log4j中再次看ThreadLocal用法

   关于ThreadLocal坛子里面ThreadLocal的帖子很多，说法也有好多不一致
每次举例都是hibernate里面的session，这周使用log4j做一些东东，发现log4j的代码用这个ThreadLocal可是清晰多了，而且很有意思。

  log4j里的MDC说起，这个是个键值对存储的容器，在里面加你的键值对，在配置文件配置你输出的格式，需要输出的内容时候取里面的键
如：

            MDC.put("usr_id", usr_id);  
            MDC.put("log_title", "网站访问记录");  
            MDC.put("log_type", "记录");  
            MDC.put("log_title", "网站访问记录");  
            MDC.put("log_datetime", format.format(now));  
            MDC.put("log_ip", log_ip);  

配置文件里面


这个实例MDC.put那个logger实例也MDC.put，里面不是会很崩溃，会覆盖一些东西，越来越大之类的

直接上代码：

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

  /**
     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);
      } 
    }
  }


  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();
      } 
    }
  }

}

重点看里面的put方法

  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);
    }
  }

先调用ThreadLocalMap.get()获取，注意此处ThreadLocalMap是log4j自己写的继承自ThreadLocal的子类，但get，set方法还是没有覆盖的。
如下代码所示：

   public T get() {
    
        Thread t = Thread.currentThread();
        
        ThreadLocalMap map = getMap(t);
        if (map != null) {
            ThreadLocalMap.Entry e = map.getEntry(this);
            if (e != null)
                return (T)e.value;
        }
        return setInitialValue();
    }

Thread t = Thread.currentThread();-->>取出当前线程
ThreadLocalMap map = getMap(t);--->>根据当前线程取出里面的变量return t.threadLocals;

  ThreadLocalMap getMap(Thread t) {
        return t.threadLocals;
    }

Thread类里面定义

ThreadLocal.ThreadLocalMap threadLocals = null;

可以看出ThreadLocalMap是ThreadLocal的一个内部类，不过这个六百多行的内部类确实不容易看
map.getEntry(this)-->> 在map不为空时候，取出里面存放的实体，为什么传this？
到这一步，已经从当前线程 当前对象 这两个纬度锁定到了这个map，对static class ThreadLocalMap这样一个静态内部类来说， 已经可以控制每个线程分配一个独立占用的虚拟内存地带了，如果其它线程要操作这个，就用这两个纬度去获取

再理一下这种方式的思路
[size=medium]
每个线程有一个ThreadLocalMap，这个是在线程本身就定义的，Thread里面有

ThreadLocal.ThreadLocalMap threadLocals = null;

那么每个线程里面可根据当前对象取出里面的实体。[/size]

这个实体看你自己使用什么就可以指定什么，反正是个object，MDC里面指定为hashtable

最终调用

  public void set(T value) {
        Thread t = Thread.currentThread();
        ThreadLocalMap map = getMap(t);
        if (map != null)
            map.set(this, value);
        else
            createMap(t, value);
    }

log4j使用上述方式，保证调用MDC的线程实例，通过二个纬度确定里面有唯一一份object（hashtable）