springboot Configuration用法

在springboot中我们构建应用不用在像spring中那么繁琐，必须声明bean，然后才能使用，在springboot只要引入了相关的jar包，则对应的功能会自动加载扩展。要想理解springboot的自动装配原理，我们首先要知道在spring中中比较重要的几个注解：

• @Import
• @Configuration

比如大家常见的@EnableWebMvc

@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
@Documented
@Import(DelegatingWebMvcConfiguration.class)
public @interface EnableWebMvc {
}
@Configuration
public class DelegatingWebMvcConfiguration extends WebMvcConfigurationSupport {
...
}

则是由这几个注解组成。

这里我们必须先说下spring在后面提供的基于注解的启动方式，我们可以直接基于注解，实现EnableXXX :

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

@Configuration
public class HelloWorldConfiguration {
    @Bean
    public String helloWorld(){
        return "hello world";
    }
}
@Configuration
@EnableHelloWorld
public class BootstrapApp {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext context =
                new AnnotationConfigApplicationContext();
        context.register(BootstrapApp.class);
        context.refresh();
        String helloWorld = context.getBean("helloWorld",String.class);
        System.out.println("helloWorld = "+helloWorld);
        context.close();
    }
}

可以看到，这里我们就实现了自动装在HelloWorldConfiguration 的功能了，还可以通过ImportSelector接口：

public interface MyServer {
    public void startup();

    public void shutdown();

    public enum Type{
        HTTP,TCP;
    }
}
public class MyHTTPServer implements MyServer {
    @Override
    public void startup() {
        System.out.println("http server startup");
    }

    @Override
    public void shutdown() {
        System.out.println("http server shutdown");
    }
}
public class MyTCPServer implements MyServer {
    @Override
    public void startup() {
        System.out.println("tcp server startup");
    }

    @Override
    public void shutdown() {
        System.out.println("tcp server shutdown");
    }
}

public class MyServerImportSelector implements ImportSelector {
    @Override
    public String[] selectImports(AnnotationMetadata annotationMetadata) {
        Map<String, Object>  attrs = annotationMetadata.getAnnotationAttributes(EnableMyServer.class.getName());
        MyServer.Type serverType = (MyServer.Type)attrs.get("type");
        String importClassName = "";
        switch (serverType){
            case HTTP:
                importClassName = MyHTTPServer.class.getName();
                break;
            case TCP:
                importClassName = MyTCPServer.class.getName();
                break;
        }
        return new String[]{importClassName};
    }
}
@Configuration
@EnableMyServer(type = MyServer.Type.HTTP)
public class MyServerBootstrap {
    public static void main(String[] args) {
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
        applicationContext.register(MyServerBootstrap.class);
        applicationContext.refresh();
        MyServer server = applicationContext.getBean(MyServer.class);
        server.startup();
        server.shutdown();
        applicationContext.close();
    }
}

如上，基于 @Import @Configuration和ImportSelector 接口，实现了Enable功能。
这里我们基于AnnotationConfigApplicationContext来进行说明

public class AnnotationConfigApplicationContext extends GenericApplicationContext implements AnnotationConfigRegistry {
	private final AnnotatedBeanDefinitionReader reader;
	private final ClassPathBeanDefinitionScanner scanner;
	public AnnotationConfigApplicationContext() {
		this.reader = new AnnotatedBeanDefinitionReader(this);
		this.scanner = new ClassPathBeanDefinitionScanner(this);
	}

	public AnnotationConfigApplicationContext(DefaultListableBeanFactory beanFactory) {
		super(beanFactory);
		this.reader = new AnnotatedBeanDefinitionReader(this);
		this.scanner = new ClassPathBeanDefinitionScanner(this);
	}
	public void register(Class<?>... componentClasses) {
		Assert.notEmpty(componentClasses, "At least one component class must be specified");
		this.reader.register(componentClasses);
	}
...
}

spring实现基于注解的自动装载，实现就在AnnotatedBeanDefinitionReader

public class AnnotatedBeanDefinitionReader {
	private final BeanDefinitionRegistry registry;
	private BeanNameGenerator beanNameGenerator = new AnnotationBeanNameGenerator();
	private ScopeMetadataResolver scopeMetadataResolver = new AnnotationScopeMetadataResolver();
	private ConditionEvaluator conditionEvaluator;
	public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry) {
		this(registry, getOrCreateEnvironment(registry));
	}
	public AnnotatedBeanDefinitionReader(BeanDefinitionRegistry registry, Environment environment) {
		Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
		Assert.notNull(environment, "Environment must not be null");
		this.registry = registry;
		this.conditionEvaluator = new ConditionEvaluator(registry, environment, null);
		AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
	}
	public final BeanDefinitionRegistry getRegistry() {
		return this.registry;
	}
	public void setEnvironment(Environment environment) {
		this.conditionEvaluator = new ConditionEvaluator(this.registry, environment, null);
	}
	public void setBeanNameGenerator(@Nullable BeanNameGenerator beanNameGenerator) {
		this.beanNameGenerator = (beanNameGenerator != null ? beanNameGenerator : new AnnotationBeanNameGenerator());
	}
 void setScopeMetadataResolver(@Nullable ScopeMetadataResolver scopeMetadataResolver) {
		this.scopeMetadataResolver =
				(scopeMetadataResolver != null ? scopeMetadataResolver : new AnnotationScopeMetadataResolver());
	}
	public void register(Class<?>... componentClasses) {
		for (Class<?> componentClass : componentClasses) {
			registerBean(componentClass);
		}
	}
	public void registerBean(Class<?> beanClass) {
		doRegisterBean(beanClass, null, null, null);
	}
	public <T> void registerBean(Class<T> beanClass, @Nullable Supplier<T> instanceSupplier) {
		doRegisterBean(beanClass, instanceSupplier, null, null);
	}
	public <T> void registerBean(Class<T> beanClass, String name, @Nullable Supplier<T> instanceSupplier) {
		doRegisterBean(beanClass, instanceSupplier, name, null);
	}
	@SuppressWarnings("unchecked")
	public void registerBean(Class<?> beanClass, Class<? extends Annotation>... qualifiers) {
		doRegisterBean(beanClass, null, null, qualifiers);
	}
	@SuppressWarnings("unchecked")
	public void registerBean(Class<?> beanClass, String name, Class<? extends Annotation>... qualifiers) {
		doRegisterBean(beanClass, null, name, qualifiers);
	}
	<T> void doRegisterBean(Class<T> beanClass, @Nullable Supplier<T> instanceSupplier, @Nullable String name,
			@Nullable Class<? extends Annotation>[] qualifiers, BeanDefinitionCustomizer... definitionCustomizers) {
		AnnotatedGenericBeanDefinition abd = new AnnotatedGenericBeanDefinition(beanClass);
		if (this.conditionEvaluator.shouldSkip(abd.getMetadata())) {
			return;
		}
		abd.setInstanceSupplier(instanceSupplier);
		ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(abd);
		abd.setScope(scopeMetadata.getScopeName());
		String beanName = (name != null ? name : this.beanNameGenerator.generateBeanName(abd, this.registry));
		AnnotationConfigUtils.processCommonDefinitionAnnotations(abd);
		if (qualifiers != null) {
			for (Class<? extends Annotation> qualifier : qualifiers) {
				if (Primary.class == qualifier) {
					abd.setPrimary(true);
				}
				else if (Lazy.class == qualifier) {
					abd.setLazyInit(true);
				}
				else {
					abd.addQualifier(new AutowireCandidateQualifier(qualifier));
				}
			}
		}
		for (BeanDefinitionCustomizer customizer : definitionCustomizers) {
			customizer.customize(abd);
		}

		BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(abd, beanName);
		definitionHolder = AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
		BeanDefinitionReaderUtils.registerBeanDefinition(definitionHolder, this.registry);
	}
	private static Environment getOrCreateEnvironment(BeanDefinitionRegistry registry) {
		Assert.notNull(registry, "BeanDefinitionRegistry must not be null");
		if (registry instanceof EnvironmentCapable) {
			return ((EnvironmentCapable) registry).getEnvironment();
		}
		return new StandardEnvironment();
	}

}

其中，在构造初始化的时候，有如下逻辑：

AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);

对应的代码逻辑如下：

public static void registerAnnotationConfigProcessors(BeanDefinitionRegistry registry) {
		registerAnnotationConfigProcessors(registry, null);
	}
public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
			BeanDefinitionRegistry registry, @Nullable Object source) {

		DefaultListableBeanFactory beanFactory = unwrapDefaultListableBeanFactory(registry);
		if (beanFactory != null) {
			if (!(beanFactory.getDependencyComparator() instanceof AnnotationAwareOrderComparator)) {
				beanFactory.setDependencyComparator(AnnotationAwareOrderComparator.INSTANCE);
			}
			if (!(beanFactory.getAutowireCandidateResolver() instanceof ContextAnnotationAutowireCandidateResolver)) {
				beanFactory.setAutowireCandidateResolver(new ContextAnnotationAutowireCandidateResolver());
			}
		}
		Set<BeanDefinitionHolder> beanDefs = new LinkedHashSet<>(8);
		if (!registry.containsBeanDefinition(CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(ConfigurationClassPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, CONFIGURATION_ANNOTATION_PROCESSOR_BEAN_NAME));
		}
		if (!registry.containsBeanDefinition(AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(AutowiredAnnotationBeanPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, AUTOWIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
		}
		if (jsr250Present && !registry.containsBeanDefinition(COMMON_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(CommonAnnotationBeanPostProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, COMMON_ANNOTATION_PROCESSOR_BEAN_NAME));
		}
		if (jpaPresent && !registry.containsBeanDefinition(PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition();
			try {
				def.setBeanClass(ClassUtils.forName(PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME,
						AnnotationConfigUtils.class.getClassLoader()));
			}
			catch (ClassNotFoundException ex) {
				throw new IllegalStateException(
						"Cannot load optional framework class: " + PERSISTENCE_ANNOTATION_PROCESSOR_CLASS_NAME, ex);
			}
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, PERSISTENCE_ANNOTATION_PROCESSOR_BEAN_NAME));
		}
		if (!registry.containsBeanDefinition(EVENT_LISTENER_PROCESSOR_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(EventListenerMethodProcessor.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_PROCESSOR_BEAN_NAME));
		}
		if (!registry.containsBeanDefinition(EVENT_LISTENER_FACTORY_BEAN_NAME)) {
			RootBeanDefinition def = new RootBeanDefinition(DefaultEventListenerFactory.class);
			def.setSource(source);
			beanDefs.add(registerPostProcessor(registry, def, EVENT_LISTENER_FACTORY_BEAN_NAME));
		}
		return beanDefs;
	}

可以看到，这步主要是注册了下面几个bean：

• ConfigurationClassPostProcessor
• AutowiredAnnotationBeanPostProcessor
• CommonAnnotationBeanPostProcessor
• EventListenerMethodProcessor
• DefaultEventListenerFactory
  其中 ConfigurationClassPostProcessor就是用来处理 @Configuration相关逻辑，其集成结构如下：
  
  其实现了BeanDefinitionRegistryPostProcessor，spring容器在bean的定义注册后会调用postProcessBeanDefinitionRegistry,在ConfigurationClassPostProcessor实现如下：

// ConfigurationClassPostProcessor.java
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);

		processConfigBeanDefinitions(registry);
	}
public void processConfigBeanDefinitions(BeanDefinitionRegistry registry) {
		List<BeanDefinitionHolder> configCandidates = new ArrayList<>();
		String[] candidateNames = registry.getBeanDefinitionNames();

		for (String beanName : candidateNames) {
			BeanDefinition beanDef = registry.getBeanDefinition(beanName);
			if (ConfigurationClassUtils.isFullConfigurationClass(beanDef) ||
					ConfigurationClassUtils.isLiteConfigurationClass(beanDef)) {
				if (logger.isDebugEnabled()) {
					logger.debug("Bean definition has already been processed as a configuration class: " + beanDef);
				}
			}
			else if (ConfigurationClassUtils.checkConfigurationClassCandidate(beanDef, this.metadataReaderFactory)) {
				configCandidates.add(new BeanDefinitionHolder(beanDef, beanName));
			}
		}
		if (configCandidates.isEmpty()) {
			return;
		}
		configCandidates.sort((bd1, bd2) -> {
			int i1 = ConfigurationClassUtils.getOrder(bd1.getBeanDefinition());
			int i2 = ConfigurationClassUtils.getOrder(bd2.getBeanDefinition());
			return Integer.compare(i1, i2);
		});
		SingletonBeanRegistry sbr = null;
		if (registry instanceof SingletonBeanRegistry) {
			sbr = (SingletonBeanRegistry) registry;
			if (!this.localBeanNameGeneratorSet) {
				BeanNameGenerator generator = (BeanNameGenerator) sbr.getSingleton(CONFIGURATION_BEAN_NAME_GENERATOR);
				if (generator != null) {
					this.componentScanBeanNameGenerator = generator;
					this.importBeanNameGenerator = generator;
				}
			}
		}

		if (this.environment == null) {
			this.environment = new StandardEnvironment();
		}
		ConfigurationClassParser parser = new ConfigurationClassParser(
				this.metadataReaderFactory, this.problemReporter, this.environment,
				this.resourceLoader, this.componentScanBeanNameGenerator, registry);
		Set<BeanDefinitionHolder> candidates = new LinkedHashSet<>(configCandidates);
		Set<ConfigurationClass> alreadyParsed = new HashSet<>(configCandidates.size());
		do {
			parser.parse(candidates);
			parser.validate();
			Set<ConfigurationClass> configClasses = new LinkedHashSet<>(parser.getConfigurationClasses());
			configClasses.removeAll(alreadyParsed);
			if (this.reader == null) {
				this.reader = new ConfigurationClassBeanDefinitionReader(
						registry, this.sourceExtractor, this.resourceLoader, this.environment,
						this.importBeanNameGenerator, parser.getImportRegistry());
			}
			this.reader.loadBeanDefinitions(configClasses);
			alreadyParsed.addAll(configClasses);
			candidates.clear();
			if (registry.getBeanDefinitionCount() > candidateNames.length) {
				String[] newCandidateNames = registry.getBeanDefinitionNames();
				Set<String> oldCandidateNames = new HashSet<>(Arrays.asList(candidateNames));
				Set<String> alreadyParsedClasses = new HashSet<>();
				for (ConfigurationClass configurationClass : alreadyParsed) {
					alreadyParsedClasses.add(configurationClass.getMetadata().getClassName());
				}
				for (String candidateName : newCandidateNames) {
					if (!oldCandidateNames.contains(candidateName)) {
						BeanDefinition bd = registry.getBeanDefinition(candidateName);
						if (ConfigurationClassUtils.checkConfigurationClassCandidate(bd, this.metadataReaderFactory) &&
								!alreadyParsedClasses.contains(bd.getBeanClassName())) {
							candidates.add(new BeanDefinitionHolder(bd, candidateName));
						}
					}
				}
				candidateNames = newCandidateNames;
			}
		}
		while (!candidates.isEmpty());
		if (sbr != null && !sbr.containsSingleton(IMPORT_REGISTRY_BEAN_NAME)) {
			sbr.registerSingleton(IMPORT_REGISTRY_BEAN_NAME, parser.getImportRegistry());
		}

		if (this.metadataReaderFactory instanceof CachingMetadataReaderFactory) {
			// Clear cache in externally provided MetadataReaderFactory; this is a no-op
			// for a shared cache since it'll be cleared by the ApplicationContext.
			((CachingMetadataReaderFactory) this.metadataReaderFactory).clearCache();
		}
	}

可以看到，这里会对当前所有注册的BeanDefinition进行判断，如果beanDefinition满足条件

1. 有Configuration注解，则是FullConfigurationCandidate
2. 有Component、ComponentScan、Import、ImportResource注解，则是LiteConfigurationCandidate

同时根据上述属性，设置Bedefinition对应属性：

beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_LITE);
// 或者
beanDef.setAttribute(CONFIGURATION_CLASS_ATTRIBUTE, CONFIGURATION_CLASS_FULL);

当获取到满足条件BeanDefinition后，通过ConfigurationClassParser来进行实际的解析：

// ConfigurationClassParser.java
public void parse(Set<BeanDefinitionHolder> configCandidates) {
		for (BeanDefinitionHolder holder : configCandidates) {
			BeanDefinition bd = holder.getBeanDefinition();
			try {
				if (bd instanceof AnnotatedBeanDefinition) {
					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());
				}
			}
			catch (BeanDefinitionStoreException ex) {
				throw ex;
			}
			catch (Throwable ex) {
				throw new BeanDefinitionStoreException(
						"Failed to parse configuration class [" + bd.getBeanClassName() + "]", ex);
			}
		}
		this.deferredImportSelectorHandler.process();
	}
protected void processConfigurationClass(ConfigurationClass configClass) throws IOException {
		if (this.conditionEvaluator.shouldSkip(configClass.getMetadata(), ConfigurationPhase.PARSE_CONFIGURATION)) {
			return;
		}
		ConfigurationClass existingClass = this.configurationClasses.get(configClass);
		if (existingClass != null) {
			if (configClass.isImported()) {
				if (existingClass.isImported()) {
					existingClass.mergeImportedBy(configClass);
				}
				return;
			}
			else {
				this.configurationClasses.remove(configClass);
				this.knownSuperclasses.values().removeIf(configClass::equals);
			}
		}
		SourceClass sourceClass = asSourceClass(configClass);
		do {
			sourceClass = doProcessConfigurationClass(configClass, sourceClass);
		}
		while (sourceClass != null);

		this.configurationClasses.put(configClass, configClass);
	}
protected final SourceClass doProcessConfigurationClass(ConfigurationClass configClass, SourceClass sourceClass)
			throws IOException {

		if (configClass.getMetadata().isAnnotated(Component.class.getName())) {
			// Recursively process any member (nested) classes first
			processMemberClasses(configClass, sourceClass);
		}
		for (AnnotationAttributes propertySource : AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), PropertySources.class,
				org.springframework.context.annotation.PropertySource.class)) {
			if (this.environment instanceof ConfigurableEnvironment) {
				processPropertySource(propertySource);
			}
			else {
				logger.info("Ignoring @PropertySource annotation on [" + sourceClass.getMetadata().getClassName() +
						"]. Reason: Environment must implement ConfigurableEnvironment");
			}
		}
		Set<AnnotationAttributes> componentScans = AnnotationConfigUtils.attributesForRepeatable(
				sourceClass.getMetadata(), ComponentScans.class, ComponentScan.class);
		if (!componentScans.isEmpty() &&
				!this.conditionEvaluator.shouldSkip(sourceClass.getMetadata(), ConfigurationPhase.REGISTER_BEAN)) {
			for (AnnotationAttributes componentScan : componentScans) {
				Set<BeanDefinitionHolder> scannedBeanDefinitions =
						this.componentScanParser.parse(componentScan, sourceClass.getMetadata().getClassName());
				for (BeanDefinitionHolder holder : scannedBeanDefinitions) {
					BeanDefinition bdCand = holder.getBeanDefinition().getOriginatingBeanDefinition();
					if (bdCand == null) {
						bdCand = holder.getBeanDefinition();
					}
					if (ConfigurationClassUtils.checkConfigurationClassCandidate(bdCand, this.metadataReaderFactory)) {
						parse(bdCand.getBeanClassName(), holder.getBeanName());
					}
				}
			}
		}
		// 处理 @Import注解
		processImports(configClass, sourceClass, getImports(sourceClass), true);
		AnnotationAttributes importResource =
				AnnotationConfigUtils.attributesFor(sourceClass.getMetadata(), ImportResource.class);
		if (importResource != null) {
		// 处理@ImportResource注解
			String[] resources = importResource.getStringArray("locations");
			Class<? extends BeanDefinitionReader> readerClass = importResource.getClass("reader");
			for (String resource : resources) {
				String resolvedResource = this.environment.resolveRequiredPlaceholders(resource);
				configClass.addImportedResource(resolvedResource, readerClass);
			}
		}
		// 处理@Bean注解
		Set<MethodMetadata> beanMethods = retrieveBeanMethodMetadata(sourceClass);
		for (MethodMetadata methodMetadata : beanMethods) {
			configClass.addBeanMethod(new BeanMethod(methodMetadata, configClass));
		}
		processInterfaces(configClass, sourceClass);
		if (sourceClass.getMetadata().hasSuperClass()) {
			String superclass = sourceClass.getMetadata().getSuperClassName();
			if (superclass != null && !superclass.startsWith("java") &&
					!this.knownSuperclasses.containsKey(superclass)) {
				this.knownSuperclasses.put(superclass, configClass);
				return sourceClass.getSuperClass();
			}
		}
		return null;
	}

通过上述步骤，将相关注解需要处理的类和方法提取出来，然后通过ConfigurationClassBeanDefinitionReader注册到IOC容器中：

// ConfigurationClassBeanDefinitionReader.java
public void loadBeanDefinitions(Set<ConfigurationClass> configurationModel) {
		TrackedConditionEvaluator trackedConditionEvaluator = new TrackedConditionEvaluator();
		for (ConfigurationClass configClass : configurationModel) {
			loadBeanDefinitionsForConfigurationClass(configClass, trackedConditionEvaluator);
		}
	}
private void loadBeanDefinitionsForConfigurationClass(
			ConfigurationClass configClass, TrackedConditionEvaluator trackedConditionEvaluator) {

		if (trackedConditionEvaluator.shouldSkip(configClass)) {
			String beanName = configClass.getBeanName();
			if (StringUtils.hasLength(beanName) && this.registry.containsBeanDefinition(beanName)) {
				this.registry.removeBeanDefinition(beanName);
			}
			this.importRegistry.removeImportingClass(configClass.getMetadata().getClassName());
			return;
		}

		if (configClass.isImported()) {
			registerBeanDefinitionForImportedConfigurationClass(configClass);
		}
		for (BeanMethod beanMethod : configClass.getBeanMethods()) {
			loadBeanDefinitionsForBeanMethod(beanMethod);
		}

		loadBeanDefinitionsFromImportedResources(configClass.getImportedResources());
		loadBeanDefinitionsFromRegistrars(configClass.getImportBeanDefinitionRegistrars());
	}

这样，便将@Configuration注解相关的类都注册到了IOC容器中，那么spring是怎么获取需要扫描哪些bean呢 ？还记得开篇说的的AnnotationConfigApplicationContext吗，奥秘就在这个类构造时：

// AnnotationConfigApplicationContext.java

public class AnnotationConfigApplicationContext extends GenericApplicationContext implements AnnotationConfigRegistry {
	private final AnnotatedBeanDefinitionReader reader;
	private final ClassPathBeanDefinitionScanner scanner;
	public AnnotationConfigApplicationContext() {
		this.reader = new AnnotatedBeanDefinitionReader(this);
		this.scanner = new ClassPathBeanDefinitionScanner(this);
	}
	public void scan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		this.scanner.scan(basePackages);
	}

// ClassPathBeanDefinitionScanner.java
public int scan(String... basePackages) {
		int beanCountAtScanStart = this.registry.getBeanDefinitionCount();
		doScan(basePackages);
		// Register annotation config processors, if necessary.
		if (this.includeAnnotationConfig) {
			AnnotationConfigUtils.registerAnnotationConfigProcessors(this.registry);
		}

		return (this.registry.getBeanDefinitionCount() - beanCountAtScanStart);
	}
protected Set<BeanDefinitionHolder> doScan(String... basePackages) {
		Assert.notEmpty(basePackages, "At least one base package must be specified");
		Set<BeanDefinitionHolder> beanDefinitions = new LinkedHashSet<>();
		for (String basePackage : basePackages) {
			Set<BeanDefinition> candidates = findCandidateComponents(basePackage);
			for (BeanDefinition candidate : candidates) {
				ScopeMetadata scopeMetadata = this.scopeMetadataResolver.resolveScopeMetadata(candidate);
				candidate.setScope(scopeMetadata.getScopeName());
				String beanName = this.beanNameGenerator.generateBeanName(candidate, this.registry);
				if (candidate instanceof AbstractBeanDefinition) {
					postProcessBeanDefinition((AbstractBeanDefinition) candidate, beanName);
				}
				if (candidate instanceof AnnotatedBeanDefinition) {
					AnnotationConfigUtils.processCommonDefinitionAnnotations((AnnotatedBeanDefinition) candidate);
				}
				if (checkCandidate(beanName, candidate)) {
					BeanDefinitionHolder definitionHolder = new BeanDefinitionHolder(candidate, beanName);
					definitionHolder =
							AnnotationConfigUtils.applyScopedProxyMode(scopeMetadata, definitionHolder, this.registry);
					beanDefinitions.add(definitionHolder);
					registerBeanDefinition(definitionHolder, this.registry);
				}
			}
		}
		return beanDefinitions;
	}
// AnnotatedBeanDefinitionReader.java
	public void register(Class<?>... componentClasses) {
		Assert.notEmpty(componentClasses, "At least one component class must be specified");
		this.reader.register(componentClasses);
	}
public void register(Class<?>... componentClasses) {
		for (Class<?> componentClass : componentClasses) {
			registerBean(componentClass);
		}
	}
	public void registerBean(Class<?> beanClass) {
		doRegisterBean(beanClass, null, null, null);
	}

就是通过AnnotatedBeanDefinitionReader读取需要扫描的类，通过ClassPathBeanDefinitionScanner扫描指定的包名，这里就是Spring中实现基于注解方式的@Configuration方式

在springboot中一般springboot应用可以如下：

import java.io.File;
@SpringBootApplication(exclude= {DataSourceAutoConfiguration.class},scanBasePackages ="xxxxx" )
public class MiddlewareApplication
{
    public static void main(String[] args) throws Exception
    {
        SpringApplication.run(MiddlewareApplication.class, args);
    }

}

我们可以看下@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 {
	@AliasFor(annotation = EnableAutoConfiguration.class)
	Class<?>[] exclude() default {};
	@AliasFor(annotation = EnableAutoConfiguration.class)
	String[] excludeName() default {};
	@AliasFor(annotation = ComponentScan.class, attribute = "basePackages")
	String[] scanBasePackages() default {};
	@AliasFor(annotation = ComponentScan.class, attribute = "basePackageClasses")
	Class<?>[] scanBasePackageClasses() default {};

}

可以看到这里@EnableAutoConfiguration就是springboot自动装配的核心。

@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 {};

}

我们看看AutoConfigurationImportSelector.selectImports逻辑：

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());
		Assert.notEmpty(configurations, "No auto configuration classes found in META-INF/spring.factories. If you "
				+ "are using a custom packaging, make sure that file is correct.");
		return configurations;
	}
	protected Class<?> getSpringFactoriesLoaderFactoryClass() {
		return EnableAutoConfiguration.class;
	}

可以看到，AutoConfigurationImportSelector.selectImports主要逻辑是读取META-INF/spring.factories中key为org.springframework.boot.autoconfigure.EnableAutoConfiguration的配置列表，然后按照条件对列表进行一定过滤，返回一个集合，spring会加载这个集合。

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,\
org.springframework.boot.autoconfigure.cassandra.CassandraAutoConfiguration,\
org.springframework.boot.autoconfigure.cloud.CloudServiceConnectorsAutoConfiguration,\
org.springframework.boot.autoconfigure.context.ConfigurationPropertiesAutoConfiguration,\
org.springframework.boot.autoconfigure.context.MessageSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.context.PropertyPlaceholderAutoConfiguration,\
org.springframework.boot.autoconfigure.couchbase.CouchbaseAutoConfiguration,\
org.springframework.boot.autoconfigure.dao.PersistenceExceptionTranslationAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.cassandra.CassandraRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.couchbase.CouchbaseRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.elasticsearch.ElasticsearchRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jdbc.JdbcRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.jpa.JpaRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.ldap.LdapRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoReactiveRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.mongo.MongoRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jDataAutoConfiguration,\
org.springframework.boot.autoconfigure.data.neo4j.Neo4jRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.solr.SolrRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.data.redis.RedisRepositoriesAutoConfiguration,\
org.springframework.boot.autoconfigure.data.rest.RepositoryRestMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.data.web.SpringDataWebAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.jest.JestAutoConfiguration,\
org.springframework.boot.autoconfigure.elasticsearch.rest.RestClientAutoConfiguration,\
org.springframework.boot.autoconfigure.flyway.FlywayAutoConfiguration,\
org.springframework.boot.autoconfigure.freemarker.FreeMarkerAutoConfiguration,\
org.springframework.boot.autoconfigure.gson.GsonAutoConfiguration,\
org.springframework.boot.autoconfigure.h2.H2ConsoleAutoConfiguration,\
org.springframework.boot.autoconfigure.hateoas.HypermediaAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastAutoConfiguration,\
org.springframework.boot.autoconfigure.hazelcast.HazelcastJpaDependencyAutoConfiguration,\
org.springframework.boot.autoconfigure.http.HttpMessageConvertersAutoConfiguration,\
org.springframework.boot.autoconfigure.http.codec.CodecsAutoConfiguration,\
org.springframework.boot.autoconfigure.influx.InfluxDbAutoConfiguration,\
org.springframework.boot.autoconfigure.info.ProjectInfoAutoConfiguration,\
org.springframework.boot.autoconfigure.integration.IntegrationAutoConfiguration,\
org.springframework.boot.autoconfigure.jackson.JacksonAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JdbcTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.JndiDataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.XADataSourceAutoConfiguration,\
org.springframework.boot.autoconfigure.jdbc.DataSourceTransactionManagerAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JmsAutoConfiguration,\
org.springframework.boot.autoconfigure.jmx.JmxAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.JndiConnectionFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.activemq.ActiveMQAutoConfiguration,\
org.springframework.boot.autoconfigure.jms.artemis.ArtemisAutoConfiguration,\
org.springframework.boot.autoconfigure.groovy.template.GroovyTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.jersey.JerseyAutoConfiguration,\
org.springframework.boot.autoconfigure.jooq.JooqAutoConfiguration,\
org.springframework.boot.autoconfigure.jsonb.JsonbAutoConfiguration,\
org.springframework.boot.autoconfigure.kafka.KafkaAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.embedded.EmbeddedLdapAutoConfiguration,\
org.springframework.boot.autoconfigure.ldap.LdapAutoConfiguration,\
org.springframework.boot.autoconfigure.liquibase.LiquibaseAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderAutoConfiguration,\
org.springframework.boot.autoconfigure.mail.MailSenderValidatorAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.embedded.EmbeddedMongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoAutoConfiguration,\
org.springframework.boot.autoconfigure.mongo.MongoReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.mustache.MustacheAutoConfiguration,\
org.springframework.boot.autoconfigure.orm.jpa.HibernateJpaAutoConfiguration,\
org.springframework.boot.autoconfigure.quartz.QuartzAutoConfiguration,\
org.springframework.boot.autoconfigure.reactor.core.ReactorCoreAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.UserDetailsServiceAutoConfiguration,\
org.springframework.boot.autoconfigure.security.servlet.SecurityFilterAutoConfiguration,\
org.springframework.boot.autoconfigure.security.reactive.ReactiveSecurityAutoConfiguration,\
org.springframework.boot.autoconfigure.security.reactive.ReactiveUserDetailsServiceAutoConfiguration,\
org.springframework.boot.autoconfigure.sendgrid.SendGridAutoConfiguration,\
org.springframework.boot.autoconfigure.session.SessionAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.servlet.OAuth2ClientAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.client.reactive.ReactiveOAuth2ClientAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.resource.servlet.OAuth2ResourceServerAutoConfiguration,\
org.springframework.boot.autoconfigure.security.oauth2.resource.reactive.ReactiveOAuth2ResourceServerAutoConfiguration,\
org.springframework.boot.autoconfigure.solr.SolrAutoConfiguration,\
org.springframework.boot.autoconfigure.task.TaskExecutionAutoConfiguration,\
org.springframework.boot.autoconfigure.task.TaskSchedulingAutoConfiguration,\
org.springframework.boot.autoconfigure.thymeleaf.ThymeleafAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.TransactionAutoConfiguration,\
org.springframework.boot.autoconfigure.transaction.jta.JtaAutoConfiguration,\
org.springframework.boot.autoconfigure.validation.ValidationAutoConfiguration,\
org.springframework.boot.autoconfigure.web.client.RestTemplateAutoConfiguration,\
org.springframework.boot.autoconfigure.web.embedded.EmbeddedWebServerFactoryCustomizerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.HttpHandlerAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.ReactiveWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.WebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.error.ErrorWebFluxAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.function.client.ClientHttpConnectorAutoConfiguration,\
org.springframework.boot.autoconfigure.web.reactive.function.client.WebClientAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.DispatcherServletAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.ServletWebServerFactoryAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.error.ErrorMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.HttpEncodingAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.MultipartAutoConfiguration,\
org.springframework.boot.autoconfigure.web.servlet.WebMvcAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.reactive.WebSocketReactiveAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketServletAutoConfiguration,\
org.springframework.boot.autoconfigure.websocket.servlet.WebSocketMessagingAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.WebServicesAutoConfiguration,\
org.springframework.boot.autoconfigure.webservices.client.WebServiceTemplateAutoConfiguration

可以看到，默认情况下，只要我们开启了EnableAutoConfiguration注解（直接或间接使用），springboot默认会为我们加载这么多的配置类，那么是不是这么多类都加载到IOC容器呢 ？这里我们就需要说下springboot中提供的ConditionalOnBean、ConditionalOnMissingBean、ConditionalOnClass、ConditionalOnMissingClass等条件注解。还记得我们在开头说的AnnotationConfigApplicationContext在register(componentClasses);的时候，会调用

AnnotatedBeanDefinitionReader.doRegisterBean
会调用：
ConditionEvaluator.shouldSkip

// ConditionEvaluator.java
public boolean shouldSkip(@Nullable AnnotatedTypeMetadata metadata, @Nullable ConfigurationPhase phase) {
		if (metadata == null || !metadata.isAnnotated(Conditional.class.getName())) {
			return false;
		}
		if (phase == null) {
			if (metadata instanceof AnnotationMetadata &&
					ConfigurationClassUtils.isConfigurationCandidate((AnnotationMetadata) metadata)) {
				return shouldSkip(metadata, ConfigurationPhase.PARSE_CONFIGURATION);
			}
			return shouldSkip(metadata, ConfigurationPhase.REGISTER_BEAN);
		}
		List<Condition> conditions = new ArrayList<>();
		for (String[] conditionClasses : getConditionClasses(metadata)) {
			for (String conditionClass : conditionClasses) {
				Condition condition = getCondition(conditionClass, this.context.getClassLoader());
				conditions.add(condition);
			}
		}
		AnnotationAwareOrderComparator.sort(conditions);
		for (Condition condition : conditions) {
			ConfigurationPhase requiredPhase = null;
			if (condition instanceof ConfigurationCondition) {
				requiredPhase = ((ConfigurationCondition) condition).getConfigurationPhase();
			}
			if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
				return true;
			}
		}
		return false;
	}
	private List<String[]> getConditionClasses(AnnotatedTypeMetadata metadata) {
		MultiValueMap<String, Object> attributes = metadata.getAllAnnotationAttributes(Conditional.class.getName(), true);
		Object values = (attributes != null ? attributes.get("value") : null);
		return (List<String[]>) (values != null ? values : Collections.emptyList());
	}
	private Condition getCondition(String conditionClassName, @Nullable ClassLoader classloader) {
		Class<?> conditionClass = ClassUtils.resolveClassName(conditionClassName, classloader);
		return (Condition) BeanUtils.instantiateClass(conditionClass);
	}

可以看到，这里会获取@Conditional注解上的value属性，这里value属性是一个实现了Condition接口的列表，然后实例化这些实现了Condition接口的。可以看到，这里如果bean满足如下条件，则不会注册：

if ((requiredPhase == null || requiredPhase == phase) && !condition.matches(this.context, metadata)) {
				return true;
			}

springboot中的条件注解@ConditionalOnXXX和ConditionalOnMissingXXX正是基于spring提供的
@Conditional注解和Condition接口实现。比如@ConditionalOnBean和@ConditionalOnMissingBean :

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnBeanCondition.class)
public @interface ConditionalOnBean {
	Class<?>[] value() default {};
	String[] type() default {};
	Class<? extends Annotation>[] annotation() default {};

	String[] name() default {};
	SearchStrategy search() default SearchStrategy.ALL;
	Class<?>[] parameterizedContainer() default {};

}

@Target({ ElementType.TYPE, ElementType.METHOD })
@Retention(RetentionPolicy.RUNTIME)
@Documented
@Conditional(OnBeanCondition.class)
public @interface ConditionalOnMissingBean {
	Class<?>[] value() default {};
	String[] type() default {};
	Class<?>[] ignored() default {};
	String[] ignoredType() default {};
	Class<? extends Annotation>[] annotation() default {};
	String[] name() default {};
	SearchStrategy search() default SearchStrategy.ALL;
	Class<?>[] parameterizedContainer() default {};

}

可有看到@ConditionalOnBean和@ConditionalOnMissingBean 的实现都是通过OnBeanCondition实现。

主要提取逻辑在：

// OnBeanCondition.java
public ConditionOutcome getMatchOutcome(ConditionContext context, AnnotatedTypeMetadata metadata) {
		ConditionMessage matchMessage = ConditionMessage.empty();
		if (metadata.isAnnotated(ConditionalOnBean.class.getName())) {
			BeanSearchSpec spec = new BeanSearchSpec(context, metadata, ConditionalOnBean.class);
			MatchResult matchResult = getMatchingBeans(context, spec);
			if (!matchResult.isAllMatched()) {
				String reason = createOnBeanNoMatchReason(matchResult);
				return ConditionOutcome
						.noMatch(ConditionMessage.forCondition(ConditionalOnBean.class, spec).because(reason));
			}
			matchMessage = matchMessage.andCondition(ConditionalOnBean.class, spec).found("bean", "beans")
					.items(Style.QUOTE, matchResult.getNamesOfAllMatches());
		}
		if (metadata.isAnnotated(ConditionalOnSingleCandidate.class.getName())) {
			BeanSearchSpec spec = new SingleCandidateBeanSearchSpec(context, metadata,
					ConditionalOnSingleCandidate.class);
			MatchResult matchResult = getMatchingBeans(context, spec);
			if (!matchResult.isAllMatched()) {
				return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnSingleCandidate.class, spec)
						.didNotFind("any beans").atAll());
			}
			else if (!hasSingleAutowireCandidate(context.getBeanFactory(), matchResult.getNamesOfAllMatches(),
					spec.getStrategy() == SearchStrategy.ALL)) {
				return ConditionOutcome.noMatch(ConditionMessage.forCondition(ConditionalOnSingleCandidate.class, spec)
						.didNotFind("a primary bean from beans")
						.items(Style.QUOTE, matchResult.getNamesOfAllMatches()));
			}
			matchMessage = matchMessage.andCondition(ConditionalOnSingleCandidate.class, spec)
					.found("a primary bean from beans").items(Style.QUOTE, matchResult.getNamesOfAllMatches());
		}
		if (metadata.isAnnotated(ConditionalOnMissingBean.class.getName())) {
			BeanSearchSpec spec = new BeanSearchSpec(context, metadata, ConditionalOnMissingBean.class);
			MatchResult matchResult = getMatchingBeans(context, spec);
			if (matchResult.isAnyMatched()) {
				String reason = createOnMissingBeanNoMatchReason(matchResult);
				return ConditionOutcome
						.noMatch(ConditionMessage.forCondition(ConditionalOnMissingBean.class, spec).because(reason));
			}
			matchMessage = matchMessage.andCondition(ConditionalOnMissingBean.class, spec).didNotFind("any beans")
					.atAll();
		}
		return ConditionOutcome.match(matchMessage);
	}

可以看到，这里@ConditionalOnBean和@ConditionalOnMissingBean 实现本质上是一样的，就是获取注解上对应的bean是否存在，只不过两者在判断结果的时候，判断逻辑相反，而已。

如果@ConditionalOnBean和@ConditionalOnMissingBean 没有指定name和value，且当前是在方法上的注解，则会获取方法的返回类型，相当于指定的value值。

到这里的话，我们就知道，虽然springtboot基于EnableAutoConfiguration注解从META-INF/spring.factories加载了这么多类，但是这些类上都有相关条件注解，只有在条件满足的时候才能装载，这也就是springboot自动装载的核心。