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
引用
log4j.appender.project-util-db.sql=insert into user_log (usr_id,log_title,log_category,log_type,log_datetime,log_ip)
VALUES ('%X{usr_id}','%X{log_title}','%X{log_type}','%X{log_title}','%X{log_datetime}','%X{log_ip}')
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:
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* 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
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* http://www.apache.org/licenses/LICENSE-2.0
*
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* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
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* limitations under the License.
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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 Gülcü */
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)