【源码解析】Spring Bean定义常见错误

案例1 隐式扫描不到Bean的定义

@RestController
public class HelloWorldController {

    @RequestMapping(path = "/hiii",method = RequestMethod.GET)
    public String hi() {
        return "hi hellowrd";
    }

}
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@SpringBootApplication
@RestController
public class ApplicationContext {

    public static void main(String[] args) {
        SpringApplication.run(ApplicationContext.class,args);
    }
}
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发现不在同一级的包路径，这个URL访问失败，那么是什么原因呢
其实就在这个main所对应的类的注解上，SpringBootApplication有一个对应的注解那就是ComponentScan

@Target({ElementType.TYPE})
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@SpringBootConfiguration
@EnableAutoConfiguration
@ComponentScan(
    excludeFilters = {@Filter(
    type = FilterType.CUSTOM,
    classes = {TypeExcludeFilter.class}
), @Filter(
    type = FilterType.CUSTOM,
    classes = {AutoConfigurationExcludeFilter.class}
)}
)
public @interface SpringBootApplication {
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ComponentScan有一个属性是扫描对应的路径注解，

	/**
	 * Base packages to scan for annotated components.
	 */
	@AliasFor("value")
	String[] basePackages() default {};
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ComponentScanAnnotationParser.parse的方法，declaringClass所在的包其实就是主方法的包，也就是com.qxlx

好了，我们找到问题所在了，加一行这个自定义路径就可以了。

@ComponentScan("com.qxlx")
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定义的Bean缺少隐式依赖

@Service
public class UserService {

    private String serviceName;

    public UserService(String serviceName) {
        this.serviceName = serviceName;
    }
}
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一启动的时候，就发现异常了。

Parameter 0 of constructor in com.qxlx.service.UserService required a bean of type 'java.lang.Integer' that could not be found.
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 @Bean
 public String serviceName() {
     return "qxlx";
 }
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添加如下bean就可以修复。启动正常。