/*
* Licensed to the Apache Software Foundation (ASF) under one or more
* contributor license agreements. See the NOTICE file distributed with
* this work for additional information regarding copyright ownership.
* The ASF licenses this file to You under the Apache License, Version 2.0
* (the "License"); you may not use this file except in compliance with
* the License. You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
// Contributors: Dan Milstein
// Ray Millard
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-simultaneously.
<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>.
<p><li><b>When exiting a thread make sure to call {@link #remove
NDC.remove()}</b>.
</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. 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.
<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 0.7.0
*/
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() {}
/**
* Get NDC stack for current thread.
* @return NDC stack for current thread.
*/
private static
Stack getCurrentStack() {
if (
ht != null) {
return (
Stack)
ht.
get(
Thread.
currentThread());
}
return null;
}
/**
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.
<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 =
getCurrentStack();
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() {
Stack stack =
getCurrentStack();
if(
stack == null)
return null;
else {
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);
}
/**
<font color="#FF4040"><b>Never use this method directly, use the {@link
org.apache.log4j.spi.LoggingEvent#getNDC} method instead</b></font>.
*/
static
public
String get() {
Stack s =
getCurrentStack();
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 =
getCurrentStack();
if(
stack == null)
return 0;
else
return
stack.
size();
}
private
static
void
lazyRemove() {
if (
ht == null) return;
// 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 enumeration =
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(
enumeration.
hasMoreElements() && (
misses <= 4)) {
Thread t = (
Thread)
enumeration.
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() {
Stack stack =
getCurrentStack();
if(
stack != null && !
stack.
isEmpty())
return ((
DiagnosticContext)
stack.
pop()).
message;
else
return "";
}
/**
Looks at the last diagnostic context at the top of this NDC
without removing it.
<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 peek() {
Stack stack =
getCurrentStack();
if(
stack != null && !
stack.
isEmpty())
return ((
DiagnosticContext)
stack.
peek()).
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) {
Stack stack =
getCurrentStack();
if(
stack == null) {
DiagnosticContext dc = new
DiagnosticContext(
message, null);
stack = new
Stack();
Thread key =
Thread.
currentThread();
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 #push} should call this method before exiting. Otherwise,
the memory used by 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
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 <code>remove</code> sometime. If
you never call it, then your application is sure to run out of
memory.
*/
static
public
void
remove() {
if (
ht != null) {
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 =
getCurrentStack();
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;
}
}
}
}