The NDC class implements
nested diagnostic contexts as
defined by Neil Harrison in the article "Patterns for Logging
Diagnostic Messages" part of the book "
Pattern Languages of
Program Design 3" edited by Martin et al.
A Nested Diagnostic Context, or NDC in short, is an instrument
to distinguish interleaved log output from different sources. Log
output is typically interleaved when a server handles multiple
clients near-simultaneously.
Interleaved log output can still be meaningful if each log entry
from different contexts had a distinctive stamp. This is where NDCs
come into play.
Note that NDCs are managed on a per thread
basis. NDC operations such as
push push,
.pop,
clear,
getDepth and
setMaxDepth
affect the NDC of the
current thread only. NDCs of other
threads remain unaffected.
For example, a servlet can build a per client request NDC
consisting the clients host name and other information contained in
the the request.
Cookies are another source of distinctive
information. To build an NDC one uses the
push push
operation. Simply put,
- Contexts can be nested.
- When entering a context, call
NDC.push
. As a
side effect, if there is no nested diagnostic context for the
current thread, this method will create it.
- When leaving a context, call
NDC.pop
.
- When exiting a thread make sure to call
remove NDC.remove().
There is no penalty for forgetting to match each
push
operation with a corresponding
pop
,
except the obvious mismatch between the real application context
and the context set in the NDC.
If configured to do so,
PatternLayout and
TTCCLayout instances automatically retrieve the nested diagnostic
context for the current thread without any user intervention.
Hence, even if a servlet is serving multiple clients
simultaneously, the logs emanating from the same code (belonging to
the same category) can still be distinguished because each client
request will have a different NDC tag.
Heavy duty systems should call the
remove method when
leaving the run method of a thread. This ensures that the memory
used by the thread can be freed by the Java garbage
collector. There is a mechanism to lazily remove references to dead
threads. In practice, this means that you can be a little sloppy
and sometimes forget to call
remove before exiting a
thread.
A thread may inherit the nested diagnostic context of another
(possibly parent) thread using the
inherit inherit
method. A thread may obtain a copy of its NDC with the
.cloneStack cloneStack method and pass the reference to any other
thread, in particular to a child.
NDC.push
. As a side effect, if there is no nested diagnostic context for the current thread, this method will create it.NDC.pop
.push
operation with a correspondingpop
, except the obvious mismatch between the real application context and the context set in the NDC. If configured to do so, PatternLayout and TTCCLayout instances automatically retrieve the nested diagnostic context for the current thread without any user intervention. Hence, even if a servlet is serving multiple clients simultaneously, the logs emanating from the same code (belonging to the same category) can still be distinguished because each client request will have a different NDC tag. Heavy duty systems should call the remove method when leaving the run method of a thread. This ensures that the memory used by the thread can be freed by the Java garbage collector. There is a mechanism to lazily remove references to dead threads. In practice, this means that you can be a little sloppy and sometimes forget to call remove before exiting a thread. A thread may inherit the nested diagnostic context of another (possibly parent) thread using the inherit inherit method. A thread may obtain a copy of its NDC with the .cloneStack cloneStack method and pass the reference to any other thread, in particular to a child.