org.apache.log4j
public class NDC extends Object
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 {@link #push push}, {@link #pop}, {@link #clear}, {@link #getDepth} and {@link #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 {@link #push push} operation. Simply put,
NDC.push
. As a
side effect, if there is no nested diagnostic context for the
current thread, this method will create it.
NDC.pop
.
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, {@link PatternLayout} and {@link 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 {@link #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 {@link #remove} before exiting a thread.
A thread may inherit the nested diagnostic context of another (possibly parent) thread using the {@link #inherit inherit} method. A thread may obtain a copy of its NDC with the {@link #cloneStack cloneStack} method and pass the reference to any other thread, in particular to a child.
Since: 0.7.0
Method Summary | |
---|---|
static void | clear()
Clear any nested diagnostic information if any. |
static Stack | cloneStack()
Clone the diagnostic context for the current thread.
|
static String | get()
Never use this method directly, use the {@link
org.apache.log4j.spi.LoggingEvent#getNDC} method instead. |
static int | getDepth()
Get the current nesting depth of this diagnostic context.
|
static void | inherit(Stack stack)
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(String message)
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. |
This method is equivalent to calling the {@link #setMaxDepth}
method with a zero maxDepth
argument.
Since: 0.8.4c
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 {@link #inherit inherit} method to inherit the parent's diagnostic context.
Returns: Stack A clone of the current thread's diagnostic context.
Since: 0.7.5
See Also: NDC
The parent thread can obtain a reference to its diagnostic context using the {@link #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.
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.
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.
The contents of the message
parameter is
determined solely by the client.
Parameters: message The new diagnostic context information.
Each thread that created a diagnostic context by calling {@link #push} 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 {@link #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.
maxDepth
, then no
action is taken.
This method is a convenient alternative to multiple {@link
#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.
Since: 0.7.5
See Also: NDC