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 threadLocal =new ThreadLocal();


    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SimpleDateFormat 这样创建费时间又线程不安全的数据。

了解更多内容：请参考 http://www.cnblogs.com/dolphin0520/p/3920407.html

转载于：https://blog.csdn.net/liuyifeng1920/article/details/53186116