src/main/java/org/apache/log4j/MDC.java

/*
 * 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.
 */

package org.apache.log4j;

import java.lang.reflect.InvocationTargetException;
import java.lang.reflect.Method;
import java.util.Hashtable;
import org.apache.log4j.helpers.Loader;
import org.apache.log4j.helpers.ThreadLocalMap;

/**
   The MDC class is similar to the {@link NDC} class except that it is
   based on a map instead of a stack. It provides <em>mapped
   diagnostic contexts</em>. A <em>Mapped Diagnostic Context</em>, or
   MDC in short, is an instrument for distinguishing interleaved log
   output from different sources. Log output is typically interleaved
   when a server handles multiple clients near-simultaneously.

   <p><b><em>The MDC is managed on a per thread basis</em></b>. A
   child thread automatically inherits a <em>copy</em> of the mapped
   diagnostic context of its parent.
  
   <p>The MDC class requires JDK 1.2 or above. Under JDK 1.1 the MDC
   will always return empty values but otherwise will not affect or
   harm your application.
   
   @since 1.2

   @author Ceki G&uuml;lc&uuml; 
*/
public class MDC {
  
  final static MDC mdc = new MDC();
  
  static final int HT_SIZE = 7;

  boolean java1;
  
  Object tlm;

  private Method removeMethod;

  private
  MDC() {
    java1 = Loader.isJava1();
    if(!java1) {
      tlm = new ThreadLocalMap();
    }

    try {
      removeMethod = ThreadLocal.class.getMethod("remove", null);
    } catch (NoSuchMethodException e) {
      // don't do anything - java prior 1.5
    }
  }

  /**
     Put a context value (the <code>o</code> parameter) as identified
     with the <code>key</code> parameter into the current thread's
     context map.

     <p>If the current thread does not have a context map it is
     created as a side effect.
    
   */
  static
  public
  void put(String key, Object o) {
     if (mdc != null) {
         mdc.put0(key, o);
     }
  }
  
  /**
     Get the context identified by the <code>key</code> parameter.

     <p>This method has no side effects.
   */
  static 
  public
  Object get(String key) {
    if (mdc != null) {
        return mdc.get0(key);
    }
    return null;
  }

  /**
     Remove the the context identified by the <code>key</code>
     parameter.

  */
  static 
  public
  void remove(String key) {
    if (mdc != null) {
        mdc.remove0(key);
    }
  }


  /**
   * Get the current thread's MDC as a hashtable. This method is
   * intended to be used internally.  
   * */
  public static Hashtable getContext() {
    if (mdc != null) {
        return mdc.getContext0();
    } else {
        return null;
    }
  }

  /**
   *  Remove all values from the MDC.
   *  @since 1.2.16
  */
  public static void clear() {
    if (mdc != null) {
        mdc.clear0();
    }
  }


  private
  void put0(String key, Object o) {
    if(java1 || tlm == null) {
      return;
    } else {
      Hashtable ht = (Hashtable) ((ThreadLocalMap)tlm).get();
      if(ht == null) {
        ht = new Hashtable(HT_SIZE);
        ((ThreadLocalMap)tlm).set(ht);
      }    
      ht.put(key, o);
    }
  }
  
  private
  Object get0(String key) {
    if(java1 || tlm == null) {
      return null;
    } else {       
      Hashtable ht = (Hashtable) ((ThreadLocalMap)tlm).get();
      if(ht != null && key != null) {
        return ht.get(key);
      } else {
        return null;
      }
    }
  }

  private
  void remove0(String key) {
    if(!java1 && tlm != null) {
      Hashtable ht = (Hashtable) ((ThreadLocalMap)tlm).get();
      if(ht != null) {
        ht.remove(key);
        // clean up if this was the last key
        if (ht.isEmpty()) {
          clear0();
        }
      } 
    }
  }


  private
  Hashtable getContext0() {
     if(java1 || tlm == null) {
      return null;
    } else {       
      return (Hashtable) ((ThreadLocalMap)tlm).get();
    }
  }

  private
  void clear0() {
    if(!java1 && tlm != null) {
      Hashtable ht = (Hashtable) ((ThreadLocalMap)tlm).get();
      if(ht != null) {
        ht.clear();
      }
      if(removeMethod != null) {
          // java 1.3/1.4 does not have remove - will suffer from a memory leak
          try {
            removeMethod.invoke(tlm, null);
          } catch (IllegalAccessException e) {
            // should not happen
          } catch (InvocationTargetException e) {
            // should not happen
          }
      }
    }
  }

}