java陷阱--------神奇的初始化

public class DaoFactory{
  private static DaoFactory instance = new DaoFactory();
  private static UserDao userDao = null;
  
  private DaoFactory(){
    try{
     Properties prop = new Properties();
     InputStream inStream = new FileInputStream(new File("src/daoconfig.properties"));
     prop.load(inStream);
     String userDaoClass = prop.getProperty("userDaoClass");
     Class clazz = Class.forName(userDaoClass);
     userDao = (UserDao)clazz.newInstance();
     System.out.println(userDao);
    }catch(Throwable e){
      throw new ExceptionIninitiallizerError(e);
    }
  }
  
  public static DaoFactory getInstance(){
    return instance;
  }
  
  public static UserDao getUserDao(){
  System.out.println(userDao);
    return userDao;
  }

} 

下面我们调用这段代码：
DaoFactory.getInstance().getUserDao();


//注意：报错了哈


原因。。。。
1。执行：
private static DaoFactory instance = new DaoFactory();

2。在静态方法里面，也就是一加载到JVM的时候就执行构造函数
所以private DaoFactory(){
    try{
     Properties prop = new Properties();
     InputStream inStream = new FileInputStream(new File("src/daoconfig.properties"));
     prop.load(inStream);
     String userDaoClass = prop.getProperty("userDaoClass");
     Class clazz = Class.forName(userDaoClass);
     userDao = (UserDao)clazz.newInstance();
     System.out.println(userDao);
    }catch(Throwable e){
      throw new ExceptionIninitiallizerError(e);
    }
  }
被执行，并且我们能看到userDao 被赋值了，

3。private static UserDao userDao = null;
在userDao被赋值之后，他又被赋值为空了。。。


如何改呢。。？
 private static UserDao userDao = null;
private static DaoFactory instance = new DaoFactory();
换顺序。。也就是说把所有的准备工作都做好之后再去new 他