工厂方法模式在java中使用泛型，怎么办？ [英] Factory method pattern in java using generics, how to?

问题描述

我的代码如下：

public interface BaseDAO{
// marker interface
}

public interface CustomerDAO extends BaseDAO{
public void createCustomer();
public void deleteCustomer();
public Customer getCustomer(int id);
// etc
}

public abstract class DAOFactory {
public BaseDAO getCustomerDAO();
public static DAOFactory getInstance(){
  if(system.getProperty("allowtest").equals("yes")) {
  return new TestDAOFactory();
  }
  else return new ProdDAOFactory();
}

public class TestDAOFactory extends DAOFactory{
public BaseDAO getCustomerDAO() {
  return new TestCustomerDAO(); // this is a concrete implementation
  //that extends CustomerDAO
  //and this implementation has dummy code on methods
}

public class ProdDAOFactory extends DAOFactory {
public BaseDAO getCustomerDAO() {
  return new ProdCustomerDAO(); // this implementation would have 
  // code that would connect to the database and do some stuff..
}
}

现在，我知道这段代码闻起来有很多原因。但是，这段代码也是这样的：
http：// java。 sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html ，请参考9.8

Now, I do know that this code smells.. for many reasons. However, this code is here too: http://java.sun.com/blueprints/corej2eepatterns/Patterns/DataAccessObject.html, refer 9.8

我打算做的是这样的：
1）开关我的DAO实现在运行时基于环境（系统属性）。
2）使用java泛型使我可以避免类型转换...
例如做这样的事情：

What I intend to do is this: 1) Switch my DAOs implementations at runtime based on environment (system properties). 2) Make use of java generics so that I can avoid type casting... for instance does something like this:

CustomerDAO dao = factory.getCustomerDAO();
dao.getCustomer();

相反：

CustomerDAO dao = (CustomerDAO) factory.getCustomerDAO();
dao.getCustomer();

请提供您的想法和建议。

Your thoughts and suggestions, please.

推荐答案

你应该定义这样的工厂：

You should define the factory like that:

public abstract class DAOFactory<DAO extends BaseDAO> {
public DAO getCustomerDAO();
public static <DAO extends BaseDAO> DAOFactory<DAO> getInstance(Class<DAO> typeToken){
  // instantiate the the proper factory by using the typeToken.
  if(system.getProperty("allowtest").equals("yes")) {
  return new TestDAOFactory();
  }
  else return new ProdDAOFactory();
}

getInstance应该返回一个正确键入的DAOFactory。

getInstance should return a proper typed DAOFactory.

工厂变量将具有以下类型：

The factory variable will have the type:

DAOFactory<CustomerDAO> factory = DAOFactory<CustomerDAO>.getInstance(CustomerDAO.class);

，使用情况将正确输入：

and the usage will be properly typed:

CustomerDAO dao = factory.getCustomerDAO();
dao.getCustomer();

唯一的问题可能是getInstance方法中需要的转换。

the only problem will probably be a cast required inside the getInstance methods.

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