java---懒汉式的线程安全问题

/*
 * 关于懒汉式的线程安全问题，使用同步机制
 * 对于一般的方法内，使用同步代码块，
 */
public class TestSingleton {
public static void main(String[] args) {
Singleton s1 = Singleton.getInstance();
Singleton s2 = Singleton.getInstance();
System.out.println(s1 == s2);
Class cl = Singleton.class;
}
}
//懒汉
class Singleton{
private Singleton(){

}

private static Singleton instance = null;

public static Singleton getInstance(){
if(instance == null){
synchronized (Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
}