springboot enable註解自動配置流程

自動配置流程

核心只有4步

@EnableCircuitBreaker
@MapperScan("com.github.chengzhx76.service.order.dao")
public class OrderServiceApplication {
    public static void main(String[] args) {
        SpringApplication.run(OrderServiceApplication.class, args);
    }
}

1、@Enable***一般是@import註解的簡寫，@import是用來匯入某個bean到spring容器的

@Import(EnableCircuitBreakerImportSelector.class)
public @interface EnableCircuitBreaker {

}

2、selector根據條件選擇需要匯入的配置類。

selector是用來匯入meta-inf/spring.factories配置的，繼承SpringFactoryImportSelector

@Order(Ordered.LOWEST_PRECEDENCE - 100)
public class EnableCircuitBreakerImportSelector extends
      SpringFactoryImportSelector<EnableCircuitBreaker> {

   @Override
   protected boolean isEnabled() {
      return new RelaxedPropertyResolver(getEnvironment()).getProperty(
            "spring.cloud.circuit.breaker.enabled", Boolean.class, Boolean.TRUE);
   }

}

3、SpringFactoriesLoader載入spring.factories配置對應的bean

selector通過父類的selectImports呼叫SpringFactoriesLoader載入對應的spring.factories類註冊到beandefinition registry 容器實現ioc

@Override
public String[] selectImports(AnnotationMetadata metadata) {
   if (!isEnabled()) {
      return new String[0];
   }
   AnnotationAttributes attributes = AnnotationAttributes.fromMap(
         metadata.getAnnotationAttributes(this.annotationClass.getName(), true));

   Assert.notNull(attributes, "No " + getSimpleName() + " attributes found. Is "
         + metadata.getClassName() + " annotated with @" + getSimpleName() + "?");

   // Find all possible auto configuration classes, filtering duplicates
   List<String> factories = new ArrayList<>(new LinkedHashSet<>(SpringFactoriesLoader
         .loadFactoryNames(this.annotationClass, this.beanClassLoader)));

   if (factories.isEmpty() && !hasDefaultFactory()) {
      throw new IllegalStateException("Annotation @" + getSimpleName()
            + " found, but there are no implementations. Did you forget to include a starter?");
   }

   if (factories.size() > 1) {
      // there should only ever be one DiscoveryClient, but there might be more than
      // one factory
      log.warn("More than one implementation " + "of @" + getSimpleName()
            + " (now relying on @Conditionals to pick one): " + factories);
   }

   return factories.toArray(new String[factories.size()]);
}

factories檔案舉例

org.springframework.cloud.client.circuitbreaker.EnableCircuitBreaker=\
org.springframework.cloud.netflix.hystrix.HystrixCircuitBreakerConfiguration

4、例項化被載入配置的類

HystrixCircuitBreakerConfiguration這個bean就是一個配置bean，所以通過以上流程spring最終實現了自動找到配置bean的目的。
我理解是個spi外掛化的過程，spring標準化了元件接入的流程。

@Configuration
public class HystrixCircuitBreakerConfiguration {

   @Bean
   public HystrixCommandAspect hystrixCommandAspect() {
      return new HystrixCommandAspect();
   }

   @Bean
   public HystrixShutdownHook hystrixShutdownHook() {
      return new HystrixShutdownHook();
   }

   @Bean
   public HasFeatures hystrixFeature() {
      return HasFeatures.namedFeatures(new NamedFeature("Hystrix", HystrixCommandAspect.class));
   }

   @Configuration
   @ConditionalOnProperty(value = "hystrix.stream.endpoint.enabled", matchIfMissing = true)
   @ConditionalOnWebApplication
   @ConditionalOnClass({ Endpoint.class, HystrixMetricsStreamServlet.class })
   protected static class HystrixWebConfiguration {

      @Bean
      public HystrixStreamEndpoint hystrixStreamEndpoint() {
         return new HystrixStreamEndpoint();
      }

      @Bean
      public HasFeatures hystrixStreamFeature() {
         return HasFeatures.namedFeature("Hystrix Stream Servlet", HystrixStreamEndpoint.class);
      }
   }

自定義starter

1、寫bean配置類

@Configuration
@ConditionalOnClass(ExampleService.class)
//屬性配置類
@EnableConfigurationProperties(ExampleServiceProperties.class)
public class ExampleAutoConfigure {

    @Autowired
    private ExampleServiceProperties properties;

    @Bean
    @ConditionalOnMissingBean
    @ConditionalOnProperty(prefix = "example.service",value = "enabled",havingValue = "true")
    ExampleService exampleService (){
        return  new ExampleService(properties.getPrefix(),properties.getSuffix());
    }

}

2、寫屬性配置類

如果有需要配置的屬性，可以加一個屬性配置類，properties的屬性會自動注入到配置類中。

@ConfigurationProperties("example.service")
public class ExampleServiceProperties {
    private String prefix;
    private String suffix;
    //省略 getter setter

3、pom檔案

<modelVersion>4.0.0</modelVersion>
    <groupId>com.example</groupId>
    <artifactId>example-spring-boot-starter</artifactId>
    <version>1.0-SNAPSHOT</version>
    <dependencies>
        <dependency>
            <groupId>org.springframework.boot</groupId>
            <artifactId>spring-boot-autoconfigure</artifactId>
        </dependency>
    </dependencies>

4、新增spring .factories

org.springframework.boot.autoconfigure.EnableAutoConfiguration=com.example.autocinfigure.ExampleAutoConfigure

5、打包

mvn package

springboot啟動原理

@springbootapplication註解

@springbootapplication包含了很多註解，如下圖所示。核心在於自動配置註解@EnableAutoConfiguration，原理如上文所述

springbootappliction.run()

run方法就是初始化ioc容器和建立各種bean的過程，核心在refresh()