springboot自動配置原理
- 2019 年 10 月 3 日
- 筆記
從main函數說起
一切的開始要從SpringbootApplication註解說起。
@SpringBootApplication public class MyBootApplication { public static void main(String[] args) { SpringApplication.run(MyBootApplication.class); } } @SpringBootConfiguration @EnableAutoConfiguration @ComponentScan public @interface SpringBootApplication { }其中最重要的就是EnableAutoConfiguration註解，開啟自動配置。
@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 {}; }通過Import註解導入AutoConfigurationImportSelector。在這個類中載入/META-INF/spring.factories文件的資訊，然後篩選出以EnableAutoConfiguration為key的數據，載入到IOC容器中，實現自動配置功能。
@Target(ElementType.TYPE) @Retention(RetentionPolicy.RUNTIME) @Documented @Inherited @Import(AutoConfigurationPackages.Registrar.class) public @interface AutoConfigurationPackage { } 從表面看就是自動配置包，主要使用了Import註解，導入了Registrar類。這裡Registrar類的registerBeanDefinitions方法導包，也就是導入當前main函數所在路徑的包地址，我這裡是com.zhangfei。
怎麼自動裝配其他N個類
Import({AutoConfigurationImportSelector.class})該註解給當前配置類導入另外N個自動配置類。
這裡既然導入N個自動配置類，那麼都導入哪些類呢？
//AutoConfigurationImportSelector實現DeferredImportSelector介面，而DeferredImportSelector介面又繼承了ImportSelector public interface ImportSelector { String[] selectImports(AnnotationMetadata var1); }
AutoConfigurationImportSelector通過實現介面ImportSelector的selectImports方法返回需要導入的組件，selectImports方法返回一個全類名字元串數組。
主角上場
//AutoConfigurationImportSelector.java @Override public 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; } 這裡又開始調用SpringFactoriesLoader.loadFactoryNames。
SpringFactoriesLoader.loadFactoryNames方法中關鍵的三步：
(1)從當前項目的類路徑中獲取所有 META-INF/spring.factories 這個文件下的資訊.
(2)將上面獲取到的資訊封裝成一個 Map 返回，EnableAutoConfiguration為key。
(3)從返回的Map中通過剛才傳入的 EnableAutoConfiguration.class參數，獲取該 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); } } }自動配置都有哪些內容呢？
# 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, ...其他省略XXXAutoConfiguration和XXProperties
在spring.factories文件中看到的都是自動配置類，那麼自動配置用到的屬性值在那裡呢？我們拿出redis為例
@Configuration @ConditionalOnClass(RedisOperations.class) //判斷當前項目有沒有這個類RedisOperations.class @EnableConfigurationProperties(RedisProperties.class) //啟用配置屬性，這裡看到了熟悉的XXXProperties @Import({ LettuceConnectionConfiguration.class, JedisConnectionConfiguration.class }) //導入這兩個類 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; } } //這裡則保存redis初始化時的屬性 @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; }