实现线程安全的 单例模式

package com.testThread;

/**
 * @author shkstart
 * @create 2021-08-06 10:07
 */
public class 实现线程安全的单例模式 {
    public static void main(String[] args) {
        Window4 w1 = Window4.getInstance();
        Window4 w2 = Window4.getInstance();
        System.out.println(w1==w2);
    }
}

 class Window4{
    private static Window4 win = null;
    public static Window4 getInstance(){
        if(win == null){
            synchronized (Window4.class){
                if (win == null){
                    win = new Window4();
                }
            }
        }
        return win;
    }
}