使用 @ComponentScan 进行元注释的 Spring 自定义 @Enable 注释

Question

我正在尝试为Spring框架编写自己的@Enable注解，应该使用如下：

package com.example.package.app;

@SpringBootApplication
@com.example.annotations.EnableCustom("com.example.package.custom")
public class MyApplication {}

我关注了Component scan using custom annotation，但这带来了一些限制：

1. 我无法使基础包属性动态化，即无法传递"com.example.package.base"，但需要在配置时预先定义包。

   我查看了@AliasFor，但无法正常工作。

2. 当我省略基本包时，扫描从注释的定义包开始，而不是从注释类的包开始。在上面的示例中，它只会扫描和创建 com.example.annotations 中的类的 bean，而不是 com.example.package.*。

   我查看了EntityScanPackages.Registrar.class，它是在@EntityScan 注释中导入的，但它是一个内部类，我的注释无法导入。

如果我将@ComponentScan(includeFilters = @ComponentScan.Filter(type = FilterType.ANNOTATION, value = MyAnnotation.class)) 放在MyApplication 类上，一切正常，但当它移到@EnableCustom 的元注释时停止工作。如何告诉 Spring Framework 将 @EnableCustom 视为使用一些默认值指定 @ComponentScan 的不同方式。我尝试使用 @Configuration、@Component 和其他人对我的注释进行元注释，但无济于事：

@Configuration
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@ComponentScan(
        includeFilters = @ComponentScan.Filter(
                type = FilterType.ANNOTATION,
                value = ApplicationService.class))
public @interface EnableApplicationServices {
    @AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
    String[] value() default {};
}

我在哪里可以找到这方面的文档，或者您会推荐什么起点？我的长期目标是拥有一个可供众多项目使用的 Spring Boot 启动器。

---

可以在以下存储库中找到一个 M(N)WE：https://github.com/knittl/stackoverflow/tree/spring-enable-annotation

以下是包结构的概要：

// com.example.annotations.EnableCustom.java
@Configuration
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
// this annotation is never honored:
@ComponentScan(
        includeFilters = @ComponentScan.Filter(
                type = FilterType.ANNOTATION,
                value = MyAnnotation.class))
//@Import(EnableCustom.EnableCustomConfiguration.class)
public @interface EnableCustom {
    // this annotation works in combination with @Import, but scans the wrong packages.
    @ComponentScan(
            includeFilters = @ComponentScan.Filter(
                    type = FilterType.ANNOTATION,
                    value = MyAnnotation.class))
    class EnableCustomConfiguration {}
}

// file:com.example.app.Application.java
@SpringBootApplication
@EnableCustom("com.example.app.custom.services")
// @ComponentScan(
//         includeFilters = @ComponentScan.Filter(
//                 type = FilterType.ANNOTATION,
//                 value = MyAnnotation.class)) // <- this would work, but I want to move it to a custom annotation
public class Application {
}

// file:com.example.app.custom.services.MyService
@MyAnnotation
public class MyService {
    public MyService() {
        System.out.println("Look, I'm a bean now!");
    }
}

// file:com.example.annotations.services.WrongService.java
@MyAnnotation
public class WrongService {
    public WrongService() {
        System.out.println("I'm in the wrong package, I must not be instantiated");
    }
}

Answer 1

在Fabio Formosa's answer的帮助下，this answer中的缺失位，以及@EntityScan注释的一些启发，我终于设法让它工作了。在https://github.com/knittl/stackoverflow/tree/spring-enable-annotation-working 可以找到一个可编译的工作示例。

简而言之：基于 Fabio 的回答，重要的是正确配置带有包含过滤器的 ClassPathScanningCandidateComponentProvider 实例，然后针对所有提供的基类运行它。 @AliasFor(annotation = Import.class, …) 似乎不是必需的，可以别名为另一个属性，例如basePackages 的同一个注解。

最低实现如下：

@Configuration
@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Import(EnableCustom.EnableCustomConfiguration.class)
public @interface EnableCustom {
    @AliasFor(attribute = "basePackages")
    String[] value() default {};

    @AliasFor(attribute = "value")
    String[] basePackages() default {};

    class EnableCustomConfiguration implements ImportBeanDefinitionRegistrar, EnvironmentAware {
        private static final BeanNameGenerator BEAN_NAME_GENERATOR = AnnotationBeanNameGenerator.INSTANCE;
        private Environment environment;

        @Override
        public void setEnvironment(final Environment environment) {
            this.environment = environment;
        }

        @Override
        public void registerBeanDefinitions(
                final AnnotationMetadata metadata,
                final BeanDefinitionRegistry registry) {
            final AnnotationAttributes annotationAttributes = new AnnotationAttributes(
                    metadata.getAnnotationAttributes(EnableCustom.class.getCanonicalName()));

            final ClassPathScanningCandidateComponentProvider provider
                    = new ClassPathScanningCandidateComponentProvider(false, environment);
            provider.addIncludeFilter(new AnnotationTypeFilter(MyAnnotation.class, true));

            final Set<String> basePackages
                    = getBasePackages((StandardAnnotationMetadata) metadata, annotationAttributes);

            for (final String basePackage : basePackages) {
                for (final BeanDefinition beanDefinition : provider.findCandidateComponents(basePackage)) {
                    final String beanClassName = BEAN_NAME_GENERATOR.generateBeanName(beanDefinition, registry);
                    if (!registry.containsBeanDefinition(beanClassName)) {
                        registry.registerBeanDefinition(beanClassName, beanDefinition);
                    }
                }
            }
        }

        private static Set<String> getBasePackages(
                final StandardAnnotationMetadata metadata,
                final AnnotationAttributes attributes) {
            final String[] basePackages = attributes.getStringArray("basePackages");
            final Set<String> packagesToScan = new LinkedHashSet<>(Arrays.asList(basePackages));

            if (packagesToScan.isEmpty()) {
                // If value attribute is not set, fallback to the package of the annotated class
                return Collections.singleton(metadata.getIntrospectedClass().getPackage().getName());
            }

            return packagesToScan;
        }
    }
}

Answer 2

使用带有 basePackages 属性的自定义注解 @EnableAnnotation

@EnableAnnotation(basePackages =  "write-here-a-base-package")
@Configuration
@EnableAutoConfiguration
@ComponentScan
public class SampleSimpleApplication implements CommandLineRunner {

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

@EnableAnnotation 是这样定义的：

@Retention(RUNTIME)
@Target(TYPE)
@Import(EnableAnnotationConfigRegistrar.class)
public @interface EnableAnnotation {

  String[] basePackages() default "*";

  @AliasFor(annotation = Import.class, attribute = "value")
  Class<?>[] value() default { EnableAnnotationConfigRegistrar.class };

}

最后，EnableAnnotationConfigRegistrar.class 以编程方式扫描：

public class EnableAnnotationConfigRegistrar implements ImportBeanDefinitionRegistrar {

 @Override
 public void registerBeanDefinitions(AnnotationMetadata enableAnnotationMetadata,
  BeanDefinitionRegistry registry) {
   AnnotationAttributes enableAnnotationAttributes = new AnnotationAttributes(
   enableAnnotationMetadata.getAnnotationAttributes(EnableAnnotation.class.getName()));

   String[] basePackages = enableAnnotationAttributes.getStringArray("basePackages");
   AnnotationConfigApplicationContext annotationConfigApplicationContext = new AnnotationConfigApplicationContext(basePackages);
   }

}