SpringBoot是如何做到自動裝配的

背景

眾所周知，如下即可啟動一個最簡單的Spring應用。檢視@SpringBootApplication註解的原始碼，發現這個註解上有一個重要的註解@EnableAutoConfiguration，而這個註解就是SpringBoot實現自動裝配的基礎

import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;

@SpringBootApplication
public class DemoApplication {
	public static void main(String[] args) {
		SpringApplication.run(DemoApplication.class, args);
	}
}

@EnableAutoConfiguration

EnableAutoConfiguration註解上通過@Import引入了兩個類，org.springframework.boot.autoconfigure.AutoConfigurationImportSelector及org.springframework.boot.autoconfigure.AutoConfigurationPackages.Registrar。通過@Import標註的類，會在解析@Import所在的配置類時，將標註類引入容器解析，並進行註冊。

有眾多的元件都是通過在配置類上加@EnableAutoConfiguration註解將元件引入的

• ImportBeanDefinitionRegistrar實現了org.springframework.context.annotation.ImportBeanDefinitionRegistrar及org.springframework.boot.context.annotation.DeterminableImports

• AutoConfigurationImportSelector實現了org.springframework.context.annotation.DeferredImportSelector

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@AutoConfigurationPackage
@Import(AutoConfigurationImportSelector.class)
public @interface EnableAutoConfiguration {
    ....
}

@Target(ElementType.TYPE)
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Inherited
@Import(AutoConfigurationPackages.Registrar.class)
public @interface AutoConfigurationPackage {
    ...
}

解析

起始

通過BeanFactoryPostProcessor對需要註冊的Bean進行解析。即org.springframework.context.support.AbstractApplicationContext#refresh，在AbstractApplicationContext#invokeBeanFactoryPostProcessors方法呼叫時，就開始了對服務配置bean的解析，為物件的生成做準備

        @Override
	public void refresh() throws BeansException, IllegalStateException {
		synchronized (this.startupShutdownMonitor) {
			...

			try {
				...
                    
				invokeBeanFactoryPostProcessors(beanFactory);

				...
			}

			catch (BeansException ex) {
				...
			}

			finally {
				...
			}
		}
	}

具體解析

呼叫org.springframework.context.support.PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors，通過獲取到的BeanFactoryPostProcessor實現類對各種配置類進行解析，具體的BeanFactoryPostProcessor解析後面我們在具體分析。

這裡有一個很重要的類org.springframework.context.annotation.ConfigurationClassPostProcessor，首先會呼叫postProcessBeanDefinitionRegistry方法

	// ConfigurationClassPostProcessor類部門原始碼

	/**
	 * Derive further bean definitions from the configuration classes in the registry.
	 */
	@Override
	public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
		int registryId = System.identityHashCode(registry);
		if (this.registriesPostProcessed.contains(registryId)) {
			throw new IllegalStateException(
					"postProcessBeanDefinitionRegistry already called on this post-processor against " + registry);
		}
		if (this.factoriesPostProcessed.contains(registryId)) {
			throw new IllegalStateException(
					"postProcessBeanFactory already called on this post-processor against " + registry);
		}
		this.registriesPostProcessed.add(registryId);
	
                // 處理配置類
		processConfigBeanDefinitions(registry);
	}


	/**
	 * Build and validate a configuration model based on the registry of
	 * {@link Configuration} classes.
	 */
	public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
		...
		// Parse each @Configuration class
		ConfigurationClassParser parser = new ConfigurationClassParser(
				this.metadataReaderFactory, this.problemReporter, this.environment,
				this.resourceLoader, this.componentScanBeanNameGenerator, registry);

                // configCandidates為待解析的Configuration類，如配置了@SpringBootApplication的類
		Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
		Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
		do {
			StartupStep processConfig = this.applicationStartup.start("spring.context.config-classes.parse");
                        // 開始解析
			parser.parse(candidates);
			parser.validate();

			...
		}
		while (!candidates.isEmpty());
		...
	}

通過原始碼可知，具體的解析操作是在org.springframework.context.annotation.ConfigurationClassParser類中

public void parse(Set<BeanDefinitionHolder> configCandidates) {
		for (BeanDefinitionHolder holder : configCandidates) {
			BeanDefinition bd = holder.getBeanDefinition();
			try {
				if (bd instanceof AnnotatedBeanDefinition) {
                                        // 將配置類進行解析。以當前配置類為原配置類，解析@PropertySource、@ComponentScan、@Import、@ImportResource、					 
                                        // @Bean等標註的類或方法，生成對應的
					parse(((AnnotatedBeanDefinition) bd).getMetadata(), holder.getBeanName());
				}
				else if (bd instanceof AbstractBeanDefinition && ((AbstractBeanDefinition) bd).hasBeanClass()) {
					parse(((AbstractBeanDefinition) bd).getBeanClass(), holder.getBeanName());
				}
				else {
					parse(bd.getBeanClassName(), holder.getBeanName());
				}
			}
			...
		}

   		// 解析通過@Import引入的配置類，自動配置類的解析也在於此
		this.deferredImportSelectorHandler.process();
	}

	public void processGroupImports() {
			for (DeferredImportSelectorGrouping grouping : this.groupings.values()) {
				Predicate<String> exclusionFilter = grouping.getCandidateFilter();
                                // grouping.getImports()方法獲取到了所有配置的可用自動配置類，然後遍歷，以配置類原點又開始一輪解析。自動裝配就是在此處
				grouping.getImports().forEach(entry -> {
					ConfigurationClass configurationClass = this.configurationClasses.get(entry.getMetadata());
					try {
                                                // import的解析
						processImports(configurationClass, asSourceClass(configurationClass, exclusionFilter),
								Collections.singleton(asSourceClass(entry.getImportClassName(), exclusionFilter)),
								exclusionFilter, false);
					}
					catch (BeanDefinitionStoreException ex) {
						throw ex;
					}
					catch (Throwable ex) {
						throw new BeanDefinitionStoreException(
								"Failed to process import candidates for configuration class [" +
										configurationClass.getMetadata().getClassName() + "]", ex);
					}
				});
			}
		}

通過DeferredImportSelectorGrouping.getImports()方法解析。在org.springframework.boot.autoconfigure.AutoConfigurationImportSelector#getAutoConfigurationEntry方法中開始了autoConfiguration的解析。

	/**
	 * Return the {@link AutoConfigurationEntry} based on the {@link AnnotationMetadata}
	 * of the importing {@link Configuration @Configuration} class.
	 * @param annotationMetadata the annotation metadata of the configuration class
	 * @return the auto-configurations that should be imported
	 */
	protected AutoConfigurationEntry getAutoConfigurationEntry(AnnotationMetadata annotationMetadata) {
		if (!isEnabled(annotationMetadata)) {
			return EMPTY_ENTRY;
		}
                // 解析@EnableAutoConfiguration註解中的屬性exclude、excludeName
		AnnotationAttributes attributes = getAttributes(annotationMetadata);
                // 使用SpringFactoriesLoader獲取META-INF/spring.properties中配置的EnableAutoConfiguration實現類，獲取所有配置的自動裝配類
		List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
                // 去重
		configurations = removeDuplicates(configurations);
                // 獲取需要排除的自動裝配類
		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
		checkExcludedClasses(configurations, exclusions);
		configurations.removeAll(exclusions);
                //getConfigurationClassFilter()方法就是獲取spring.factories中配置的AutoConfigurationImportFilter實現類。然後呼叫filter		//法對自動裝配類進行有效性校驗
		configurations = getConfigurationClassFilter().filter(configurations);
		fireAutoConfigurationImportEvents(configurations, exclusions);
		return new AutoConfigurationEntry(configurations, exclusions);
	}

再繼續看org.springframework.boot.autoconfigure.AutoConfigurationImportSelector.ConfigurationClassFilter#filter

	List<String> filter(List<String> configurations) {
			long startTime = System.nanoTime();
			String[] candidates = StringUtils.toStringArray(configurations);
			boolean skipped = false;
			for (AutoConfigurationImportFilter filter : this.filters) {
                                // autoConfigurationMetadata為通過META-INF/spring-autoconfigure-metadata.properties配置檔案的內容
                                // 使用filter及autoConfigurationMetadata對candidates進行校驗
				boolean[] match = filter.match(candidates, this.autoConfigurationMetadata);
				for (int i = 0; i < match.length; i++) {
					if (!match[i]) {
						candidates[i] = null;
						skipped = true;
					}
				}
			}
			if (!skipped) {
				return configurations;
			}
        	...
			return result;
		}

再繼續看match方法，org.springframework.boot.autoconfigure.condition.FilteringSpringBootCondition#match

	@Override
	public boolean[] match(String[] autoConfigurationClasses, AutoConfigurationMetadata autoConfigurationMetadata) {
		ConditionEvaluationReport report = ConditionEvaluationReport.find(this.beanFactory);
                // 抽象方法，不同的filter進行不同的處理。這裡會獲取每一個自動裝配類的條件判斷情況
		ConditionOutcome[] outcomes = getOutcomes(autoConfigurationClasses, autoConfigurationMetadata);
		boolean[] match = new boolean[outcomes.length];
		for (int i = 0; i < outcomes.length; i++) {
			match[i] = (outcomes[i] == null || outcomes[i].isMatch());
			if (!match[i] && outcomes[i] != null) {
				logOutcome(autoConfigurationClasses[i], outcomes[i]);
				if (report != null) {
					report.recordConditionEvaluation(autoConfigurationClasses[i], this, outcomes[i]);
				}
			}
		}
		return match;
	}

通過match方法，經過多種filter的過濾，返回的就是每一個自動配置類是否可用

結論

• SpringBoot專案有一個子專案org.springframework.boot:spring-boot-autoconfigure:xx，這個子專案主要就是做自動裝配的。SpringBoot提前配置了眾多已經實現自動配置功能的配置類(org.springframework.boot.autoconfigure.EnableAutoConfiguration介面的實現類)。當容器啟動的時候，通過 SpringFactoriesLoader將配置類載入進容器中
• 啟動中，容器通過BeanFactoryPostProcessor介面解析、修改物件的定義。有一個很重要的配置解析實現類org.springframework.context.annotation.ConfigurationClassPostProcessor，用來解析專案中標註@Configuration 的類
• 在進行配置類解析時(即解析配置了@SpringBootApplication註解的類)，需要經過解析類的 @PropertySource、@ComponentScan、@Import、@ImportResource、@Bean、介面預設實現、父類等（org.springframework.context.annotation.ConfigurationClassParser#doProcessConfigurationClass）。對於自動裝配來說，最重要的就是解析@Import
• 通過@Import引入了org.springframework.boot.autoconfigure.AutoConfigurationImportSelector，在進行解析@Import引入的配置類時，org.springframework.boot.autoconfigure.AutoConfigurationImportSelector#getAutoConfigurationEntry獲取到所有配置的自動裝配類(通過META-INF/spring.factories檔案配置EnableAutoConfiguration實現類)，通過org.springframework.context.annotation.Condition定義過濾器，判斷自動裝配置是否需要自動裝配。預設的過濾器有OnClassCondition、OnWebApplicationCondition、OnBeanCondition，對應常見的condition註解ConditionalOnClass、ConditionalOnBean、@ConditionalOnWebApplication。
• 通過過濾判斷，將需要自動配置的類進行configuration解析，從而將需要配置的類轉換成對應的BeanDefinition進行註冊

備註

• SpringBoot將自動裝配類及過濾條件通過配置檔案的形式放在了META-INF目錄下，META-INF/spring.factories和META-INF/spring-autoconfigure-metadata.properties

• 在BeanFactoryPostProcessor進行呼叫時，有兩種處理。首先是通過BeanDefinitionRegistryPostProcessor#postProcessBeanDefinitionRegistry解析更多的BeanDefinition，在這裡就包含了所有標註類的掃描解析，自動裝配類的解析，自動裝配類引入類的解析。在進行BeanFactoryPostProcessor#postProcessBeanFactory呼叫，進行CGLIB-enhanced配置類。這裡最重要的一個類就是org.springframework.context.annotation.ConfigurationClassPostProcessor，以下為此類的繼承關係

  
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