ThreadLocal的使用，保证每一个线程都有自己的共享变量。

来源：互联网发布：如何学好高中化学知乎编辑：程序博客网时间：2024/05/22 03:47

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{@Overridepublic 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 blocke.printStackTrace();}}}}public static  class ThreadB extends Thread{@Overridepublic 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 blocke.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。

0 0