在生成ApplicationContext过程中，AbstractRefreshableApplicationContext.refreshBeanFactory()完成旧BeanFactory关闭，创建新的BeanFactory，即new DefaultListableBeanFactory(…)。然后bean实例化时调用本类的preInstantiateSingletons方法。
类作用
BeanFactory作用是初始化和管理普通bean，因此DefaultListableBeanFactory是实现框架核心功能（IoC)的核心类。
类关系图
DefaultListableBeanFactory类关系图比较复杂，我们从顶向下梳理：
接口定义说明
AliasRegistry：顶层接口，别名注册表接口，定义了管理bean的别名和名称映射关系的基本方法，如注册registerAlias(String name, String alias)、获取（getAliases(String name)）和销毁（removeAlias(String alias)）。
SingletonBeanRegistry：顶层接口，单实例bean注册表接口，定义了管理单实例Bean的基本方法，如注册（registerSingleton(String beanName, Object singletonObject)）和获取（getSingleton(String beanName)。
BeanFactory：顶层接口，定义了访问bean容器中bean的基本方法，只包括get或判断（如is、contain）类方法。完全不包括BeanFactory是如何初始化并管理bean的方法。从使用者来说，只是按需使用即可，不关注如何实现。
HierarchicalBeanFactory：继承BeanFactory，仅提供访问继承关系BeanFactory的方法，包括etParentBeanFactory()和containsLocalBean(String name)（当前BeanFactory（不是父BeanFactory）是否包含名为name的bean)。
ListableBeanFactory：是BeanFactory接口扩展，定义通过枚举其所有bean实例的方法来访问bean，而不是通过getBean(String beanName)这种单个按名称查找bean。
AutowireCapableBeanFactory：是BeanFactory接口扩展，定义具备自动装配能力的BeanFactory的实现方法。
ConfigurableBeanFactory：定义普通配置模式下BeanFactory的实现方法，如解析bean配置和bean的初始化。
ConfigurableListableBeanFactory：继承了接口ListableBeanFactory, AutowireCapableBeanFactory, ConfigurableBeanFactory，并增加定义了分析和修改bean定义以及预实例化单实例bean的方法。
BeanDefinitionRegistry：BeanDefinition注册表接口，定义对BeanDefinition（即bean的元数据）进行管理的方法。
SimpleAliasRegistry
实现接口AliasRegistry（bean alias存取及相关操作）。
DefaultSingletonBeanRegistry
主要实现单实例bean的缓存池，包括bean依赖关系管理（关系注册和销毁）。本类方法大量采用了synchronized存取缓存池（集合）。
继承于SimpleAliasRegistry，并实现接口SingletonBeanRegistry（单实例bean的存取及相关操作）
FactoryBeanRegistrySupport
单实例bean管理中用到的关于factoryBean的相关方法实现。
AbstractBeanFactory
AbstractBeanFactory主要实现ConfigurableBeanFactory接口功能。BeanFactory大多数功能在该类实现，但核心方法createBean需在子类实现。
AbstractAutowireCapableBeanFactory
继承AbstractBeanFactory，并实现接口AutowireCapableBeanFactory，即自动装配相关功能在该类实现。
核心方法createBean在该类实现。
DefaultListableBeanFactory
继承AbstractAutowireCapableBeanFactory，主要实现接口ConfigurableListableBeanFactory, BeanDefinitionRegistry, Serializable。其中ConfigurableListableBeanFactory接口中的AutowireCapableBeanFactory, ConfigurableBeanFactory均在祖先类实现了，因此只需实现ListableBeanFactory接口方法。BeanDefinitionRegistry接口全部在这里实现。
类实现
DefaultListableBeanFactory继承于AbstractAutowireCapableBeanFactory，大多数bean的创建、初始化等实现已在祖先类实现。本类主要实现BeanDefinitionRegistry接口和ListableBeanFactory接口。
DefaultListableBeanFactory(@Nullable BeanFactory parentBeanFactory)
DefaultListableBeanFactory构建类，实际构建在父类AbstractAutowireCapableBeanFactory完成，核心动作是动态生成子类的对象instantiationStrategy（CglibSubclassingInstantiationStrategy 用cgilib动态创建子类）。
DefaultListableBeanFactory构建代码：
public DefaultListableBeanFactory(@Nullable BeanFactory parentBeanFactory) {
super(parentBeanFactory);
}
父类AbstractAutowireCapableBeanFactory构建代码：
public AbstractAutowireCapableBeanFactory(@Nullable BeanFactory parentBeanFactory) {
this();
setParentBeanFactory(parentBeanFactory);
}
public AbstractAutowireCapableBeanFactory() {
super();
ignoreDependencyInterface(BeanNameAware.class);
ignoreDependencyInterface(BeanFactoryAware.class);
ignoreDependencyInterface(BeanClassLoaderAware.class);
//bean实例化器
this.instantiationStrategy = new CglibSubclassingInstantiationStrategy();
}
注1：CglibSubclassingInstantiationStrategy是Bean真正初始化类，详见https://blog.csdn.net/davidwkx/article/details/131053503
preInstantiateSingletons
该方法是启动过程中spring对单实例bean初始化的入口。
@Override
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
//所有bean定义都统一成RootBeanDefinition 注1
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
//非抽象类、非延迟加载且是Singleton的bean进行初始化
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//FactoryBean初始化
if (isFactoryBean(beanName)) {
//getBean通过有无前缀FACTORY_BEAN_PREFIX 识别是否FactoryBean 注2
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
//FactoryBean中bean默认是延迟创建的，如果FactoryBean实现了SmartFactoryBean，重载isEagerInit()返回true，则表示立即对FactoryBean中的bean进行创建 注3
if (bean instanceof SmartFactoryBean<?> smartFactoryBean && smartFactoryBean.isEagerInit()) {
//FactoryBean创建bean
getBean(beanName);
}
}
//普通bean初始化
else {
getBean(beanName);
}
}
}
//如果bean实现了SmartInitializingSingleton接口，则在初始化后回调afterSingletonsInstantiated
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton smartSingleton) {
StartupStep smartInitialize = this.getApplicationStartup().start("spring.beans.smart-initialize")
.tag("beanName", beanName);
smartSingleton.afterSingletonsInstantiated();
smartInitialize.end();
}
}
}
注1：bean定义相关见：
注2：getBean(beanName)在AbstractBeanFactory实现，实际是调用AbstractBeanFactory.doGetBean(beanName, null, null, false)，详见：https://blog.csdn.net/davidwkx/article/details/130963899
注3：如果beanName是FactoryBean的名称，getBean(“&”+beanName) 是获取 FactoryBean 本身，而getBean(beanName) 获取的是 FactoryBean 创建出来的 bean 实例。文章来源：https://www.toymoban.com/news/detail-479367.html
resolveDependency / doResolveDependency
解析指定beanName对应的属性，并将解析到的属性名放入autowiredBeanNames，根据类型查找依赖关系文章来源地址https://www.toymoban.com/news/detail-479367.html
@Override
@Nullable
public Object resolveDependency(DependencyDescriptor descriptor, @Nullable String requestingBeanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
descriptor.initParameterNameDiscovery(getParameterNameDiscoverer());
if (Optional.class == descriptor.getDependencyType()) {
// Optional类型的注入处理
return createOptionalDependency(descriptor, requestingBeanName);
}
else if (ObjectFactory.class == descriptor.getDependencyType() ||
ObjectProvider.class == descriptor.getDependencyType()) {
// ObjectFactory类型的注入处理
return new DependencyObjectProvider(descriptor, requestingBeanName);
}
else if (javaxInjectProviderClass == descriptor.getDependencyType()) {
// Jsr330注解的注入处理
return new Jsr330Factory().createDependencyProvider(descriptor, requestingBeanName);
}
else {
Object result = getAutowireCandidateResolver().getLazyResolutionProxyIfNecessary(
descriptor, requestingBeanName);
if (result == null) {
//非延迟加载的常规注入处理
result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
}
return result;
}
}
@Nullable
public Object doResolveDependency(DependencyDescriptor descriptor, @Nullable String beanName,
@Nullable Set<String> autowiredBeanNames, @Nullable TypeConverter typeConverter) throws BeansException {
InjectionPoint previousInjectionPoint = ConstructorResolver.setCurrentInjectionPoint(descriptor);
try {
Object shortcut = descriptor.resolveShortcut(this);
if (shortcut != null) {
return shortcut;
}
// @Value注解的注入
Class<?> type = descriptor.getDependencyType();
Object value = getAutowireCandidateResolver().getSuggestedValue(descriptor);
if (value != null) {
if (value instanceof String strValue) {
// 占位符解析
String resolvedValue = resolveEmbeddedValue(strValue);
BeanDefinition bd = (beanName != null && containsBean(beanName) ?
getMergedBeanDefinition(beanName) : null);
// EL表达式
value = evaluateBeanDefinitionString(resolvedValue, bd);
}
// 转换类型
TypeConverter converter = (typeConverter != null ? typeConverter : getTypeConverter());
try {
return converter.convertIfNecessary(value, type, descriptor.getTypeDescriptor());
}
catch (UnsupportedOperationException ex) {
// A custom TypeConverter which does not support TypeDescriptor resolution...
return (descriptor.getField() != null ?
converter.convertIfNecessary(value, type, descriptor.getField()) :
converter.convertIfNecessary(value, type, descriptor.getMethodParameter()));
}
}
//解析多个bean，例如List、set、Map
Object multipleBeans = resolveMultipleBeans(descriptor, beanName, autowiredBeanNames, typeConverter);
if (multipleBeans != null) {
return multipleBeans;
}
// 依赖查询
Map<String, Object> matchingBeans = findAutowireCandidates(beanName, type, descriptor);
if (matchingBeans.isEmpty()) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
return null;
}
String autowiredBeanName;
Object instanceCandidate;
// 如果有多个匹配的Bean，就根据方法名称进行匹配
if (matchingBeans.size() > 1) {
autowiredBeanName = determineAutowireCandidate(matchingBeans, descriptor);
if (autowiredBeanName == null) {
if (isRequired(descriptor) || !indicatesMultipleBeans(type)) {
return descriptor.resolveNotUnique(descriptor.getResolvableType(), matchingBeans);
}
else {
// In case of an optional Collection/Map, silently ignore a non-unique case:
// possibly it was meant to be an empty collection of multiple regular beans
// (before 4.3 in particular when we didn't even look for collection beans).
return null;
}
}
instanceCandidate = matchingBeans.get(autowiredBeanName);
}
else {
// We have exactly one match.
Map.Entry<String, Object> entry = matchingBeans.entrySet().iterator().next();
autowiredBeanName = entry.getKey();
instanceCandidate = entry.getValue();
}
if (autowiredBeanNames != null) {
autowiredBeanNames.add(autowiredBeanName);
}
if (instanceCandidate instanceof Class) {
instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
}
Object result = instanceCandidate;
if (result instanceof NullBean) {
if (isRequired(descriptor)) {
raiseNoMatchingBeanFound(type, descriptor.getResolvableType(), descriptor);
}
result = null;
}
if (!ClassUtils.isAssignableValue(type, result)) {
throw new BeanNotOfRequiredTypeException(autowiredBeanName, type, instanceCandidate.getClass());
}
return result;
}
finally {
ConstructorResolver.setCurrentInjectionPoint(previousInjectionPoint);
}
}
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