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
,
pop()
,
clear()
,
getDepth()
and
setMaxDepth(int)
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
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
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
method. A thread may obtain a copy of its NDC with the
cloneStack
method and pass the reference to any other
thread, in particular to a child.
clear
public static void clear()
Clear any nested diagnostic information if any. This method is
useful in cases where the same thread can be potentially used
over and over in different unrelated contexts.
This method is equivalent to calling the
setMaxDepth(int)
method with a zero
maxDepth
argument.
cloneStack
public static Stack cloneStack()
Clone the diagnostic context for the current thread.
Internally a diagnostic context is represented as a stack. A
given thread can supply the stack (i.e. diagnostic context) to a
child thread so that the child can inherit the parent thread's
diagnostic context.
The child thread uses the
inherit
method to
inherit the parent's diagnostic context.
- Stack A clone of the current thread's diagnostic context.
get
public static String get()
getDepth
public static int getDepth()
Get the current nesting depth of this diagnostic context.
inherit
public static void inherit(Stack stack)
Inherit the diagnostic context of another thread.
The parent thread can obtain a reference to its diagnostic
context using the
cloneStack()
method. It should
communicate this information to its child so that it may inherit
the parent's diagnostic context.
The parent's diagnostic context is cloned before being
inherited. In other words, once inherited, the two diagnostic
contexts can be managed independently.
In java, a child thread cannot obtain a reference to its
parent, unless it is directly handed the reference. Consequently,
there is no client-transparent way of inheriting diagnostic
contexts. Do you know any solution to this problem?
stack
- The diagnostic context of the parent thread.
peek
public static String peek()
Looks at the last diagnostic context at the top of this NDC
without removing it.
The returned value is the value that was pushed last. If no
context is available, then the empty string "" is returned.
- String The innermost diagnostic context.
pop
public static String pop()
Clients should call this method before leaving a diagnostic
context.
The returned value is the value that was pushed last. If no
context is available, then the empty string "" is returned.
- String The innermost diagnostic context.
push
public static void push(String message)
Push new diagnostic context information for the current thread.
The contents of the
message
parameter is
determined solely by the client.
message
- The new diagnostic context information.
remove
public static void remove()
Remove the diagnostic context for this thread.
Each thread that created a diagnostic context by calling
push(String)
should call this method before exiting. Otherwise,
the memory used by the
thread cannot be reclaimed by the
VM.
As this is such an important problem in heavy duty systems and
because it is difficult to always guarantee that the remove
method is called before exiting a thread, this method has been
augmented to lazily remove references to dead threads. In
practice, this means that you can be a little sloppy and
occasionally forget to call
remove()
before exiting a
thread. However, you must call
remove
sometime. If
you never call it, then your application is sure to run out of
memory.
setMaxDepth
public static void setMaxDepth(int maxDepth)
Set maximum depth of this diagnostic context. If the current
depth is smaller or equal to
maxDepth
, then no
action is taken.
This method is a convenient alternative to multiple
pop()
calls. Moreover, it is often the case that at the end of
complex call sequences, the depth of the NDC is
unpredictable. The
setMaxDepth
method circumvents
this problem.
For example, the combination
void foo() {
int depth = NDC.getDepth();
... complex sequence of calls
NDC.setMaxDepth(depth);
}
ensures that between the entry and exit of foo the depth of the
diagnostic stack is conserved.