踩坑記:根據型別獲取Spring容器中的Bean
在專案開發中遇到了這樣的一個問題:有同事在BeanFactoryPostProcessor的實現方法中寫了類似這樣的一段程式碼:
@Component
public class BeanFactoryPostProcessorImpl implements BeanFactoryPostProcessor {
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
//就是這一段程式碼
beanFactory.getBean(GetBeanByType.class);
}
}
然後導致了一些實現了FactoryBean介面的Bean被提前建立了,當時感覺很奇怪,我只是根據傳入的Class會獲取Bean,為什麼會導致一些實現了FactoryBean介面的Bean也被建立呢?其實即使我們是根據型別從Spring容器中獲取Bean,但是Spring容器在實現的時候,最底層還是會呼叫getBean(java.lang.String, java.lang.Class, java.lang.Object…)這個方法獲取Bean,那麼當我們根據型別從Spring容器中獲取Bean的時候就先找到這個型別對應的在Spring容器中的beanName,問題就出在獲取這個beanName的地方。真正出問題的是AbstractBeanFactory#isTypeMatch(java.lang.String, org.springframework.core.ResolvableType)這個方法。下面我們來分一下獲取beanName的這個過程。
其入口方法為:DefaultListableBeanFactory#getBeanNamesForType(java.lang.Class
public String[] getBeanNamesForType(Class<?> type) {
//這裡傳入的三個引數 type是bean的Class 第二個引數的意思是 是否包含非單例的bean
//第三個引數很重要 這裡傳入的值為true 這裡傳入true 代表允許提前初始化Bean
//如果這個值為false的話 代表不允許提前初始化Bean 問題就出在這個值為true!!!
return getBeanNamesForType(type, true, true);
}
@Override
public String[] getBeanNamesForType(Class<?> type, boolean includeNonSingletons, boolean allowEagerInit) {
//如果是在容器啟動過程中 這個值為false 如果在容器啟動完成之後 這個值會變為true 這個值的更改在
//DefaultListableBeanFactory#freezeConfiguration 可以看一下它的呼叫關係
//下面的內容就很簡單了 我們主要分析doGetBeanNamesForType這個方法
if (!isConfigurationFrozen() || type == null || !allowEagerInit) {
return doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, allowEagerInit);
}
//這裡是從快取中獲取,includeNonSingletons是否包含非單例的方法
Map<Class<?>, String[]> cache =
(includeNonSingletons ? this.allBeanNamesByType : this.singletonBeanNamesByType);
String[] resolvedBeanNames = cache.get(type);
if (resolvedBeanNames != null) {
return resolvedBeanNames;
}
resolvedBeanNames = doGetBeanNamesForType(ResolvableType.forRawClass(type), includeNonSingletons, true);
if (ClassUtils.isCacheSafe(type, getBeanClassLoader())) {
cache.put(type, resolvedBeanNames);
}
return resolvedBeanNames;
}
DefaultListableBeanFactory#doGetBeanNamesForType
private String[] doGetBeanNamesForType(ResolvableType type, boolean includeNonSingletons, boolean allowEagerInit) {
List<String> result = new ArrayList<String>();
// Check all bean definitions.
// 注意這裡是迴圈Spring容器中所有的BeanDefinition
for (String beanName : this.beanDefinitionNames) {
// Only consider bean as eligible if the bean name
// is not defined as alias for some other bean.
//beanName不是別名
if (!isAlias(beanName)) {
try {
//這裡你可以暫時理解為根據BeanName獲取BeanDefinition
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
// Only check bean definition if it is complete.
//下面這個判斷比較長 一個一個的來說一下
//mbd.isAbstract()判斷這個類是不是抽象類 預設是false
//allowEagerInit 是否允許提前初始化 這個值是傳入的值 我們這裡傳入的值是true 即允許提前初始化
//mbd.hasBeanClass() 是否已經有BeanClass了
//mbd.isLazyInit()是否允許延遲初始化 預設false
//isAllowEagerClassLoading() 是否允許提前載入 上面這四個值是或的關係 只有有一個為true就可以
//requiresEagerInitForType 檢查特殊的Bean是否需要提前被初始化 這裡傳入的是工廠方法的名字
//我們把這個條件總結一下 如果這個Bean不是一個抽象類,並且(被允許提前初始化 或者 (這個Bean的
//BeanDefinition設定了BeanClass了 或者 這個Bean沒有設定延遲載入 或者 即使這個Bean標註為延遲載入了,也可以被提前載入(全域性變數) 並且 BeanDefinition中的FactoryBeanName是否允許被提前初始化))
//根據我們前面的分析 allowEagerInit 的值為true 這個Bean也不是個抽象類,所以會繼續下面的流程
if (!mbd.isAbstract() && (allowEagerInit ||
((mbd.hasBeanClass() || !mbd.isLazyInit() || isAllowEagerClassLoading())) &&
!requiresEagerInitForType(mbd.getFactoryBeanName()))) {
// In case of FactoryBean, match object created by FactoryBean.
// 判斷是不是FactoryBean
boolean isFactoryBean = isFactoryBean(beanName, mbd);
BeanDefinitionHolder dbd = mbd.getDecoratedDefinition();
// 這個地方是判斷是不是找到匹配的beanName 這裡關鍵的是isTypeMatch這個方法
// 在這個方法中會導致一些FactoryBean被提前例項化 所以這裡給我們的一個提示是:
// 在Spring容器的啟動階段,呼叫isTypeMatch這個方法去判斷Bean的型別的時候要慎重一些。
// 我們要重點分析isTypeMatch這個方法
boolean matchFound =
(allowEagerInit || !isFactoryBean ||
(dbd != null && !mbd.isLazyInit()) || containsSingleton(beanName)) &&
(includeNonSingletons ||
(dbd != null ? mbd.isSingleton() : isSingleton(beanName))) &&
isTypeMatch(beanName, type);
//如果型別不匹配 並且還是FactoryBean型別的BeanDefinition
if (!matchFound && isFactoryBean) {
// In case of FactoryBean, try to match FactoryBean instance itself next.
//這裡講beanName變為&beanName
beanName = FACTORY_BEAN_PREFIX + beanName;
//繼續判斷型別匹配不匹配
matchFound = (includeNonSingletons || mbd.isSingleton()) && isTypeMatch(beanName, type);
}
//如果型別相匹配的話 則將beanName放入到結果集中
if (matchFound) {
result.add(beanName);
}
}
}
catch (CannotLoadBeanClassException ex) {
}
catch (BeanDefinitionStoreException ex) {
}
}
}
//這一部分的處理流程大致相同
// Check manually registered singletons too.
for (String beanName : this.manualSingletonNames) {
try {
// In case of FactoryBean, match object created by FactoryBean.
if (isFactoryBean(beanName)) {
if ((includeNonSingletons || isSingleton(beanName)) && isTypeMatch(beanName, type)) {
result.add(beanName);
// Match found for this bean: do not match FactoryBean itself anymore.
continue;
}
// In case of FactoryBean, try to match FactoryBean itself next.
beanName = FACTORY_BEAN_PREFIX + beanName;
}
// Match raw bean instance (might be raw FactoryBean).
if (isTypeMatch(beanName, type)) {
result.add(beanName);
}
}
catch (NoSuchBeanDefinitionException ex) {
}
}
return StringUtils.toStringArray(result);
}
下面我們來看看isTypeMatch這個方法的內容
@Override
public boolean isTypeMatch(String name, ResolvableType typeToMatch) throws NoSuchBeanDefinitionException {
//這裡是對beanName進行轉換 如:如果傳入的beanName是&beanName,則將&去掉。如果傳入的beanName有別名的話,則替換為別名
String beanName = transformedBeanName(name);
// Check manually registered singletons.
//根據beanName獲取bean的例項 前提是其對應的bean已經被例項化了 或者正在例項化中
Object beanInstance = getSingleton(beanName, false);
if (beanInstance != null) {
//如果是FactoryBean型別的例項
if (beanInstance instanceof FactoryBean) {
//是否要獲取在FactoryBean中建立的Bean例項 呼叫 getObject getObjectType方法
//這個地方的判斷條件是 傳入的beanName是否以&開頭 如果以&開頭,則返回true
if (!BeanFactoryUtils.isFactoryDereference(name)) {
//這裡是呼叫FactoryBean的getObjectType方法 來獲取bean的型別
Class<?> type = getTypeForFactoryBean((FactoryBean<?>) beanInstance);
return (type != null && typeToMatch.isAssignableFrom(type));
}
else {
//直接獲取FactoryBean型別的例項
return typeToMatch.isInstance(beanInstance);
}
}
//判斷beanName是否以&開頭,如果以&開頭的話 則返回true
else if (!BeanFactoryUtils.isFactoryDereference(name)) {
//如果beanName不是以&開頭 則直接進行型別比較
if (typeToMatch.isInstance(beanInstance)) {
// Direct match for exposed instance?
return true;
}
//泛型的處理
else if (typeToMatch.hasGenerics() && containsBeanDefinition(beanName)) {
// Generics potentially only match on the target class, not on the proxy...
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
Class<?> targetType = mbd.getTargetType();
if (targetType != null && targetType != ClassUtils.getUserClass(beanInstance) &&
typeToMatch.isAssignableFrom(targetType)) {
// Check raw class match as well, making sure it's exposed on the proxy.
Class<?> classToMatch = typeToMatch.resolve();
return (classToMatch == null || classToMatch.isInstance(beanInstance));
}
}
}
return false;
}
else if (containsSingleton(beanName) && !containsBeanDefinition(beanName)) {
// null instance registered
return false;
}
// No singleton instance found -> check bean definition.
//這裡是從父BeanFactory中進行查詢匹配 查詢匹配的過程是一樣的。
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// No bean definition found in this factory -> delegate to parent.
return parentBeanFactory.isTypeMatch(originalBeanName(name), typeToMatch);
}
// Retrieve corresponding bean definition.
//根據beanName重新檢索BeanDefinition 這裡返回一個RootBeanDefinition 如果父子容器中存在相同型別的bean會進行合併
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
//解析出 class的型別
Class<?> classToMatch = typeToMatch.resolve();
if (classToMatch == null) {
classToMatch = FactoryBean.class;
}
//如果傳入的class型別為FactoryBean型別 則直接返回FactoryBean型別的陣列 否則會將傳入的class的型別和FactoryBean組裝在一起
Class<?>[] typesToMatch = (FactoryBean.class == classToMatch ?
new Class<?>[] {classToMatch} : new Class<?>[] {FactoryBean.class, classToMatch});
// Check decorated bean definition, if any: We assume it'll be easier
// to determine the decorated bean's type than the proxy's type.
//這個地方是獲取最原始的BeanDefinition 不是組裝之後的 RootBeanDefinition
BeanDefinitionHolder dbd = mbd.getDecoratedDefinition();
//如果dbd 存在並且 beanName不是以&開頭
if (dbd != null && !BeanFactoryUtils.isFactoryDereference(name)) {
RootBeanDefinition tbd = getMergedBeanDefinition(dbd.getBeanName(), dbd.getBeanDefinition(), mbd);
//這裡是取BeanDefinition中的bean型別
Class<?> targetClass = predictBeanType(dbd.getBeanName(), tbd, typesToMatch);
if (targetClass != null && !FactoryBean.class.isAssignableFrom(targetClass)) {
return typeToMatch.isAssignableFrom(targetClass);
}
}
//獲取目標型別 通常是去BeanDefinition中的resolvedTargetType
Class<?> beanType = predictBeanType(beanName, mbd, typesToMatch);
if (beanType == null) {
return false;
}
// Check bean class whether we're dealing with a FactoryBean.
//如果BeanDefinition中的beanType是FactoryBean型別
if (FactoryBean.class.isAssignableFrom(beanType)) {
if (!BeanFactoryUtils.isFactoryDereference(name)) {
// If it's a FactoryBean, we want to look at what it creates, not the factory class.
//就是這個地方 會將FactoryBean型別的Bean進行例項化
//我們來分析這個方法
beanType = getTypeForFactoryBean(beanName, mbd);
if (beanType == null) {
return false;
}
}
}
else if (BeanFactoryUtils.isFactoryDereference(name)) {
// Special case: A SmartInstantiationAwareBeanPostProcessor returned a non-FactoryBean
// type but we nevertheless are being asked to dereference a FactoryBean...
// Let's check the original bean class and proceed with it if it is a FactoryBean.
//這個地方需要注意的是 SmartInstantiationAwareBeanPostProcessor型別的BeanPostProcessor會對返回的Bean型別做修改
beanType = predictBeanType(beanName, mbd, FactoryBean.class);
if (beanType == null || !FactoryBean.class.isAssignableFrom(beanType)) {
return false;
}
}
ResolvableType resolvableType = mbd.targetType;
if (resolvableType == null) {
//工廠方法建立的Bean
resolvableType = mbd.factoryMethodReturnType;
}
if (resolvableType != null && resolvableType.resolve() == beanType) {
return typeToMatch.isAssignableFrom(resolvableType);
}
return typeToMatch.isAssignableFrom(beanType);
}
isTypeMatch這個方法就是進行各種Bean型別的判斷,如是已經例項化的FactoryBean可能會呼叫它的getObjectType方法獲取Bean的型別,或者從BeanDefinition中獲取Bean的型別,並且如果是未例項化的FactoryBean,為了進行Bean型別的判斷會導致FactoryBean的例項化。
下面我們來看看org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#getTypeForFactoryBean這個方法的內容:
protected Class<?> getTypeForFactoryBean(String beanName, RootBeanDefinition mbd) {
//工程Bean和工廠方法名 根據工廠方法建立Bean
String factoryBeanName = mbd.getFactoryBeanName();
String factoryMethodName = mbd.getFactoryMethodName();
if (factoryBeanName != null) {
if (factoryMethodName != null) {
// Try to obtain the FactoryBean's object type from its factory method declaration
// without instantiating the containing bean at all.
BeanDefinition fbDef = getBeanDefinition(factoryBeanName);
if (fbDef instanceof AbstractBeanDefinition) {
AbstractBeanDefinition afbDef = (AbstractBeanDefinition) fbDef;
if (afbDef.hasBeanClass()) {
Class<?> result = getTypeForFactoryBeanFromMethod(afbDef.getBeanClass(), factoryMethodName);
if (result != null) {
return result;
}
}
}
}
// If not resolvable above and the referenced factory bean doesn't exist yet,
// exit here - we don't want to force the creation of another bean just to
// obtain a FactoryBean's object type...
if (!isBeanEligibleForMetadataCaching(factoryBeanName)) {
return null;
}
}
// Let's obtain a shortcut instance for an early getObjectType() call...
//getSingletonFactoryBeanForTypeCheck和getNonSingletonFactoryBeanForTypeCheck這兩個方法是建立FactoryBean的例項了
FactoryBean<?> fb = (mbd.isSingleton() ?
getSingletonFactoryBeanForTypeCheck(beanName, mbd) :
getNonSingletonFactoryBeanForTypeCheck(beanName, mbd));
if (fb != null) {
// Try to obtain the FactoryBean's object type from this early stage of the instance.
//呼叫getObjectType方法
Class<?> result = getTypeForFactoryBean(fb);
if (result != null) {
return result;
}
else {
// No type found for shortcut FactoryBean instance:
// fall back to full creation of the FactoryBean instance.
return super.getTypeForFactoryBean(beanName, mbd);
}
}
if (factoryBeanName == null && mbd.hasBeanClass()) {
// No early bean instantiation possible: determine FactoryBean's type from
// static factory method signature or from class inheritance hierarchy...
if (factoryMethodName != null) {
return getTypeForFactoryBeanFromMethod(mbd.getBeanClass(), factoryMethodName);
}
else {
return GenericTypeResolver.resolveTypeArgument(mbd.getBeanClass(), FactoryBean.class);
}
}
return null;
}
上面這個方法的內容是:如果是以工廠方法來建立Bean的話,則從工廠方法中返回bean型別,如果是FactoryBean型別的Bean的話,會例項化FactoryBean型別的Bean。例項化是在getSingletonFactoryBeanForTypeCheck或getNonSingletonFactoryBeanForTypeCheck方法中完成的。
private FactoryBean<?> getSingletonFactoryBeanForTypeCheck(String beanName, RootBeanDefinition mbd) {
synchronized (getSingletonMutex()) {
//先從快取中獲取
BeanWrapper bw = this.factoryBeanInstanceCache.get(beanName);
if (bw != null) {
return (FactoryBean<?>) bw.getWrappedInstance();
}
//這個bean是不是正在建立中
if (isSingletonCurrentlyInCreation(beanName) ||
(mbd.getFactoryBeanName() != null && isSingletonCurrentlyInCreation(mbd.getFactoryBeanName()))) {
return null;
}
Object instance = null;
try {
// Mark this bean as currently in creation, even if just partially.
//判斷是不是正在建立中的bean
beforeSingletonCreation(beanName);
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
//Bean例項化之前的一些處理 由Spring容器中InstantiationAwareBeanPostProcessor型別的例項來完成
instance = resolveBeforeInstantiation(beanName, mbd);
if (instance == null) {
//建立Bean的例項 看到這裡就很明瞭了
bw = createBeanInstance(beanName, mbd, null);
instance = bw.getWrappedInstance();
}
}
finally {
// Finished partial creation of this bean.
afterSingletonCreation(beanName);
}
FactoryBean<?> fb = getFactoryBean(beanName, instance);
if (bw != null) {
this.factoryBeanInstanceCache.put(beanName, bw);
}
return fb;
}
}
按照我們上面的分析,在doGetBeanNamesForType這個方法中,
private String[] doGetBeanNamesForType(ResolvableType type, boolean includeNonSingletons, boolean allowEagerInit)
allowEagerInit是一個非常重要的引數,這個引數可以控制要不要提前例項化一些Bean。為什麼這樣說呢?因為要不要呼叫isTypeMatch這個方法在很大程度上是由這個條件控制的
if (!mbd.isAbstract() && (allowEagerInit ||((mbd.hasBeanClass() || !mbd.isLazyInit() || isAllowEagerClassLoading())) &&!requiresEagerInitForType(mbd.getFactoryBeanName())))
通常我們的BeanDefinition都不是抽象類的,所以第一個條件為true,我們來看第二個條件,就是後面的那一大堆
(allowEagerInit ||((mbd.hasBeanClass() || !mbd.isLazyInit() || isAllowEagerClassLoading())) &&!requiresEagerInitForType(mbd.getFactoryBeanName()))
這個條件分為兩部分,是一個或的條件。如果allowEagerInit 為true的話,則整個條件為true,如果allowEagerInit 為false呢?mbd.hasBeanClass() || !mbd.isLazyInit() || isAllowEagerClassLoading()這三個條件基本上也是為true但是這個地方是一個與的條件,那麼還要判斷一下requiresEagerInitForType(mbd.getFactoryBeanName())這個方法是返回的true還是false。
private boolean requiresEagerInitForType(String factoryBeanName) {
//factoryBeanName不為null isFactoryBean(factoryBeanName) 是否為FactoryBeanBean
//containsSingleton(factoryBeanName)) 如果這個Bean已經被例項化了 這個返回的值為true
return (factoryBeanName != null && isFactoryBean(factoryBeanName) && !containsSingleton(factoryBeanName));
}
因為我們在這篇文章中的分析是根據型別獲取Bean會導致FactoryBean型別的Bean被提前例項化,所以factoryBeanName不為null,isFactoryBean(factoryBeanName) 為true,!containsSingleton(factoryBeanName)同樣為true。綜合requiresEagerInitForType這個方法返回true,那麼
((mbd.hasBeanClass() || !mbd.isLazyInit() || isAllowEagerClassLoading())) &&!requiresEagerInitForType(mbd.getFactoryBeanName())
會返回false,那這個這一大段的邏輯也就返回false了。
所以我們在根據型別獲取Bean的時候要謹慎一點,如果整個Spring容器已經啟動完成之後,根據型別獲取Bean是沒有問題的,如果是在Spring容器的啟動過程中根據型別獲取Bean可能會導致一些意想不到的結果。