Hi Steve,
Your question really should be directed to the SLF4J mailing list [1], as
Log4j already provides what you need. Your question deals with "why doesn't
SLF4J provide what Log4j provides", which the Log4j team cannot
authoritatively answer.
[1] http://slf4j.org/mailing-lists.html
Jake
On Tue, 01 Nov 2011 22:04:24 -0500
Steve Cohen <sco...@javactivity.org> wrote:
I haven't used MDC's in awhile but today I thought I saw a way to put them
to advantage for an application I'm working on. In doing so, though ,I
stumbled upon an incompatibility between log4j's MDC and slf4j's MDC that
I've not seen much comment on.
The signatures for the get() and put() methods in the two MDC classes are
incompatible.
In log4j, it's
public static Object get(String key)
and
public static void put(String key,
Object o)
while in slf4j (and logback), it's
public static String get(String key)
throws IllegalArgumentException
and
public static void put(String key,
String val)
throws IllegalArgumentException
I'm not concerned about the exceptions as much as I am about the restriction
to Strings.
It so happens that the use case I came up with today much prefers objects to
Strings as things to put on an MDC. Admittedly, my use case is a bit
non-standard. Basically I want to defer to append-time in the case of one
appender the decision as to whether to emit or not emit a log message. We
are interfacing to a legacy system and want to use a JMS Queue Appender to
send messages to the server for this legacy system. This system uses its own
system of bitmapped filters to control output and we have to use it. We'd
like to restrict logging to this appender, whereas, possibly not restrict it
in the case of other optional local appenders on the client side, by this
method. The way I hit upon to do that was to defer the go/no-go decision to
the appender in the case of the JMS Queue appender. But to do that I need to
put non-Strings on the MDC.
Converting these longs to and from a String seems like a somewhat heavy
penalty to pay just to be able to use slf4j instead of log4j which I'd much
prefer to do otherwise. With non-Strings in the MDC, I just OR two longs
together at the appender and abort the log.
I suppose I could just or them together at logging time and store true or
false in the MDC which only the JMS appender would look at. That's probably
the best thing to do. But why was the ability to store non-Strings removed
between log4j and slf4j in the first place?
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