@Autowired 注解在构造器上的使用规则（字段注入也挺好的）

news2025/3/16 18:36:31

背景

在看Spring Framework官方文档时，看到这样一段描述：

As of Spring Framework 4.3, an @Autowired annotation on such a constructor is no longer necessary if the target bean defines only one constructor to begin with. However, if several constructors are available and there is no primary/default constructor, at least one of the constructors must be annotated with @Autowired in order to instruct the container which one to use. See the discussion on constructor resolution for details.

为了更好地理解这段话，做了一些实践，写成文章分享给大家。

‍

实践

第一句

“As of Spring Framework 4.3, an @Autowired annotation on such a constructor is no longer necessary if the target bean defines only one constructor to begin with.”

含义：从 Spring Framework 4.3 版本开始，如果一个目标 Bean 只定义了一个构造器，那么在该构造器上标注 @Autowired 注解就不再是必须的。

@RestController
public class HelloController {

    private HelloService helloService;

    // 只有一个构造器，可以不写@Autowired
    public HelloController(HelloService helloService) {
        this.helloService = helloService;
    }

    @GetMapping("/hello")
    public String hello() {
        return helloService.sayHello();
    }
}

第二句

“However, if several constructors are available and there is no primary/default constructor, at least one of the constructors must be annotated with @Autowired in order to instruct the container which one to use.”

含义：然而，如果一个类有多个构造器，并且没有一个被标记为“主要构造器”或默认构造器，那么至少需要在一个构造器上标注 @Autowired，以便告诉 Spring 容器应该使用哪一个构造器。

@RestController
public class HelloController {

    private HelloService helloService;

//    @Autowired 不写@Autowired，helloSService会为null
    public HelloController(HelloService helloService) {
        this.helloService = helloService;
    }

    public HelloController() {

    }

    @GetMapping("/hello")
    public String hello() {
        return helloService.sayHello();
    }
}


Cannot invoke "com.forrest.learn.spring.autowired.example1.HelloService.sayHello()" because "this.helloService" is null

@RestController
public class HelloController {

    private HelloService helloService;

    @Autowired
    public HelloController(HelloService helloService) {
        this.helloService = helloService;
    }

    public HelloController() {

    }

    @GetMapping("/hello")
    public String hello() {
        return helloService.sayHello();
    }
}

想法

在实际开发中，很少会采用构造器注入，因为写起来太麻烦了…

那我们需要搞清楚，为什么不推荐使用注入？

Why field injection is not recommended?
- （1）不方便写单测（不赞同，挺方便的）
- （2）不能被final修饰（确实有风险，但完全属于人为造成的风险…）
- （3）字段注入隐藏了类的依赖项（不赞同，根据字段注入，也可以知道当前类依赖了哪些类）
- （4）字段注入依赖于 Spring 框架在构建对象后填充依赖项。(下文解释)
- （5）字段注入使类依赖于用于注入依赖项的框架。（不赞同，@Autowired是Spring的注解，确实依赖了框架，但我们可以用Java的@Resource进行依赖注入）

不能被final修饰（确实有风险，但helloService被恶意修改了，在运行时可以被测出来，因为运行结果不符合预期了）

@RestController
public class HelloController {

    @Autowired
    private HelloService helloService;

    @GetMapping("/hello")
    public String hello() {
        return helloService.sayHello();
    }
}

由于helloService不是final的，因此有被修改的风险：

@RestController
public class HelloController {

    @Autowired
    private HelloService helloService;

    @Autowired
    private ApplicationContext applicationContext;

    @GetMapping("/hello")
    public String hello() {
        HelloController helloController = applicationContext.getBean(HelloController.class);
        Field field = null;
        try {
            field = HelloController.class.getDeclaredField("helloService");
        } catch (NoSuchFieldException e) {
            throw new RuntimeException(e);
        }
        field.setAccessible(true);

        try {
            field.set(helloController, null);
        } catch (IllegalAccessException e) {
            throw new RuntimeException(e);
        }

        return helloService.sayHello();
    }
}

helloService注入时不为null，但被修改成了null，导致了npe…

不方便写单测（挺方便的）

public class HelloControllerTest {

    private MockMvc mockMvc;

    @Mock
    private HelloService helloService;

    @InjectMocks
    private HelloController helloController;

    @BeforeEach
    public void setup() {
        MockitoAnnotations.initMocks(this);
        mockMvc = MockMvcBuilders.standaloneSetup(helloController).build();
    }

    @Test
    public void hello_WhenCalled_ReturnsHelloMessage() throws Exception {
        String expectedMessage = "Hello, World!";
        when(helloService.sayHello()).thenReturn(expectedMessage);

        mockMvc.perform(get("/hello"))
                .andExpect(status().isOk())
                .andExpect(content().string(expectedMessage));
    }
}

字段注入依赖于 Spring 框架在构建对象后填充依赖项。

‍

@Service
public class HelloServiceImpl implements HelloService {

    private final HelloRepository helloRepository;
//
//    public HelloServiceImpl() {
//
//    }

    @Autowired
    public HelloServiceImpl(HelloRepository helloRepository) {
        this.helloRepository = helloRepository;
        helloRepository.initialize();
    }

    @Override
    public String sayHello() {
        return "Hello World";
    }
}