一.SpringBoot的自动装配

1.@Import（AutoConfigurationImportSelector.class）

public class AutoConfigurationImportSelector implements DeferredImportSelector, BeanClassLoaderAware,
		ResourceLoaderAware, BeanFactoryAware, EnvironmentAware, Ordered

2.DeferredImprotSelector

public interface DeferredImportSelector extends ImportSelector

• DeferredImprotSelector是ImprotSelector的一种扩展
• 在处理完所有的 @Configuration类型的Bean之后运行
• 当导入@Conditional时，这种类型选择器特别有用
• 实现还可以扩展Ordered接口，或使用@Order注解指示对其DeferredImportSelector优先级

总结:

• 执行时机：在@configuration注解中其他逻辑被处理完毕之后（包括对@ImprotResource,@Bean）再执行
• DeferredImportSelector执行时机比ImprotSelector更晚

中其

二.AutoConfigurationImportSelector的核心

@override
public String[] selectImports(AnnotationMetadata annotationMetadata){
   if(!isEmabled(annotationMetadata)){
       return NO_IMPORTS;
   
   }
   AutoconfigurationMetadata autoConfigurationMetadata=AutoconfigurationMetadataLoder.loadMetadata(this.beanClassLoader);
  // 加载自动配置类
    AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata, 
            annotationMetadata);
 
   return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());
}

三.getAutoConfigurationEntry(autoConfigurationMetadata, annotationMetadata)

protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata, AnnotationMetadata annotationMetadata){
        if(!isEnabled(annotationMetadata)){
            return EMPTY_ENTRY;
        }
        AnnotationAttributes attributes=getAttributes(annotationMetadata);
        //核心-加载候选的自动配置类
        List configurations=getCandidateConfigurations(annotationMetadata, attribute);
        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);
    }

四.SpringBoot使用的工厂机制

 1.java的SPI

• SPI全称Service Provider Interface
• jdk内置服务提供发现机制---动态替换发现的机制

 2.SpringFramework的SpringFactoriesLoader

 loadFactoryNames方法：

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

3.loadSpringFactories的实现，其中有缓存的写法值得学习

 
    public static final String FACTORIES_RESOURCE_LOCATION="META-INF/spring.factories";
 
    private static final Map> cache=new ConcurrentReferenceHashMap<>();
 
    private static Map> loadSpringFactories(@Nullable ClassLoader classLoader){
 
        Map> result=cache.get(classLoader);
        if(result!=null){
            return result;
        }
        try{
            //获取当前 classpath 下所有jar包中有的 spring.factories 文件，并将它们加载到内存中
            Enumeration urls=(classLoader != null ?
                    classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            //每一个文件，并用 Properties 方式加载文件，之后把这个文件中每一组键值对都加载出来，放入 MultiValueMap 中
            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 );
        }
 
    }

五.总结

• AutoconfigurationImprotSelector 配合SpringFactoriseLoader可加载“META-INF/spring.factories”中配置@EnableAutoconfiguration对应的自动配置类
• deferredImportSelector的执行时机比ImprotSelector更晚
• SpringFramework 实现了自己的SPI技术，相比较于Java原生的SPI更灵活。