putCloseable should not remove the key from MDC if it was previously set to a different value

[mdpearson]

Our team wrote some code that broadly looked like this:

MDC.put("color", "red");

try(ignored = MDC.putCloseable("color", "green")) {
...
}

assertTrue(MDC.get("color", "green")) // fails, the key is deleted

It should be an easy patch to allow callers to nest putCloseable calls and get expected results; in fact, I'll go try to write one now.

Thank you very much, by the way, for SLF4j -- it's a wonderful tool. Cheers

[huxi]

We had the same issue and addressed it by writing our own NestedMDCClosable, essentially wrapping the MDC.

This should also be taken into account for the MDCAmbit proposal in #377 and #372.

Our use case are correlationIds in Services where a UUID is supposed to turn up in every log statement of a method call.

This works perfectly in case of a single service but breaks with the current MDCClosable implementation once one such service is directly calling another such service or methods of itself.