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

org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.env.EnvironmentPostProcessorApplicationListener

Class.forName("com.mysql.cj.jdbc.Driver");

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\
org.mybatis.spring.boot.autoconfigure.MybatisLanguageDriverAutoConfiguration,\
org.mybatis.spring.boot.autoconfigure.MybatisAutoConfiguration

public SpringApplication(ResourceLoader resourceLoader, Class<?>... primarySources) {
    this.resourceLoader = resourceLoader;
    Assert.notNull(primarySources, "PrimarySources must not be null");
    this.primarySources = new LinkedHashSet<>(Arrays.asList(primarySources));
    this.webApplicationType = WebApplicationType.deduceFromClasspath();
    // 下面三行都調(diào)用了 getSpringFactoriesInstances 方法，從方法名可以大致理解到作用是獲取 factories 文件中某個接口的實(shí)現(xiàn)類
    this.bootstrapRegistryInitializers = new ArrayList<>(
        getSpringFactoriesInstances(BootstrapRegistryInitializer.class));
    setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));
    setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
    this.mainApplicationClass = deduceMainApplicationClass();
}

private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
    ClassLoader classLoader = getClassLoader();
    // loadFactoryNames 方法獲取 spring.factories 定義的對應(yīng)接口的所有實(shí)現(xiàn)類的全限定名
    Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
    // 根據(jù)全限定名，和參數(shù)類型，實(shí)例化上面獲取到的那些類
    List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
    // 根據(jù) @Order 注解排序
    AnnotationAwareOrderComparator.sort(instances);
    return instances;
}

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
    ClassLoader classLoaderToUse = classLoader;
    if (classLoaderToUse == null) {
        classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
    }
    String factoryTypeName = factoryType.getName();
    // loadSpringFactories 是 spring.factories 加載的核心方法
    // 返回值是一個 Map，包含 spring.factories 所有的鍵值對，key 是接口名，value 是實(shí)現(xiàn)類的名字?jǐn)?shù)組
    // 這里獲取到所有鍵值對后，根據(jù)需要的接口名稱，獲取到相應(yīng)的實(shí)現(xiàn)數(shù)組
    return loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList());
}

private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
    // cache 是這個類的靜態(tài)變量，所以是唯一的
    Map<String, List<String>> result = cache.get(classLoader);
    // 如果緩存中已經(jīng)有了，那么直接返回
    if (result != null) {
        return result;
    }

    result = new HashMap<>();
    try {
        // 使用 classLoader 從 classpath 讀取文件
        // 這個地址常量在上面已經(jīng)貼出來過了，也就是 META-INF/spring.factories
        // 如果引用了第三方庫，那么就會獲取到很多 url
        Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
        // 所以要循環(huán)遍歷每一個庫里的 spring.factories 文件
        while (urls.hasMoreElements()) {
            URL url = urls.nextElement();
            UrlResource resource = new UrlResource(url);
            // 用 properties 解析器讀取文件
            Properties properties = PropertiesLoaderUtils.loadProperties(resource);
            // 下面的內(nèi)容就是把文件中的每一行數(shù)據(jù)，變成 result 數(shù)據(jù)，也就是 key(文件#String) 變成 key(Map#String)
            // value(文件#String) 變成 value(Map#List<String>)
            for (Map.Entry<?, ?> entry : properties.entrySet()) {
                String factoryTypeName = ((String) entry.getKey()).trim();
                // 這個 commaDelimitedListToStringArray 就是以逗號為分隔符，解析成字符串?dāng)?shù)組
                String[] factoryImplementationNames =
                    StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
                for (String factoryImplementationName : factoryImplementationNames) {
                    result.computeIfAbsent(factoryTypeName, key -> new ArrayList變成
                        .add(factoryImplementationName.trim());
                }
            }
        }

        // Replace all lists with unmodifiable lists containing unique elements
        result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
                          .collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
        cache.put(classLoader, result);
    }
    catch (IOException ex) {
        throw new IllegalArgumentException("Unable to load factories from location [" +
                                           FACTORIES_RESOURCE_LOCATION + "]", ex);
    }
    return result;
}