线程安全的单例模式

public class Singleton { 

    private volatile static Singleton uniqueInstance;
 
    private Singleton() {}
        
    public static Singleton getInstance() { 
	if(uniqueInstance == null) { //(1)
        //只有第一次才彻底执行这里的代码
	   synchronized() {
	      //再检查一次
	      if(uniqueInstance == null)
		uniqueInstance = new Singleton();
   	   }
	}
         return uniqueInstance;
    }
}