从main函数说起
一切的开始要从SpringbootApplication注解说起。
@SpringBootApplicationpublic class MyBootApplication { public static void main(String[] args) { SpringApplication.run(MyBootApplication.class); } }@SpringBootConfiguration@EnableAutoConfiguration@ComponentScanpublic @interface SpringBootApplication { }
其中最重要的就是EnableAutoConfiguration注解，开启自动配置。
@Target({ElementType.TYPE})@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@AutoConfigurationPackage@Import({AutoConfigurationImportSelector.class})public @interface EnableAutoConfiguration { String ENABLED_OVERRIDE_PROPERTY = "spring.boot.enableautoconfiguration"; Class<?>[] exclude() default {}; String[] excludeName() default {};}
通过Import注解导入AutoConfigurationImportSelector。在这个类中加载/META-INF/spring.factories文件的信息，然后筛选出以EnableAutoConfiguration为key的数据，加载到IOC容器中，实现自动配置功能。
@Target(ElementType.TYPE)@Retention(RetentionPolicy.RUNTIME)@Documented@Inherited@Import(AutoConfigurationPackages.Registrar.class)public @interface AutoConfigurationPackage {}
从表面看就是自动配置包，主要使用了Import注解，导入了Registrar类。这里Registrar类的registerBeanDefinitions方法导包，也就是导入当前main函数所在路径的包地址，我这里是com.zhangfei。
怎么自动装配其他N个类
Import({AutoConfigurationImportSelector.class})该注解给当前配置类导入另外N个自动配置类。
这里既然导入N个自动配置类，那么都导入哪些类呢？
//AutoConfigurationImportSelector实现DeferredImportSelector接口，而DeferredImportSelector接口又继承了ImportSelectorpublic interface ImportSelector { String[] selectImports(AnnotationMetadata var1);}
AutoConfigurationImportSelector通过实现接口ImportSelector的selectImports方法返回需要导入的组件，selectImports方法返回一个全类名字符串数组。
主角上场
//AutoConfigurationImportSelector.java@Overridepublic String[] selectImports(AnnotationMetadata annotationMetadata) { if (!isEnabled(annotationMetadata)) { return NO_IMPORTS; } AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader); AutoConfigurationEntry autoConfigurationEntry = getAutoConfigurationEntry(autoConfigurationMetadata,annotationMetadata); return StringUtils.toStringArray(autoConfigurationEntry.getConfigurations());}protected AutoConfigurationEntry getAutoConfigurationEntry(AutoConfigurationMetadata autoConfigurationMetadata,AnnotationMetadata annotationMetadata) { if (!isEnabled(annotationMetadata)) { return EMPTY_ENTRY; } AnnotationAttributes attributes = getAttributes(annotationMetadata); List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes); configurations = removeDuplicates(configurations); Set<String> exclusions = getExclusions(annotationMetadata, attributes); checkExcludedClasses(configurations, exclusions); configurations.removeAll(exclusions); configurations = filter(configurations, autoConfigurationMetadata); fireAutoConfigurationImportEvents(configurations, exclusions); return new AutoConfigurationEntry(configurations, exclusions);}protected List<String> getCandidateConfigurations(AnnotationMetadata metadata, AnnotationAttributes attributes) { List<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(),getBeanClassLoader()); return configurations;}
这里又开始调用SpringFactoriesLoader.loadFactoryNames。SpringFactoriesLoader.loadFactoryNames方法中关键的三步：(1)从当前项目的类路径中获取所有 META-INF/spring.factories 这个文件下的信息.(2)将上面获取到的信息封装成一个 Map 返回，EnableAutoConfiguration为key。(3)从返回的Map中通过刚才传入的 EnableAutoConfiguration.class参数，获取该 key 下的所有值。
public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) { String factoryClassName = factoryClass.getName(); return (List)loadSpringFactories(classLoader).getOrDefault(factoryClassName, Collections.emptyList());}private static Map<String, List<String>> loadSpringFactories(@Nullable ClassLoader classLoader) { MultiValueMap<String, String> result = (MultiValueMap)cache.get(classLoader); if (result != null) { return result; } else { try { Enumeration<URL> urls = classLoader != null ? classLoader.getResources("META-INF/spring.factories") : ClassLoader.getSystemResources("META-INF/spring.factories"); LinkedMultiValueMap result = new LinkedMultiValueMap(); while(urls.hasMoreElements()) { URL url = (URL)urls.nextElement(); UrlResource resource = new UrlResource(url); Properties properties = PropertiesLoaderUtils.loadProperties(resource); Iterator var6 = properties.entrySet().iterator(); while(var6.hasNext()) { Entry<?, ?> entry = (Entry)var6.next(); String factoryClassName = ((String)entry.getKey()).trim(); String[] var9 = StringUtils.commaDelimitedListToStringArray((String)entry.getValue()); int var10 = var9.length; for(int var11 = 0; var11 < var10; ++var11) { String factoryName = var9[var11]; result.add(factoryClassName, factoryName.trim()); } } } cache.put(classLoader, result); return result; } catch (IOException var13) { throw new IllegalArgumentException("Unable to load factories from location [META-INF/spring.factories]", var13); } }}
自动配置都有哪些内容呢？
# Auto Configureorg.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,\...其他省略
XXXAutoConfiguration和XXProperties
在spring.factories文件中看到的都是自动配置类，那么自动配置用到的属性值在那里呢？我们拿出redis为例
@Configuration@ConditionalOnClass(RedisOperations.class) //判断当前项目有没有这个类RedisOperations.class@EnableConfigurationProperties(RedisProperties.class) //启用配置属性，这里看到了熟悉的XXXProperties@Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class }) //导入这两个类public class RedisAutoConfiguration { @Bean @ConditionalOnMissingBean(name = "redisTemplate") public RedisTemplate<Object, Object> redisTemplate(RedisConnectionFactory redisConnectionFactory) throws UnknownHostException { RedisTemplate<Object, Object> template = new RedisTemplate<>(); template.setConnectionFactory(redisConnectionFactory); return template; } @Bean @ConditionalOnMissingBean public StringRedisTemplate stringRedisTemplate(RedisConnectionFactory redisConnectionFactory) throws UnknownHostException { StringRedisTemplate template = new StringRedisTemplate(); template.setConnectionFactory(redisConnectionFactory); return template; }} //这里则保存redis初始化时的属性@ConfigurationProperties(prefix = "spring.redis")public class RedisProperties { private int database = 0; private String url; private String host = "localhost"; private String password; private int port = 6379; private boolean ssl;}
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本文标题: 浅谈springboot自动配置原理
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