可以在一个旧项目中开始编写单元测试了吗?

我想开始为我们的项目编写单元测试(JUnit).它将J2EE 1.4与Hibernate 3.1结合使用，并且连接代码和服务层之间存在紧密的联系(如果正确，请使用servlet?请纠正我！).

I would like to start writing unit test (JUnit) for our project. This uses J2EE 1.4 with Hibernate 3.1 and there is a tight coupling between connection code and service layer(servlets if I am right? correct me!).

因此，假设我具有保留某些表单值的功能.结构是这样的，

So suppose I have functionality to persist some form values. The structure is something like,

MyServlet.java

MyServlet.java

public void doGet(ServletRequest request, ServletResponse response)
{
    T_Donation instance - new T_Donation();

    instance.setName(request.getParameter("name"));
    instance.setAmount(request.getParameter("amount"));

    MyDAO dao = new MyDAO();
    Boolean b = dao.persistInstance(instance);

    if(b.booleanValue())
    {
        // forward to .jsp file by means of RequestDispatcher
    } 

}

T_Donation模型

Model T_Donation

public class T_Donation implements Serializable
{
   private String name;
   private String amount;

   // getters, setters
   // equals, hashcode
}

DAO类

public class MyDAO
{
    public boolean persistInstance(T_Donation instance)
    {
        Session session = HibernateUtil.getSessionFactory().openSession();
        Transaction tx = null; 
        try
        {
           tx = sesion.beginTransation();
           // setters again if needed
           session.save(instance);           
           tx.commit();
        }
        catch(Exception ex)
        {
             ex.printStackTrace();
             return false;
        } 
        finally
        {
             session.close();
        }
        return true; 
    }

}

这是代码库的方式.我正在学习DAO模式，我认为这里也错误地实现了DAO模式.

This is how the codebase is. I am learning DAO pattern and I think that too is wrongly implemented here.

所以我的问题是，考虑到这一小功能，我将如何开始编写单元测试?它将需要多少?第一步对我来说真的很难.

So my question is, considering this small functionality, how would I start writing Unit test? And how many it would require? The first steps are really hard for me.

此外，欢迎提供有关更简洁的代码实践的意见.

Also, comments about cleaner code practice are welcome.