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在初始化ClassPathXmlApplicatonContext过程中,核心初始化逻辑在AbstractApplicationContext的refresh函数中:
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// 供子类拓展,添加创建前必需属性,校验如果必需属性不存在则抛出MissingRequiredPropertiesException已成
prepareRefresh();
// 调用子类实现方法获取(创建或刷新)BeanFacotry容器,对于ClassPathXmlApplicationContext,主要调用了AbstractRefreshableApplicationContext中实现的方法
// 在这里,将xml配置文件中 的Bean解析成了一个个BeanDefinition,建立一个beanName-> beanDefinition 的 map
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// 设定类加载器,spel解析器,属性编辑解析器等,忽略特定接口的依赖注册(在特定时刻相关Bean再完成注入),注册一些系统Bean供依赖注入使用。
prepareBeanFactory(beanFactory);
try {
// BeanFactory创建完的后置处理。当前为空实现,供子类拓展
postProcessBeanFactory(beanFactory);
// 调用BeanFacotry的相关后置处理器,如果实现了Order相关接口,会先进行排序。
invokeBeanFactoryPostProcessors(beanFactory);
// 注册相关BeanPostProcessor,供Bean生成前后调用。
registerBeanPostProcessors(beanFactory);
// 初始化国际化信息源
initMessageSource();
// 初始化Spring相关上下文时间广播器
initApplicationEventMulticaster();
// 模版方法供子类实现,用于初始化一些特殊Bean配置等
onRefresh();
// 注册实现了ApplicationListener接口的事件监听器,用于后续广播器广播事件
registerListeners();
// BeanFactory初始化完成时调用,初始ConversionService Bean,冻结beanFactory配置,并开始创建BeanFactory中所有非懒加载的单例Bean
finishBeanFactoryInitialization(beanFactory);
// 初始化Lifecycle处理器,调用onRefresh方法,广播ContextRefreshedEvent。
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;
}
}
}
其中有一行代码,调用了finishBeanFactoryInitialization(beanFactory)
。在这一步之前,已经完成了BeanFactory对象初始化、xml配置文件解析成BeanDefinition、BeanPostProcessor初始化与注册等操作。
而在完成BeanFactory初始化之时,会初始化容器内所有单例非懒加载对象,供后续业务逻辑进行依赖注入等使用,具体实现在finishBeanFactoryInitialization(beanFactory)
内部的最后一行代码:
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
//…… 省略前面代码
// 预实例化所有非懒加载单例Bean
beanFactory.preInstantiateSingletons();
}
具体的preInstantiateSingletons实现如下:
public void preInstantiateSingletons() throws BeansException {
if (this.logger.isInfoEnabled()) {
this.logger.info("Pre-instantiating singletons in " + this);
}
List<String> beanNames;
synchronized (this.beanDefinitionMap) {
// 获取容器内加载的所有BeanDefinition
beanNames = new ArrayList<String>(this.beanDefinitionNames);
}
// 遍历初始化所有非懒加载单例Bean
for (String beanName : beanNames) {
// Bean定义公共的抽象类是AbstractBeanDefinition,普通的Bean在Spring加载Bean定义的时候,实例化出来的是GenericBeanDefinition
// 而Spring上下文包括实例化所有Bean用的AbstractBeanDefinition是RootBeanDefinition
// 这时候就使用getMergedLocalBeanDefinition方法做了一次转化,将非RootBeanDefinition转换为RootBeanDefinition以供后续操作。
// 注意如果当前BeanDefinition存在父BeanDefinition,会基于父BeanDefinition生成一个RootBeanDefinition,然后再将调用OverrideFrom子BeanDefinition的相关属性覆写进去。
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
// 如果Bean不是抽象的,是单例的,不是懒加载的,则开始创建单例对象通过调用getBean(beanName)方法初始化
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
// 判断当前Bean是否实现了FactoryBean接口,如果实现了,判断是否要立即初始化
// 判断是否需要立即初始化,根据Bean是否实现了SmartFactoryBean并且重写的内部方法isEagerInit放回true
if (isFactoryBean(beanName)) {
final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
boolean isEagerInit;
if (System.getSecurityManager() != null && factory instanceof SmartFactoryBean) {
isEagerInit = AccessController.doPrivileged(new PrivilegedAction<Boolean>() {
public Boolean run() {
return ((SmartFactoryBean<?>) factory).isEagerInit();
}
}, getAccessControlContext());
}
else {
isEagerInit = (factory instanceof SmartFactoryBean &&
((SmartFactoryBean<?>) factory).isEagerInit());
}
if (isEagerInit) {
getBean(beanName);
}
}
else {
getBean(beanName);
}
}
}
}
// 具体getBean函数实现如下所示:
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
/*
进一步调用了如下方法,其中有参数:
requiredType=null: 一般情况用不到,如果获取到的字符串,但requiredType是Integer,会在最后进行类型转换。
args=null: 在获取prototype对象时传入,用来初始化原型对象
typeCheckOnly=false: 如果为false,会将Bean标志为已创建,记录在alreadyCreated变量中。
*/
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
// 如果是FactoryBean,会去掉Bean开头的&符号
// 可能存在传入别名且别名存在多重映射的情况,这里会返回最终的名字,如存在多层别名映射A->B->C->D,传入D,最终会返回A
final String beanName = transformedBeanName(name);
Object bean;
// 这里先尝试从缓存中获取,获取不到再走后面创建的流程
// 获取到有两种情况,一种是Bean创建完成存储到最终的缓存中。
// 另一种是未创建完成,但先预存到一个单独的缓存中,这种是针对可能存在循环引用的情况的处理。
// 如A引用B,B又引用了A,因而在初始化A时,A会先调用构造函数创建出一个实例,在依赖注入B之前,现将A实例缓存起来
// 然后在初始化A时,依赖注入阶段,会触发初始化B,B创建后需要依赖注入A时,先从缓存中获取A(这个时候的A是不完整的),避免循环依赖的问题出现。
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isDebugEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.debug("Returning cached instance of singleton bean '" + beanName + "'");
}
}
//这里主要处理实现了FactoryBean的情况,需要调用重写的getObject()方法来获取实际的Bean实例。
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// 原型对象不允许循环创建,如果是原型对象则抛异常
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 如果存在父容器,且Bean在父容器中有定义,则通过父容器返回
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
// 进行已创建标记
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
// 根据名字获取合并过的对应的RootBeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
// 检查mbd是否为抽象的或mbd为单例,但存在args的情况(args只有初始化原型对象才允许存在)
checkMergedBeanDefinition(mbd, beanName, args);
// 确保当前Bean依赖的相关Bean先完成初始化工作
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dependsOnBean : dependsOn) {
getBean(dependsOnBean);
registerDependentBean(dependsOnBean, beanName);
}
}
// 前面获取失败,开始创建
if (mbd.isSingleton()) {
// 会先尝试从缓存中获取,获取失败就通过ObjectFactory的createBean方法创建
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
public Object getObject() throws BeansException {
try {
// 创建单例对象
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
}
});
//这里主要处理实现了FactoryBean的情况,需要调用重写的getObject()方法来获取实际的Bean实例。
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
// 将正在创建的原型对象进行记录
beforePrototypeCreation(beanName);
// 创建原型对象
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
// 移除正在创建的原型对象记录
afterPrototypeCreation(beanName);
}
//这里主要处理实现了FactoryBean的情况,需要调用重写的getObject()方法来获取实际的Bean实例。
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
public Object getObject() throws BeansException {
beforePrototypeCreation(beanName);
try {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
}
});
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new BeanCreationException(beanName,
"Scope '" + scopeName + "' is not active for the current thread; " +
"consider defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
}
}
}
catch (BeansException ex) {
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// 检查是否为要求的类型,如果不是则尝试进行类型转换
if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {
try {
return getTypeConverter().convertIfNecessary(bean, requiredType);
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type [" +
ClassUtils.getQualifiedName(requiredType) + "]", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
getSingleton的方法的实现在父类DefaultSingletonBeanRegistry中
// 实现如下:
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "'beanName' must not be null");
synchronized (this.singletonObjects) {
//双重判定从缓存中获取
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while the singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
// 创建前置检查,默认实现是记录当前beanName正在注册中
beforeSingletonCreation(beanName);
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<Exception>();
}
try {
// 调用签名定义的内部类进行创建,内部调用了createBean(String beanName, RootBeanDefinition mbd, Object[] args)
singletonObject = singletonFactory.getObject();
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
// 创建前置检查,默认实现是移除当前beanName正在注册状态的记录
afterSingletonCreation(beanName);
}
addSingleton(beanName, singletonObject);
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
}
createBean方法位于AbstractAutowireCapableBeanFactory中,具体实现如下:
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
// 确保对应BeanClass完成解析,具体表现是进行了ClassLoder.loadClass或Class.forName完成了类加载
resolveBeanClass(mbd, beanName);
try {
// 准备方法覆盖,主要为lookup-method,replace-method等配置准备
mbd.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// 供特定后置处理器拓展,如果直接生成了一个Bean,就直接返回不走正常创建流程。
// 具体逻辑是判断当前Spring容器是否注册了实现了InstantiationAwareBeanPostProcessor接口的后置处理器
// 如果有,则依次调用其中的applyBeanPostProcessorsBeforeInstantiation方法,如果中间任意一个方法返回不为null,直接结束调用。
// 然后依次所有注册的BeanPostProcessor的postProcessAfterInitialization方法(同样如果任意一次返回不为null,即终止调用。
Object bean = resolveBeforeInstantiation(beanName, mbd);
// 如果不为空,说明提前生成了实例,直接返回
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
// 具体创建Bean逻辑
Object beanInstance = doCreateBean(beanName, mbd, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
具体创建Bean的逻辑如下所示:
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
// BeanWrapper封装了具体的Bean实例,然后可以很方便地通过调用getPropertyValue和setPropertyValue等方法反射读写Bean的具体属性
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
// 先尝试从缓存中取
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
// 调用构造方法创建一个空实例对象,并用BeanWrapper进行包装
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
// 获取所有的后置处理器,如果后置处理器实现了MergedBeanDefinitionPostProcessor接口,则一次调用其postProcessMergedBeanDefinition方法
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
mbd.postProcessed = true;
}
}
// 如果满足循环引用缓存条件,先缓存具体对象
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
// 提前暴露一个单例工厂方法,确保其他Bean能引用到此bean
// 具体内部会遍历后置处理器,判断是否有SmartInstantiationAwareBeanPostProcessor的实现类,然后调用里面getEarlyBeanReference覆盖当前Bean
// 默认不做任何操作返回当前Bean,作为拓展,这里比如可以供AOP来创建代理类
addSingletonFactory(beanName, new ObjectFactory<Object>() {
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// 开始对Bean实例进行初始化
Object exposedObject = bean;
try {
// 对bean进行填充,在这里面完成依赖注入的相关内容
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
// 完成属性依赖注入后,进一步初始化Bean
// 具体进行了以下操作:
// 1.若实现了BeanNameAware, BeanClassLoaderAware,BeanFactoryAwareAware等接口,则注入相关对象
// 2.遍历后置处理器,调用实现的postProcessBeforeInitialization方法,
// 3.如果实现了initialzingBean,调用实现的 afterPropertiesSet()
// 4.如果配置了init-mothod,调用相应的init方法
// 5.遍历后置处理器,调用实现的postProcessAfterInitialization
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException && beanName.equals(((BeanCreationException) ex).getBeanName())) {
throw (BeanCreationException) ex;
}
else {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
}
}
// 忽略部分代码……
// 如果实现了Disposable接口,会在这里进行注册,最后在销毁的时候调用相应的destroy方法
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
在讲解过程中,跳过了具体依赖注入相关属性的具体代码讲解,这一部分代码非常复杂,后续会单独在一篇文章内进行分析。
至此,完成了单例非懒加载Bean初始化流程分析,实际上,几乎所有的Bean都通过getBean
方法进行获取,在获取过程中,会先尝试从缓存中获取,在获取失败后,则会调用createBean
方法进行创建。