SpringBoot 源碼getSpringFactoriesInstances（）方法詳解

一、getSpringFactoriesInstances（）方法詳解，一般都是解析ＭETA-INF/spring.factories文件下

ApplicationContextInitializer、ApplicationListener、SpringBootExceptionReporter、SpringApplicationRunListeners等接口的實現類，基於ＳＰＩ可插拔

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

二、主要是調用SpringFactoriesLoader.loadFactoryNames，然后調用createSpringFactoriesInstances。

    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();
        // Use names and ensure unique to protect against duplicates
        Set<String> names = new LinkedHashSet<>(SpringFactoriesLoader.loadFactoryNames(type, classLoader));
        List<T> instances = createSpringFactoriesInstances(type, parameterTypes, classLoader, args, names);
        AnnotationAwareOrderComparator.sort(instances);
        return instances;
    }

三、SpringFactoriesLoader.loadFactoryNames（）方法解析，FACTORIES_RESOURCE_LOCATION＝"META-INF/spring.factories";

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) {
　　　　　//主要獲取spring.factories中的key,key對應接口全名
        String factoryTypeName = factoryType.getName();
        return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList());//篩選Map中key為factoryTypeName對應放到list返回
    }
　　//會把ｓpring.factories文件中所有鍵值對放到Map中
    private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) {
        MultiValueMap<String, String> result = cache.get(classLoader);
        if (result != null) {
            return result;
        }

        try {
            Enumeration<URL> urls = (classLoader != null ?
                    classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            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 factoryTypeName = ((String) entry.getKey()).trim();
                    for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
                        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);
        }
    }

三、createSpringFactoriesInstances（）方法執行過程,主要根據names中的value值反射創建相應的實例，然后講instance實例集合賦值給ＳpringApplicatin對應的屬性

@SuppressWarnings("unchecked")
    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 {
                Class<?> instanceClass = ClassUtils.forName(name, classLoader);
                Assert.isAssignable(type, instanceClass);
                Constructor<?> constructor = instanceClass.getDeclaredConstructor(parameterTypes);
                T instance = (T) BeanUtils.instantiateClass(constructor, args);
                instances.add(instance);
            }
            catch (Throwable ex) {
                throw new IllegalArgumentException("Cannot instantiate " + type + " : " + name, ex);
            }
        }
        return instances;
    }