springboot启动流程 (3) 自动装配 |
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springboot启动流程 (3) 自动装配
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在SpringBoot中，EnableAutoConfiguration注解用于开启自动装配功能。 本文将详细分析该注解的工作流程。 EnableAutoConfiguration注解启用SpringBoot自动装配功能，尝试猜测和配置可能需要的组件Bean。 自动装配类通常是根据类路径和定义的Bean来应用的。例如，如果类路径上有tomcat-embedded.jar，那么可能需要一个TomcatServletWebServerFactory(除非已经定义了自己的Servlet WebServerFactory Bean)。 自动装配试图尽可能地智能化，并将随着开发者定义自己的配置而取消自动装配相冲突的配置。开发者可以使用exclude()排除不想使用的配置，也可以通过spring.autoconfig.exclude属性排除这些配置。自动装配总是在用户定义的Bean注册之后应用。 用@EnableAutoConfiguration注解标注的类所在包具有特定的意义，通常用作默认扫描的包。通常建议将@EnableAutoConfiguration(如果没有使用@SpringBootApplication注解)放在根包中，以便可以搜索所有子包和类。 自动装配类是普通的Spring @Configuration类，使用SpringFactoriesLoader机制定位。通常使用@Conditional方式装配，最常用的是@ConditionalOnClass和@ConditionalOnMissingBean注解。 @AutoConfigurationPackage @Import(AutoConfigurationImportSelector.class) public @interface EnableAutoConfiguration { /** * Exclude specific auto-configuration classes such that they will never be applied. */ Class[] exclude() default {}; /** * Exclude specific auto-configuration class names such that they will never be * applied. * 当类路径下没有指定的类时，可以使用这个属性指定排除的类 */ String[] excludeName() default {}; }该注解Import了AutoConfigurationImportSelector类，AutoConfigurationImportSelector类实现了DeferredImportSelector接口。 Import注解和DeferredImportSelector接口在之前的"Spring @Import注解源码分析"中详细分析过，此处在介绍它们，只分析AutoConfigurationImportSelector的工作流程。 AutoConfigurationImportSelector类 DeferredImportSelector接口A variation of ImportSelector that runs after all @Configuration beans have been processed. This type of selector can be particularly useful when the selected imports are @Conditional. Implementations can also extend the org.springframework.core.Ordered interface or use the org.springframework.core.annotation.Order annotation to indicate a precedence against other DeferredImportSelectors. Implementations may also provide an import group which can provide additional sorting and filtering logic across different selectors. AutoConfigurationGroup类AutoConfigurationImportSelector的getImportGroup方法返回了AutoConfigurationGroup类。 private static class AutoConfigurationGroup implements DeferredImportSelector.Group, BeanClassLoaderAware, BeanFactoryAware, ResourceLoaderAware { private final Map entries = new LinkedHashMap(); private final List autoConfigurationEntries = new ArrayList(); // ... 略 @Override public void process( AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) { // AutoConfigurationEntry类使用List保存Configuration类 AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector) .getAutoConfigurationEntry(annotationMetadata); this.autoConfigurationEntries.add(autoConfigurationEntry); for (String importClassName : autoConfigurationEntry.getConfigurations()) { this.entries.putIfAbsent(importClassName, annotationMetadata); } } @Override public Iterable selectImports() { // 查找排除的配置类 Set allExclusions = this.autoConfigurationEntries.stream() .map(AutoConfigurationEntry::getExclusions) .flatMap(Collection::stream) .collect(Collectors.toSet()); // 所有配置类 Set processedConfigurations = this.autoConfigurationEntries.stream() .map(AutoConfigurationEntry::getConfigurations) .flatMap(Collection::stream) .collect(Collectors.toCollection(LinkedHashSet::new)); // 将排除的配置类移除掉 processedConfigurations.removeAll(allExclusions); // 排序 return sortAutoConfigurations(processedConfigurations, getAutoConfigurationMetadata()).stream() .map((importClassName) -> new Entry(this.entries.get(importClassName), importClassName)) .collect(Collectors.toList()); } // ... 略 }从上面的代码可以看出，查找自动装配类的逻辑在getAutoConfigurationEntry方法中。 getAutoConfigurationEntry方法从META-INF/spring.factories文件解析EnableAutoConfiguration配置。 META-INF/spring.factories文件示例：
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 loadFactoryNames(Class factoryType, ClassLoader classLoader) { String factoryTypeName = factoryType.getName(); return loadSpringFactories(classLoader).getOrDefault(factoryTypeName, Collections.emptyList()); } private static Map loadSpringFactories(ClassLoader classLoader) { MultiValueMap result = cache.get(classLoader); if (result != null) { return result; } try { // FACTORIES_RESOURCE_LOCATION = "META-INF/spring.factories" // 从类路径下查找META-INF/spring.factories文件 Enumeration 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); // 获取properties配置 Properties properties = PropertiesLoaderUtils.loadProperties(resource); for (Map.Entry entry : properties.entrySet()) { String factoryTypeName = ((String) entry.getKey()).trim(); for (String factoryImplementationName : StringUtils.commaDelimitedListToStringArray((String) entry.getValue())) { result.add(factoryTypeName, factoryImplementationName.trim()); } } } // 把配置添加缓存 cache.put(classLoader, result); return result; } catch (IOException ex) { throw new IllegalArgumentException("Unable to load factories from location [" + FACTORIES_RESOURCE_LOCATION + "]", ex); } } |
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