SpringBoot中的自動裝載

（1）ImportSelector接口

ImportSelector接口是Spring導(dǎo)入外部配置的核心接口，在SpringBoot的自動化配置和@EnableXXX(功能性注解)中起到了決定性的作用。當(dāng)在@Configuration標注的Class上使用@Import引入了一個ImportSelector實現(xiàn)類后，會把ImportSelector實現(xiàn)類中返回的Class名稱都定義為bean。

public interface ImportSelector {String[] selectImports(AnnotationMetadata var1); }

DeferredImportSelector接口繼承ImportSelector,他和ImportSelector的區(qū)別在于裝載bean的時機上,DeferredImportSelector需要等所有的@Configuration都執(zhí)行完畢后才會進行裝載

public interface DeferredImportSelector extends ImportSelector {//...省略 }

（2）模擬SpringBoot自動裝配的實現(xiàn)

接下來我們寫一個小例子,看下ImportSelector接口的用法

通過模擬實現(xiàn)ImportSelector接口來還原SpringBoot自動裝配的原理（SpringBoot的自動裝配原理就是通過封裝ImportSelector接口等一些列操作來實現(xiàn)的）

1）定義Bean對象

這個類模擬的是我們要自動專配在SpringBoot容器中的Bean實例

public class User {private String username;private Integer age;//省略.. }

2）定義配置類Configuration

//定義一個configuration //注意這里并沒有使用spring注解 //spring掃描的時候并不會裝載該類 public class UserConfiguration {@Beanpublic User getUser() {return new User("張三",18);} }

3 ) 定義ImportSelector接口的實現(xiàn)類

ImportSelector 接口中的selectImports方法會將加載的類當(dāng)作一個配置類來初始化，就是相當(dāng)于@Configuration注解的功能。

public class UserImportSelector implements ImportSelector {@Overridepublic String[] selectImports(AnnotationMetadata importingClassMetadata) {//獲取配置類名稱return new String[]{UserConfiguration.class.getName()};} }

4） 定義EnableXXX注解

在SpringBoot在自動裝配正式通過注解@EnableAutoCofiguration注解實現(xiàn)的，這里通過自定義EnableXXX注解模擬SpringBoot中的@EnableAutoConfiguration注解實現(xiàn)自動裝配。

@SpringBoot源碼：

@Retention(RetentionPolicy.RUNTIME) @Documented @Target(ElementType.TYPE) @Import(UserImportSelector.class) public @interface EnableUserBean { }

5 ) 測試

/** * 通過在類上聲明@EnableUserBean，會自動的加載所有對象 */ @EnableUserBean public class TestApplication {public static void main(String[] args) {//獲取sprinboot注解驅(qū)動的容器對象AnnotationConfigApplicationContext applicationContext =new AnnotationConfigApplicationContext(TestApplication.class);//加載通過實現(xiàn)ImportSelector接口模擬SpringBoot自動裝載功能，將聲明的User對象的實例裝配到SpringBoot的容器中//獲取SpringBoot容器中的User的實例對象User user = applicationContext.getBean(User.class);System.out.println(user);} }

由此可見，User對象并沒有使用Spring的對象創(chuàng)建注解聲明（@Controller，@Service，@Repostiroty），而是使用編程的方式動態(tài)的載入bean。

這個接口在哪里調(diào)用呢？我們可以來看一下ConfigurationClassParser這個類的processImports方法

private void processImports(ConfigurationClass configClass, SourceClass currentSourceClass,Collection<SourceClass> importCandidates, boolean checkForCircularImports) {if (importCandidates.isEmpty()) {return;}if (checkForCircularImports && isChainedImportOnStack(configClass)) {this.problemReporter.error(new CircularImportProblem(configClass, this.importStack));}else {this.importStack.push(configClass);try {for (SourceClass candidate : importCandidates) { //對ImportSelector的處理if (candidate.isAssignable(ImportSelector.class)) {// Candidate class is an ImportSelector -> delegate to it to determine importsClass<?> candidateClass = candidate.loadClass();ImportSelector selector = BeanUtils.instantiateClass(candidateClass, ImportSelector.class);ParserStrategyUtils.invokeAwareMethods(selector, this.environment, this.resourceLoader, this.registry);if (this.deferredImportSelectors != null && selector instanceof DeferredImportSelector) { //如果為延遲導(dǎo)入處理 則加入集合當(dāng)中this.deferredImportSelectors.add(new DeferredImportSelectorHolder(configClass, (DeferredImportSelector) selector));}else { //根據(jù)ImportSelector方法 的返回值來進行遞歸操作String[] importClassNames = selector.selectImports(currentSourceClass.getMetadata());Collection<SourceClass> importSourceClasses = asSourceClasses(importClassNames);processImports(configClass, currentSourceClass, importSourceClasses, false);}}else if (candidate.isAssignable(ImportBeanDefinitionRegistrar.class)) {// Candidate class is an ImportBeanDefinitionRegistrar - >// delegate to it to register additional bean definitionsClass<?> candidateClass = candidate.loadClass();ImportBeanDefinitionRegistrar registrar =BeanUtils.instantiateClass(candidateClass, ImportBeanDefinitionRegistrar.class);ParserStrategyUtils.invokeAwareMethods(registrar, this.environment, this.resourceLoader, this.registry);configClass.addImportBeanDefinitionRegistrar(registrar, currentSourceClass.getMetadata());}else { // 如果當(dāng)前的類既不是 ImportSelector也不是ImportBeanDefinitionRegistar就進行@Configuration的解析處理// Candidate class not an ImportSelector or ImportBeanDefinitionRegistrar ->// process it as an @Configuration classthis.importStack.registerImport(currentSourceClass.getMetadata(), candidate.getMetadata().getClassName());processConfigurationClass(candidate.asConfigClass(configClass));}}}catch (BeanDefinitionStoreException ex) {throw ex;}catch (Throwable ex) {throw new BeanDefinitionStoreException("Failed to process import candidates for configuration class [" +configClass.getMetadata().getClassName() + "]", ex);}finally {this.importStack.pop();}}}

（3）springBoot自動裝載原理解析

SpringBoot開箱即用的特點，很大程度上歸功于ImportSelector接口。接下來我們看下springBoot是如何在spring的基礎(chǔ)上做擴展的。
在SpringBoot中最重要的一個注解SpringBootApplication

@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 {//... }

在SpringBootApplication注解中聲明了一個 @EnableAutoConfiguration注解

@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 {}; }

在EnableAutoConfiguration接口中通過@Import注解引入了SpringBoot定義的AutoConfigurationImportSelector類，這個類內(nèi)容比較多，我們只需看下最主要的邏輯代碼即可

public class AutoConfigurationImportSelectorimplements DeferredImportSelector, BeanClassLoaderAware, ResourceLoaderAware,BeanFactoryAware, EnvironmentAware, Ordered {@Overridepublic String[] selectImports(AnnotationMetadata annotationMetadata) {if (!isEnabled(annotationMetadata)) {return NO_IMPORTS;}AutoConfigurationMetadata autoConfigurationMetadata = AutoConfigurationMetadataLoader.loadMetadata(this.beanClassLoader);//主要邏輯在getAutoConfigurationEntry這個方法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);//通過getCandidateConfigurations方法獲取所有需要加載的beanList<String> configurations = getCandidateConfigurations(annotationMetadata,attributes);//去重處理configurations = removeDuplicates(configurations);//獲取不需要加載的bean,這里我們可以通過spring.autoconfigure.exclude人為配置Set<String> exclusions = getExclusions(annotationMetadata, attributes);checkExcludedClasses(configurations, exclusions);configurations.removeAll(exclusions);configurations = filter(configurations, autoConfigurationMetadata);//發(fā)送事件,通知所有的AutoConfigurationImportListener進行監(jiān)聽fireAutoConfigurationImportEvents(configurations, exclusions);return new AutoConfigurationEntry(configurations, exclusions);}//這里是獲取bean渠道的地方,重點看SpringFactoriesLoader#loadFactoryNamesprotected List<String> getCandidateConfigurations(AnnotationMetadata metadata,AnnotationAttributes attributes) {//這里的getSpringFactoriesLoaderFactoryClass()最終返回 EnableAutoConfiguration.classList<String> configurations = SpringFactoriesLoader.loadFactoryNames(getSpringFactoriesLoaderFactoryClass(), getBeanClassLoader());Assert.notEmpty(configurations,"No auto configuration classes found in METAINF/spring.factories. If you "+ "are using a custom packaging, make sure that file is correct.");return configurations;} }

從上面的邏輯可以看出,最終獲取bean的渠道在SpringFactoriesLoader.loadFactoryNames

public final class SpringFactoriesLoader {public static final String FACTORIES_RESOURCE_LOCATION = "METAINF/spring.factories";private static final Log logger = LogFactory.getLog(SpringFactoriesLoader.class);private static final Map<ClassLoader, MultiValueMap<String, String>> cache = new ConcurrentReferenceHashMap();public static List<String> loadFactoryNames(Class<?> factoryClass, @Nullable ClassLoader classLoader) {String factoryClassName = factoryClass.getName();//通過factoryClassName獲取相應(yīng)的bean全稱//上面?zhèn)魅氲膄actoryClass是EnableAutoConfiguration.classreturn (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 {//獲取工程中所有META-INF/spring.factories文件,將其中的鍵值組合成MapEnumeration<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);}}}private static <T> T instantiateFactory(String instanceClassName, Class<T> factoryClass, ClassLoader classLoader) {try {Class<?> instanceClass = ClassUtils.forName(instanceClassName, classLoader);if (!factoryClass.isAssignableFrom(instanceClass)) {throw new IllegalArgumentException("Class [" + instanceClassName + "] is not assignable to [" + factoryClass.getName() + "]");} else {return ReflectionUtils.accessibleConstructor(instanceClass, new Class[0]).newInstance();}} catch (Throwable var4) {throw new IllegalArgumentException("Unable to instantiate factory class: " + factoryClass.getName(), var4);}} }

每個jar都可以定義自己的META-INF/spring.factories ,jar被加載的同時 spring.factories里面定義的bean就可以自動被加載。

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