kotlin logging with lambda parameters

Question

In log4j2 we have a handy feature which is described as

// Java-8 style optimization: no need to explicitly check the log level:
// the lambda expression is not evaluated if the TRACE level is not enabled
logger.trace("Some long-running operation returned {}", () -> expensiveOperation());

my attempt to use this in kotlin

log.debug("random {}", { UUID.randomUUID() })

which will print

random Function0<java.util.UUID>

How do we use lambda argument logging with kotlin? Or how do we explicitly tell kotlin what method to call?

Answer 1

The problem is that debug() is overloaded, and has another method taking a vararg Object as argument. Kotlin chooses that overload rather than the one taking a Supplier<?> as argument, because it doesn't know that the lambda expression is supposed to be a Supplier.

Just specify it as a Supplier:

log.debug("random {}", Supplier { UUID.randomUUID() })