Add MDC variables in JsonLayout - log4j2

Question

How to add MDC variables in the json log generated by JsonLayout of log4j2. I've used KeyValuePair tag to add properties like host name into the log, but I didn't found any way to add MDC variables into it. In pattern layout I used %X{traceId} but I'm sure JsonLayout can't parse those conversion chars(As far as I know conversion chars are used by pattern layout only). I went into source code of JsonLayout but didn't found function which actually puts all of the data into the log message.

Thank you.

Answer 1

What you're looking for is a log4j2 lookup. It sounds like you're interested specifically in the Context Map Lookup as you mentioned MDC (which is now called ThreadContext in log4j2 by the way).

Here is a simple example:

package example;

import org.apache.logging.log4j.LogManager;
import org.apache.logging.log4j.Logger;
import org.apache.logging.log4j.ThreadContext;

public class ThreadContextExample {

    private static final Logger log = LogManager.getLogger();

    public static void main(String[] args) {
        ThreadContext.put("myKey", "myValue");
        log.info("Here's a message!");
    }
}

Here is the log4j2.xml configuration:

<?xml version="1.0" encoding="UTF-8"?>
<Configuration status="WARN">
    <Appenders>
        <Console name="Console" target="SYSTEM_OUT">
            <JsonLayout compact="false" eventEol="false" stacktraceAsString="true">
                <KeyValuePair key="myJsonKey" value="${ctx:myKey}"/>
            </JsonLayout>
        </Console>

    </Appenders>

    <Loggers>
        <Root level="debug">
            <AppenderRef ref="Console"/>
        </Root>
    </Loggers>
</Configuration>

and finally some sample output (shortened for readability):

{
  "thread" : "main",
  "level" : "INFO",
  "loggerName" : "example.ThreadContextExample",
  "message" : "Here's a message!",
  ...
  "myJsonKey" : "myValue"
}