Method resolution during logging: Why is ()->String.format(...) not recoginzed as Supplier<String>?

Question

I have made a call to the log4j-v2-API like this:

Logger logger = LogManager.getLogger("MyLogger");

/**
 * syntactic sugar as part of a facade
 */
private void logAtLevel(String level, Supplier<String> messageSupplier, Throwable thrown){
      Level priority = Level.toLevel(level, Level.ALL);
      if (null == thrown) {
         logger.log(priority, messageSupplier);
      } else {
         logger.log(priority, messageSupplier, thrown);
      }

}

/**
 * method calling logger
 */
private void callLogging(Object value){
   logAtLevel("Debug", ()->String.format("something or other: %s", value), null);

}

My expectations would have been that the above call creates a log entry "something or other: <object.toString()>" however I got "lambda@1223454" instead.

This suggests to me that the method that was executed is log(level, object) rather than the expected log(level, supplier)

Why are the methods resolved the way they are and how can I prevent this (preferably without casting)?

Answer 1

Your Supplier is not Log4j2's Supplier!

java.util.function.Supplier was introduced in Java 8. At around the same time (version 2.4, cf. LOG4J2-599) Log4j2 introduced org.apache.logging.log4j.util.Supplier so that the library can be used on Java 7.

That is why your code does not call log(Level, Supplier<?>, Throwable) but log(Level, Object, Throwable) and you end up with Supplier#toString() being logged instead of Supplier#get().

This is probably something that should change and I filed a wish list bug about it (cf. #1262).

Remark: Wrapping well established logging APIs like Log4j2 API and SLF4J into custom wrappers is not a good idea (cf. this question). While it is not rocket science to write a good wrapper, there are many details you should consider. For example your wrapper breaks location information.