来思考个问题，为什么引入 spring-boot-starter-data-redis 依赖后就可以使用 @Autowired 注入 RedisTemplate ?
无疑是 SpringBoot 自动装配，下面就从源码角度探索下其实现。
SpringBoot 自动装配是 SpringBoot 较 springMVC 最大的亮点。
SpringBootApplication 注解
SpringBoot 项目的入口类一般会标注 @SpringBootApplication，其继承的注解有以下几个，也就是加入了注解就自动开始了自动装配，component 扫描，同时入口类也是一个配置类 Configuration。
@SpringBootApplication包含如下注解@SpringBootConfiguration 包含 @Configuration@EnableAutoConfiguration@ComponentScan
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何时加载
从 main 入口 SpringApplication.run 进行跟踪，在 refreshContext(context)进行加载 ApplicationContext 上下文，继续查找会找到处理 @Configuration 的类，再继续会找到 AutoConfigurationImportSelector 来处理自动装配。
SpringApplication.run方法中refreshContext(context);⬇️AbstractApplicationContextinvokeBeanFactoryPostProcessors(beanFactory);⬇️// 用来处理标记@Configuration的类ConfigurationClassPostProcessorprocessConfigBeanDefinitions(registry);⬇️//实现DeferredImportSelector用来处理 auto-configurationAutoConfigurationImportSelectorprocess()// AutoConfigurationEntry getAutoConfigurationEntry(...){... // 获取配置 List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes); configurations = removeDuplicates(configurations); // 排除项 Set<String> exclusions = getExclusions(annotationMetadata, attributes); checkExcludedClasses(configurations, exclusions); configurations.removeAll(exclusions); // 过滤配置 AutoConfigurationImportFilter configurations = filter(configurations, autoConfigurationMetadata); // 广播事件 fireAutoConfigurationImportEvents(configurations, exclusions);...}⬇️protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) { List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader()); Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you " + "are using a custom packaging, make sure that file is correct."); return configurations; }⬇️// 真正指定EnableAutoConfiguration的地方protected Class<?> getSpringFactoriesLoaderFactoryClass() { return EnableAutoConfiguration.class; }
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从 META-INF/spring.factories 加载 EnableAutoConfiguration 指定的类
/** * 从 META-INF/spring.factories 加载类名称 * classloader ==> AppClassLoader * Load the fully qualified class names of factory implementations of the given type from * "META-INF/spring.factories", using the given class loader. */ public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) { String factoryTypeName = factoryType.getName(); return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList()); }
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SpringFactoriesLoader 类 获取加载工厂类
private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) { ... try { // FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories"; 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()) { // key 作为factoryTypeName String factoryTypeName = ((String) entry.getKey()).trim(); for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) { result.add(factoryTypeName, factoryImplementationName.trim()); } } } ... }
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调试获取到 13 个配置项， V2.2.5 版本
"org.springframework.boot.diagnostics.FailureAnalysisReporter" -> {LinkedList@4979} size = 1"org.springframework.boot.diagnostics.FailureAnalyzer" -> {LinkedList@4981} size = 18"org.springframework.boot.SpringBootExceptionReporter" -> {LinkedList@4983} size = 1"org.springframework.boot.SpringApplicationRunListener" -> {LinkedList@4985} size = 1"org.springframework.context.ApplicationListener" -> {LinkedList@4987} size = 11"org.springframework.boot.env.PropertySourceLoader" -> {LinkedList@4989} size = 2"org.springframework.context.ApplicationContextInitializer" -> {LinkedList@4991} size = 7"org.springframework.boot.env.EnvironmentPostProcessor" -> {LinkedList@4993} size = 4"org.springframework.boot.autoconfigure.AutoConfigurationImportListener" -> {LinkedList@4995} size = 1"org.springframework.boot.autoconfigure.AutoConfigurationImportFilter" -> {LinkedList@4997} size = 3"org.springframework.boot.autoconfigure.EnableAutoConfiguration" -> {LinkedList@4999} size = 124"org.springframework.boot.autoconfigure.template.TemplateAvailabilityProvider" -> {LinkedList@5001} size = 5"org.springframework.beans.BeanInfoFactory" -> {LinkedList@5003} size = 1
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AutoConfigurationImportFilter 类型有 3 个，来进行配置过滤。
0 = "org.springframework.boot.autoconfigure.condition.OnBeanCondition"1 = "org.springframework.boot.autoconfigure.condition.OnClassCondition"2 = "org.springframework.boot.autoconfigure.condition.OnWebApplicationCondition"
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ConfigurationClassParser 是加载配置的核心类
在 doProcessConfigurationClass 方法处理了如下注解及父类处理
@PropertySource@ComponentScan@Import@Bean methods
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总结
自动装配的核心类有 ConfigurationClassPostProcessor、ConfigurationClassParser、AutoConfigurationImportSelector。
在 ConfigurationClassPostProcessor 配置类处理器，创建了 ConfigurationClassParser 对象对 @Configuration 注解的类进行处理，ConfigurationClassParser 类持有 DeferredImportSelector(即实现类 AutoConfigurationImportSelector)对象实现对配置类的加载、过滤、排除最终获取到配置类。
SpringFactoriesLoader 会加载 META-INF/spring.factories 下的所有类型并以 key-value 的形式缓存。自动配置获取以 EnableAutoConfiguration 为 key 的 value 即定义的配置类。
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