依赖注入(Dependency Injection)----DI

Constructor-based Dependency Injection(基于构造器的依赖注入)

public class UserServiceImpl implements UserService {
    private UserDao userDao;

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