初始化基本流程

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setInitializers设置初始化器
我们来看看这一句话做了什么：

setInitializers((Collection) getSpringFactoriesInstances(ApplicationContextInitializer.class));

getSpringFactoriesInstances获取Spring工厂实例
好像是获取某个类型的集合，类型就是ApplicationContextInitializer，兴趣加载一堆名字，然后创建实例，然后排序返回，我们看看具体怎么做的：

    private <T> Collection<T> getSpringFactoriesInstances(Class<T> type) {
        return getSpringFactoriesInstances(type, new Class<?>[] {});
    }
    
    private <T> Collection<T> getSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes, Object... args) {
        ClassLoader classLoader = getClassLoader();//获取系统加载器，就是线程上下文加载器
        // Use names and ensure unique to protect against duplicates
        Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));//获取type实现类的名字集合
        List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
        AnnotationAwareOrderComparator.sort(instances);
        return instances;
    }

SpringFactoriesLoader的loadFactoryNames

其实这里就是最开始说的，去加载jar包下的META-INF/spring.factories加载相应属性的值，这里就是org.springframework.context.ApplicationContextInitialize属性。

    public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
        String factoryTypeName = factoryType.getName();//获取类的全限定名，org.springframework.context.ApplicationContextInitialize
        return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());//获取org.springframework.context.ApplicationContextInitialize的值的数组
    }

loadSpringFactories加载META-INF/spring.factories

先从缓存里取一个MultiValueMap，这个是一个一对多的映射集合，比如说一个接口，对应多个实现啦。然后获取所有依赖的jar包下的META-INF/spring.factories的url，再去加载相应的数据，然后把值用逗号分隔符分开，然后把值去空格后和键一起都放入MultiValueMap中，顺便放入缓存里。

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

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

        try {//获取URL路径
            Enumeration<URL> urls = (classLoader != null ?
                    classLoader.getResources(FACTORIES_RESOURCE_LOCATION) ://META-INF/spring.factories
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            result = new LinkedMultiValueMap<>();
            while (urls.hasMoreElements()) {//变量URL，获取配置属性，加到result里
                URL url = urls.nextElement();
                UrlResource resource = new UrlResource(url);
                Properties properties = PropertiesLoaderUtils.loadProperties(resource);
                for (Map.Entry<?, ?> entry : properties.entrySet()) {
                    String factoryTypeName = ((String) entry.getKey()).trim();
                    for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {//值用逗号分割负分开，都放入result中
                        result.add(factoryTypeName, factoryImplementationName.trim());
                    }
                }
            }
            cache.put(classLoader, result);
            return result;
        }
        catch (IOException ex) {
            throw new IllegalArgumentException("Unable to load factories from location [" +
                    FACTORIES_RESOURCE_LOCATION + "]", ex);
        }
    }

PropertiesLoaderUtils的loadProperties加载资源文件

获取输入流，获取资源名字，处理的是xml的或者其他的类型。

    private static final String XML_FILE_EXTENSION = ".xml";

public static void fillProperties(Properties props, Resource resource) throws IOException {
        InputStream is = resource.getInputStream();
        try {
            String filename = resource.getFilename();
            if (filename != null && filename.endsWith(XML_FILE_EXTENSION)) {
                props.loadFromXML(is);
            }
            else {
                props.load(is);
            }
        }
        finally {
            is.close();
        }
    }

从org/springframework/boot/spring-boot/2.2.6.RELEASE/spring-boot-2.2.6.RELEASE.jar!/META-INF/spring.factories里
解析出这些，其实就是键值对，有些值是多个，用逗号分割的：

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最后你会看到那么多：

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SpringApplication的createSpringFactoriesInstances创建相关类型的实例

进行反射创建实例并返回。

    private <T> List<T> createSpringFactoriesInstances(Class<T> type, Class<?>[] parameterTypes,
            ClassLoader classLoader, Object[] args, Set<String> names) {
        List<T> instances = new ArrayList<>(names.size());
        for (String name : names) {
            try {//进行反射实例化
                Class<?> instanceClass = ClassUtils.forName(name, classLoader);
                Assert.isAssignable(type, instanceClass);//判断instanceClass是type的类型或者子类
                Constructor<?> constructor = instanceClass.getDeclaredConstructor(parameterTypes);
                T instance = (T) BeanUtils.instantiateClass(constructor, args);//实例化
                instances.add(instance);
            }
            catch (Throwable ex) {
                throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name, ex);
            }
        }
        return instances;
    }

AnnotationAwareOrderComparator的sort根据优先级排序

初始化可能有优先顺序的，所以要进行排序，如果没有实现Ordered接口的一律优先级最低的，内部是这样排序的，PriorityOrdered优先，然后是Ordered：

    private int doCompare(@Nullable Object o1, @Nullable Object o2, @Nullable OrderSourceProvider sourceProvider) {
        boolean p1 = (o1 instanceof PriorityOrdered);
        boolean p2 = (o2 instanceof PriorityOrdered);
        if (p1 && !p2) {
            return -1;
        }
        else if (p2 && !p1) {
            return 1;
        }

        int i1 = getOrder(o1, sourceProvider);
        int i2 = getOrder(o2, sourceProvider);
        return Integer.compare(i1, i2);
    }

排序前：

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排序后：

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其实如果你自己写的想早点初始化，就可以实现PriorityOrdered或者Ordered，返回小点的值就好啦。

SpringApplication的setInitializers

最后把他们放进ArrayList里，后面肯定会有用，后面再讲吧。

    public void setInitializers(Collection<? extends ApplicationContextInitializer<?>> initializers) {
        this.initializers = new ArrayList<>(initializers);
    }

至此设置初始化器讲完了