java靜態(tài)內(nèi)部類的加載順序：https://www.cnblogs.com/maohuidong/p/7843807.html
內(nèi)部類靜態(tài)單例模式：https://blog.csdn.net/a303549861/article/details/53028349
public class Singleton {
private Singleton(){
}
private static class SingletonHolder{
private final static Singleton instance=new Singleton();
}
public static Singleton getInstance(){
return SingletonHolder.instance;
}
}
靜態(tài)內(nèi)部類在使用時(shí)才加載, 因此保證了lazy loading.
