单例模式的一种实现方式，但很多人会忽略volatile关键字，因为没有该关键字，程序也可以很好的运行，只不过代码的稳定性总不是100%，说不定在未来的某个时刻，隐藏的bug就出来了。
双重校验锁
class Singleton {
private volatile static Singleton instance;
private Singleton (){}
public static Singleton getInstance() {
if (instance == null) {
syschronized(Singleton.class) {
if (instance == null) {
instance = new Singleton();
}
}
}
return instance;
}
}
懒加载的优雅写法静态内部类
public class Singleton {
private Singleton (){}
public static Singleton getInstance(){
return SingletonHolder.instance;
}
static class SingletonHolder {
static Singleton instance = new Singleton();
}
}
转http://www.jianshu.com/p/195ae7c77afe
http://cantellow.iteye.com/blog/838473