// Copyright 1996-1999, International Business Machines
// Corporation. All Rights Reserved.
//
// See the LICENCE file for the terms of usage and distribution.
package org.apache.log4j;
import java.util.Hashtable;
import java.util.Stack;
import java.util.Enumeration;
import java.util.Vector;
import org.apache.log4j.helpers.LogLog;
/**
The NDC class implements <i>nested diagnostic contexts</i> as
defined by Neil Harrison in the article "Patterns for Logging
Diagnostic Messages" part of the book "<i>Pattern Languages of
Program Design 3</i>" edited by Martin et al.
<p>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-simulatanously.
<p>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.
<p><em><b>Note that NDCs are managed on a per thread
basis</b></em>. NDC operations such as {@link #push push}, {@link
#pop}, {@link #clear}, {@link #getDepth} and {@link #setMaxDepth}
affect the NDC of the <em>current</em> thread only. NDCs of other
threads remain unaffected.
<p>For example, a servlet can build a per client request NDC
consisting the clients host name and other information contained in
the the request. <em>Cookies</em> are another source of distinctive
information. To build an NDC one uses the {@link #push push}
operation. Simply put,
<p><ul>
<li>Contexts can be nested.
<p><li>When entering a context, call <code>NDC.push</code>. As a
side effect, if there is no nested diagnostic context for the
current thread, this method will create it.
<p><li>When leaving a context, call <code>NDC.pop</code>.
</ul>
<p>There is no penalty for forgetting to match each
<code>push</code> operation with a corresponding <code>pop</code>,
except the obvious mismatch between the real application context
and the context set in the NDC.
<p>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.
<p>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. In
version 0.8.5, we have added a meachanism 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.
<p>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.
@author Ceki Gülcü
@since log4j v0.7.0
*/
public class NDC {
// The synchronized keyword is not used in this class. This may seem
// dangerous, especially since the class will be used by
// multiple-threads. In particular, all threads share the same
// hashtable (the "ht" variable). This is OK since java hashtables
// are thread safe. Same goes for Stacks.
// More importantly, when inheriting diagnostic contexts the child
// thread is handed a clone of the parent's NDC. It follows that
// each thread has its own NDC (i.e. stack).
static Hashtable ht = new Hashtable();
static int pushCounter = 0; // the number of times push has been called
// after the latest call to lazyRemove
// The number of times we allow push to be called before we call lazyRemove
// 5 is a relatively small number. As such, lazyRemove is not called too
// frequently. We thus avoid the cost of creating an Enumeration too often.
// The higher this number, the longer is the avarage period for which all
// logging calls in all threads are blocked.
static final int REAP_THRESHOLD = 5;
// No instances allowed.
private NDC() {}
/**
Clear any nested disgnostic information if any. This method is
useful in cases where the same thread can be potentially used
over and over in different unrelated contexts.
<p>This method is equivalent to calling the {@link #setMaxDepth}
method with a zero <code>maxDepth</code> argument.
@since 0.8.4c */
public
static
void clear() {
Stack stack = (Stack) ht.get(Thread.currentThread());
if(stack != null)
stack.setSize(0);
}
/**
Clone the diagnostic context for the current thread.
<p>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.
<p>The child thread uses the {@link #inherit inherit} method to
inherit the parent's diagnostic context.
@return Stack A clone of the current thread's diagnostic context.
*/
public
static
Stack cloneStack() {
Object o = ht.get(Thread.currentThread());
if(o == null)
return null;
else {
Stack stack = (Stack) o;
return (Stack) stack.clone();
}
}
/**
Inherit the diagnostic context of another thread.
<p>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.
<p>The parent's diagnostic context is cloned before being
inherited. In other words, once inherited, the two diagnostic
contexts can be managed independently.
<p>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?
@param stack The diagnostic context of the parent thread.
*/
public
static
void inherit(Stack stack) {
if(stack != null)
ht.put(Thread.currentThread(), stack);
}
/**
Used when printing the diagnostic context.
*/
static
public
String get() {
Stack s = (Stack) ht.get(Thread.currentThread());
if(s != null && !s.isEmpty())
return ((DiagnosticContext) s.peek()).fullMessage;
else
return null;
}
/**
Get the current nesting depth of this diagnostic context.
@see #setMaxDepth
@since 0.7.5
*/
public
static
int getDepth() {
Stack stack = (Stack) ht.get(Thread.currentThread());
if(stack == null)
return 0;
else
return stack.size();
}
private
static
void lazyRemove() {
// The synchronization on ht is necessary to prevent JDK 1.2.x from
// throwing ConcurrentModificationExceptions at us. This sucks BIG-TIME.
// One solution is to write our own hashtable implementation.
Vector v;
synchronized(ht) {
// Avoid calling clean-up too often.
if(++pushCounter >= REAP_THRESHOLD) {
return; // We release the lock ASAP.
} else {
pushCounter = 0; // OK let's do some work.
}
int misses = 0;
v = new Vector();
Enumeration enum = ht.keys();
// We give up after 4 straigt missses. That is 4 consecutive
// inspected threads in 'ht' that turn out to be alive.
// The higher the proportion on dead threads in ht, the higher the
// chances of removal.
while(enum.hasMoreElements() && (misses <= 4)) {
Thread t = (Thread) enum.nextElement();
if(t.isAlive()) {
misses++;
} else {
misses = 0;
v.addElement(t);
}
}
} // synchronized
int size = v.size();
for(int i = 0; i < size; i++) {
Thread t = (Thread) v.elementAt(i);
LogLog.debug("Lazy NDC removal for thread [" + t.getName() + "] ("+
ht.size() + ").");
ht.remove(t);
}
}
/**
Clients should call this method before leaving a diagnostic
context.
<p>The returned value is the value that was pushed last. If no
context is available, then the empty string "" is returned.
@return String The innermost diagnostic context.
*/
public
static
String pop() {
Thread key = Thread.currentThread();
Stack stack = (Stack) ht.get(key);
if(stack != null && !stack.isEmpty())
return ((DiagnosticContext) stack.pop()).message;
else
return "";
}
/**
Push new diagnostic context information for the current thread.
<p>The contents of the <code>message</code> parameter is
determined solely by the client.
@param message The new diagnostic context information. */
public
static
void push(String message) {
Thread key = Thread.currentThread();
Stack stack = (Stack) ht.get(key);
if(stack == null) {
DiagnosticContext dc = new DiagnosticContext(message, null);
stack = new Stack();
ht.put(key, stack);
stack.push(dc);
} else if (stack.isEmpty()) {
DiagnosticContext dc = new DiagnosticContext(message, null);
stack.push(dc);
} else {
DiagnosticContext parent = (DiagnosticContext) stack.peek();
stack.push(new DiagnosticContext(message, parent));
}
}
/**
Remove the diagnostic context for this thread.
<p>Each thread that created a diagnostic context by calling
{@link #pop} should call this method before exiting. Otherwise,
the memory used by the diagnostic context for the <b>thread</b>
cannot be reclaimed by the VM.
<p>As this is such an important problem in heavy duty systems and
because it is difficult to always guarantee that the remove
mehtod 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.
*/
static
public
void remove() {
ht.remove(Thread.currentThread());
// Lazily remove dead-thread references in ht.
lazyRemove();
}
/**
Set maximum depth of this diagnostic context. If the current
depth is smaller or equal to <code>maxDepth</code>, then no
action is taken.
<p>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 <code>setMaxDepth</code> method circumvents
this problem.
<p>For example, the combination
<pre>
void foo() {
int depth = NDC.getDepth();
... complex sequence of calls
NDC.setMaxDepth(depth);
}
</pre>
ensures that between the entry and exit of foo the depth of the
diagnostic stack is conserved.
@see #getDepth
@since 0.7.5 */
static
public
void setMaxDepth(int maxDepth) {
Stack stack = (Stack) ht.get(Thread.currentThread());
if(stack != null && maxDepth < stack.size())
stack.setSize(maxDepth);
}
// =====================================================================
private static class DiagnosticContext {
String fullMessage;
String message;
DiagnosticContext(String message, DiagnosticContext parent) {
this.message = message;
if(parent != null) {
fullMessage = parent.fullMessage + ' ' + message;
} else {
fullMessage = message;
}
}
}
}