Spring boot扩展机制Spring Factories

Spring Cloud SPI

SPI英文为Service Provider Interface单从字面可以理解为Service提供者接口，正如从SPI的名字去理解SPI就是Service提供者接口；我对SPI的定义：提供给服务提供厂商与扩展框架功能的开发者使用的接口。

SPI是供给框架开发者以方便的对框架进行额外拓展的；API是提供给框架使用者的，使用者可以实现API接口来定义自己的功能组件。

SPI思想

很多框架都使用了java的SPI机制，如java.sql.Driver的SPI实现（MySQL驱动、oracle驱动等）、common-logging的日志接口实现、dubbo的扩展实现等等框架；

在传统spring开发中，大量使用配置文件，将bean进行统一组装。spring boot使用SPI技术将bean的一些初始化配置信息分散到各包中的/META-INF/spring.factories文件中，通过配置进行自动装载bean。

以spring boot的spring.factories如下：

# PropertySource Loaders
org.springframework.boot.env.PropertySourceLoader=\
org.springframework.boot.env.PropertiesPropertySourceLoader,\
org.springframework.boot.env.YamlPropertySourceLoader

# Run Listeners
org.springframework.boot.SpringApplicationRunListener=\
org.springframework.boot.context.event.EventPublishingRunListener

# Error Reporters
org.springframework.boot.SpringBootExceptionReporter=\
org.springframework.boot.diagnostics.FailureAnalyzers

# Application Context Initializers
org.springframework.context.ApplicationContextInitializer=\
org.springframework.boot.context.ConfigurationWarningsApplicationContextInitializer,\
org.springframework.boot.context.ContextIdApplicationContextInitializer,\
org.springframework.boot.context.config.DelegatingApplicationContextInitializer,\
org.springframework.boot.web.context.ServerPortInfoApplicationContextInitializer

# Application Listeners
org.springframework.context.ApplicationListener=\
org.springframework.boot.ClearCachesApplicationListener,\
org.springframework.boot.builder.ParentContextCloserApplicationListener,\
org.springframework.boot.context.FileEncodingApplicationListener,\
org.springframework.boot.context.config.AnsiOutputApplicationListener,\
org.springframework.boot.context.config.ConfigFileApplicationListener,\
org.springframework.boot.context.config.DelegatingApplicationListener,\
org.springframework.boot.context.logging.ClasspathLoggingApplicationListener,\
org.springframework.boot.context.logging.LoggingApplicationListener,\
org.springframework.boot.liquibase.LiquibaseServiceLocatorApplicationListener

# Environment Post Processors
org.springframework.boot.env.EnvironmentPostProcessor=\
org.springframework.boot.cloud.CloudFoundryVcapEnvironmentPostProcessor,\
org.springframework.boot.env.SpringApplicationJsonEnvironmentPostProcessor,\
org.springframework.boot.env.SystemEnvironmentPropertySourceEnvironmentPostProcessor

# Failure Analyzers
org.springframework.boot.diagnostics.FailureAnalyzer=\
org.springframework.boot.diagnostics.analyzer.BeanCurrentlyInCreationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanDefinitionOverrideFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BeanNotOfRequiredTypeFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.BindValidationFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.UnboundConfigurationPropertyFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ConnectorStartFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoSuchMethodFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.NoUniqueBeanDefinitionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.PortInUseFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.ValidationExceptionFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyNameFailureAnalyzer,\
org.springframework.boot.diagnostics.analyzer.InvalidConfigurationPropertyValueFailureAnalyzer

# FailureAnalysisReporters
org.springframework.boot.diagnostics.FailureAnalysisReporter=\
org.springframework.boot.diagnostics.LoggingFailureAnalysisReporter

spring boot加载Spring Factories

SpringApplication中Spring Factories处理

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;
}

@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;
}

根据SpringApplication中的SpringFactories中的加载机制，最终是通过SpringFactoriesLoader类进行加载包中/META-INF/spring.factories文件。

SpringFactoriesLoader类的信息如下：

public final class SpringFactoriesLoader {

	/**
	 * The location to look for factories.
	 * <p>Can be present in multiple JAR files.
	 */
	public static final String FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories";


	private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);

	private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap<>();


	private SpringFactoriesLoader() {
	}

	public static <T> List<T> loadFactories(Class<T> factoryClass, @Nullable ClassLoader classLoader) {
		Assert.notNull(factoryClass, "'factoryClass' must not be null");
		ClassLoader classLoaderToUse = classLoader;
		if (classLoaderToUse == null) {
			classLoaderToUse = SpringFactoriesLoader.class.getClassLoader();
		}
		List<String> factoryNames = loadFactoryNames(factoryClass, classLoaderToUse);
		if (logger.isTraceEnabled()) {
			logger.trace("Loaded [" + factoryClass.getName() + "] names: " + factoryNames);
		}
		List<T> result = new ArrayList<>(factoryNames.size());
		for (String factoryName : factoryNames) {
			result.add(instantiateFactory(factoryName, factoryClass, classLoaderToUse));
		}
		AnnotationAwareOrderComparator.sort(result);
		return result;
	}


	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);
		}
	}


	@SuppressWarnings("unchecked")
	private static <T> T instantiateFactory(String instanceClassName, Class<T> factoryClass, ClassLoader classLoader) {
		try {
			Class<?> instanceClass = ClassUtils.forName(instanceClassName, classLoader);
			if (!factoryClass.isAssignableFrom(instanceClass)) {
				throw new IllegalArgumentException(
						"Class [" + instanceClassName + "] is not assignable to [" + factoryClass.getName() + "]");
			}
			return (T) ReflectionUtils.accessibleConstructor(instanceClass).newInstance();
		}
		catch (Throwable ex) {
			throw new IllegalArgumentException("Unable to instantiate factory class: " + factoryClass.getName(), ex);
		}
	}

}

SpringFactoriesLoader是在spring3.2版本提供的，通过该类可以加载spring.propertis配置文件。properties的加载时通过PropertiesLoaderUtils.loadProperties(resource)方法进行解析的。

dudu

Spring boot扩展机制Spring Factories

Spring Cloud SPI

SPI思想

spring boot加载Spring Factories