依赖注入的方式有几种，各是什么？

面试

Spring 里常见的依赖注入方式，主要有 3 种：

1. 构造器注入

@Service
public class UserService {

    private final UserDao userDao;

    public UserService(UserDao userDao) {
        this.userDao = userDao;
    }
}

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2. Setter 注入

@Service
public class UserService {

    private UserDao userDao;

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

---

3. 属性注入（字段注入）

@Service
public class UserService {

    @Autowired
    private UserDao userDao;
}