/*
 * 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 G&uuml;lc&uuml;
 * @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() {
       &nbsp;  int depth = NDC.getDepth();
    
       &nbsp;  ... complex sequence of calls
    
       &nbsp;  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;
	    }
	}
    }
}
