========
通用工厂
========

1 更加通用的工厂类

在一个工厂类中，通过不同的工厂方法创建不同的实现类对象：

public class BeanFactory {
    ...

    public static UserDao getUserDao() {
        UserDao userDao = null;

        try {
            userDao = (UserDao) Class.forName(
                properties.getProperty("userDao")).newInstance();
        }
        catch (InstantiationException e) {
            e.printStackTrace();
        }
        catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        catch (ClassNotFoundException e) {
            e.printStackTrace();
        }

        return userDao;
    }

    public static UserService getUserService() {
        UserService userService = null;

        try {
            userService = (UserService) Class.forName(
                properties.getProperty("userService")).newInstance();
        }
        catch (InstantiationException e) {
            e.printStackTrace();
        }
        catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        catch (ClassNotFoundException e) {
            e.printStackTrace();
        }

        return userService;
    }
}

如果有更多的实现类，还需要再增加更多的创建方法，而且这些方法中大部分代码都是相似的。

设计一个通用的工厂，只提供一个工厂方法，所有实现类的对象都由该方法创建。

/src/main/java/cn/tedu/commonfactory/BeanFactory.java：

public class BeanFactory {
    private static Properties properties = new Properties();

    static {
        try {
            InputStream inputStream =
                BeanFactory.class.getResourceAsStream(
                    "/applicationContext.properties");
            properties.load(inputStream);
            inputStream.close();
        }
        catch (IOException e) {
            e.printStackTrace();
        }
    }

    public static Object getBean(String key) {
        Object object = null;

        try {
            object = Class.forName(
                properties.getProperty(key)).newInstance();
        }
        catch (InstantiationException e) {
            e.printStackTrace();
        }
        catch (IllegalAccessException e) {
            e.printStackTrace();
        }
        catch (ClassNotFoundException e) {
            e.printStackTrace();
        }

        return object;
    }
}

2 通过通用工厂创建对象

/src/main/java/cn/tedu/commonfactory/UserServiceImpl.java：

public class UserServiceImpl implements UserService {
    private UserDao userDao = (UserDao) BeanFactory.getBean("userDao");

    @Override
    public void register(User user) {
        userDao.save(user);
    }

    @Override
    public User login(String username, String password) {
        return userDao.query(username, password);
    }
}

/src/test/java/cn/tedu/commonfactory/UserServiceTest.java：

class UserServiceTest {
    @org.junit.jupiter.api.Test
    void testRegister() {
        UserService userService =
            (UserService) BeanFactory.getBean("userService");
        userService.register(new User("zhangfei", "123456"));
    }

    @org.junit.jupiter.api.Test
    void testLogin() {
        UserService userService =
            (UserService) BeanFactory.getBean("userService");
        userService.login("zhangfei", "123456");
    }
}

例程：CommonFactory

- 简单工厂存在大量的代码冗余，扩展性差
- 通用工厂提供通用的工厂方法，创建一切