Question

When you do something like LOG.debug("Exported {}.", product) in slf4j it will eventually call toString() on the arguments, e.g. product.

For certain reasons, I can’t override toString() on all the classes I want to use as arguments. Some classes come from third party jars, others will have their toString() called in other contexts, too, where the information I want to print in my log statement isn’t available.

However, I have a class for debugging purposes which has a method DebugFormatter.format(Object) that has a long cascade of instanceofs that selects the routine to find some useful debugging information about that object.

My question is: Is it possible to configure slf4j so that it calls such a static method instead of toString()?

Of course, I could call my format method on the object before passing it as a parameter to Logger.debug() but then it would be executed even when the respective logger is not enabled. So I had to surround it with if (LOG.isDebugEnabled()) which means that the whole point of having arguments in debug() was missed.

Answer 1

You could create a shim, taking your object and calling DebugFormatter.format() from its toString() function. Something like this:

class DebugFormatObject {
  private final Object o;

  public static DebugFormatObject forDebug(Object o) {
    return new DebugFormatObject(o);
  }

  private DebugFormatObject(Object o) {
    this.o = o;
  }

  @Override
  public String toString() {
    return DebugFormatter.format(o);
  }
}

With the appropriate static import, your logging statement becomes this:

LOG.debug("Exported {}.", forDebug(product));

This does have slightly more overhead than passing the object in straight, but it’s a small, constant overhead — and the object created will be very short-lived.