[logback-user] Associating a per-class instance property with a Logger

[George C. Hawkins]

Hallo --

We have a system where we have what I'll call groups.

Each group has a numeric identifier and when a new group is created a
corresponding large set of heterogeneous objects are created.

These objects are not shared between groups.

Whenever any of these objects log anything we always want to see the
numeric identifier (of the group to which they belong) in the log message.

Ideally I'd create all loggers as instance variables (rather than as
static) and at the point of creation somehow associate a group
identifier with them.

E.g.

public class Foo
{
     private final Logger logger = LoggerFactory.getLogger(Foo.class);

     public Foo(long groupId)
     {
         logger.setProperty("groupId", groupId);
     }
}

So far the best I can come up with is to create a helper class like so:

     public static class GroupId
     {
         private final String id;

         public GroupId(long id) { this.id = Long.toString(id); }

         public Object[] args(Object... o)
         {
             MDC.put("group", id);
             return o;
         }
     }

I then make one of these a member of each class that also has a logger -
and then require people to only use the Logger methods that take an
Object array and use them like so:

logger.error("expected [0, 1] but received [{}, {}]", groupId.args(x, y));

[ If I do this I don't have to move from static to instance variables
for my loggers ]

This approach means I have to update MDC every call (I thought of
maintaining a lastId ThreadLocal value but it didn't look as if the cost
of using this to avoid not calling put on MDC every time would really
save anything). And it also means people have to use the GroupId
instance even if they have no arguments for a given log message, e.g.:

logger.error("limit exceeded", groupId.args());

I'm inclined to think using Markers would be better. I would create a
Marker instance in every class where I also had a logger, and initialize
it such that its name would return something like "groupId=" + groupId.

However comments like the following on stackoverflow suggest this is an
inappropriate use of Markers:

http://stackoverflow.com/a/4167298/245602

At the moment though I think I'll ignore this and uses Markers unless
someone can suggest a better approach?

Regards,

/George


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