Class NDC
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.
- Since:
- 0.7.0
- Author:
- Ceki Gülcü
-
Method Summary
Modifier and TypeMethodDescriptionstatic void
clear()
Clear any nested diagnostic information if any.static Stack
Clone the diagnostic context for the current thread.static String
get()
Never use this method directly, use theLoggingEvent.getNDC()
method instead.static int
getDepth()
Get the current nesting depth of this diagnostic context.static void
Inherit the diagnostic context of another thread.static String
peek()
Looks at the last diagnostic context at the top of this NDC without removing it.static String
pop()
Clients should call this method before leaving a diagnostic context.static void
Push new diagnostic context information for the current thread.static void
remove()
Remove the diagnostic context for this thread.static void
setMaxDepth
(int maxDepth) Set maximum depth of this diagnostic context.
-
Method Details
-
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 zeromaxDepth
argument.- Since:
- 0.8.4c
-
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.- Returns:
- Stack A clone of the current thread's diagnostic context.
-
inherit
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?
- Parameters:
stack
- The diagnostic context of the parent thread.
-
get
Never use this method directly, use theLoggingEvent.getNDC()
method instead. -
getDepth
public static int getDepth()Get the current nesting depth of this diagnostic context.- Since:
- 0.7.5
- See Also:
-
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.
- Returns:
- String The innermost diagnostic context.
-
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.
- Returns:
- String The innermost diagnostic context.
-
push
Push new diagnostic context information for the current thread.The contents of the
message
parameter is determined solely by the client.- Parameters:
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(java.lang.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 callremove
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 tomaxDepth
, 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. ThesetMaxDepth
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.- Since:
- 0.7.5
- See Also:
-