单例类，线程安全懒加载。

单例模式的七种写法：
http://cantellow.iteye.com/blog/838473

双重校验锁：

public class Singleton {

	private volatile static Singleton singleton;  
    private Singleton (){}  
    public static Singleton getSingleton() {  
	    if (singleton == null) {  
	        synchronized (Singleton.class) {  
		        if (singleton == null) {  
		            singleton = new Singleton();  
		        }  
	        }  
	    }  
	    return singleton;  
	}  
}