Spring boot append structured json log with a new property

Question

My application is using structured json logging and log4j. My goal is to add a new field to json log if a specific throwable is passed into log method as an argument

With mdc this is easy

} catch (SpecificException ex)
   MDC.put(key, ex.getData())
   logger.error("something bad happened", ex)
   MDC.remove(key)
} catch (OtherException ex) {
  logger.error("something bad happened", ex)
}

However I would prefer to find a solution that would work project wide possibly without mdc api optimally on a level of log appender. Is anything like this possible to achieve with log4j?

user30673520 · Accepted Answer · 2025-06-06 06:00:12Z

1

May I suggest you to try net.logstash.logback.argument.StructuredArguments

Here is the doc:

From there, you can try:

log.info("Your message {}", keyValue("key", yourValue));

which will produce something like:

{
  "@timestamp": "2019-04-17T18:27:01.099+02:00",
  "@version": "1",
  "message": "our message key=123",
  "logger_name": "MyApplication",
  "thread_name": "main",
  "level": "INFO",
  "level_value": 20000,
  "key": "123"
}

Or even something like:

log.info("Some object with ID {} is processed on {}.",
  StructuredArguments.fields(someObjectId),
  "someOtherArgument" // etc
);

answered Jun 6 at 6:00

user30673520

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