Display thread id instead thread name in log

Question

I have a Struts application with log4j to display information about application.

The pattern to format log's output is as follows:

log4j.appender.RALL.layout.ConversionPattern=[%p] %d{dd/MM/yyyy HH:mm:ss} [THREAD ID=%t] [CLASS=(%C{1}:%L)] %m%n  

I need to show the thread id instead the thread name in log. The conversion character that display the thread name is %t. I don't see in log4j documentation the way to get it.

Can anyone help me??

Answer 1

It is possible but not so easy as just using some preconfigured patterns.

Log4j 1.X and Log4j 2.x don't have any preconfigured patterns for printing Thread ID but you can always use some "magic trick".

PatternLayout is using PatternParser class which is mark as final class and has static map of "patterns" as keys and Converters classes as values. Everytime when Parses finds pattern using for logging pattern format starting with % it uses converter matched with this pattern key in map.

You cannot add your own rule to that map, but you can still write your own MyOwnPatternLayout:

public class MyOwnPatternLayout extends PatternLayout 

which will in it's format method do such trick:

public String format(LoggingEvent event) {    String log = super.format(event);    /*    Now you just have to replace with regex all occurences of %i or     any mark you would like to use as mark to represent Thread ID     with Thread ID value.    Only thing you have to be sure to not use any mark as your Thread ID    that already is defined by PatterParser class    */    return log.replaceAll("%i", someThreadID); } 

The only problem is that you have to get that thread ID in some way. Sometimes all you have to do is to parse Thread name which can you easily collect:

String threadName = event.getThreadName(); 

For example Apache-Tomcat put thread ID at the end of thread name http-nio-/127.0.0.1-8084"-exec-41.

To be sure that thread ID is correct you can also make your own subclass of LogginEvent and Logger (MyLoggingEvent and MyLogger) and inside MyLogger create MyLoggingEvent witch will also take as argument Thread ID not only Thread Name. Then you can easly collect it in code above.