SpringBoot自动配置原理分析

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

	//获得自动配置的实体
    protected AutoConfigurationImportSelector.AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {		/
        if (!this.isEnabled(annotationMetadata)) {
            return EMPTY_ENTRY;
        } else {
            AnnotationAttributes attributes = this.getAttributes(annotationMetadata);
            List<String> configurations = this.getCandidateConfigurations(annotationMetadata, attributes);
            configurations = this.removeDuplicates(configurations);
            Set<String> exclusions = this.getExclusions(annotationMetadata, attributes);
            this.checkExcludedClasses(configurations, exclusions);
            configurations.removeAll(exclusions);
            configurations = this.getConfigurationClassFilter().filter(configurations);
            this.fireAutoConfigurationImportEvents(configurations, exclusions);
            return new AutoConfigurationImportSelector.AutoConfigurationEntry(configurations, exclusions);
        }
    }

	//获取候选的配置
    protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) {
        List<String> configurations = SpringFactoriesLoader.loadFactoryNames(this.getSpringFactoriesLoaderFactoryClass(), this.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;
    }

	//获得主启动类加载的所有组件
    protected Class<?> getSpringFactoriesLoaderFactoryClass() {
        return EnableAutoConfiguration.class;
    }

}

public final class SpringFactoriesLoader {
	
	//获取所有的加载配件
    public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
        String factoryTypeName = factoryType.getName();
        //这里调用了下面的 loadSpringFactories 方法
        return (List)loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());
    }

    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader);
        if (result != null) {
            return result;
        } else {
            try {
           	    //获得classLoader ， 我们返回可以看到这里得到的就是EnableAutoConfiguration标注的类本身
                Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories");
                LinkedMultiValueMap result = new LinkedMultiValueMap();

				//将读取到的资源遍历，封装成为一个Properties
                while(urls.hasMoreElements()) {
                    URL url = (URL)urls.nextElement();
                    UrlResource resource = new UrlResource(url);
                    Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                    Iterator var6 = properties.entrySet().iterator();

                    while(var6.hasNext()) {
                        Entry<?, ?> entry = (Entry)var6.next();
                        String factoryTypeName = ((String)entry.getKey()).trim();
                        String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue());
                        int var10 = var9.length;

                        for(int var11 = 0; var11 < var10; ++var11) {
                            String factoryImplementationName = var9[var11];
                            result.add(factoryTypeName, factoryImplementationName.trim());
                        }
                    }
                }

                cache.put(classLoader, result);
                return result;
            } catch (IOException var13) {
                throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13);
            }
        }
    }

}

springboot所有自动配置都是在启动的时候扫描并加载: 所有的自动配置类都在spring.factories里面，但是不一定生效
判断条件是否成立,只要导入了对应的start，就有对应的启动器了，有了启动器，我们自动装配就会生效，然后就配置成功!

自动配置真正实现是从classpath中搜寻所有的META-INF/spring.factories配置文件 ，并将其中对应org.springframework.boot.autoconfigure包下的配置项，通过反射实例化为对应标注了 @Configuration的JavaConfig形式的IOC容器配置类 ， 然后将这些都汇总成为一个实例并加载到IOC容器中。

1. SpringBoot在启动的时候从类路径下的META-INF/spring.factories中获取EnableAutoConfiguration指定的值
2. 将这些值作为自动配置类导入容器 ， 自动配置类就生效 ， 帮我们进行自动配置工作；
3. 整个J2EE的整体解决方案和自动配置都在springboot-autoconfigure的jar包中；
4. 它会把所有需要导入的组件，以类名的方式返回，这些组件就会被添加到容器;
5. 有了自动配置类 ， 免去了我们手动编写配置注入功能组件等的工作；