[SOLVED] Extend Log4j to capture log arguments

Extend Log4j to capture log arguments

I aim to extend log4j in Spring Boot 3 to intercept log arguments and hide sensitive data when logging automatically. Since I had a difficult time to find a solution, I am opening and answering my own question, hoping it will help someone else.

Solution

Add the dependency org.apache.logging.log4j:log4j-api:2.24.2.

Then create a custom LogFactory class by extending org.apache.logging.log4j.message.MessageFactory2 and implement all the methods (yes, they are many but implementation is easy). Example:

class MyLogFactory : MessageFactory2 {

  override fun newMessage(charSequence: CharSequence?): Message {
    return charSequence?.let {
      ParameterizedMessage(it.toString())
    } ?: ParameterizedMessage("")
  }

  override fun newMessage(message: String?, p0: Any?): Message {
    return ParameterizedMessage(
      message,
      hideSensitiveData(arrayOf(p0))
    )
  }

  override fun newMessage(message: String?, p0: Any?, p1: Any?): Message {
    return ParameterizedMessage(
      message,
      hideSensitiveData(arrayOf(p0, p1))
    )
  }

  // and so on
}

Then when creating the logger, specific to use the custom LogFactory:

Logger log = LogManager.getLogger(MyCurrentClass.class, new MyLogFactory());

If you prefer to use the custom log factory by default for any class, then you can specify it by defining a file under resources named log4j2.properties:

log4j2.loggerFactory=com.example.MyLogFactory