Display thread id instead thread name in log

I implemented thread ID and thread priority for the upcoming 2.6. Tracking here: https://issues.apache.org/jira/browse/LOG4J2-1299

You can pick up a 2.6-SNAPSHOT build from the Apache snapshots repository: https://repository.apache.org/content/repositories/snapshots/

One way you can do it is to add it yourself using log4j MDC. We use it for adding the username for web requests. We do this in a filter at the start of each request. Eg.

import org.apache.log4j.MDC;

...

  // Add username to MDC
  String username = ...;
  MDC.put("user", username);

Then add [%X{user}] to your conversion pattern.

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.