Logging in Java can be messy. Especially the dreaded isDebugEnabled. Look – a class that calculates something:

import org.slf4j.LoggerFactory;

class SomeCalculator implements Calculator {

  private final log = LoggerFactory.getLogger(getClass());

  public Result caclulate(Input input) {

    if (log.isDebugEnabled()) {

      log.debug("Input is " + input);

    }

    Result result = input.times(64).shift(LEFT);

    if (log.isDebugEnabled()) {

      log.debug("Result is " + result);

    }

    return result;

  }

}

We have a class which has one line that actually does something and loads of lines that do logging. This is kind of simple, but in more complex cases it really messes your head up.

Now, consider this, in Scala:

class SomeCalculator extends Calculator with Logging {

  override def calculate(input:Input) : Result = {

    debug("Input is " + input)

    val result = input.times(64).shift(LEFT);

    debug("Result is " + result)

    result

  }

}

Why is this possible? Because I’ve made myself a nice little Logging trait, which looks something like this:

package com.supplesoftware.scala.log

import org.slf4j.LoggerFactory

trait Logging {

  val log = LoggerFactory.getLogger(getClass)

  def debug(msg: => String) = if (log.isDebugEnabled) log.debug(msg)

  ....

  def error(msg: => String, e:Throwable) = if (log.isErrorEnabled) log.error(msg,e)

}

So, as long as I add the logging trait to my Calculator class, I can just say debug(“something”). Note that the argument to debug() will not get evaluated unless it’s necessary – the argument is actually a function, not the actual evaluated string. So doing one isDebugEnabled in the actual Logging trait is fine and does not cause evaluation of the string.

In case that sounds weird, consider the debug method written like this:

def debug(msg: () => String) = if (log.isDebugEnabled) log.debug(msg())

I.e. the argument is not a by-name argument, but an actual function (which I guess is pretty much the same), taking no arguments and returning String. If debug is enabled, the VM will execute that function, to get the result – the log message.

Thanks for the folks at StackOverflow for the great suggestions.