线程安全

//正确的双重检查锁
public class Singleton {
    private volatile static Singleton singleton;

    private Singleton() {
    }

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