ThreadLocal的使用，保證每一個線程都有自己的共享變量。

變量的共享可以使用public static 修飾，所有的線程都使用同一個public static 變量，那如何實現每一個線程都有自己的共享變量呢，ThradLocal類就是幹這個用的。

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();
	}
	
}

結果爲：

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

雖然兩個線程同時對threadlocal 進行存儲，但是還是能分別get出屬於自己線程的變量值。

底層原理：

 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;
    }

當前的每個線程自帶一個map，當前線程對象作爲key存到map（具體是ThreadLocal.ThreadLocalMap）裏，要存的真實value作爲map的v。

使用場景：

可以爲每個線程存儲不需要共享的數據，比如鏈接或者session，又或者存儲像像SimpleDateFormat 這樣創建費時間又線程不安全的數據。