关于SpringBoot中spring.factories文件的常识

关于SpringBoot中spring.factories文件的常识

1. 此文件结构类似于Map<String, List<String>>,一个键可以对应多个值。如：

   org.springframework.boot.autoconfigure.AutoConfigurationImportFilter=\
   org.springframework.boot.autoconfigure.condition.OnBeanCondition,\
   org.springframework.boot.autoconfigure.condition.OnClassCondition,\
   org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition
   

2. 此文件的加载时间最初为SpringBoot创建SpringApplication时调用，具体调用栈为：SpringApplication(ResourceLoader, Class<?>...) ->getSpringFactoriesInstances(Class<T>)->SpringFactoriesLoader.loadFactoryNames(...)->``SpringFactoriesLoader.loadSpringFactories(...)`。引用代码如下：

   private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
   		Map<String, List<String>> result = cache.get(classLoader);
   		if (result != null) {
   			return result;
   		}
   
   		result = new HashMap<>();
   		try {
   			Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
   			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();
   					String[] factoryImplementationNames =
   							StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
   					for (String factoryImplementationName : factoryImplementationNames) {
   						result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
   								.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;
   	}
   

注意：spring.factories其实是Spring SPI机制的实现。