题目：
请用程序解析ThreadLocal原理，并用程序展示其作用。
答：
/**
*
*/
package mythread;
import java.util.Collections;
import java.util.Map;
import java.util.HashMap;
import java.util.Random;
/**
* Those code show you muti threads use thread local to keep his own variable or object,
* and make sure his variable keep the same all the time!
*
*/
public class MyThreadLocal {
/**
* Main Enter
* @param args
*/
public static void main(String[] args) {
ThreadTest tt=new ThreadTest();
Thread t1=new Thread(tt,"thread 1");
Thread t2=new Thread(tt,"thread 2");
Thread t3=new Thread(tt,"thread 3");
t1.setPriority(Thread.MAX_PRIORITY);
t1.start();
t2.start();
t3.start();
}
}
/**
* This thread have use thread local implements MyThreadLocalImpl
* to keep mutil thread's variable, this is a key
*
*/
class ThreadTest implements Runnable{
private static MyThreadLocalImpl threadlocal=new MyThreadLocalImpl();
@Override
public void run() {
String currentThread=Thread.currentThread().getName();
System.out.println(currentThread+" runing");
VO vo = getVO();
Random random = new Random();
int age = random.nextInt(100);
String value=String.valueOf(age);
vo.setAge(value);
System.out.println(currentThread+"set age: "+vo.getAge());
System.out.println(currentThread+" first time get age: "+vo.getAge());
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println(currentThread+" second get age: "+vo.getAge());
try {
Thread.sleep(2000);
} catch (InterruptedException e) {
// TODO Auto-generated catch block
e.printStackTrace();
}
System.out.println(currentThread+" third get age: "+vo.getAge());
//clear up
clearVO();
}
/**
* Get object from threadlocal
* @return
*/
public static VO getVO(){
VO vo= (VO) threadlocal.get();
if (vo==null) {
vo=new VO();
threadlocal.set(vo);
}
return vo;
}
/**
* Clear up threadlocal
*/
public static void clearVO(){
VO vo= (VO) threadlocal.get();
threadlocal.set(null);
if (vo!=null) {
vo=null;
}
}
}
/**
* Here I have use java.util.Map, java.util.Collections to implements an simple threadlocal Object
*
*/
class MyThreadLocalImpl{
private Map<Thread, Object> valueMap=Collections.synchronizedMap(new HashMap<Thread, Object>());
public void set(Object newValue){
valueMap.put(Thread.currentThread(), newValue);
}
public Object get(){
Thread curThread=Thread.currentThread();
Object obj=valueMap.get(curThread);
if (obj==null && !valueMap.containsKey(curThread)) {
obj=initialValue();
valueMap.put(curThread, obj);
}
return obj;
}
public Object initialValue(){
return null;
}
}
/**
* VO, demo object
*
*/
class VO{
private String age;
public void setAge(String age){
this.age=age;
}
public String getAge(){
return age;
}
}
责任编辑：小草
