ThreadLocal的使用,保證每一個執行緒都有自己的共享變數。
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(); } }
結果為:
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
雖然兩個執行緒同時對threadlocal 進行儲存,但是還是能分別get出屬於自己執行緒的變數值。
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; }
當前的每個執行緒自帶一個map,當前執行緒物件作為key存到map(具體是ThreadLocal.ThreadLocalMap)裡,要存的真實value作為map的v。
使用場景:
可以為每個執行緒儲存不需要共享的資料,比如連結或者session,又或者儲存像像SimpleDateFormat 這樣建立費時間又執行緒不安全的資料。