看Springboot源码的时候，SpringFactoriesLoader类中loadFactoryNames方法里：

public static List<String> loadFactoryNames(Class<?> factoryClass, ClassLoader classLoader) {
        String factoryClassName = factoryClass.getName();
        try {
            Enumeration<URL> urls = (classLoader != null ? classLoader.getResources(FACTORIES_RESOURCE_LOCATION) :
                    ClassLoader.getSystemResources(FACTORIES_RESOURCE_LOCATION));
            List<String> result = new ArrayList<String>();
            while (urls.hasMoreElements()) {
                URL url = urls.nextElement();
                Properties properties = PropertiesLoaderUtils.loadProperties(new UrlResource(url));
                String factoryClassNames = properties.getProperty(factoryClassName);
                result.addAll(Arrays.asList(StringUtils.commaDelimitedListToStringArray(factoryClassNames)));
            }
            return result;
        }

当添加了springboot-devtools依赖后classLoader.getResources(FACTORIES_RESOURCE_LOCATION)加载的是spring-boot-devtools包中的META-INF/spring.factories文件，而去掉devtools的依赖之后加载的是spring-boot包中的META-INF/spring.factories文件，这是怎么实现的？为什么会这样？