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自动装配原理springboot 版本

发布于：2022-03-16 09:09:53  栏目：技术文档

自动装配原理springboot 版本：2.4.3

SpringBootApplicationspringboot启动类必须要加@SpringBootApplication注解，那这个注解是什么意思呢？

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@SpringBootConfiguration@EnableAutoConfiguration@ComponentScan(excludeFilters = { @Filter(type = FilterType.CUSTOM, classes = TypeExcludeFilter.class), @Filter(type = FilterType.CUSTOM, classes = AutoConfigurationExcludeFilter.class) })public @interface SpringBootApplication {12345678910抛开元数据注解来说，SpringBootApplication注解主要由@SpringBootConfiguration、@EnableAutoConfiguration、@ComponentScan组成。这三个又有不同的作用如下：

@SpringBootConfiguration:被@Configuration标记，表示这是个springboot配置，支持JavaConfig的方式来进行配置。@EnableAutoConfiguration：表示开启自动装配(重点介绍)@ComponentScan：扫描注解，扫描basePackages包下的bean并将他们注入到IOC容器中，比如：@Service、@Controller、@Component等注解。EnableAutoConfiguration真正开启自动配置的还是@EnableAutoConfiguration注解，来看下EnableAutoConfiguration注解源码：

@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@AutoConfigurationPackage@Import(AutoConfigurationImportSelector.class)public @interface EnableAutoConfiguration {1234567@EnableAutoConfiguration 又是由@AutoConfigurationPackage和@Import注解组成。

@AutoConfigurationPackage是一个复合注解的，它在内部使用@Import(AutoConfigurationPackages.Registrar.class)注解，Registrar是AutoConfigurationPackages的一个内部类，它的作用就是注册一个springboot启动类所在的包名，这个包名可以供列如JPA的使用。

AutoConfigurationImportSelector通过selectImports方法将配置类导入，从而完成bean的装配

AutoConfigurationImportSelectorAutoConfigurationImportSelector实现了DeferredImportSelector接口，DeferredImportSelector是ImportSelector的变种，它是一个延迟选择器。实现了DeferredImportSelector接口的子类如果重新了getImportGroup方法并返回DeferredImportSelector内部接口Group的子类，DeferredImportSelector接口的子类的子类将不会调用selectImports而是调用Group的selectImports方法。

接下来看看AutoConfigurationImportSelector重写了getImportGroup方法并返回一个内部类AutoConfigurationGroup，AutoConfigurationGroup#selectImports方法只是对配置数组进行排序筛选，真正处理自动配置的流程的是process方法。process方法源码如下：

@Override public void process(AnnotationMetadata annotationMetadata, DeferredImportSelector deferredImportSelector) { Assert.state(deferredImportSelector instanceof AutoConfigurationImportSelector, () -> String.format(“Only %s implementations are supported, got %s”, AutoConfigurationImportSelector.class.getSimpleName(), deferredImportSelector.getClass().getName())); AutoConfigurationEntry autoConfigurationEntry = ((AutoConfigurationImportSelector) deferredImportSelector) .getAutoConfigurationEntry(annotationMetadata); this.autoConfigurationEntries.add(autoConfigurationEntry); for (String importClassName : autoConfigurationEntry.getConfigurations()) { this.entries.putIfAbsent(importClassName, annotationMetadata); } }

1234567891011121314process方法就是对AutoConfigurationGroup一些属性的填充，起作用的还是AutoConfigurationImportSelector.getAutoConfigurationEntry方法。getAutoConfigurationEntry方法中经过各种判断过滤、去重等操作，最后返回AutoConfigurationEntry对象。源码如下

protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) { if (!isEnabled(annotationMetadata)) { return EMPTY_ENTRY; } //获取EnableAutoConfiguration注解的exclude和excludeName属性 AnnotationAttributes attributes = getAttributes(annotationMetadata); //获取所有配置类 List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes); //去除重复配置类 configurations = removeDuplicates(configurations); Set<String> exclusions = getExclusions(annotationMetadata, attributes); checkExcludedClasses(configurations, exclusions); //移除 exclude的配置类 configurations.removeAll(exclusions); configurations = getConfigurationClassFilter().filter(configurations); fireAutoConfigurationImportEvents(configurations, exclusions); return new AutoConfigurationEntry(configurations, exclusions); }

123456789101112131415161718getAutoConfigurationEntry 筛选的自动配置类：

getAutoConfigurationEntry方法中要重点分析的是getCandidateConfigurations方法。getCandidateConfigurations的作用是获取所有自动装配的配置类的全限定名。

来看下getCandidateConfigurations方法源码：

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 “


 
  1. + "are using a custom packaging, make sure that file is correct.");
  2. return configurations;
  3. }

protected Class<?> getSpringFactoriesLoaderFactoryClass() { return EnableAutoConfiguration.class; }1234567891011这里面用到了SpringFactoriesLoader是spring提供的一种加载配置的方式，它会将类从配置文件中读取到，然后利用反射将bean加载到IOC容器中。

SpringFactoriesLoader.loadFactoryNames中会加载META-INF/spring.factories自动配置类。这些配置类在spring.factories文件中是以key=value的形式存储的，来看下部分自动配置类：

Auto Configure

org.springframework.boot.autoconfigure.EnableAutoConfiguration=\org.springframework.boot.autoconfigure.admin.SpringApplicationAdminJmxAutoConfiguration,\org.springframework.boot.autoconfigure.aop.AopAutoConfiguration,\org.springframework.boot.autoconfigure.amqp.RabbitAutoConfiguration,\org.springframework.boot.autoconfigure.batch.BatchAutoConfiguration,\org.springframework.boot.autoconfigure.cache.CacheAutoConfiguration,\org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\org.springframework.boot.autoconfigure.cloud.CloudAutoConfiguration,\org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\

12345678910111213141516来看下SpringFactoriesLoader.loadFactoryNames源码：

public static final String FACTORIES_RESOURCE_LOCATION = “META-INF/spring.factories”;

public static List<String> loadFactoryNames(Class<?> factoryType, @Nullable ClassLoader classLoader) { ClassLoader classLoaderToUse = classLoader; if (classLoaderToUse == null) { classLoaderToUse = SpringFactoriesLoader.class.getClassLoader(); } String factoryTypeName = factoryType.getName(); return loadSpringFactories(classLoaderToUse).getOrDefault(factoryTypeName, Collections.emptyList()); }


 
  1. private static Map<String, List<String>> loadSpringFactories(ClassLoader classLoader) {
  2. Map<String, List<String>> result = cache.get(classLoader);
  3. if (result != null) {
  4. return result;
  5. }
  6.  
  7. result = new HashMap<>();
  8. try {
  9. Enumeration<URL> urls = classLoader.getResources(FACTORIES_RESOURCE_LOCATION);
  10. while (urls.hasMoreElements()) {
  11. URL url = urls.nextElement();
  12. UrlResource resource = new UrlResource(url);
  13. Properties properties = PropertiesLoaderUtils.loadProperties(resource);
  14. for (Map.Entry<?, ?> entry : properties.entrySet()) {
  15. String factoryTypeName = ((String) entry.getKey()).trim();
  16. String[] factoryImplementationNames =
  17. StringUtils.commaDelimitedListToStringArray((String) entry.getValue());
  18. for (String factoryImplementationName : factoryImplementationNames) {
  19. result.computeIfAbsent(factoryTypeName, key -> new ArrayList<>())
  20. .add(factoryImplementationName.trim());
  21. }
  22. }
  23. }
  24.  
  25. // Replace all lists with unmodifiable lists containing unique elements
  26. result.replaceAll((factoryType, implementations) -> implementations.stream().distinct()
  27. .collect(Collectors.collectingAndThen(Collectors.toList(), Collections::unmodifiableList)));
  28. cache.put(classLoader, result);
  29. }
  30. catch (IOException ex) {
  31. throw new IllegalArgumentException("Unable to load factories from location [" +
  32. FACTORIES_RESOURCE_LOCATION + "]", ex);
  33. }
  34. return result;
  35. }

12345678910111213141516171819202122232425262728293031323334353637383940414243444546loadFactoryNames方法只是传递一个org.springframework.boot.autoconfigure.EnableAutoConfiguration作为key，然后取到对应的自动配置类列表。最终调用的是loadSpringFactories方法，loadSpringFactories会从jar包中找到spring.factories文件然后将其中的自动配置类存到一个map中，从下图可以看到map中存在很多bean，loadFactoryNames方法在加载自动配置类时只取了一个key。弱水三千，只取一瓢。

loadSpringFactories返回结果：

自动装配流程图大致如下：

总结EnableAutoConfiguration注解开启自动装配，其上的标记的@Import(AutoConfigurationImportSelector.class)注解中导入配置类AutoConfigurationImportSelector实现DeferredImportSelector接口，并重写了getImportGroup方法并返回AutoConfigurationImportSelector.AutoConfigurationGroup，AutoConfigurationImportSelector.AutoConfigurationGroup.process开始处理自动配置流程。AutoConfigurationImportSelector.getCandidateConfigurations获取所有配置类getAutoConfigurationEntry方法筛选，去重、移除不符合条件的自动配置类。SpringFactoriesLoader.loadSpringFactories从jar包中找到所有META-INF/spring.factories文件并读取自动配置类，存放到map中， loadFactoryNames方法通过全限定名org.springframework.boot.autoconfigure.EnableAutoConfiguration找到自动配置类。最后经过层层筛选，去重、移除不符合条件的bean，由ConfigurationClassPostProcessor#processConfigBeanDefinitions注册所有的自动配置类。能力一般，水平有限，如有错误，请多指出。如果对你有用点个关注给个赞呗————————————————版权声明：本文为CSDN博主「码猿笔记」的原创文章，遵循CC 4.0 BY-SA版权协议，转载请附上原文出处链接及本声明。原文链接：https://blog.csdn.net/qq_39654841/article/details/123352176

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