Spring原始碼分析4 — spring bean建立和初始化
1 介紹
建立並初始化spring容器中,refresh()方法中解析xml配置檔案,註冊容器後處理器,bean後處理器,初始化MessageSource,ApplicationEventMulticaster廣播器,註冊完ApplicationListener監聽器後,關鍵一步就是建立和初始化其他非lazy-init的singleton beans。這樣在容器初始化好的時候,這些singleton beans就已經建立和初始化好了,可以大大提高bean的訪問效率。這個過程比較複雜,本文將詳細分析整個流程。先看涉及到的關鍵類。
AbstractApplicationContext: 定義了spring容器初始化的大部分流程方法,子類必須遵循這個流程,但可以修改流程中的方法,典型的模板模式。bean建立的入口方法finishBeanFactoryInitialization也在這個方法中。
DefaultListableBeanFactory:一種BeanFactory容器實現,實現了ConfigurableListableBeanFactory介面
BeanDefinition:描述bean結構,對應XML中的或者註解中的@Component
AbstractBeanFactory:繼承了BeanFactory容器,主要負責getBean建立Bean例項。
2 流程
2.1 finishBeanFactoryInitialization
初始化spring容器中的refresh()方法中,會呼叫finishBeanFactoryInitialization()方法,它是建立和初始化其他非lazy-init的singleton的bean的入口。下面從這個方法開始分析。
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// 初始化conversionService型別轉換bean,它可以服務於其他bean的型別轉換
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));
}
// 註冊字串解析器,用來解析註解中的屬性
if (!beanFactory.hasEmbeddedValueResolver()) {
beanFactory.addEmbeddedValueResolver(strVal -> getEnvironment().resolvePlaceholders(strVal));
}
// 初始化LoadTimeWeaverAware bean
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// 停止使用臨時的ClassLoader,
beanFactory.setTempClassLoader(null);
beanFactory.freezeConfiguration();
// 這兒才是最關鍵的一步,建立和初始化非lazy-init的singleton beans
beanFactory.preInstantiateSingletons();
}
finishBeanFactoryInitialization()做了初始化conversionService型別轉換器等的工作,這些不是關鍵點。關鍵點在preInstantiateSingletons()方法中,它會做建立和初始化singleton bean的工作。下面接著分析
public void preInstantiateSingletons() throws BeansException {
// 獲取XML配置檔案解析時,解析到的所有beanname
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// 遍歷所有沒有標註lazy-init的singleton的beanname,建立bean
for (String beanName : beanNames) {
// 利用beanname獲取BeanDefinition,在XML解析時會生成BeanDefinition物件,將XML中的各屬性新增到BeanDefinition的相關標誌位中,比如abstractFlag,scope等
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
// 非abstract,非lazy-init的singleton bean才需要在容器初始化階段建立
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
// 處理FactoryBean
if (isFactoryBean(beanName)) {
// 獲取FactoryBean例項,FactoryBean前面會加一個&符號
final FactoryBean<?> factory = (FactoryBean<?>) getBean(FACTORY_BEAN_PREFIX + beanName);
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);
}
}
// 非Factorybean,直接呼叫getBean方法,關鍵所在,後續分析
else {
getBean(beanName);
}
}
}
// bean建立後,對SmartInitializingSingleton回撥afterSingletonsInstantiated()方法,這兒不用太care
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();
}
}
}
}
preInstantiateSingletons流程稍微複雜點,主要有
- 獲取XML解析時的beanNames
- 遍歷beanNames,獲取BeanDefinition。對非abstract,非lazy-init的singleton bean的進行例項化
- 如果是FactoryBean,則需要判斷isEagerInit,來確定是否呼叫getBean建立對應的bean。
- 如果不是,則直接呼叫getBean建立對應bean
- bean建立後,對SmartInitializingSingleton回撥afterSingletonsInstantiated()方法。
2.2 getBean 建立和初始化bean例項
下面我們著重來分析bean的建立,也就是getBean()方法。
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
// beanname轉換,去掉FactoryBean的&字首,處理alias宣告。細節可自行分析
final String beanName = transformedBeanName(name);
Object bean;
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
// 判斷singleton bean是否已經建立好了,建立好了則直接從記憶體取出。
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
} else {
// 之前沒建立的,則需要建立。
// 正在建立,則直接異常返回
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 檢查是否有beanname對應的BeanDefinition
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// 沒有找到BeanDefinition,看看parent工廠中有沒有,呼叫parent工廠的getBean
// 獲取原始的name,包含了FactoryBean字首,&符號
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
// 找到了beanname對應的BeanDefinition,合併parent的BeanDefinition(XML中的parent屬性)
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// 處理dependsOn屬性
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
// 遍歷所有的dependOn bean,要先註冊和建立依賴的bean
for (String dep : dependsOn) {
// check是否兩個bean是迴圈依賴,spring不能出現bean的迴圈依賴
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 註冊並建立依賴的bean
registerDependentBean(dep, beanName);
getBean(dep);
}
}
// 處理scope屬性
if (mbd.isSingleton()) {
// singleton, 必須保證執行緒安全情況下建立bean,保證單例
sharedInstance = getSingleton(beanName, () -> {
try {
// 反射建立bean例項,這個過程很複雜,稍後分析
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// 異常處理,清除掉bean
destroySingleton(beanName);
throw ex;
}
});
// 獲取bean例項
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// prototype,建立一個全新的例項
Object prototypeInstance = null;
try {
// 建立前的回撥
beforePrototypeCreation(beanName);
// 反射建立bean例項,稍後詳細分析
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
// 建立後的回撥,清除inCreation的標誌
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
// 其他scope值
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
// scope屬性不能接收空值
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, () -> {
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;
}
}
// check 建立的bean是否是requiredType指明的型別。如果不是,先做轉換,轉換不成的話只能型別不匹配丟擲異常了
if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
try {
// 嘗試將建立的bean轉換為requiredType指明的型別
return getTypeConverter().convertIfNecessary(bean, requiredType);
} catch (TypeMismatchException ex) {
// 轉換不成功,丟擲異常
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
doGetBean概括了bean建立和初始化的主要流程,十分複雜,步驟主要為
- beanname轉換,去掉FactoryBean的&字首,處理alias宣告
- 判斷singleton bean是否已經建立好了,建立好了則直接從記憶體取出
- 沒有建立好,則檢查是否有beanname對應的BeanDefinition,沒有則到parent工廠中查詢,命中則使用parent工廠再次呼叫getBean以及doGetBean建立
- 有BeanDefinition,則合併parent屬性指向的中的屬性,這主要是處理bean的parent屬性。子bean會繼承parent bean的屬性。
- 處理dependsOn屬性。必須先建立好所有的依賴的bean
- 處理scope屬性,如果是singleton的,則必須保證執行緒安全情況下建立單例。如果是prototype,則必須保證建立一個全新的bean。建立bean通過createBean()反射建立。
2.3 createBean 反射建立bean例項
下面來分析createBean()方法,這個過程也是相當複雜的。
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
RootBeanDefinition mbdToUse = mbd;
// 拷貝一個新的RootBeanDefinition供建立bean使用
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// 處理bean中定義的覆蓋方法,主要是xml:lookup-method或replace-method。標記override的方法為已經載入過的,避免不必要的引數檢查開銷。這兒不詳細展開了。
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
// 呼叫BeanPostProcessors bean後處理器,使得bean後處理器可以返回一個proxy bean,從而代替我們要建立的bean。回撥後處理器的postProcessBeforeInstantiation()方法,如果這個方法中返回了一個bean,也就是使用了proxy,則再回調postProcessAfterInitialization()方法。之後返回這個Proxy bean即可。
try {
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
// doCreateBean建立bean例項,後面詳細分析
try {
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
return beanInstance;
}
// 各種異常,省略
...
}
createBean()方法大概步驟如下
- 拷貝一個新的RootBeanDefinition供建立bean使用
- 處理lookup-method或replace-method
- 呼叫BeanPostProcessors後處理器
- doCreateBean建立bean例項
下面我們重點分析doCreateBean方法
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// 建立bean例項,如果是singleton,先嚐試從快取中取,取不到則建立
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
// 反射建立bean例項,後面詳細說
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
mbd.resolvedTargetType = beanType;
// 回撥MergedBeanDefinitionPostProcessor.postProcessMergedBeanDefinition,它可以修改bean屬性
if (beanType != null) {
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
}
// 曝光單例物件的引用,主要是為了解決單例間的迴圈依賴問題,以及依賴的bean比較複雜時的初始化效能問題
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// 初始化bean,後面詳細介紹
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
}
// 省略異常處理
// 單例曝光物件的處理,不用太在意
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
// 丟擲異常,省略程式碼
}
}
}
}
// 註冊bean為可銷燬的bean,bean銷燬時,會回撥destroy-method
if (bean != null) {
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
}
return exposedObject;
}
doCreateBean方法主要流程為
- createBeanInstance() 建立bean例項
- 回撥postProcessMergedBeanDefinition(), 可以修改bean屬性
- initializeBean() 初始化bean例項,包括後處理器的呼叫,init-method的呼叫等
- 註冊bean為可銷燬的,這樣在bean銷燬時,就可以回撥到destroy-method.
2.3.1 createBeanInstance 反射建立bean例項
我們先分析如何建立bean例項的。
個protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// 先建立class物件,反射的套路。利用bean的class屬性進行反射,所以class屬性一定要是bean的實現類
Class<?> beanClass = resolveBeanClass(mbd, beanName);
// class如果不是public的,則丟擲異常。因為沒法進行例項化
if (beanClass != null && !Modifier.isPublic(beanClass.getModifiers()) && !mbd.isNonPublicAccessAllowed()) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isn't public, and non-public access not allowed: " + beanClass.getName());
}
//
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
// 使用FactoryBean的factory-method來建立,支援靜態工廠和例項工廠
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// 無引數情況時,建立bean。呼叫無參構造方法
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
// autoWire建立 自動裝配
return autowireConstructor(beanName, mbd, null, null);
}
else {
// 普通建立
return instantiateBean(beanName, mbd);
}
}
// 有引數情況時,建立bean。先利用引數個數,型別等,確定最精確匹配的構造方法。
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
// 有引數時,又沒獲取到構造方法,則只能呼叫無參構造方法來建立例項了(兜底方法)
return instantiateBean(beanName, mbd);
}
instantiateBean,使用無參構造方法,反射建立bean例項程式碼如下
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
try {
Object beanInstance;
final BeanFactory parent = this;
if (System.getSecurityManager() != null) {
beanInstance = AccessController.doPrivileged((PrivilegedAction<Object>) () ->
getInstantiationStrategy().instantiate(mbd, beanName, parent),
getAccessControlContext());
}
else {
// 建立例項,關鍵點,其他都不用care
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
這個方法沒什麼要注意的,關鍵點在instantiate方法
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
if (bd.getMethodOverrides().isEmpty()) {
Constructor<?> constructorToUse;
// 保證執行緒安全情況下,獲取Constructor
synchronized (bd.constructorArgumentLock) {
// 獲取構造方法或factory-method
constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null) {
// BeanDefinition中如果沒有Constructor或者factory-method,則直接使用預設無參構造方法。
final Class<?> clazz = bd.getBeanClass();
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
if (System.getSecurityManager() != null) {
constructorToUse = AccessController.doPrivileged(
(PrivilegedExceptionAction<Constructor<?>>) () ->
clazz.getDeclaredConstructor());
}
else {
// 獲取預設無參構造方法
constructorToUse = clazz.getDeclaredConstructor();
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
// 使用上一步得到的Constructor,反射獲取bean例項
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
instantiate方法主要做兩件事
- 確定Constructor或者factory-method
- 利用Constructor,反射建立bean例項。
分析到這兒Bean的建立就結束了,這個過程實在是太複雜了!
2.3.2 initializeBean 初始化bean例項
bean建立完後,容器會對它進行初始化,包括後處理的呼叫,init-method的呼叫等。請看下面詳解。
protected Object initializeBean(final String beanName, final Object bean, @Nullable RootBeanDefinition mbd) {
// 回撥各種aware method,如BeanNameAware, BeanFactoryAware等
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
invokeAwareMethods(beanName, bean);
return null;
}, getAccessControlContext());
}
else {
invokeAwareMethods(beanName, bean);
}
// 回撥beanPostProcessor的postProcessBeforeInitialization()方法
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
// init-method 和 postProcessAfterInitialization
if (wrappedBean != null) {
try {
// 回撥init-method
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
// 回撥beanPostProcessor的postProcessAfterInitialization()方法
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
}
return wrappedBean;
}
由此可見,initializeBean(),也就是bean的初始化流程為
- 回撥各種aware method,如BeanNameAware,將容器中相關引用注入到bean中,供bean使用
- 回撥beanPostProcessor的postProcessBeforeInitialization(), 後處理器的初始化前置呼叫
- 回撥init-method, 註解和XML中都可以宣告
- 回撥beanPostProcessor的postProcessAfterInitialization()方法,後處理器的初始化後置呼叫。
從這個流程,我們也能清晰的分析出容器後處理器兩個方法的呼叫時機。分析原始碼可以大大加深我們對spring API的理解。
3 總結
Bean例項的建立和初始化流程還是十分複雜的。從原始碼中可以清晰的分析出spring bean的各種特性。如factory-method, BeanPostProcessor等。有助於我們spring bean行為的理解。所以分析原始碼還是十分值得的。