ThreadLocal的使用，保证每一个线程都有自己的共享变量。

变量的共享可以使用public static 修饰，所有的线程都使用同一个public static 变量，那如何实现每一个线程都有自己的共享变量呢，ThradLocal类就是干这个用的。

package com.wupao.controller.test;

public class ThreadLocalTest {

	static ThreadLocal<String> threadLocal =new ThreadLocal<String>();
	
	
	public static  class ThreadA extends Thread{
		@Override
		public void run() {
			for (int i = 0; i < 20; i++) {
				threadLocal.set("ThreadA-"+i);
				System.out.println("ThreadA:"+threadLocal.get());
				try {
					Thread.sleep(100);
				} catch (InterruptedException e) {
					// TODO Auto-generated catch block
					e.printStackTrace();
				}
			}
		
		}
	}
	public static  class ThreadB extends Thread{
		@Override
		public void run() {
			for (int i = 0; i < 20; i++) {
				threadLocal.set("ThreadB-"+i);
			System.out.println("ThreadB:"+threadLocal.get());
			try {
				Thread.sleep(100);
			} catch (InterruptedException e) {
				// TODO Auto-generated catch block
				e.printStackTrace();
			}
			}
		
		}
	}
	
	public static void main(String[] args) {
		ThreadA a =new ThreadA();
	
		ThreadB b =new ThreadB();
		a.start();
		b.start();
	}
	
}
           

结果为：

ThreadB:ThreadB-0

ThreadA:ThreadA-0

ThreadB:ThreadB-1

ThreadA:ThreadA-1

ThreadB:ThreadB-2

ThreadA:ThreadA-2

ThreadB:ThreadB-3

ThreadA:ThreadA-3

ThreadB:ThreadB-4

ThreadA:ThreadA-4

ThreadB:ThreadB-5

ThreadA:ThreadA-5

ThreadA:ThreadA-6

ThreadB:ThreadB-6

ThreadB:ThreadB-7

ThreadA:ThreadA-7

ThreadA:ThreadA-8

ThreadB:ThreadB-8

ThreadA:ThreadA-9

ThreadB:ThreadB-9

ThreadB:ThreadB-10

ThreadA:ThreadA-10

ThreadB:ThreadB-11

ThreadA:ThreadA-11

ThreadA:ThreadA-12

ThreadB:ThreadB-12

ThreadB:ThreadB-13

ThreadA:ThreadA-13

ThreadA:ThreadA-14

ThreadB:ThreadB-14

ThreadB:ThreadB-15

ThreadA:ThreadA-15

ThreadA:ThreadA-16

ThreadB:ThreadB-16

ThreadA:ThreadA-17

ThreadB:ThreadB-17

ThreadB:ThreadB-18

ThreadA:ThreadA-18

ThreadA:ThreadA-19

ThreadB:ThreadB-19

虽然两个线程同时对threadlocal 进行存储，但是还是能分别get出属于自己线程的变量值。

底层原理：

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

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

当前的每个线程自带一个map，当前线程对象作为key存到map（具体是ThreadLocal.ThreadLocalMap）里，要存的真实value作为map的v。

使用场景：

可以为每个线程存储不需要共享的数据，比如链接或者session，又或者存储像像SimpleDateFormat 这样创建费时间又线程不安全的数据。