java线程封闭 ThreadLocal的使用

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    当访问共享的可变数据时，通常需要使用同步，一种避免使用同步的方式就是不共享数据。Java提供了ThreadLocal类来方便开发者们实现线程封闭，在这个ThreadLocal类中的某个值与保存值的对象关联起来。ThreadlLocal提供了get与set方法，这些方法为每一个使用该变量的线程都存着一份独立的副本，因此get总是返回当前执行线程再调用set时设置的最新值。

package com.zy.charter3;

import java.util.Date;
import java.util.concurrent.TimeUnit;

public class ThreadLocalDemo {
    public static void main(String[] args) {
        SafeTask task = new SafeTask();

//

UnSafeTask task = new UnSafeTask();//可以试试这个

for (int i = 0; i < 10; i++) { 
    Thread thread = new Thread(task); 
    thread.start(); 
    try { 
        TimeUnit.SECONDS.sleep(2); 
    } catch (InterruptedException e) { 
    // TODO Auto-generated catch block 
        e.printStackTrace(); 
    } 
    }
 }
}

class UnSafeTask implements Runnable { 
    private Date startDate; 
    @Override public void run() { 
        startDate = new Date(); 
        System.out.println("Start Thread:" + Thread.currentThread().getName() + "time:" + startDate);
        try { 
            TimeUnit.SECONDS.sleep((int)Math.rint(Math.random()*10)); 
        } catch (InterruptedException e) { 
            // TODO Auto-generated catch block 
            e.printStackTrace(); 
        } 
        System.out.println("Thread finished:" + Thread.currentThread().getName() + "time:" + startDate); 
        }
}
class SafeTask implements Runnable { 
    private static ThreadLocal<Date> startDate = new ThreadLocal<Date>() {
     protected Date initialValue() { return new Date(); } 
    }; 
    @Override public void run() { 
        System.out.println("Start Thread:" + Thread.currentThread().getName() + "time:" + startDate.get()); 
        try { 
            TimeUnit.SECONDS.sleep((int)Math.rint(Math.random()*10)); 
        } catch (InterruptedException e) { 
            // TODO Auto-generated catch block 
            e.printStackTrace(); 
        } System.out.println("Thread finished:" + Thread.currentThread().getName() + "time:" + startDate.get()); }}

这里面有两个demo，一个用了ThreadLocal，一个没用