随着spring功能的扩充，以及spring封装了很多底层实现的细节，使得我们在学习spring的源码的时候经常会比较困惑，本节开始研究spring容器启动的相关过程，暂时先从整体大流程上分析，比较细节的部分后续再补充说明，spring的版本为spring-framework-5.1.15.RELEASE，我们先从后台spring应用开始说明，web相关后续补充。
以ClassPathXmlApplicationContext启动说明，其类继承结构如下：
当我们实例化一个 ClassPathXmlApplicationContext的时候，实际上最终会调用这个方法：
public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}
而这里面refresh方法就是启动spring容器的方法，最终是通过：
AbstractAppliationContext.refresh实现
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
//获取新的beanFactory，销毁原来的beanFactory，
// 通过AbstractRefreshableApplicationContext获取到 DefaultListableBeanFactory，
// 在这一步获取到beanFactory的时候回调用 loadBeanDefinitions，
// AbstractRefreshableApplicationContext并没有实现它，模板方法，子类自己实现逻辑
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
// 对beanFactory进行一些配置
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
//模板方法，子类继承实现,对beanFactory的后置处理
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
// 调用注册实现了BeanFactoryPostProcessor接口类的postProcessBeanDefinitionRegistry方法，
// 主要有是两个类 BeanFactoryPostProcessor和BeanDefinitionRegistryPostProcessor，
// 会通过beanFactory.getBean方法实例化BeanFactoryPostProcessor，然后执行postProcessBeanDefinitionRegistry
// 这一步处理的时候实际上BeanFactory已经准备好了，因此如果想往beanFactory中增加bean可以在这个地方进行处理，
// 如ConfigurationClassPostProcessor就是实现了BeanDefinitionRegistryPostProcessor接口，在这里将相关bean加入到了IOC容器中
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
//向beanFactory中注册实现了BeanPostProcessor的bean
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
//初始化国际化工具类
initMessageSource();
// Initialize event multicaster for this context.
//初始化事件广播器
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
//模板方法，子类自己去实现相应逻辑
onRefresh();
// Check for listener beans and register them.
// 注册监听器
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
//初始化所有的单例bean，不包含lazy-init
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 {
resetCommonCaches();
}
}
}
如上是refresh的实现逻辑步骤，我们一项一项分析
首先是：
（1）刷新容器前的准备工作prepareRefresh
AbstractApplicationContext.prepareRefresh启动容器前的一些准备工作，包含设置状态位，验证一些必要的属性是否存在等
具体方法如下：
/**
* Prepare this context for refreshing, setting its startup date and
* active flag as well as performing any initialization of property sources.
*/
protected void prepareRefresh() {
// Switch to active.
this.startupDate = System.currentTimeMillis();
this.closed.set(false);
this.active.set(true);
if (logger.isDebugEnabled()) {
if (logger.isTraceEnabled()) {
logger.trace("Refreshing " + this);
}
else {
logger.debug("Refreshing " + getDisplayName());
}
}
// Initialize any placeholder property sources in the context environment.
// 初始化任意属性，子类可以实现该方法
initPropertySources();
// Validate that all properties marked as required are resolvable:
// see ConfigurablePropertyResolver#setRequiredProperties
getEnvironment().validateRequiredProperties();
// Store pre-refresh ApplicationListeners...
if (this.earlyApplicationListeners == null) {
this.earlyApplicationListeners = new LinkedHashSet<>(this.applicationListeners);
}
else {
// Reset local application listeners to pre-refresh state.
this.applicationListeners.clear();
this.applicationListeners.addAll(this.earlyApplicationListeners);
}
// Allow for the collection of early ApplicationEvents,
// to be published once the multicaster is available...
this.earlyApplicationEvents = new LinkedHashSet<>();
}
其中initPropertySources如下三个类中有实现，基本上都是web相关
准备工作完成之后，接下来就开始实例化BeanFactory：
(2)实例化BeanFactory，obtainFreshBeanFactory
AbstractApplicationContext.obtainFreshBeanFactory();
其实现在
AbstractRefreshableApplicationContext.refreshBeanFactory
中，其代码逻辑如下：
protected final void refreshBeanFactory() throws BeansException {
//如果已经有了BeanFactory先销毁关闭它
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
//直接new 了一个 DefaultListableBeanFactory，
// 并且在其继承父类中 AbstractAutowireCapableBeanFactory配置忽略自动装配的配置
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
//这是重要的步骤，加载所有bean的配置，这是一个模板方法，由具体子类去实现
loadBeanDefinitions(beanFactory);
synchronized (this.beanFactoryMonitor) {
this.beanFactory = beanFactory;
}
}
catch (IOException ex) {
throw new ApplicationContextException("I/O error parsing bean definition source for " + getDisplayName(), ex);
}
}
可以看到，主要做了如下事情：
（1）关闭销毁如果已经存在的BeanFactory
（2）实例化一个新的beanFactory实例（DefaultListableBeanFactory）以及将之前的一些属性赋值到新的beanFactory上
（3）加载bean的配置，这是最重要的一步
loadBeanDefinitions是一个模板方法，子类实现，通过idea，我们发现有如下几种实现方式，
这块后面再详细描述。
这样通过obtainFreshBeanFactory我们获得了一个beanFactory，并且已经加载了所有bean的信息，下面是对beanFactory进行一些前置处理：
（3）beanFactory的前置准备工作，prepareBeanFactory
AbstractApplicationContext.prepareBeanFactory
其代码逻辑如下：
protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
// Tell the internal bean factory to use the context's class loader etc.
// 设置beanFactory的类加载器
beanFactory.setBeanClassLoader(getClassLoader());
// 设置EL表达式处理类，bean在初始化后填充属性会用到
beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
//设置属性表达式处理类
beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));
// Configure the bean factory with context callbacks.
beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
// 设置忽略自动装配的bean，当bean实现这些接口，不要Autowired自动装配
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.
// 会将下面 beanFactory（DefaultListableBeanFactory）和
// 当前类（可以理解为我们常用的ApplicationContext）放入到 DefaultListableBeanFactory的resolvableDependencies 这个map中
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.
// 这里就是对AspectJ的支持，LOAD_TIME_WEAVER 类加载时候编织
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());
}
}
可以看到，这块主要是对beanFactory做一些前置处理为了接下来加载实例化做准备。
接下来是beanFactory准备工作的后置处理，
(4)beanFactory准备工作的后置处理，postProcessBeanFactory
AbstractApplicationContext.postProcessBeanFactory
这是一个模板方法，由子类自己去实现
当前ClasspathXmlApplicationContext没有相关实现，可以发现一般web相关容器会实现该方法，后续研究web相关容器我们在讨论。
到这一步，我们的beanFactory基本上就准备好了，当beanFactory准备好了之后，我们需要进行beanFactory准备好之后的后置方法，这样供相关子类在这个阶段：在beanFactory准备好了之后但是还没开始任何bean实例初始化的时候进行一些操作。
(5)执行BeanDefinitionRegistryPostProcessor.postProcessBeanDefinitionRegistry和BeanFactoryPostProcessor.postProcessBeanFactory方法，通过invokeBeanFactoryPostProcessors
AbstractApplicationContext. invokeBeanFactoryPostProcessors
具体代码实现如下：
protected void invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory beanFactory) {
PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());
// Detect a LoadTimeWeaver and prepare for weaving, if found in the meantime
// (e.g. through an @Bean method registered by ConfigurationClassPostProcessor)
if (beanFactory.getTempClassLoader() == null && beanFactory.containsBean(LOAD_TIME_WEAVER_BEAN_NAME)) {
beanFactory.addBeanPostProcessor(new LoadTimeWeaverAwareProcessor(beanFactory));
beanFactory.setTempClassLoader(new ContextTypeMatchClassLoader(beanFactory.getBeanClassLoader()));
}
}
主要分为两步：
（1）调用PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors来处理beanFactoryPostProcessor相关逻辑
（2）织入AspectJ相关处理
这里加入的LoadTimeWeaverAwareProcessor后置处理器，只处理LoadTimeWeaverAware类型bean，而实现这个接口的只有如下几个类：
这块后面讲AspectJ我们在细谈。
在PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());对两类BeanFactoryPostProcessor和BeanDefinitionRegistryPostProcessor进行处理，首先会获取自定义添加的
public List<BeanFactoryPostProcessor> getBeanFactoryPostProcessors() {
return this.beanFactoryPostProcessors;
}
可以发现，我们可以通过调用:
public void addBeanFactoryPostProcessor(BeanFactoryPostProcessor postProcessor) {
Assert.notNull(postProcessor, "BeanFactoryPostProcessor must not be null");
this.beanFactoryPostProcessors.add(postProcessor);
}
可以手动添加BeanFactoryPostProcessor.
在
PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());中，首先会处理自定义添加的：
实现了BeanFactoryPostProcessor接口的postProcessBeanDefinitionRegistry方法。然后在从容器中获取实现了这个接口bean并调用postProcessBeanDefinitionRegistry。执行完这个处理后，接下来会处理BeanFactoryPostProcessor.postProcessBeanFactory。这两步处理中，如果对应的bean没有实例化，通过getBean实例化Bean，但是自定义的添加进去的是已经实例化后的bean。
(6)注册BeanPostProcessor，通过registerBeanPostProcessors
AbstractApplicationContext.registerBeanPostProcessors
将容器中BeanPostProcessor类型的bean的信息从beanFactory中获取到并通过getBean(String name, Class<T> requiredType)方法实例化这些Bean，然后将其注册到beanFactory中，这里加入的都是BeanPostProcessor的实例。
private static void registerBeanPostProcessors(
ConfigurableListableBeanFactory beanFactory, List<BeanPostProcessor> postProcessors) {
for (BeanPostProcessor postProcessor : postProcessors) {
beanFactory.addBeanPostProcessor(postProcessor);
}
}
(7)初始化国际化相关工具类，通过registerBeanPostProcessors实现
AbstractApplicationContext.initMessageSource
初始化国际化相关信息
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.isTraceEnabled()) {
logger.trace("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.isTraceEnabled()) {
logger.trace("No '" + MESSAGE_SOURCE_BEAN_NAME + "' bean, using [" + this.messageSource + "]");
}
}
}
（8）初始化事件广播器
AbstractApplicationContext.initApplicationEventMulticaster
初始化事件广播器
protected void initApplicationEventMulticaster() {
ConfigurableListableBeanFactory beanFactory = getBeanFactory();
if (beanFactory.containsLocalBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME)) {
this.applicationEventMulticaster =
beanFactory.getBean(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, ApplicationEventMulticaster.class);
if (logger.isTraceEnabled()) {
logger.trace("Using ApplicationEventMulticaster [" + this.applicationEventMulticaster + "]");
}
}
else {
this.applicationEventMulticaster = new SimpleApplicationEventMulticaster(beanFactory);
beanFactory.registerSingleton(APPLICATION_EVENT_MULTICASTER_BEAN_NAME, this.applicationEventMulticaster);
if (logger.isTraceEnabled()) {
logger.trace("No '" + APPLICATION_EVENT_MULTICASTER_BEAN_NAME + "' bean, using " +
"[" + this.applicationEventMulticaster.getClass().getSimpleName() + "]");
}
}
}
可以看到如果没有当前beanFactory中不包含名为applicationEventMulticaster的bean，则会默认注入SimpleApplicationEventMulticaster当做当前applicationContext的事件广播器
（9）执行模板方法onRefresh
AbstractApplicationContext.onRefresh
这是一个模板方法，当前类是一个空的实现，子类可以复写这块的逻辑，可以看到实际实现：
按照spring官方给出的注释：
Initialize other special beans in specific context subclasses.
可以在这个方法中实例化某个自定义的applicationContext中特殊的bean的处理，我们也可以看到，这块主要是web相关的application在应用，我们在讲解web相关处理的时候回详细说明。
（10）注册监听器，registerListeners
AbstractApplicationContext.registerListeners
注册监听器
在（8）步的时候，我们初始化了事件广播器，这里我们将注册的ApplicationListener加入到（8）步中注册的ApplicationEventMulticaster中去，这样后续当ApplicationEventMulticaster收到相关的事件的时候，通过调用SimpleApplicationEventMulticaster.multicastEvent继而调用SimpleApplicationEventMulticaster.invokeListener来通知监听器监听相关事件（这是以默认的SimpleApplicationEventMulticaster举例说明）
（11）初始化所有的单例bean（不包含lazy-init)，finishBeanFactoryInitialization
AbstractApplicationContext.finishBeanFactoryInitialization
这一步就开始初始化我们自定义的相关bean，实现如下：
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
}
// Register a default embedded value resolver if no bean post-processor
// (such as a PropertyPlaceholderConfigurer bean) registered any before:
// at this point, primarily for resolution in annotation attribute values.
if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
}
// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// Stop using the temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(null);
// Allow for caching all bean definition metadata, not expecting further changes.
beanFactory.freezeConfiguration();
// Instantiate all remaining (non-lazy-init) singletons.
beanFactory.preInstantiateSingletons();
}
- 首先是设置当前applicationContext的
ConversionService.ConversionService主要是用来类型转换。
public interface ConversionService {
boolean canConvert(@Nullable Class<?> sourceType, Class<?> targetType);
boolean canConvert(@Nullable TypeDescriptor sourceType, TypeDescriptor targetType);
<T> T convert(@Nullable Object source, Class<T> targetType);
Object convert(@Nullable Object source, @Nullable TypeDescriptor sourceType, TypeDescriptor targetType);
}
其默认实现DefaultConversionService继承如下：
- 注册默认的值替换器，
比如我们经常在属性上进行值注入：
@Value("${name}")
private String name;
这里注册默认的值解决类。
- 实例化Aspect相关类
- 缓存当前bean的配置，并冻结配置不允许更改
- 实例化剩下的单例，通过
DeaultListableBeanFactory.preInstantiateSingletons来预实例化单例
我们可以看下DeaultListableBeanFactory.preInstantiateSingletons的实现。
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) {
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean) {
final FactoryBean<?> factory = (FactoryBean<?>) bean;
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged((PrivilegedAction<Boolean>)
((SmartFactoryBean<?>) factory)::isEagerInit,
getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
}
else {
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton) {
final SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
smartSingleton.afterSingletonsInstantiated();
}
}
}
}
主要是两步，如果bean对应的是一个OjectFactory类型，则通过getBean获取实例，如果ObjectFactory是SmartFactoryBean且isEagerInit为true时，提前实例化对应单例。
如果是普通单例，直接通过getBean获取。
getBean这个方法后面会着重描述
(12)结束容器刷新，执行相关方法，finishRefresh
protected void finishRefresh() {
// Clear context-level resource caches (such as ASM metadata from scanning).
clearResourceCaches();
// Initialize lifecycle processor for this context.
initLifecycleProcessor();
// Propagate refresh to lifecycle processor first.
getLifecycleProcessor().onRefresh();
// Publish the final event.
publishEvent(new ContextRefreshedEvent(this));
// Participate in LiveBeansView MBean, if active.
LiveBeansView.registerApplicationContext(this);
}
首先是清空相关资源的缓存。
然后初始化运行周期相关处理类，默认使用DefaultLifecycleProcessor
初始化生命周期处理类之后，执行onRefresh生命周期相关方法
发布容器刷新完成事件消息
如果开启了JMX，执行JMX相关处理。
至此，一个spring容器启动成功。