一、感受IOC

dao下

//dao
public interface UserDao {
    public void getUser();
}

//dao
public class UserDaoImpl implements UserDao {
    @Override
    public void getUser() {
        System.out.println("获取用户");
    }
}

service下

//service
public interface UserService {
    public void getUser();
}

//service
public class UserServiceImpl implements UserService {
    private UserDao userDao = new UserDaoImpl();
   
    @Override
    public void getUser() {
        userDao.getUser();
    }
}

test下

//test
public class MyTest {
    public static void main(String[] args) {
        UserService userService = new UserServiceImpl();
        userService.getUser();
    }
}

思考一个问题，现在只要一个dao需要，那如果又多出来两个数据库信息获取呢？

//dao
public class UserDaoMysqlImpl implements UserDao {
    @Override
    public void getUser() {
        System.out.println("从musql获取数据");
    }
}

//dao
public class UserDaoOracleImpl implements UserDao {
    @Override
    public void getUser() {
        System.out.println("从oracle中获取数据");
    }
}

难道要对service每次修改，然后在test中挨个实现吗？
不行，这不现实
那么应该怎么做呢？
可以这样

//service
public class UserServiceImpl implements UserService {
    //private UserDao userDao = new UserDaoImpl();

    private UserDao userDao;
    public void setUserDao(UserDao userDao){
        this.userDao = userDao;
    }

    @Override
    public void getUser() {
        userDao.getUser();
    }
}

//test
public class MyTest {
    public static void main(String[] args) {
        //UserService userService = new UserServiceImpl();

        UserServiceImpl userService = new UserServiceImpl();

        UserDaoMysqlImpl userDaoMysql = new UserDaoMysqlImpl();
        userService.setUserDao(userDaoMysql);
        userService.getUser();

        System.out.println("----------------");

        UserDaoOracleImpl userDaoOracle = new UserDaoOracleImpl();
        userService.setUserDao(userDaoOracle);
        userService.getUser();
    }
}