SpringBoot是如何做到自动装配的

背景

众所周知，如下即可启动一个最简单的Spring应用。查看@SpringBootApplication注解的源码，发现这个注解上有一个重要的注解@EnableAutoConfiguration，而这个注解就是SpringBoot实现自动装配的基础

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注解上通过@Import引入了两个类，org.springframework.boot.autoconfigure.AutoConfigurationImportSelector及org.springframework.boot.autoconfigure.AutoConfigurationPackages.Registrar。通过@Import标注的类，会在解析@Import所在的配置类时，将标注类引入容器解析，并进行注册。

有众多的组件都是通过在配置类上加@EnableAutoConfiguration注解将组件引入的

• ImportBeanDefinitionRegistrar实现了org.springframework.context.annotation.ImportBeanDefinitionRegistrar及org.springframework.boot.context.annotation.DeterminableImports

• AutoConfigurationImportSelector实现了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

通过BeanFactoryPostProcessor对需要注册的Bean进行解析。即org.springframework.context.support.AbstractApplicationContext#refresh，在AbstractApplicationContext#invokeBeanFactoryPostProcessors方法调用时，就开始了对服务配置bean的解析，为对象的生成做准备

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

			try {
				...

				invokeBeanFactoryPostProcessors(beanFactory);

				...
			}

			catch (BeansException ex) {
				...
			}

			finally {
				...
			}
		}
	}

具体解析

调用org.springframework.context.support.PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors，通过获取到的BeanFactoryPostProcessor实现类对各种配置类进行解析，具体的BeanFactoryPostProcessor解析后面我们在具体分析。

这里有一个很重要的类org.springframework.context.annotation.ConfigurationClassPostProcessor，首先会调用postProcessBeanDefinitionRegistry方法

	// ConfigurationClassPostProcessor类部门源码

	/**
	 * 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);

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

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

通过源码可知，具体的解析操作是在org.springframework.context.annotation.ConfigurationClassParser类中

public void parse(Set<BeanDefinitionHolder> configCandidates) {
		for (BeanDefinitionHolder holder : configCandidates) {
			BeanDefinition bd = holder.getBeanDefinition();
			try {
				if (bd instanceof AnnotatedBeanDefinition) {
                                        // 将配置类进行解析。以当前配置类为原配置类，解析@PropertySource、@ComponentScan、@Import、@ImportResource、
                                        // @Bean等标注的类或方法，生成对应的
					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());
				}
			}
			...
		}

   		// 解析通过@Import引入的配置类，自动配置类的解析也在于此
		this.deferredImportSelectorHandler.process();
	}

	public void processGroupImports() {
			for (DeferredImportSelectorGrouping grouping : this.groupings.values()) {
				Predicate<String> exclusionFilter = grouping.getCandidateFilter();
                                // grouping.getImports()方法获取到了所有配置的可用自动配置类，然后遍历，以配置类原点又开始一轮解析。自动装配就是在此处
				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);
					}
				});
			}
		}

通过DeferredImportSelectorGrouping.getImports()方法解析。在org.springframework.boot.autoconfigure.AutoConfigurationImportSelector#getAutoConfigurationEntry方法中开始了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注解中的属性exclude、excludeName
		AnnotationAttributes attributes = getAttributes(annotationMetadata);
                // 使用SpringFactoriesLoader获取META-INF/spring.properties中配置的EnableAutoConfiguration实现类，获取所有配置的自动装配类
		List<String> configurations = getCandidateConfigurations(annotationMetadata, attributes);
                // 去重
		configurations = removeDuplicates(configurations);
                // 获取需要排除的自动装配类
		Set<String> exclusions = getExclusions(annotationMetadata, attributes);
		checkExcludedClasses(configurations, exclusions);
		configurations.removeAll(exclusions);
                //getConfigurationClassFilter()方法就是获取spring.factories中配置的AutoConfigurationImportFilter实现类。然后调用filter		//法对自动装配类进行有效性校验
		configurations = getConfigurationClassFilter().filter(configurations);
		fireAutoConfigurationImportEvents(configurations, exclusions);
		return new AutoConfigurationEntry(configurations, exclusions);
	}

再继续看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为通过META-INF/spring-autoconfigure-metadata.properties配置文件的内容
                                // 使用filter及autoConfigurationMetadata对candidates进行校验
				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;
		}

再继续看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进行不同的处理。这里会获取每一个自动装配类的条件判断情况
		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;
	}

通过match方法，经过多种filter的过滤，返回的就是每一个自动配置类是否可用

结论

• SpringBoot项目有一个子项目org.springframework.boot:spring-boot-autoconfigure:xx，这个子项目主要就是做自动装配的。SpringBoot提前配置了众多已经实现自动配置功能的配置类(org.springframework.boot.autoconfigure.EnableAutoConfiguration接口的实现类)。当容器启动的时候，通过 SpringFactoriesLoader将配置类加载进容器中
• 启动中，容器通过BeanFactoryPostProcessor接口解析、修改对象的定义。有一个很重要的配置解析实现类org.springframework.context.annotation.ConfigurationClassPostProcessor，用来解析项目中标注@Configuration 的类
• 在进行配置类解析时(即解析配置了@SpringBootApplication注解的类)，需要经过解析类的 @PropertySource、@ComponentScan、@Import、@ImportResource、@Bean、接口默认实现、父类等（org.springframework.context.annotation.ConfigurationClassParser#doProcessConfigurationClass）。对于自动装配来说，最重要的就是解析@Import
• 通过@Import引入了org.springframework.boot.autoconfigure.AutoConfigurationImportSelector，在进行解析@Import引入的配置类时，org.springframework.boot.autoconfigure.AutoConfigurationImportSelector#getAutoConfigurationEntry获取到所有配置的自动装配类(通过META-INF/spring.factories文件配置EnableAutoConfiguration实现类)，通过org.springframework.context.annotation.Condition定义过滤器，判断自动装配置是否需要自动装配。默认的过滤器有OnClassCondition、OnWebApplicationCondition、OnBeanCondition，对应常见的condition注解ConditionalOnClass、ConditionalOnBean、@ConditionalOnWebApplication。
• 通过过滤判断，将需要自动配置的类进行configuration解析，从而将需要配置的类转换成对应的BeanDefinition进行注册

备注

• SpringBoot将自动装配类及过滤条件通过配置文件的形式放在了META-INF目录下，META-INF/spring.factories和META-INF/spring-autoconfigure-metadata.properties

• 在BeanFactoryPostProcessor进行调用时，有两种处理。首先是通过BeanDefinitionRegistryPostProcessor#postProcessBeanDefinitionRegistry解析更多的BeanDefinition，在这里就包含了所有标注类的扫描解析，自动装配类的解析，自动装配类引入类的解析。在进行BeanFactoryPostProcessor#postProcessBeanFactory调用，进行CGLIB-enhanced配置类。这里最重要的一个类就是org.springframework.context.annotation.ConfigurationClassPostProcessor，以下为此类的继承关系

  
  
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