SpringIOC容器的依赖来源都有哪些？这次一起捋清楚~

一、依赖查找的依赖来源

1、自定义的BeanDefinition

1、使用xml方式配置的

<bean id="user" class="org.geekbang...User">

2、使用@Bean

@Bean public User user(){...}

3、使用BeanDefinitionBuilder自己构建的

总之，一切用户自己定义的bean。（下面详细介绍）

2、外部化配置的单体对象

就是已初始化的外部 Java 对象，在 Spring 容器中是唯一的。（下面详细介绍）

3、Spring 內建 BeanDefintion

使用AnnotationConfigUtils定义的BeanDefintion

Bean 名称	Bean 实例	使用场景
org.springframework.context.annotation.internalConfigurationAnnotationProcessor	ConfigurationClassPostProcessor 对象	处理 Spring 配置类
org.springframework.context.annotation.internalAutowiredAnnotationProcessor	AutowiredAnnotationBeanPostProcessor 对象	处理 @Autowired 以及 @Value注解
org.springframework.context.annotation.internalCommonAnnotationProcessor	CommonAnnotationBeanPostProcessor 对象	（条件激活）处理 JSR-250 注解，如 @PostConstruct 等
org.springframework.context.event.internalEventListenerProcessor	EventListenerMethodProcessor对象	处理标注 @EventListener 的Spring 事件监听方法

以下是AnnotationConfigUtils的registerAnnotationConfigProcessors方法定义这些BeanDefintion的过程：

// AnnotationConfigUtils定义的BeanDefintion
// org.springframework.context.annotation.AnnotationConfigUtils#registerAnnotationConfigProcessors(org.springframework.beans.factory.support.BeanDefinitionRegistry, java.lang.Object)
public static Set<BeanDefinitionHolder> registerAnnotationConfigProcessors(
		BeanDefinitionRegistry registry, 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<BeanDefinitionHolder>(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 (!registry.containsBeanDefinition(REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME)) {
		RootBeanDefinition def = new RootBeanDefinition(RequiredAnnotationBeanPostProcessor.class);
		def.setSource(source);
		beanDefs.add(registerPostProcessor(registry, def, REQUIRED_ANNOTATION_PROCESSOR_BEAN_NAME));
	}

	// Check for JSR-250 support, and if present add the CommonAnnotationBeanPostProcessor.
	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));
	}

	// Check for JPA support, and if present add the PersistenceAnnotationBeanPostProcessor.
	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;
}

使用AbstractApplicationContext定义的BeanDefinition

Bean 名称	Bean 实例	使用场景
environment	Environment 对象	外部化配置以及 Profiles
systemProperties	java.util.Properties 对象	Java 系统属性
systemEnvironment	java.util.Map 对象	操作系统环境变量
messageSource	MessageSource 对象	国际化文案
lifecycleProcessor	LifecycleProcessor 对象	Lifecycle Bean 处理器
applicationEventMulticaster	ApplicationEventMulticaster 对象	Spring 事件广播器

以下是AbstractApplicationContext的prepareBeanFactory、initMessageSource方法定义这些BeanDefintion的过程：

// 注册一些beanDefinatiton
// org.springframework.context.support.AbstractApplicationContext#prepareBeanFactory
protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
	// Tell the internal bean factory to use the context's class loader etc.
	beanFactory.setBeanClassLoader(getClassLoader());
	beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
	beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));

	// Configure the bean factory with context callbacks.
	beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
	beanFactory.ignoreDependencyInterface(EnvironmentAware.class);
	beanFactory.ignoreDependencyInterface(EmbeddedValueResolverAware.class);
	beanFactory.ignoreDependencyInterface(ResourceLoaderAware.class);
	beanFactory.ignoreDependencyInterface(ApplicationEventPublisherAware.class);
	beanFactory.ignoreDependencyInterface(MessageSourceAware.class);
	beanFactory.ignoreDependencyInterface(ApplicationContextAware.class);

	// BeanFactory interface not registered as resolvable type in a plain factory.
	// MessageSource registered (and found for autowiring) as a bean.
	beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
	beanFactory.registerResolvableDependency(ResourceLoader.class, this);
	beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
	beanFactory.registerResolvableDependency(ApplicationContext.class, this);

	// Register early post-processor for detecting inner beans as ApplicationListeners.
	beanFactory.addBeanPostProcessor(new ApplicationListenerDetector(this));

	// Detect a LoadTimeWeaver and prepare for weaving, if found.
	if (beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
		beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
		// Set a temporary ClassLoader for type matching.
		beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
	}

	// Register default environment beans.
	if (!beanFactory.containsLocalBean(ENVIRONMENT_BEAN_NAME)) {
		beanFactory.registerSingleton(ENVIRONMENT_BEAN_NAME, getEnvironment());
	}
	if (!beanFactory.containsLocalBean(SYSTEM_PROPERTIES_BEAN_NAME)) {
		beanFactory.registerSingleton(SYSTEM_PROPERTIES_BEAN_NAME, getEnvironment().getSystemProperties());
	}
	if (!beanFactory.containsLocalBean(SYSTEM_ENVIRONMENT_BEAN_NAME)) {
		beanFactory.registerSingleton(SYSTEM_ENVIRONMENT_BEAN_NAME, getEnvironment().getSystemEnvironment());
	}
}

// 注册国际化相关
// org.springframework.context.support.AbstractApplicationContext#initMessageSource
protected void initMessageSource() {
	ConfigurableListableBeanFactory beanFactory = getBeanFactory();
	if (beanFactory.containsLocalBean(MESSAGE_SOURCE_BEAN_NAME)) {
		this.messageSource = beanFactory.getBean(MESSAGE_SOURCE_BEAN_NAME, MessageSource.class);
		// Make MessageSource aware of parent MessageSource.
		if (this.parent != null && this.messageSource instanceof HierarchicalMessageSource) {
			HierarchicalMessageSource hms = (HierarchicalMessageSource) this.messageSource;
			if (hms.getParentMessageSource() == null) {
				// Only set parent context as parent MessageSource if no parent MessageSource
				// registered already.
				hms.setParentMessageSource(getInternalParentMessageSource());
			}
		}
		if (logger.isDebugEnabled()) {
			logger.debug("Using MessageSource [" + this.messageSource + "]");
		}
	}
	else {
		// Use empty MessageSource to be able to accept getMessage calls.
		DelegatingMessageSource dms = new DelegatingMessageSource();
		dms.setParentMessageSource(getInternalParentMessageSource());
		this.messageSource = dms;
		beanFactory.registerSingleton(MESSAGE_SOURCE_BEAN_NAME, this.messageSource);
		if (logger.isDebugEnabled()) {
			logger.debug("Unable to locate MessageSource with name '" + MESSAGE_SOURCE_BEAN_NAME +
					"': using default [" + this.messageSource + "]");
		}
	}
}

加载注册好的bean-容器的refresh()方法

beandefination定义完之后，在AbstractApplicationContext的refresh()调用invokeBeanFactoryPostProcessors，将bean初始化。

// org.springframework.context.support.AbstractApplicationContext#refresh
@Override
public void refresh() throws BeansException, IllegalStateException {
	synchronized (this.startupShutdownMonitor) {
		// Prepare this context for refreshing.
		prepareRefresh();

		// Tell the subclass to refresh the internal bean factory.
		ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();

		// Prepare the bean factory for use in this context.
		prepareBeanFactory(beanFactory);

		try {
			// Allows post-processing of the bean factory in context subclasses.
			postProcessBeanFactory(beanFactory);

			// Invoke factory processors registered as beans in the context.
			invokeBeanFactoryPostProcessors(beanFactory);

			// Register bean processors that intercept bean creation.
			registerBeanPostProcessors(beanFactory);

			// Initialize message source for this context.
			initMessageSource();

			// Initialize event multicaster for this context.
			initApplicationEventMulticaster();

			// Initialize other special beans in specific context subclasses.
			onRefresh();

			// Check for listener beans and register them.
			registerListeners();

			// Instantiate all remaining (non-lazy-init) singletons.
			finishBeanFactoryInitialization(beanFactory);

			// Last step: publish corresponding event.
			finishRefresh();
		}

		catch (BeansException ex) {
			if (logger.isWarnEnabled()) {
				logger.warn("Exception encountered during context initialization - " +
						"cancelling refresh attempt: " + ex);
			}

			// Destroy already created singletons to avoid dangling resources.
			destroyBeans();

			// Reset 'active' flag.
			cancelRefresh(ex);

			// Propagate exception to caller.
			throw ex;
		}

		finally {
			// Reset common introspection caches in Spring's core, since we
			// might not ever need metadata for singleton beans anymore...
			resetCommonCaches();
		}
	}
}

详情请看springIOC容器经典启动过程：spring系列-注解驱动原理及源码-spring容器创建流程

二、依赖注入的依赖来源

1、依赖查找的所有来源

同上

2、非Spring容器管理的对象

注入时机

在springIOC启动的refresh()中，调用prepareBeanFactory方法，prepareBeanFactory中注册了这样几个依赖：

// BeanFactory interface not registered as resolvable type in a plain factory.
// MessageSource registered (and found for autowiring) as a bean.
beanFactory.registerResolvableDependency(BeanFactory.class, beanFactory);
beanFactory.registerResolvableDependency(ResourceLoader.class, this);
beanFactory.registerResolvableDependency(ApplicationEventPublisher.class, this);
beanFactory.registerResolvableDependency(ApplicationContext.class, this);

使用时机

在依赖注入的DefaultListableBeanFactory类的doResolveDependency方法中：

// 关键方法：findAutowireCandidates
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
if (matchingBeans.isEmpty()) {
	if (isRequired(descriptor)) {
		raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
	}
	return null;
}

// org.springframework.beans.factory.support.DefaultListableBeanFactory#findAutowireCandidates
protected Map<String, Object> findAutowireCandidates(
		String beanName, Class<?> requiredType, DependencyDescriptor descriptor) {

	String[] candidateNames = BeanFactoryUtils.beanNamesForTypeIncludingAncestors(
			this, requiredType, true, descriptor.isEager());
	Map<String, Object> result = new LinkedHashMap<String, Object>(candidateNames.length);
	// 这里的resolvableDependencies 就是上面注册的ResolvableDependency
	for (Class<?> autowiringType : this.resolvableDependencies.keySet()) {
		if (autowiringType.isAssignableFrom(requiredType)) {
			Object autowiringValue = this.resolvableDependencies.get(autowiringType);
			autowiringValue = AutowireUtils.resolveAutowiringValue(autowiringValue, requiredType);
			if (requiredType.isInstance(autowiringValue)) {
				result.put(ObjectUtils.identityToString(autowiringValue), autowiringValue);
				break;
			}
		}
	}
	for (String candidate : candidateNames) {
		if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, descriptor)) {
			addCandidateEntry(result, candidate, descriptor, requiredType);
		}
	}
	if (result.isEmpty() && !indicatesMultipleBeans(requiredType)) {
		// Consider fallback matches if the first pass failed to find anything...
		DependencyDescriptor fallbackDescriptor = descriptor.forFallbackMatch();
		for (String candidate : candidateNames) {
			if (!isSelfReference(beanName, candidate) && isAutowireCandidate(candidate, fallbackDescriptor)) {
				addCandidateEntry(result, candidate, descriptor, requiredType);
			}
		}
		if (result.isEmpty()) {
			// Consider self references as a final pass...
			// but in the case of a dependency collection, not the very same bean itself.
			for (String candidate : candidateNames) {
				if (isSelfReference(beanName, candidate) &&
						(!(descriptor instanceof MultiElementDescriptor) || !beanName.equals(candidate)) &&
						isAutowireCandidate(candidate, fallbackDescriptor)) {
					addCandidateEntry(result, candidate, descriptor, requiredType);
				}
			}
		}
	}
	return result;
}

再回过头来看prepareBeanFactory方法中注册时调用的registerResolvableDependency方法，我们发现确实是直接将k-v值放到resolvableDependencies中：

// org.springframework.beans.factory.support.DefaultListableBeanFactory#registerResolvableDependency
@Override
public void registerResolvableDependency(Class<?> dependencyType, Object autowiredValue) {
	Assert.notNull(dependencyType, "Dependency type must not be null");
	if (autowiredValue != null) {
		if (!(autowiredValue instanceof ObjectFactory || dependencyType.isInstance(autowiredValue))) {
			throw new IllegalArgumentException("Value [" + autowiredValue +
					"] does not implement specified dependency type [" + dependencyType.getName() + "]");
		}
		this.resolvableDependencies.put(dependencyType, autowiredValue);
	}
}

这四个对象没法用BeanFactory.getBean获取到？

这四个对象，虽然也是依赖注入到了IOC容器中，但是我们不能用BeanFactory.getBean的方式获取。

如果直接调用getBean，会报Bean找不到的错误-NoSuchBeanException。

实例

import org.springframework.beans.factory.BeanFactory;
import org.springframework.beans.factory.NoSuchBeanDefinitionException;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.context.ApplicationContext;
import org.springframework.context.ApplicationEventPublisher;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;
import org.springframework.core.io.ResourceLoader;
import javax.annotation.PostConstruct;

/**
 * 依赖来源示例
 */
public class DependencySourceDemo {

    // 注入在 postProcessProperties 方法执行，早于 setter注入，也早于 @PostConstruct
    @Autowired
    private BeanFactory beanFactory;

    @Autowired
    private ResourceLoader resourceLoader;

    @Autowired
    private ApplicationContext applicationContext;

    @Autowired
    private ApplicationEventPublisher applicationEventPublisher;

    // 由源码我们得出：后三个是同一个东西
    @PostConstruct
    public void initByInjection() {
        System.out.println("beanFactory == applicationContext " + (beanFactory == applicationContext));
        System.out.println("beanFactory == applicationContext.getBeanFactory() " + (beanFactory == applicationContext.getAutowireCapableBeanFactory()));
        System.out.println("resourceLoader == applicationContext " + (resourceLoader == applicationContext));
        System.out.println("ApplicationEventPublisher == applicationContext " + (applicationEventPublisher == applicationContext));
    }

    // 使用getBean 方法查找，是查不到的，只能使用@Autowired自动注入获取
    @PostConstruct
    public void initByLookup() {
        getBean(BeanFactory.class);
        getBean(ApplicationContext.class);
        getBean(ResourceLoader.class);
        getBean(ApplicationEventPublisher.class);
    }

    private <T> T getBean(Class<T> beanType) {
        try {
            return beanFactory.getBean(beanType);
        } catch (NoSuchBeanDefinitionException e) {
            System.err.println("当前类型" + beanType.getName() + " 无法在 BeanFactory 中查找!");
        }
        return null;
    }


    public static void main(String[] args) {

        // 创建 BeanFactory 容器
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
        // 注册 Configuration Class（配置类） -> Spring Bean
        applicationContext.register(DependencySourceDemo.class);

        // 启动 Spring 应用上下文
        applicationContext.refresh();

        // 依赖查找 DependencySourceDemo Bean
        DependencySourceDemo demo = applicationContext.getBean(DependencySourceDemo.class);

        // 显示地关闭 Spring 应用上下文
        applicationContext.close();
    }
}

执行结果：

beanFactory == applicationContext false
beanFactory == applicationContext.getBeanFactory() true
resourceLoader == applicationContext true
ApplicationEventPublisher == applicationContext true
当前类型org.springframework.beans.factory.BeanFactory 无法在 BeanFactory 中查找!
当前类型org.springframework.context.ApplicationContext 无法在 BeanFactory 中查找!
当前类型org.springframework.core.io.ResourceLoader 无法在 BeanFactory 中查找!
当前类型org.springframework.context.ApplicationEventPublisher 无法在 BeanFactory 中查找!
Disconnected from the target VM, address: ‘127.0.0.1:60775’, transport: ‘socket’
Process finished with exit code 0

三、各种bean的详细解释

1、自定义BeanDefinition注册流程

自定义的BeanDefinition是由BeanDefinitionRegistry进行注册的，BeanDefinitionRegistry接口提供了对BeanDefinition基本的增删改查接口：

public interface BeanDefinitionRegistry extends AliasRegistry {

	void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
			throws BeanDefinitionStoreException;

	void removeBeanDefinition(String beanName) throws NoSuchBeanDefinitionException;

	BeanDefinition getBeanDefinition(String beanName) throws NoSuchBeanDefinitionException;

	boolean containsBeanDefinition(String beanName);

	String[] getBeanDefinitionNames();

	int getBeanDefinitionCount();

	boolean isBeanNameInUse(String beanName);

}

BeanDefinitionRegistry的默认实现就是DefaultListableBeanFactory，我们看一下DefaultListableBeanFactory的bean注册方法——registerBeanDefinition：

// org.springframework.beans.factory.support.DefaultListableBeanFactory#registerBeanDefinition
@Override
public void registerBeanDefinition(String beanName, BeanDefinition beanDefinition)
		throws BeanDefinitionStoreException {

	Assert.hasText(beanName, "Bean name must not be empty");
	Assert.notNull(beanDefinition, "BeanDefinition must not be null");

	if (beanDefinition instanceof AbstractBeanDefinition) {
		try {
			// 对beanDefinition做校验，其实使用BeanDefinitionBuilder时，调用builder方法时做过校验
			((AbstractBeanDefinition) beanDefinition).validate();
		}
		catch (BeanDefinitionValidationException ex) {
			throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
					"Validation of bean definition failed", ex);
		}
	}

	// 看BeanDefinition 是否已被注册
	BeanDefinition existingDefinition = this.beanDefinitionMap.get(beanName);
	if (existingDefinition != null) {
	 	// 如果已经被注册，看BeanDefinition是否允许被覆盖，spring2.1被人为改成false
		if (!isAllowBeanDefinitionOverriding()) {
			throw new BeanDefinitionStoreException(beanDefinition.getResourceDescription(), beanName,
					"Cannot register bean definition [" + beanDefinition + "] for bean '" + beanName +
					"': There is already [" + existingDefinition + "] bound.");
		}
		// 看角色是否相等，角色其实并没有具体含义
		else if (existingDefinition.getRole() < beanDefinition.getRole()) {
			// e.g. was ROLE_APPLICATION, now overriding with ROLE_SUPPORT or ROLE_INFRASTRUCTURE
			if (logger.isWarnEnabled()) {
				logger.warn("Overriding user-defined bean definition for bean '" + beanName +
						"' with a framework-generated bean definition: replacing [" +
						existingDefinition + "] with [" + beanDefinition + "]");
			}
		}
		else if (!beanDefinition.equals(existingDefinition)) {
			if (logger.isInfoEnabled()) {
				logger.info("Overriding bean definition for bean '" + beanName +
						"' with a different definition: replacing [" + existingDefinition +
						"] with [" + beanDefinition + "]");
			}
		}
		else {
			if (logger.isDebugEnabled()) {
				logger.debug("Overriding bean definition for bean '" + beanName +
						"' with an equivalent definition: replacing [" + existingDefinition +
						"] with [" + beanDefinition + "]");
			}
		}
		// put进去
		this.beanDefinitionMap.put(beanName, beanDefinition);
	}
	else { // bean不存在
		if (hasBeanCreationStarted()) { // Bean是否已经开始创建？
			// Cannot modify startup-time collection elements anymore (for stable iteration)
			synchronized (this.beanDefinitionMap) {
				this.beanDefinitionMap.put(beanName, beanDefinition);
				List<String> updatedDefinitions = new ArrayList<String>(this.beanDefinitionNames.size() + 1);
				updatedDefinitions.addAll(this.beanDefinitionNames);
				updatedDefinitions.add(beanName);
				this.beanDefinitionNames = updatedDefinitions;
				if (this.manualSingletonNames.contains(beanName)) {
					Set<String> updatedSingletons = new LinkedHashSet<String>(this.manualSingletonNames);
					updatedSingletons.remove(beanName);
					this.manualSingletonNames = updatedSingletons;
				}
			}
		}
		else { // 存beanDefinition信息，使用map存，所以name是唯一的。
			// Still in startup registration phase
			this.beanDefinitionMap.put(beanName, beanDefinition); // 无序
			this.beanDefinitionNames.add(beanName); // 有序，bean按顺序初始化
			this.manualSingletonNames.remove(beanName);
		}
		this.frozenBeanDefinitionNames = null;
	}

	if (existingDefinition != null || containsSingleton(beanName)) {
		resetBeanDefinition(beanName);
	}
}

2、外部化配置的单体对象

外部化配置的单体对象无生命周期管理、无法实现延迟初始化Bean。

它是由SingletonBeanRegistry接口进行注册的，SingletonBeanRegistry接口实现了对singletonBean的处理，其默认实现是DefaultListableBeanFactory。

public interface SingletonBeanRegistry {

	void registerSingleton(String beanName, Object singletonObject);

	Object getSingleton(String beanName);

	boolean containsSingleton(String beanName);

	String[] getSingletonNames();

	Object getSingletonMutex();

}

我们来看registerSingleton方法的实现：

// org.springframework.beans.factory.support.DefaultListableBeanFactory#registerSingleton
@Override
public void registerSingleton(String beanName, Object singletonObject) throws IllegalStateException {
	super.registerSingleton(beanName, singletonObject); // 调用父类的注册

	if (hasBeanCreationStarted()) {
		// Cannot modify startup-time collection elements anymore (for stable iteration)
		synchronized (this.beanDefinitionMap) {
			if (!this.beanDefinitionMap.containsKey(beanName)) {
				Set<String> updatedSingletons = new LinkedHashSet<String>(this.manualSingletonNames.size() + 1);
				updatedSingletons.addAll(this.manualSingletonNames);
				updatedSingletons.add(beanName);
				this.manualSingletonNames = updatedSingletons;
			}
		}
	}
	else {
		// Still in startup registration phase
		if (!this.beanDefinitionMap.containsKey(beanName)) {
			this.manualSingletonNames.add(beanName);
		}
	}

	clearByTypeCache();
}

// org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#registerSingleton
@Override
public void registerSingleton(String beanName, Object singletonObject) throws IllegalStateException {
	Assert.notNull(beanName, "'beanName' must not be null");
	synchronized (this.singletonObjects) {
		Object oldObject = this.singletonObjects.get(beanName);
		if (oldObject != null) {
			throw new IllegalStateException("Could not register object [" + singletonObject +
					"] under bean name '" + beanName + "': there is already object [" + oldObject + "] bound");
		}
		addSingleton(beanName, singletonObject);
	}
}

// org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#addSingleton
protected void addSingleton(String beanName, Object singletonObject) {
	synchronized (this.singletonObjects) { // 加锁，可重入
		this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
		this.singletonFactories.remove(beanName);
		this.earlySingletonObjects.remove(beanName);
		this.registeredSingletons.add(beanName);
	}
}

我们再来看getBean的实现，我们发现，如果获取Singleton Bean获取到了，就不会走下面的逻辑了。如果没有获取到，就会获取普通的BeanDefination逻辑，这个逻辑会有一个将BeanDefination注册为Bean的逻辑，而Singleton Bean没有：

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
		@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {

	final String beanName = transformedBeanName(name);
	Object bean;

	// Eagerly check singleton cache for manually registered singletons.
	Object sharedInstance = getSingleton(beanName); // 获取Singleton Bean
	if (sharedInstance != null && args == null) {
		if (logger.isTraceEnabled()) {
			if (isSingletonCurrentlyInCreation(beanName)) {
				logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
						"' that is not fully initialized yet - a consequence of a circular reference");
			}
			else {
				logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
			}
		}
		bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
	}
	... 

3、非Spring容器管理的对象——ResolvableDependency

通过：ConfigurableListableBeanFactory#registerResolvableDependency 进行注册。

这种对象无生命周期管理、无法实现延迟初始化Bean、无法通过依赖查找。

import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.beans.factory.config.AutowireCapableBeanFactory;
import org.springframework.beans.factory.config.ConfigurableListableBeanFactory;
import org.springframework.context.annotation.AnnotationConfigApplicationContext;

import javax.annotation.PostConstruct;

/**
 * ResolvableDependency 作为依赖来源
 */
public class ResolvableDependencySourceDemo {

    @Autowired
    private String value;

    @PostConstruct
    public void init() {
        System.out.println(value);
    }

    public static void main(String[] args) {

        // 创建 BeanFactory 容器
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();

        // 注册 Configuration Class（配置类） -> Spring Bean
        applicationContext.register(ResolvableDependencySourceDemo.class);

        applicationContext.addBeanFactoryPostProcessor(beanFactory -> {
            // 注册 Resolvable Dependency
            beanFactory.registerResolvableDependency(String.class, "Hello,World");
        });

        // 启动 Spring 应用上下文，会有注册BeanPostProcessor的回调
        applicationContext.refresh();

        // 显示地关闭 Spring 应用上下文
        applicationContext.close();
    }

}

四、外部化配置作为依赖来源

1、实例

使用外部化的配置文件作为依赖来源，获取依赖对象时，只能通过@Value获取。

@Configuration
@PropertySource(value = "META-INF/default.properties",encoding="UTF-8")
public class ExternalConfigurationDependencySourceDemo {

    @Value("${user.id:-1}")
    private Long id;

    @Value("${usr.name}")
    private String name;

    @Value("${user.resource:classpath://default.properties}")
    private Resource resource;

    public static void main(String[] args) {

        // 创建 BeanFactory 容器
        AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext();
        // 注册 Configuration Class（配置类） -> Spring Bean
        applicationContext.register(ExternalConfigurationDependencySourceDemo.class);

        // 启动 Spring 应用上下文
        applicationContext.refresh();

        // 依赖查找 ExternalConfigurationDependencySourceDemo Bean
        ExternalConfigurationDependencySourceDemo demo = applicationContext.getBean(ExternalConfigurationDependencySourceDemo.class);

        System.out.println("demo.id = " + demo.id);
        System.out.println("demo.name = " + demo.name);
        System.out.println("demo.resource = " + demo.resource);

        // 显示地关闭 Spring 应用上下文
        applicationContext.close();
    }
}

2、原理

AutowiredAnnotationBeanPostProcessor也会处理@Value注解：

public AutowiredAnnotationBeanPostProcessor() {
	this.autowiredAnnotationTypes.add(Autowired.class);
	this.autowiredAnnotationTypes.add(Value.class);
	try {
		this.autowiredAnnotationTypes.add((Class<? extends Annotation>)
				ClassUtils.forName("javax.inject.Inject", AutowiredAnnotationBeanPostProcessor.class.getClassLoader()));
		logger.info("JSR-330 'javax.inject.Inject' annotation found and supported for autowiring");
	}
	catch (ClassNotFoundException ex) {
		// JSR-330 API not available - simply skip.
	}
}

当进行依赖注入时：

// org.springframework.beans.factory.support.DefaultListableBeanFactory#doResolveDependency
public Object doResolveDependency(DependencyDescriptor descriptor, String beanName,
		Set<String> autowiredBeanNames, TypeConverter typeConverter) throws BeansException {

	InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
	try {
		Object shortcut = descriptor.resolveShortcut(this);
		if (shortcut != null) {
			return shortcut;
		}

		Class<?> type = descriptor.getDependencyType();
		// 此处的value值，就是@Value里面的表达式：${user.id}
		Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
		if (value != null) {
			if (value instanceof String) {
				// 获取配置文件中的值
				String strVal = resolveEmbeddedValue((String) value);
				BeanDefinition bd = (beanName != null && containsBean(beanName) ? getMergedBeanDefinition(beanName) : null);
				value = evaluateBeanDefinitionString(strVal, bd);
			}

// org.springframework.beans.factory.support.AbstractBeanFactory#resolveEmbeddedValue
@Override
public String resolveEmbeddedValue(String value) {
	if (value == null) {
		return null;
	}
	String result = value;
	// 此处的StringValueResolver ，是可以通过EmbeddedValueResolverAware回调的
	for (StringValueResolver resolver : this.embeddedValueResolvers) {
		result = resolver.resolveStringValue(result);
		if (result == null) {
			return null;
		}
	}
	return result;
}

我们都知道，spring的aware接口都是一些回调接口：

public interface EmbeddedValueResolverAware extends Aware {

	/**
	 * Set the StringValueResolver to use for resolving embedded definition values.
	 */
	void setEmbeddedValueResolver(StringValueResolver resolver);

}