@Autowired 注解在构造器上的使用规则（字段注入也挺好的）

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。

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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@Autowiredpublic 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nullpublic 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;@Autowiredpublic 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…

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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 {@Autowiredprivate 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 {@Autowiredprivate HelloService helloService;@Autowiredprivate 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();}
}

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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;@Mockprivate HelloService helloService;@InjectMocksprivate HelloController helloController;@BeforeEachpublic void setup() {MockitoAnnotations.initMocks(this);mockMvc = MockMvcBuilders.standaloneSetup(helloController).build();}@Testpublic 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。

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@Service
public class HelloServiceImpl implements HelloService {private final HelloRepository helloRepository;
//
//    public HelloServiceImpl() {
//
//    }@Autowiredpublic HelloServiceImpl(HelloRepository helloRepository) {this.helloRepository = helloRepository;helloRepository.initialize();}@Overridepublic String sayHello() {return "Hello World";}
}

• 但完全可以先字段注入，然后通过@PostContruct再调用依赖的方法：

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总结

• 通过@Autowired字段注入，并未有不可接受的副作用，而且代码简洁。

• IDEA的警告挺烦的，怎么关了？

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• 或者采用@Resouce

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