppend(WriterAppender.java:302)
	at org.apache.log4j.WriterAppender.append(WriterAppender.java:160)
	at org.apache.log4j.AppenderSkeleton.doAppend(AppenderSkeleton.java:251)
	at org.apache.log4j.helpers.AppenderAttachableImpl.appendLoopOnAppenders(AppenderAttachableImpl.java:66)
	at org.apache.log4j.Category.callAppenders(Category.java:208)
	at org.apache.log4j.Category.forcedLog(Category.java:396)
	at org.apache.log4j.Category.info(Category.java:637)
	at examples.MyPatternLayout.main(MyPatternLayout.java:60)

152 fullInfo:examples.MyPatternLayout.main(MyPatternLayout.java:60)
[counter=2]60 - Hello again...