线程安全的单例模式

public class Singleton {
    //双重检查加锁
    private static volatile Singleton instance=null;
    private Singleton(){}

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