java - Spring Boot 在@SpringBootApplication 类上找不到默认构造函数

Question

我想知道为什么字段注入(inject)在 @SpringBootApplication 类中起作用，而构造函数注入(inject)却不起作用。

我的 ApplicationTypeBean 正在按预期工作，但是当我想对 CustomTypeService 进行构造函数注入(inject)时，我收到此异常:

Failed to instantiate [at.eurotours.ThirdPartyGlobalAndCustomTypesApplication$$EnhancerBySpringCGLIB$$2a56ce70]: No default constructor found; nested exception is java.lang.NoSuchMethodException: at.eurotours.ThirdPartyGlobalAndCustomTypesApplication$$EnhancerBySpringCGLIB$$2a56ce70.<init>()

它在@SpringBootApplication 类中不起作用有什么原因吗？

---

我的 SpringBootApplication 类:

@SpringBootApplication
public class ThirdPartyGlobalAndCustomTypesApplication implements CommandLineRunner{

@Autowired
ApplicationTypeBean applicationTypeBean;

private final CustomTypeService customTypeService;

@Autowired
public ThirdPartyGlobalAndCustomTypesApplication(CustomTypeService customTypeService) {
    this.customTypeService = customTypeService;
}

@Override
public void run(String... args) throws Exception {
    System.out.println(applicationTypeBean.getType());
    customTypeService.process();
}

public static void main(String[] args) {
    SpringApplication.run(ThirdPartyGlobalAndCustomTypesApplication.class, args);
}

public CustomTypeService getCustomTypeService() {
    return customTypeService;
}

我的@Service类:

@Service
public class CustomTypeService {

    public void process(){
        System.out.println("CustomType");
    }
}

我的@Component 类:

@Component
@ConfigurationProperties("application.type")
public class ApplicationTypeBean {

    private String type;

Answer 1

SpringBootApplication 是一个元注解:

// Other annotations
@Configuration
@EnableAutoConfiguration
@ComponentScan
public @interface SpringBootApplication { ... }

因此，您的 ThirdPartyGlobalAndCustomTypesApplication 也是一个 Spring Configuration 类。作为配置的javadoc状态:

@Configuration is meta-annotated with @Component, therefore @Configuration classes are candidates for component scanning (typically using Spring XML's element) and therefore may also take advantage of @Autowired/@Inject at the field and method level (but not at the constructor level).

因此您不能对Configuration 类使用构造函数注入(inject)。显然它将在 4.3 版本中修复，基于 this answer还有这个jira ticket .