springboot SPI加载机制说明

SpringSPI的加载机制相信大家一定不会陌生，它在META-INF/spring.factories文件中配置接口的实现类名称，然后在程序中读取这些配置文件并实例化。但是，它和@Bean注解实现的Bean实例化有什么区别呢？今天我们就来说明下SpringSPI的实现原理。

Spring Factories 的实现原理

Spring SPI的实现依赖于SpringFactoriesLoader类的方法来实现
具体的实现类SpringFactoriesLoader方法如下：

/**
	 * Load the fully qualified class names of factory implementations of the
	 * given type from {@value #FACTORIES_RESOURCE_LOCATION}, using the given
	 * class loader.
	 * @param factoryClass the interface or abstract class representing the factory
	 * @param classLoader the ClassLoader to use for loading resources; can be
	 * {@code null} to use the default
	 * @throws IllegalArgumentException if an error occurs while loading factory names
	 * @see #loadFactories
	 */
	public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {
		String factoryClassName = factoryClass.getName();
		return loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());
	}

	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 factoryClassName = ((String) entry.getKey()).trim();
					for (String factoryName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) {
						result.add(factoryClassName, factoryName.trim());
					}
				}
			}
			cache.put(classLoader, result);
			return result;
		}
		catch (IOException ex) {
			throw new IllegalArgumentException("Unable to load factories from location [" +
					FACTORIES_RESOURCE_LOCATION + "]", ex);
		}
	}

通过代码我们可以看到，加载spring.factories文件的方法是loadSpringFactories(@Nullable ClassLoader classLoader)，它底层是通过PropertiesLoaderUtils来获取文件里的具体值并返回Properties对象。

这时，我们再debug网上看的时候，就会发现

/**
	 * Create a new {@link SpringApplication} instance. The application context will load
	 * beans from the specified primary sources (see {@link SpringApplication class-level}
	 * documentation for details. The instance can be customized before calling
	 * {@link #run(String...)}.
	 * @param resourceLoader the resource loader to use
	 * @param primarySources the primary bean sources
	 * @see #run(Class, String[])
	 * @see #setSources(Set)
	 */
	@SuppressWarnings({ "unchecked", "rawtypes" })
	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();
		setInitializers((Collection) 
		
// 这里就是spring.factories文件的加载入口
		getSpringFactoriesInstances(ApplicationContextInitializer.class)
		);
		setListeners((Collection) getSpringFactoriesInstances(ApplicationListener.class));
		this.mainApplicationClass = deduceMainApplicationClass();
	}

参见上面代码，我们会发现，其实加载的入口是在Springboot的启动类入口方法里。
也就是说，springboot的加载还未实例化bean的情况下，就已经开始加载spring.factories文件了。

这也佐证了一点，spring.factories的加载顺序是先于@Bean的实例化的。

所以大家畅想下，我们经常依赖的 starter 组件，是不是会找到spring.factories文件，根据上面的原理，可以推理知道，为什么我只要依赖了 starter 组件就可以自动加载其中的服务了。