org.apache.log4j

Class NDC

java.lang.Object

org.apache.log4j.NDC

public static class NDC
extends Object

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 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

See Also:

Serialized Form

Method Summary

Methods inherited from class java.lang.Object

clone, equals, finalize, getClass, hashCode, notify, notifyAll, toString, wait, wait, wait