单例模式构造器中调用静态属性的方法

public class InitServlet extends HttpServlet {
private static final long serialVersionUID = 1L;

@Override
public void init() throws ServletException {
    CustomerDAOFactory.getInstance().setType("xml");
    InputStream in = 
            getServletContext().getResourceAsStream("/WEB-INF/classes/switch.properties");
    
    Properties pros = new Properties();
    try {
        pros.load(in);
        String type =  pros.getProperty("type");
        CustomerDAOFactory.getInstance().setType(type);
    } catch (Exception e) {            
        e.printStackTrace();
    }
    
}

上面的servlet设置了1

public class CustomerDAOFactory {
private static String type = null;

private static CustomerDAOFactory instance = new CustomerDAOFactory();

private Map<String,CustomerDAO> docs = new HashMap<String,CustomerDAO>();

private CustomerDAOFactory() {
    docs.put("jdbc", new CustomerDAOImpl());
    docs.put("xml", new CustomerDAOXMLImpl());
}

public static CustomerDAOFactory getInstance() {
    return instance;
}

public void setType(String type) {
    this.type = type;
}

public CustomerDAO getCustomerDAO() {
    return docs.get(type);
}

}

为什么加上static 写成private static Map<String,CustomerDAO> docs
就会在第一段代码的这个地方：CustomerDAOFactory.getInstance().setType("xml");报错