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.

Pavis

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"
}

Overarch

There is one problem though. Not all of the log messages should contain MyJsonKey. So in that case the log gets printed and it value field comes out to be "{ctx:mykey}", which looks very bad. Is there any way we can add parameters to json log dynamically ? @Overarch – Pavis

If you don't want the key printed in the log at all you probably want to look at RoutingAppender and use two different appender configurations within the Routes - one with the key, the other without. Also read the log4j2 FAQ page, there is a good example of RoutingAppender in there. – Overarch

A better way to use your value field is $${ctx:myKey:-}. The json key will exist even with empty value, however the ctx:myKey will not appears in logs, just an empty string. – Plauen

Is it necessary to do a ThreadContext.clearMap(); just like we used to do a MDC.clear() at the end? – Aigneis