隨着spring功能的擴充，以及spring封裝了很多底層實現的細節，使得我們在學習spring的源碼的時候經常會比較困惑，本節開始研究spring容器啓動的相關過程，暫時先從整體大流程上分析，比較細節的部分後續再補充說明，spring的版本爲spring-framework-5.1.15.RELEASE，我們先從後臺spring應用開始說明，web相關後續補充。
以ClassPathXmlApplicationContext啓動說明，其類繼承結構如下：
當我們實例化一個 ClassPathXmlApplicationContext的時候，實際上最終會調用這個方法：
public ClassPathXmlApplicationContext(
String[] configLocations, boolean refresh, @Nullable ApplicationContext parent)
throws BeansException {
super(parent);
setConfigLocations(configLocations);
if (refresh) {
refresh();
}
}
而這裏面refresh方法就是啓動spring容器的方法，最終是通過：
AbstractAppliationContext.refresh實現
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
//獲取新的beanFactory，銷燬原來的beanFactory，
// 通過AbstractRefreshableApplicationContext獲取到 DefaultListableBeanFactory，
// 在這一步獲取到beanFactory的時候回調用 loadBeanDefinitions，
// AbstractRefreshableApplicationContext並沒有實現它，模板方法，子類自己實現邏輯
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
// 對beanFactory進行一些配置
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
//模板方法，子類繼承實現,對beanFactory的後置處理
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
// 調用註冊實現了BeanFactoryPostProcessor接口類的postProcessBeanDefinitionRegistry方法，
// 主要有是兩個類 BeanFactoryPostProcessor和BeanDefinitionRegistryPostProcessor，
// 會通過beanFactory.getBean方法實例化BeanFactoryPostProcessor，然後執行postProcessBeanDefinitionRegistry
// 這一步處理的時候實際上BeanFactory已經準備好了，因此如果想往beanFactory中增加bean可以在這個地方進行處理，
// 如ConfigurationClassPostProcessor就是實現了BeanDefinitionRegistryPostProcessor接口，在這裏將相關bean加入到了IOC容器中
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
//向beanFactory中註冊實現了BeanPostProcessor的bean
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
//初始化國際化工具類
initMessageSource();
// Initialize event multicaster for this context.
//初始化事件廣播器
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
//模板方法，子類自己去實現相應邏輯
onRefresh();
// Check for listener beans and register them.
// 註冊監聽器
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
//初始化所有的單例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的實現邏輯步驟，我們一項一項分析
首先是：
（1）刷新容器前的準備工作prepareRefresh
AbstractApplicationContext.prepareRefresh啓動容器前的一些準備工作，包含設置狀態位，驗證一些必要的屬性是否存在等
具體方法如下：
/**
* 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.
// 初始化任意屬性，子類可以實現該方法
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如下三個類中有實現，基本上都是web相關
準備工作完成之後，接下來就開始實例化BeanFactory：
(2)實例化BeanFactory，obtainFreshBeanFactory
AbstractApplicationContext.obtainFreshBeanFactory();
其實現在
AbstractRefreshableApplicationContext.refreshBeanFactory
中，其代碼邏輯如下：
protected final void refreshBeanFactory() throws BeansException {
//如果已經有了BeanFactory先銷燬關閉它
if (hasBeanFactory()) {
destroyBeans();
closeBeanFactory();
}
try {
//直接new 了一個 DefaultListableBeanFactory，
// 並且在其繼承父類中 AbstractAutowireCapableBeanFactory配置忽略自動裝配的配置
DefaultListableBeanFactory beanFactory = createBeanFactory();
beanFactory.setSerializationId(getId());
customizeBeanFactory(beanFactory);
//這是重要的步驟，加載所有bean的配置，這是一個模板方法，由具體子類去實現
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）關閉銷燬如果已經存在的BeanFactory
（2）實例化一個新的beanFactory實例（DefaultListableBeanFactory）以及將之前的一些屬性賦值到新的beanFactory上
（3）加載bean的配置，這是最重要的一步
loadBeanDefinitions是一個模板方法，子類實現，通過idea，我們發現有如下幾種實現方式，
這塊後面再詳細描述。
這樣通過obtainFreshBeanFactory我們獲得了一個beanFactory，並且已經加載了所有bean的信息，下面是對beanFactory進行一些前置處理：
（3）beanFactory的前置準備工作，prepareBeanFactory
AbstractApplicationContext.prepareBeanFactory
其代碼邏輯如下：
protected void prepareBeanFactory(ConfigurableListableBeanFactory beanFactory) {
// Tell the internal bean factory to use the context's class loader etc.
// 設置beanFactory的類加載器
beanFactory.setBeanClassLoader(getClassLoader());
// 設置EL表達式處理類，bean在初始化後填充屬性會用到
beanFactory.setBeanExpressionResolver(new StandardBeanExpressionResolver(beanFactory.getBeanClassLoader()));
//設置屬性表達式處理類
beanFactory.addPropertyEditorRegistrar(new ResourceEditorRegistrar(this, getEnvironment()));
// Configure the bean factory with context callbacks.
beanFactory.addBeanPostProcessor(new ApplicationContextAwareProcessor(this));
// 設置忽略自動裝配的bean，當bean實現這些接口，不要Autowired自動裝配
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.
// 會將下面 beanFactory（DefaultListableBeanFactory）和
// 當前類（可以理解爲我們常用的ApplicationContext）放入到 DefaultListableBeanFactory的resolvableDependencies 這個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.
// 這裏就是對AspectJ的支持，LOAD_TIME_WEAVER 類加載時候編織
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，這塊主要是對beanFactory做一些前置處理爲了接下來加載實例化做準備。
接下來是beanFactory準備工作的後置處理，
(4)beanFactory準備工作的後置處理，postProcessBeanFactory
AbstractApplicationContext.postProcessBeanFactory
這是一個模板方法，由子類自己去實現
當前ClasspathXmlApplicationContext沒有相關實現，可以發現一般web相關容器會實現該方法，後續研究web相關容器我們在討論。
到這一步，我們的beanFactory基本上就準備好了，當beanFactory準備好了之後，我們需要進行beanFactory準備好之後的後置方法，這樣供相關子類在這個階段：在beanFactory準備好了之後但是還沒開始任何bean實例初始化的時候進行一些操作。
(5)執行BeanDefinitionRegistryPostProcessor.postProcessBeanDefinitionRegistry和BeanFactoryPostProcessor.postProcessBeanFactory方法，通過invokeBeanFactoryPostProcessors
AbstractApplicationContext. invokeBeanFactoryPostProcessors
具體代碼實現如下：
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()));
}
}
主要分爲兩步：
（1）調用PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors來處理beanFactoryPostProcessor相關邏輯
（2）織入AspectJ相關處理
這裏加入的LoadTimeWeaverAwareProcessor後置處理器，只處理LoadTimeWeaverAware類型bean，而實現這個接口的只有如下幾個類：
這塊後面講AspectJ我們在細談。
在PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());對兩類BeanFactoryPostProcessor和BeanDefinitionRegistryPostProcessor進行處理，首先會獲取自定義添加的
public List<BeanFactoryPostProcessor> getBeanFactoryPostProcessors() {
return this.beanFactoryPostProcessors;
}
可以發現，我們可以通過調用:
public void addBeanFactoryPostProcessor(BeanFactoryPostProcessor postProcessor) {
Assert.notNull(postProcessor, "BeanFactoryPostProcessor must not be null");
this.beanFactoryPostProcessors.add(postProcessor);
}
可以手動添加BeanFactoryPostProcessor.
在
PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(beanFactory, getBeanFactoryPostProcessors());中，首先會處理自定義添加的：
實現了BeanFactoryPostProcessor接口的postProcessBeanDefinitionRegistry方法。然後在從容器中獲取實現了這個接口bean並調用postProcessBeanDefinitionRegistry。執行完這個處理後，接下來會處理BeanFactoryPostProcessor.postProcessBeanFactory。這兩步處理中，如果對應的bean沒有實例化，通過getBean實例化Bean，但是自定義的添加進去的是已經實例化後的bean。
(6)註冊BeanPostProcessor，通過registerBeanPostProcessors
AbstractApplicationContext.registerBeanPostProcessors
將容器中BeanPostProcessor類型的bean的信息從beanFactory中獲取到並通過getBean(String name, Class<T> requiredType)方法實例化這些Bean，然後將其註冊到beanFactory中，這裏加入的都是BeanPostProcessor的實例。
private static void registerBeanPostProcessors(
ConfigurableListableBeanFactory beanFactory, List<BeanPostProcessor> postProcessors) {
for (BeanPostProcessor postProcessor : postProcessors) {
beanFactory.addBeanPostProcessor(postProcessor);
}
}
(7)初始化國際化相關工具類，通過registerBeanPostProcessors實現
AbstractApplicationContext.initMessageSource
初始化國際化相關信息
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）初始化事件廣播器
AbstractApplicationContext.initApplicationEventMulticaster
初始化事件廣播器
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() + "]");
}
}
}
可以看到如果沒有當前beanFactory中不包含名爲applicationEventMulticaster的bean，則會默認注入SimpleApplicationEventMulticaster當做當前applicationContext的事件廣播器
（9）執行模板方法onRefresh
AbstractApplicationContext.onRefresh
這是一個模板方法，當前類是一個空的實現，子類可以複寫這塊的邏輯，可以看到實際實現：
按照spring官方給出的註釋：
Initialize other special beans in specific context subclasses.
可以在這個方法中實例化某個自定義的applicationContext中特殊的bean的處理，我們也可以看到，這塊主要是web相關的application在應用，我們在講解web相關處理的時候回詳細說明。
（10）註冊監聽器，registerListeners
AbstractApplicationContext.registerListeners
註冊監聽器
在（8）步的時候，我們初始化了事件廣播器，這裏我們將註冊的ApplicationListener加入到（8）步中註冊的ApplicationEventMulticaster中去，這樣後續當ApplicationEventMulticaster收到相關的事件的時候，通過調用SimpleApplicationEventMulticaster.multicastEvent繼而調用SimpleApplicationEventMulticaster.invokeListener來通知監聽器監聽相關事件（這是以默認的SimpleApplicationEventMulticaster舉例說明）
（11）初始化所有的單例bean（不包含lazy-init)，finishBeanFactoryInitialization
AbstractApplicationContext.finishBeanFactoryInitialization
這一步就開始初始化我們自定義的相關bean，實現如下：
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();
}
- 首先是設置當前applicationContext的
ConversionService.ConversionService主要是用來類型轉換。
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);
}
其默認實現DefaultConversionService繼承如下：
- 註冊默認的值替換器，
比如我們經常在屬性上進行值注入：
@Value("${name}")
private String name;
這裏註冊默認的值解決類。
- 實例化Aspect相關類
- 緩存當前bean的配置，並凍結配置不允許更改
- 實例化剩下的單例，通過
DeaultListableBeanFactory.preInstantiateSingletons來預實例化單例
我們可以看下DeaultListableBeanFactory.preInstantiateSingletons的實現。
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();
}
}
}
}
主要是兩步，如果bean對應的是一個OjectFactory類型，則通過getBean獲取實例，如果ObjectFactory是SmartFactoryBean且isEagerInit爲true時，提前實例化對應單例。
如果是普通單例，直接通過getBean獲取。
getBean這個方法後面會着重描述
(12)結束容器刷新，執行相關方法，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);
}
首先是清空相關資源的緩存。
然後初始化運行週期相關處理類，默認使用DefaultLifecycleProcessor
初始化生命週期處理類之後，執行onRefresh生命週期相關方法
發佈容器刷新完成事件消息
如果開啓了JMX，執行JMX相關處理。
至此，一個spring容器啓動成功。