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    1   /*
    2    * Licensed to the Apache Software Foundation (ASF) under one or more
    3    * contributor license agreements.  See the NOTICE file distributed with
    4    * this work for additional information regarding copyright ownership.
    5    * The ASF licenses this file to You under the Apache License, Version 2.0
    6    * (the "License"); you may not use this file except in compliance with
    7    * the License.  You may obtain a copy of the License at
    8    *
    9    *      http://www.apache.org/licenses/LICENSE-2.0
    10   *
    11   * Unless required by applicable law or agreed to in writing, software
    12   * distributed under the License is distributed on an "AS IS" BASIS,
    13   * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
    14   * See the License for the specific language governing permissions and
    15   * limitations under the License.
    16   */
    17
    18  //      Contributors:      Dan Milstein
    19  //                         Ray Millard
    20
    21  package org.apache.log4j;
    22
    23  import java.util.Hashtable;
    24  import java.util.Stack;
    25  import java.util.Enumeration;
    26  import java.util.Vector;
    27
    28  import org.apache.log4j.helpers.LogLog;
    29
    30  /**
    31     The NDC class implements <i>nested diagnostic contexts</i> as
    32     defined by Neil Harrison in the article "Patterns for Logging
    33     Diagnostic Messages" part of the book "<i>Pattern Languages of
    34     Program Design 3</i>" edited by Martin et al.
    35
    36     <p>A Nested Diagnostic Context, or NDC in short, is an instrument
    37     to distinguish interleaved log output from different sources. Log
    38     output is typically interleaved when a server handles multiple
    39     clients near-simultaneously.
    40
    41     <p>Interleaved log output can still be meaningful if each log entry
    42     from different contexts had a distinctive stamp. This is where NDCs
    43     come into play.
    44
    45     <p><em><b>Note that NDCs are managed on a per thread
    46     basis</b></em>. NDC operations such as {@link #push push}, {@link
    47     #pop}, {@link #clear}, {@link #getDepth} and {@link #setMaxDepth}
    48     affect the NDC of the <em>current</em> thread only. NDCs of other
    49     threads remain unaffected.
    50
    51     <p>For example, a servlet can build a per client request NDC
    52     consisting the clients host name and other information contained in
    53     the the request. <em>Cookies</em> are another source of distinctive
    54     information. To build an NDC one uses the {@link #push push}
    55     operation. Simply put,
    56
    57     <p><ul>
    58       <li>Contexts can be nested.
    59
    60       <p><li>When entering a context, call <code>NDC.push</code>. As a
    61       side effect, if there is no nested diagnostic context for the
    62       current thread, this method will create it.
    63
    64       <p><li>When leaving a context, call <code>NDC.pop</code>.
    65
    66       <p><li><b>When exiting a thread make sure to call {@link #remove
    67       NDC.remove()}</b>.
    68     </ul>
    69
    70     <p>There is no penalty for forgetting to match each
    71     <code>push</code> operation with a corresponding <code>pop</code>,
    72     except the obvious mismatch between the real application context
    73     and the context set in the NDC.
    74
    75     <p>If configured to do so, {@link PatternLayout} and {@link
    76     TTCCLayout} instances automatically retrieve the nested diagnostic
    77     context for the current thread without any user intervention.
    78     Hence, even if a servlet is serving multiple clients
    79     simultaneously, the logs emanating from the same code (belonging to
    80     the same category) can still be distinguished because each client
    81     request will have a different NDC tag.
    82
    83     <p>Heavy duty systems should call the {@link #remove} method when
    84     leaving the run method of a thread. This ensures that the memory
    85     used by the thread can be freed by the Java garbage
    86     collector. There is a mechanism to lazily remove references to dead
    87     threads. In practice, this means that you can be a little sloppy
    88     and sometimes forget to call {@link #remove} before exiting a
    89     thread.
    90
    91     <p>A thread may inherit the nested diagnostic context of another
    92     (possibly parent) thread using the {@link #inherit inherit}
    93     method. A thread may obtain a copy of its NDC with the {@link
    94     #cloneStack cloneStack} method and pass the reference to any other
    95     thread, in particular to a child.
    96
    97     @author Ceki G&uuml;lc&uuml;
    98     @since 0.7.0
    99
    100 */
    101
    102 public class NDC {
    103
    104   // The synchronized keyword is not used in this class. This may seem
    105   // dangerous, especially since the class will be used by
    106   // multiple-threads. In particular, all threads share the same
    107   // hashtable (the "ht" variable). This is OK since java hashtables
    108   // are thread safe. Same goes for Stacks.
    109
    110   // More importantly, when inheriting diagnostic contexts the child
    111   // thread is handed a clone of the parent's NDC.  It follows that
    112   // each thread has its own NDC (i.e. stack).
    113
    114   static Hashtable ht = new Hashtable();
    115
    116   static int pushCounter = 0; // the number of times push has been called
    117                               // after the latest call to lazyRemove
    118
    119   // The number of times we allow push to be called before we call lazyRemove
    120   // 5 is a relatively small number. As such, lazyRemove is not called too
    121   // frequently. We thus avoid the cost of creating an Enumeration too often.
    122   // The higher this number, the longer is the avarage period for which all
    123   // logging calls in all threads are blocked.
    124   static final int REAP_THRESHOLD = 5;
    125
    126   // No instances allowed.
    127   private NDC() {}
    128
    129   /**
    130    *   Get NDC stack for current thread.
    131    *   @return NDC stack for current thread.
    132    */
    133   private static Stack getCurrentStack() {
    134       if (ht != null) {
    135           return (Stack) ht.get(Thread.currentThread());
    136       }
    137       return null;
    138   }
    139
    140
    141   /**
    142      Clear any nested diagnostic information if any. This method is
    143      useful in cases where the same thread can be potentially used
    144      over and over in different unrelated contexts.
    145
    146      <p>This method is equivalent to calling the {@link #setMaxDepth}
    147      method with a zero <code>maxDepth</code> argument.
    148
    149      @since 0.8.4c */
    150   public
    151   static
    152   void clear() {
    153     Stack stack = getCurrentStack();
    154     if(stack != null)
    155       stack.setSize(0);
    156   }
    157
    158
    159   /**
    160      Clone the diagnostic context for the current thread.
    161
    162      <p>Internally a diagnostic context is represented as a stack.  A
    163      given thread can supply the stack (i.e. diagnostic context) to a
    164      child thread so that the child can inherit the parent thread's
    165      diagnostic context.
    166
    167      <p>The child thread uses the {@link #inherit inherit} method to
    168      inherit the parent's diagnostic context.
    169
    170      @return Stack A clone of the current thread's  diagnostic context.
    171
    172   */
    173   public
    174   static
    175   Stack cloneStack() {
    176     Stack stack = getCurrentStack();
    177     if(stack == null)
    178       return null;
    179     else {
    180       return (Stack) stack.clone();
    181     }
    182   }
    183
    184
    185   /**
    186      Inherit the diagnostic context of another thread.
    187
    188      <p>The parent thread can obtain a reference to its diagnostic
    189      context using the {@link #cloneStack} method.  It should
    190      communicate this information to its child so that it may inherit
    191      the parent's diagnostic context.
    192
    193      <p>The parent's diagnostic context is cloned before being
    194      inherited. In other words, once inherited, the two diagnostic
    195      contexts can be managed independently.
    196
    197      <p>In java, a child thread cannot obtain a reference to its
    198      parent, unless it is directly handed the reference. Consequently,
    199      there is no client-transparent way of inheriting diagnostic
    200      contexts. Do you know any solution to this problem?
    201
    202      @param stack The diagnostic context of the parent thread.
    203
    204   */
    205   public
    206   static
    207   void inherit(Stack stack) {
    208     if(stack != null)
    209       ht.put(Thread.currentThread(), stack);
    210   }
    211
    212
    213   /**
    214      <font color="#FF4040"><b>Never use this method directly, use the {@link
    215      org.apache.log4j.spi.LoggingEvent#getNDC} method instead</b></font>.
    216   */
    217   static
    218   public
    219   String get() {
    220     Stack s = getCurrentStack();
    221     if(s != null && !s.isEmpty())
    222       return ((DiagnosticContext) s.peek()).fullMessage;
    223     else
    224       return null;
    225   }
    226
    227   /**
    228    * Get the current nesting depth of this diagnostic context.
    229    *
    230    * @see #setMaxDepth
    231    * @since 0.7.5
    232    */
    233   public
    234   static
    235   int getDepth() {
    236     Stack stack = getCurrentStack();
    237     if(stack == null)
    238       return 0;
    239     else
    240       return stack.size();
    241   }
    242
    243   private
    244   static
    245   void lazyRemove() {
    246     if (ht == null) return;
    247
    248     // The synchronization on ht is necessary to prevent JDK 1.2.x from
    249     // throwing ConcurrentModificationExceptions at us. This sucks BIG-TIME.
    250     // One solution is to write our own hashtable implementation.
    251     Vector v;
    252
    253     synchronized(ht) {
    254       // Avoid calling clean-up too often.
    255       if(++pushCounter <= REAP_THRESHOLD) {
    256 	return; // We release the lock ASAP.
    257       } else {
    258 	pushCounter = 0; // OK let's do some work.
    259       }
    260
    261       int misses = 0;
    262       v = new Vector();
    263       Enumeration enumeration = ht.keys();
    264       // We give up after 4 straigt missses. That is 4 consecutive
    265       // inspected threads in 'ht' that turn out to be alive.
    266       // The higher the proportion on dead threads in ht, the higher the
    267       // chances of removal.
    268       while(enumeration.hasMoreElements() && (misses <= 4)) {
    269 	Thread t = (Thread) enumeration.nextElement();
    270 	if(t.isAlive()) {
    271 	  misses++;
    272 	} else {
    273 	  misses = 0;
    274 	  v.addElement(t);
    275 	}
    276       }
    277     } // synchronized
    278
    279     int size = v.size();
    280     for(int i = 0; i < size; i++) {
    281       Thread t = (Thread) v.elementAt(i);
    282       LogLog.debug("Lazy NDC removal for thread [" + t.getName() + "] ("+
    283 		   ht.size() + ").");
    284       ht.remove(t);
    285     }
    286   }
    287
    288   /**
    289      Clients should call this method before leaving a diagnostic
    290      context.
    291
    292      <p>The returned value is the value that was pushed last. If no
    293      context is available, then the empty string "" is returned.
    294
    295      @return String The innermost diagnostic context.
    296
    297      */
    298   public
    299   static
    300   String pop() {
    301     Stack stack = getCurrentStack();
    302     if(stack != null && !stack.isEmpty())
    303       return ((DiagnosticContext) stack.pop()).message;
    304     else
    305       return "";
    306   }
    307
    308   /**
    309      Looks at the last diagnostic context at the top of this NDC
    310      without removing it.
    311
    312      <p>The returned value is the value that was pushed last. If no
    313      context is available, then the empty string "" is returned.
    314
    315      @return String The innermost diagnostic context.
    316
    317      */
    318   public
    319   static
    320   String peek() {
    321     Stack stack = getCurrentStack();
    322     if(stack != null && !stack.isEmpty())
    323       return ((DiagnosticContext) stack.peek()).message;
    324     else
    325       return "";
    326   }
    327
    328   /**
    329      Push new diagnostic context information for the current thread.
    330
    331      <p>The contents of the <code>message</code> parameter is
    332      determined solely by the client.
    333
    334      @param message The new diagnostic context information.  */
    335   public
    336   static
    337   void push(String message) {
    338     Stack stack = getCurrentStack();
    339
    340     if(stack == null) {
    341       DiagnosticContext dc = new DiagnosticContext(message, null);
    342       stack = new Stack();
    343       Thread key = Thread.currentThread();
    344       ht.put(key, stack);
    345       stack.push(dc);
    346     } else if (stack.isEmpty()) {
    347       DiagnosticContext dc = new DiagnosticContext(message, null);
    348       stack.push(dc);
    349     } else {
    350       DiagnosticContext parent = (DiagnosticContext) stack.peek();
    351       stack.push(new DiagnosticContext(message, parent));
    352     }
    353   }
    354
    355   /**
    356      Remove the diagnostic context for this thread.
    357
    358      <p>Each thread that created a diagnostic context by calling
    359      {@link #push} should call this method before exiting. Otherwise,
    360      the memory used by the <b>thread</b> cannot be reclaimed by the
    361      VM.
    362
    363      <p>As this is such an important problem in heavy duty systems and
    364      because it is difficult to always guarantee that the remove
    365      method is called before exiting a thread, this method has been
    366      augmented to lazily remove references to dead threads. In
    367      practice, this means that you can be a little sloppy and
    368      occasionally forget to call {@link #remove} before exiting a
    369      thread. However, you must call <code>remove</code> sometime. If
    370      you never call it, then your application is sure to run out of
    371      memory.
    372
    373   */
    374   static
    375   public
    376   void remove() {
    377     if (ht != null) {
    378         ht.remove(Thread.currentThread());
    379
    380         // Lazily remove dead-thread references in ht.
    381         lazyRemove();
    382     }
    383   }
    384
    385   /**
    386      Set maximum depth of this diagnostic context. If the current
    387      depth is smaller or equal to <code>maxDepth</code>, then no
    388      action is taken.
    389
    390      <p>This method is a convenient alternative to multiple {@link
    391      #pop} calls. Moreover, it is often the case that at the end of
    392      complex call sequences, the depth of the NDC is
    393      unpredictable. The <code>setMaxDepth</code> method circumvents
    394      this problem.
    395
    396      <p>For example, the combination
    397      <pre>
    398        void foo() {
    399        &nbsp;  int depth = NDC.getDepth();
    400
    401        &nbsp;  ... complex sequence of calls
    402
    403        &nbsp;  NDC.setMaxDepth(depth);
    404        }
    405      </pre>
    406
    407      ensures that between the entry and exit of foo the depth of the
    408      diagnostic stack is conserved.
    409
    410      @see #getDepth
    411      @since 0.7.5 */
    412   static
    413   public
    414   void setMaxDepth(int maxDepth) {
    415     Stack stack = getCurrentStack();
    416     if(stack != null && maxDepth < stack.size())
    417       stack.setSize(maxDepth);
    418   }
    419
    420   // =====================================================================
    421    private static class DiagnosticContext {
    422
    423     String fullMessage;
    424     String message;
    425
    426     DiagnosticContext(String message, DiagnosticContext parent) {
    427       this.message = message;
    428       if(parent != null) {
    429 	fullMessage = parent.fullMessage + ' ' + message;
    430       } else {
    431 	fullMessage = message;
    432       }
    433     }
    434   }
    435 }
    436