spring依赖注入的三种方式

• 变量(filed) 注入

@Autowired
 UserDao userDao;

• 构造器注入

final UserDao userDao;
@Autowired
public UserServiceImpl(UserDao userDao){
        this.userDao = userDao;
}

• set 方法注入

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

• 这三种方式的区别