SpringBoot自动配置原理是什么？从启动器到spring.factories全解析

<dependency>
    <groupid>org.springframework.boot</groupid>
    <artifactid>spring-boot-starter</artifactid>
</dependency>

@springbootapplication       //标注这个类是一个springboot的应用
public class springboot01application {
    //将springboot应用启动
    public static void main(string[] args) {
        springapplication.run(springboot01application.class, args);
    }
}

list<string> configurations = this.getcandidateconfigurations(annotationmetadata, attributes);

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;
}

protected class<?> getspringfactoriesloaderfactoryclass() {
    return enableautoconfiguration.class;
}

public static list<string> loadfactorynames(class<?> factorytype, @nullable classloader classloader) {
    classloader classloadertouse = classloader;
    if (classloader == null) {
        classloadertouse = springfactoriesloader.class.getclassloader();
    }

    string factorytypename = factorytype.getname();
    return (list)loadspringfactories(classloadertouse).getordefault(factorytypename, collections.emptylist());
}

private static map<string, list<string>> loadspringfactories(classloader classloader) {
    map<string, list<string>> result = (map)cache.get(classloader);
    if (result != null) {
        return result;
    } else {
        hashmap result = new hashmap();

        try {
            enumeration urls = classloader.getresources("meta-inf/spring.factories");

            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[] factoryimplementationnames = stringutils.commadelimitedlisttostringarray((string)entry.getvalue());
                    string[] var10 = factoryimplementationnames;
                    int var11 = factoryimplementationnames.length;

                    for(int var12 = 0; var12 < var11; ++var12) {
                        string factoryimplementationname = var10[var12];
                        ((list)result.computeifabsent(factorytypename, (key) -> {
                            return new arraylist();
                        })).add(factoryimplementationname.trim());
                    }
                }
            }

            result.replaceall((factorytype, implementations) -> {
                return (list)implementations.stream().distinct().collect(collectors.collectingandthen(collectors.tolist(), collections::unmodifiablelist));
            });
            cache.put(classloader, result);
            return result;
        } catch (ioexception var14) {
            throw new illegalargumentexception("unable to load factories from location [meta-inf/spring.factories]", var14);
        }
    }
}

@springbootapplication
public class springboot01application {
    //将springboot应用启动，该方法返回一个configurableapplicationcontext对象
    //参数一:应用入口的类,参数二:命令行参数
    public static void main(string[] args) {
        springapplication.run(springboot01application.class, args);
    }
}