/*
 * Copyright (C) The Apache Software Foundation. All rights reserved.
 *
 * This software is published under the terms of the Apache Software License
 * version 1.1, a copy of which has been included  with this distribution in
 * the LICENSE.txt file.
 */

//      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 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 {@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,

   

     
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 {@link #remove
     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, {@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.
   
   @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() {}


  /**
     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 {@link #setMaxDepth}
     method with a zero maxDepth 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.

     
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.
     
     @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.

     
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?

     @param stack The diagnostic context of the parent thread.

  */
  public
  static
  void inherit(Stack stack) {
    if(stack != null)
      ht.put(Thread.currentThread(), stack);
  }


  /**
     Never use this method directly, use the {@link
     org.apache.log4j.spi.LoggingEvent#getNDC} method instead.
  */
  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.

     
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 "";
  }

  /**
     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.
     
     @return String The innermost diagnostic context.
     
     */
  public
  static
  String peek() {
    Thread key = Thread.currentThread();
    Stack stack = (Stack) ht.get(key);
    if(stack != null && !stack.isEmpty())
      return ((DiagnosticContext) stack.peek()).message;
    else
      return "";
  }
  
  /**
     Push new diagnostic context information for the current thread.

     
The contents of the message 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.

     
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.
     
  */
  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 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.
     
     @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;
      }
    }
  }
}