Springboot 自动装配原理--浅析

首先 来看下SpringBoot的启动类上注解 @SpringBootApplication

 

 

public @interface SpringBootApplication {

	@AliasFor(annotation = EnableAutoConfiguration.class)
	Class[] exclude() default {};
	@AliasFor(annotation = EnableAutoConfiguration.class)
	@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
	String[] scanBasePackages() default {};
	@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
	Class[] scanBasePackageClasses() default {};

}

我们可以看到在SpringBootApplication 定义中又含有配置相关的注解：EnableAutoConfiguration。

Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
	String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration";
	Class[] exclude() default {};
	String[] excludeName() default {};

}

从EnableAutoConfiguration 注解定义中我们不难发现，它导入了一个AutoConfigurationImportSelector。AutoConfigurationImportSelector就是自动装配的主题，接下来来看下AutoConfigurationImportSelector的相关代码，看看他是怎么干活的？

源码如下：

protected AutoConfigurationEntry getAutoConfigurationEntry(
			AutoConfigurationMetadata autoConfigurationMetadata,
			AnnotationMetadata annotationMetadata) {
		if (!isEnabled(annotationMetadata)) {
			return EMPTY_ENTRY;
		}
		AnnotationAttributes attributes = getAttributes(annotationMetadata);
		 // 这个是关键，获取候选的配置
List configurations = getCandidateConfigurations(annotationMetadata,
				attributes);
		configurations = removeDuplicates(configurations);
		Set exclusions = getExclusions(annotationMetadata, attributes);
		checkExcludedClasses(configurations, exclusions);
		configurations.removeAll(exclusions);
		configurations = filter(configurations, autoConfigurationMetadata);
		fireAutoConfigurationImportEvents(configurations, exclusions);
		return new AutoConfigurationEntry(configurations, exclusions);
	}

跟踪：getCandidateConfigurations 方法：

	protected List getCandidateConfigurations(AnnotationMetadata metadata,
			AnnotationAttributes attributes) {
                //    关键代码： 获取候选配置
		List configurations = SpringFactoriesLoader.loadFactoryNames(
				getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());
		Assert.notEmpty(configurations,
				"No auto configuration classes found in META-INF/spring.factories. If you "
						+ "are using a custom packaging, make sure that file is correct.");
		return configurations;
	}

接着跟踪 SpringFactory.loadFactoryNames：

public static List loadFactoryNames(Class factoryClass, @Nullable ClassLoader classLoader) {
		String factoryClassName = factoryClass.getName();
		return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
	}

	private static Map> loadSpringFactories(@Nullable ClassLoader classLoader) {
		MultiValueMap result = cache.get(classLoader);
		if (result != null) {
			return result;
		}

		try {
               // 重点关注 
			Enumeration urls = (classLoader != null ?
					classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
					ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
			result = new LinkedMultiValueMap<>();
			while (urls.hasMoreElements()) {
				URL url = urls.nextElement();
				UrlResource resource = new UrlResource(url);
				Properties properties = PropertiesLoaderUtils.loadProperties(resource);
				for (Map.Entry entry : properties.entrySet()) {
					String factoryClassName = ((String) entry.getKey()).trim();
					for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
						result.add(factoryClassName, factoryName.trim());
					}
				}
			}
			cache.put(classLoader, result);
			return result;
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Unable to load factories from location [" +
					FACTORIES_RESOURCE_LOCATION + "]", ex);
		}
	}

从源码中 我们可以看到 他构建了一个资源路径，然后去这个路径下去获取属性值（需要自动装配的类）；我们来看下资源路径地址：

public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";

 定位到相应的路径下：

就这样就找到了org.springframework.boot.autoconfigure.EnableAutoConfiguration 对应的属性（配置类的全路径名），进行包装然后加入IOC，完成自动配置；

 

自动装配时机：

refreshContext 
  refresh(context) 
   invokeBeanFactoryPostProcessors(beanFactory);
    PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
      invokeBeanDefinitionRegistryPostProcessors(currentRegistryProcessors, registry);
        postProcessor.postProcessBeanDefinitionRegistry(registry);
         processConfigBeanDefinitions(registry)
          parser.parse(candidates);
           deferredImportSelectorHandler.process()
             handler.processGroupImports()
              group.selectImports() 
                group.process
               AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector)   .getAutoConfigurationEntry(getAutoConfigurationMetadata(),     annotationMetadata)

         

 

 

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