目錄
代碼塊3:doGetObjectFromFactoryBean
代碼塊4:postProcessObjectFromFactoryBean
代碼塊9:beforeSingletonCreation、afterSingletonCreation
代碼塊11:beforePrototypeCreation、afterPrototypeCreation
Spring IoC源碼學習全系列
Spring IoC源碼學習:ApplicationContext 刷新前的配置
Spring IoC源碼學習:obtainFreshBeanFactory詳解
Spring IoC源碼學習:parseDefaultElement詳解
Spring IoC源碼學習:parseCustomElement詳解
Spring IoC源碼學習:obtainFreshBeanFactory詳解
Spring IoC源碼學習:invokeBeanFactoryPostProcessors詳解
Spring IoC源碼學習:registerBeanPostProcessors詳解
Spring IoC源碼學習:finishBeanFactoryInitialization詳解
Spring IoC源碼學習:createBean詳解(上)
Spring IoC源碼學習:createBean詳解(下)
Spring IoC源碼學習:finishRefresh 詳解
前言
接着 Spring IoC:finishBeanFactoryInitialization詳解,我們正式開始學習獲取 bean 實例方法,該方法是 Spring 最核心的方法。
正文
單擊 preInstantiateSingletons 方法裏的 getBean(beanName) 代碼,進入該方法。
getBean
@Override
public Object getBean(String name) throws BeansException {
// 獲取name對應的bean實例,如果不存在,則創建一個
return doGetBean(name, null, null, false);
}
見 doGetBean 方法詳解。
doGetBean
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
// 1.解析beanName,主要是解析別名、去掉FactoryBean的前綴“&”
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 2.嘗試從緩存中獲取beanName對應的實例
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
// 3.如果beanName的實例存在於緩存中
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 + "'");
}
}
// 3.1 返回beanName對應的實例對象(主要用於FactoryBean的特殊處理,普通Bean會直接返回sharedInstance本身)
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
} else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 4.scope爲prototype的循環依賴校驗:如果beanName已經正在創建Bean實例中,而此時我們又要再一次創建beanName的實例,則代表出現了循環依賴,需要拋出異常。
// 例子:如果存在A中有B的屬性,B中有A的屬性,那麼當依賴注入的時候,就會產生當A還未創建完的時候因爲對於B的創建再次返回創建A,造成循環依賴
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 5.獲取parentBeanFactory
BeanFactory parentBeanFactory = getParentBeanFactory();
// 5.1 如果parentBeanFactory存在,並且beanName在當前BeanFactory不存在Bean定義,則嘗試從parentBeanFactory中獲取bean實例
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
// 5.2 將別名解析成真正的beanName
String nameToLookup = originalBeanName(name);
// 5.3 嘗試在parentBeanFactory中獲取bean對象實例
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);
}
}
if (!typeCheckOnly) {
// 6.如果不是僅僅做類型檢測,而是創建bean實例,這裏要將beanName放到alreadyCreated緩存
markBeanAsCreated(beanName);
}
try {
// 7.根據beanName重新獲取MergedBeanDefinition(步驟6將MergedBeanDefinition刪除了,這邊獲取一個新的)
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
// 7.1 檢查MergedBeanDefinition
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 8.拿到當前bean依賴的bean名稱集合,在實例化自己之前,需要先實例化自己依賴的bean
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
// 8.1 遍歷當前bean依賴的bean名稱集合
for (String dep : dependsOn) {
// 8.2 檢查dep是否依賴於beanName,即檢查是否存在循環依賴
if (isDependent(beanName, dep)) {
// 8.3 如果是循環依賴則拋異常
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 8.4 將dep和beanName的依賴關係註冊到緩存中
registerDependentBean(dep, beanName);
// 8.5 獲取dep對應的bean實例,如果dep還沒有創建bean實例,則創建dep的bean實例
getBean(dep);
}
}
// Create bean instance.
// 9.針對不同的scope進行bean的創建
if (mbd.isSingleton()) {
// 9.1 scope爲singleton的bean創建(新建了一個ObjectFactory,並且重寫了getObject方法)
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException { //
try {
// 9.1.1 創建Bean實例
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;
}
}
});
// 9.1.2 返回beanName對應的實例對象
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
} else if (mbd.isPrototype()) {
// 9.2 scope爲prototype的bean創建
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
// 9.2.1 創建實例前的操作(將beanName保存到prototypesCurrentlyInCreation緩存中)
beforePrototypeCreation(beanName);
// 9.2.2 創建Bean實例
prototypeInstance = createBean(beanName, mbd, args);
} finally {
// 9.2.3 創建實例後的操作(將創建完的beanName從prototypesCurrentlyInCreation緩存中移除)
afterPrototypeCreation(beanName);
}
// 9.2.4 返回beanName對應的實例對象
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
} else {
// 9.3 其他scope的bean創建,可能是request之類的
// 9.3.1 根據scopeName,從緩存拿到scope實例
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope name '" + scopeName + "'");
}
try {
// 9.3.2 其他scope的bean創建(新建了一個ObjectFactory,並且重寫了getObject方法)
Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
// 9.3.3 創建實例前的操作(將beanName保存到prototypesCurrentlyInCreation緩存中)
beforePrototypeCreation(beanName);
try {
// 9.3.4 創建bean實例
return createBean(beanName, mbd, args);
} finally {
// 9.3.5 創建實例後的操作(將創建完的beanName從prototypesCurrentlyInCreation緩存中移除)
afterPrototypeCreation(beanName);
}
}
});
// 9.3.6 返回beanName對應的實例對象
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) {
// 如果創建bean實例過程中出現異常,則將beanName從alreadyCreated緩存中移除
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
}
// Check if required type matches the type of the actual bean instance.
// 10.檢查所需類型是否與實際的bean對象的類型匹配
if (requiredType != null && bean != null && !requiredType.isInstance(bean)) {
try {
// 10.1 類型不對,則嘗試轉換bean類型
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());
}
}
// 11.返回創建出來的bean實例對象
return (T) bean;
}
1.解析 beanName,主要是解析別名、去掉 FactoryBean 的修飾符 “&”,在 Spring IoC:finishBeanFactoryInitialization詳解 中的代碼塊4已解析過。
2.嘗試從緩存中獲取 beanName 對應的實例,在 Spring IoC:finishBeanFactoryInitialization詳解 中的代碼塊7已解析過。
3.1 返回 beanName 對應的實例對象(主要用於 FactoryBean 的特殊處理,普通 bean 會直接返回 sharedInstance 本身),見代碼塊1詳解。
6.如果不是僅僅做類型檢測,而是創建 bean 實例,這裏要將 beanName 放到 alreadyCreated 緩存,見代碼塊5詳解。
7.根據 beanName 重新獲取 MergedBeanDefinition,在 Spring IoC:finishBeanFactoryInitialization詳解 中的代碼塊2已解析過。
8.2 檢查 dep 是否依賴於 beanName,即檢查是否存在循環依賴,見代碼塊6詳解。
8.4 將 dep 和 beanName 的依賴關係註冊到緩存中,見代碼塊7詳解。
9.1 scope 爲 singleton 的 bean 創建(新建了一個 ObjectFactory,並且重寫了 getObject 方法),見代碼塊8詳解。
9.1.1、9.2.2、9.3.4 創建 bean 實例,限於篇幅,在下篇文章單獨解析。
9.1.2、9.2.4、9.3.6 返回 beanName 對應的實例對象,見代碼塊1詳解。
9.2.1 scope 爲 prototype 時創建實例前的操作、9.2.3 scope 爲 prototype 時 創建實例後的操作,相對應的兩個方法,見代碼塊11詳解。
代碼塊1:getObjectForBeanInstance
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
// Don't let calling code try to dereference the factory if the bean isn't a factory.
// 1.如果name以“&”爲前綴,但是beanInstance不是FactoryBean,則拋異常
if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
// Now we have the bean instance, which may be a normal bean or a FactoryBean.
// If it's a FactoryBean, we use it to create a bean instance, unless the
// caller actually wants a reference to the factory.
// 2.1 如果beanInstance不是FactoryBean(也就是普通bean),則直接返回beanInstance
// 2.2 如果beanInstance是FactoryBean,並且name以“&”爲前綴,則直接返回beanInstance(以“&”爲前綴代表想獲取的是FactoryBean本身)
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
// 3.走到這邊,代表beanInstance是FactoryBean,但name不帶有“&”前綴,表示想要獲取的是FactoryBean創建的對象實例
Object object = null;
if (mbd == null) {
// 4.如果mbd爲空,則嘗試從factoryBeanObjectCache緩存中獲取該FactoryBean創建的對象實例
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
// 5.只有beanInstance是FactoryBean才能走到這邊,因此直接強轉
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
// 6.mbd爲空,但是該bean的BeanDefinition在緩存中存在,則獲取該bean的MergedBeanDefinition
mbd = getMergedLocalBeanDefinition(beanName);
}
// 7.mbd是否是合成的(這個字段比較複雜,mbd正常情況都不是合成的,也就是false,有興趣的可以自己查閱資料看看)
boolean synthetic = (mbd != null && mbd.isSynthetic());
// 8.從FactoryBean獲取對象實例
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
// 9.返回對象實例
return object;
}
如果對 FactoryBean 不熟悉的,可以回頭去看 Spring IoC:finishBeanFactoryInitialization詳解 中對 FactoryBean 的簡單介紹。
6.mbd 爲空,但是該 bean 的 BeanDefinition 在緩存中存在,則獲取該 bean 的 MergedBeanDefinition,在 Spring IoC:finishBeanFactoryInitialization詳解 中的代碼塊2已經解析過。
8.從 FactoryBean 獲取對象實例,見代碼塊2詳解。
代碼塊2:getObjectFromFactoryBean
protected Object getObjectFromFactoryBean(FactoryBean<?> factory, String beanName, boolean shouldPostProcess) {
// 1.如果是單例,並且已經存在於單例對象緩存中
if (factory.isSingleton() && containsSingleton(beanName)) {
synchronized (getSingletonMutex()) {
// 2.從FactoryBean創建的單例對象的緩存中獲取該bean實例
Object object = this.factoryBeanObjectCache.get(beanName);
if (object == null) {
// 3.調用FactoryBean的getObject方法獲取對象實例
object = doGetObjectFromFactoryBean(factory, beanName);
// Only post-process and store if not put there already during getObject() call above
// (e.g. because of circular reference processing triggered by custom getBean calls)
Object alreadyThere = this.factoryBeanObjectCache.get(beanName);
// 4.如果該beanName已經在緩存中存在,則將object替換成緩存中的
if (alreadyThere != null) {
object = alreadyThere;
} else {
if (object != null && shouldPostProcess) {
try {
// 5.對bean實例進行後置處理,執行所有已註冊的BeanPostProcessor的postProcessAfterInitialization方法
object = postProcessObjectFromFactoryBean(object, beanName);
} catch (Throwable ex) {
throw new BeanCreationException(beanName,
"Post-processing of FactoryBean's singleton object failed", ex);
}
}
// 6.將beanName和object放到factoryBeanObjectCache緩存中
this.factoryBeanObjectCache.put(beanName, (object != null ? object : NULL_OBJECT));
}
}
// 7.返回object對象實例
return (object != NULL_OBJECT ? object : null);
}
} else {
// 8.調用FactoryBean的getObject方法獲取對象實例
Object object = doGetObjectFromFactoryBean(factory, beanName);
if (object != null && shouldPostProcess) {
try {
// 9.對bean實例進行後置處理,執行所有已註冊的BeanPostProcessor的postProcessAfterInitialization方法
object = postProcessObjectFromFactoryBean(object, beanName);
} catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of FactoryBean's object failed", ex);
}
}
// 10.返回object對象實例
return object;
}
}
3.調用 FactoryBean 的 getObject 方法獲取對象實例,見代碼塊3詳解。
5.對 bean 實例進行後續處理,執行所有已註冊的 BeanPostProcessor 的 postProcessAfterInitialization 方法,見代碼塊4詳解。
代碼塊3:doGetObjectFromFactoryBean
private Object doGetObjectFromFactoryBean(final FactoryBean<?> factory, final String beanName)
throws BeanCreationException {
Object object;
try {
// 1.調用FactoryBean的getObject方法獲取bean對象實例
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
@Override
public Object run() throws Exception {
// 1.1 帶有權限驗證的
return factory.getObject();
}
}, acc);
} catch (PrivilegedActionException pae) {
throw pae.getException();
}
} else {
// 1.2 不帶權限
object = factory.getObject();
}
} catch (FactoryBeanNotInitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
} catch (Throwable ex) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
// Do not accept a null value for a FactoryBean that's not fully
// initialized yet: Many FactoryBeans just return null then.
// 2.getObject返回的是空值,並且該FactoryBean正在初始化中,則直接拋異常,不接受一個尚未完全初始化的FactoryBean的getObject返回的空值
if (object == null && isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
// 3.返回創建好的bean對象實例
return object;
}
很簡單的方法,就是直接調用 FactoryBean 的 getObject 方法來獲取到對象實例。
細心的同學可以發現,該方法是以 do 開頭,看過 Spring IoC:源碼學習總覽 的同學知道,我在總覽裏就特別提到以 do 開頭的方法是最終進行實際操作的方法,例如本方法就是 FactoryBean 最終實際進行創建 bean 對象實例的方法。
代碼塊4:postProcessObjectFromFactoryBean
@Override
protected Object postProcessObjectFromFactoryBean(Object object, String beanName) {
return applyBeanPostProcessorsAfterInitialization(object, beanName);
}
@Override
public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)
throws BeansException {
Object result = existingBean;
// 1.遍歷所有註冊的BeanPostProcessor實現類,調用postProcessAfterInitialization方法
for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {
// 2.在bean初始化後,調用postProcessAfterInitialization方法
result = beanProcessor.postProcessAfterInitialization(result, beanName);
if (result == null) {
// 3.如果返回null,則不會調用後續的BeanPostProcessors
return result;
}
}
return result;
}
這邊走的是 AbstractAutowireCapableBeanFactory 裏的方法。通過前面的介紹,我們知道創建的 BeanFactory 爲 DefaultListableBeanFactory,而 DefaultListableBeanFactory 繼承了 AbstractAutowireCapableBeanFactory,因此這邊會走 AbstractAutowireCapableBeanFactory 的重寫方法。
在 Spring IoC:registerBeanPostProcessors詳解 中已經學過 BeanPostProcessor,在創建完 bean 實例後,會執行 BeanPostProcessor 的 postProcessAfterInitialization 方法。
代碼塊5:markBeanAsCreated
protected void markBeanAsCreated(String beanName) {
if (!this.alreadyCreated.contains(beanName)) {
synchronized (this.mergedBeanDefinitions) {
// 1.如果alreadyCreated緩存中不包含beanName
if (!this.alreadyCreated.contains(beanName)) {
// Let the bean definition get re-merged now that we're actually creating
// the bean... just in case some of its metadata changed in the meantime.
// 2.將beanName的MergedBeanDefinition從mergedBeanDefinitions緩存中移除,
// 在之後重新獲取MergedBeanDefinition,避免BeanDefinition在創建過程中發生變化
clearMergedBeanDefinition(beanName);
// 3.將beanName添加到alreadyCreated緩存中,代表該beanName的bean實例已經創建(或即將創建)
this.alreadyCreated.add(beanName);
}
}
}
}
protected void clearMergedBeanDefinition(String beanName) {
this.mergedBeanDefinitions.remove(beanName);
}
2.這邊會將 beanName 對應的 MergedBeanDefinition 移除,然後在之後的代碼重新獲取,主要是爲了使用最新的 MergedBeanDefinition 來進行創建操作。
代碼塊6:isDependent
private boolean isDependent(String beanName, String dependentBeanName, Set<String> alreadySeen) {
// 已經檢查過的直接跳過
if (alreadySeen != null && alreadySeen.contains(beanName)) {
return false;
}
// 1.將別名解析爲真正的名稱
String canonicalName = canonicalName(beanName);
// 2.拿到依賴canonicalName的beanName集合
Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName);
// 3.如果dependentBeans爲空,則兩者必然還未確定依賴關係,返回false
if (dependentBeans == null) {
return false;
}
// 4.如果dependentBeans包含dependentBeanName,則表示兩者已確定依賴關係,返回true
if (dependentBeans.contains(dependentBeanName)) {
return true;
}
// 5.循環檢查,即檢查依賴canonicalName的所有beanName是否存在被dependentBeanName依賴的(即隔層依賴)
for (String transitiveDependency : dependentBeans) {
if (alreadySeen == null) {
alreadySeen = new HashSet<String>();
}
// 6.已經檢查過的添加到alreadySeen,避免重複檢查
alreadySeen.add(beanName);
if (isDependent(transitiveDependency, dependentBeanName, alreadySeen)) {
return true;
}
}
return false;
}
這邊引入了一個緩存 dependentBeanMap:beanName -> 所有依賴 beanName 對應的 bean 的 beanName 集合。內容比較簡單,就是檢查依賴 beanName 的集合中是否包含 dependentBeanName,隔層依賴也算。例如:A 依賴了 B,B 依賴了 C,則 A 也算依賴了 C。
代碼塊7:registerDependentBean
public void registerDependentBean(String beanName, String dependentBeanName) {
// A quick check for an existing entry upfront, avoiding synchronization...
// 1.解析別名
String canonicalName = canonicalName(beanName);
// 2.拿到依賴canonicalName的beanName集合
Set<String> dependentBeans = this.dependentBeanMap.get(canonicalName);
// 3.如果dependentBeans包含dependentBeanName,則表示依賴關係已經存在,直接返回
if (dependentBeans != null && dependentBeans.contains(dependentBeanName)) {
return;
}
// No entry yet -> fully synchronized manipulation of the dependentBeans Set
// 4.如果依賴關係還沒有註冊,則將兩者的關係註冊到dependentBeanMap和dependenciesForBeanMap緩存
synchronized (this.dependentBeanMap) {
// 4.1 將dependentBeanName添加到依賴canonicalName的beanName集合中
dependentBeans = this.dependentBeanMap.get(canonicalName);
if (dependentBeans == null) {
dependentBeans = new LinkedHashSet<String>(8);
this.dependentBeanMap.put(canonicalName, dependentBeans);
}
dependentBeans.add(dependentBeanName);
}
synchronized (this.dependenciesForBeanMap) {
// 4.2 將canonicalName添加到dependentBeanName依賴的beanName集合中
Set<String> dependenciesForBean = this.dependenciesForBeanMap.get(dependentBeanName);
if (dependenciesForBean == null) {
dependenciesForBean = new LinkedHashSet<String>(8);
this.dependenciesForBeanMap.put(dependentBeanName, dependenciesForBean);
}
dependenciesForBean.add(canonicalName);
}
}
這邊又引入了一個跟 dependentBeanMap 類似的緩存,dependenciesForBeanMap:beanName -> beanName 對應的 bean 依賴的所有 bean 的 beanName 集合。
這兩個緩存很容易搞混,舉個簡單例子:例如 B 依賴了 A,則 dependentBeanMap 緩存中應該存放一對映射:其中 key 爲 A,value 爲含有 B 的 Set;而 dependenciesForBeanMap 緩存中也應該存放一對映射:其中 key 爲:B,value 爲含有 A 的 Set。
代碼塊8:getSingleton
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "'beanName' must not be null");
// 1.加鎖,避免重複創建單例對象
synchronized (this.singletonObjects) {
// 2.首先檢查beanName對應的bean實例是否在緩存中存在,如果已經存在,則直接返回
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
// 3.beanName對應的bean實例不存在於緩存中,則進行Bean的創建
if (this.singletonsCurrentlyInDestruction) {
// 4.當bean工廠的單例處於destruction狀態時,不允許進行單例bean創建,拋出異常
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while 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 + "'");
}
// 5.創建單例前的操作
beforeSingletonCreation(beanName);
boolean newSingleton = false;
// suppressedExceptions用於記錄異常相關信息
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<Exception>();
}
try {
// 6.執行singletonFactory的getObject方法獲取bean實例
singletonObject = singletonFactory.getObject();
// 標記爲新的單例對象
newSingleton = true;
} catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
} catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
} finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
// 7.創建單例後的操作
afterSingletonCreation(beanName);
}
if (newSingleton) {
// 8.如果是新的單例對象,將beanName和對應的bean實例添加到緩存中(singletonObjects、registeredSingletons)
addSingleton(beanName, singletonObject);
}
}
// 9.返回創建出來的單例對象
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
}
5.創建單例前的操作,7.創建單例後的操作,這兩個方法是對應的,見代碼塊9詳解。
6.執行 singletonFactory 的 getObject 方法獲取 bean 實例,該方法會走文章開頭 doGetBean 方法的註釋 9.1.1。
8.如果是新的單例對象,將 beanName 和對應的單例對象添加到緩存中,見代碼塊10詳解。
代碼塊9:beforeSingletonCreation、afterSingletonCreation
protected void beforeSingletonCreation(String beanName) {
// 先校驗beanName是否爲要在創建檢查排除掉的(inCreationCheckExclusions緩存),如果不是,
// 則將beanName加入到正在創建bean的緩存中(Set),如果beanName已經存在於該緩存,會返回false拋出異常(這種情況出現在構造器的循環依賴)
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
protected void afterSingletonCreation(String beanName) {
// 先校驗beanName是否爲要在創建檢查排除掉的(inCreationCheckExclusions緩存),如果不是,
// 則將beanName從正在創建bean的緩存中(Set)移除,如果beanName不存在於該緩存,會返回false拋出異常
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
}
}
inCreationCheckExclusions 是要在創建檢查排除掉的 beanName 集合,正常爲空,可以不管。這邊主要是引入了 singletonsCurrentlyInCreation 緩存:當前正在創建的 bean 的 beanName 集合。在 beforeSingletonCreation 方法中,通過添加 beanName 到該緩存,可以預防出現構造器循環依賴的情況。
爲什麼無法解決構造器循環依賴?
我們之前在 Spring IoC:finishBeanFactoryInitialization詳解 中的代碼塊7提過,getSingleton 方法是解決循環引用的核心代碼。解決邏輯的第一句話:“我們先用構造函數創建一個 “不完整” 的 bean 實例”,從這句話可以看出,構造器循環依賴是無法解決的,因爲當構造器出現循環依賴,我們連 “不完整” 的 bean 實例都構建不出來。Spring 能解決的循環依賴有:通過 setter 注入的循環依賴、通過屬性注入的循環依賴。
代碼塊10:addSingleton
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
// 1.添加到單例對象緩存
this.singletonObjects.put(beanName, (singletonObject != null ? singletonObject : NULL_OBJECT));
// 2.將單例工廠緩存移除(已經不需要)
this.singletonFactories.remove(beanName);
// 3.將早期單例對象緩存移除(已經不需要)
this.earlySingletonObjects.remove(beanName);
// 4.添加到已經註冊的單例對象緩存
this.registeredSingletons.add(beanName);
}
}
代碼塊11:beforePrototypeCreation、afterPrototypeCreation
protected void beforePrototypeCreation(String beanName) {
// 1.拿到當前線程中正在創建的prototype的bean的beanName集合
Object curVal = this.prototypesCurrentlyInCreation.get();
// 2.如果爲空,則將ThreadLocal設置成當前的beanName
if (curVal == null) {
this.prototypesCurrentlyInCreation.set(beanName);
}
// 3.如果不爲空,並且是String類型,則代表目前只有一個beanName,將之前和當前的一起封裝成Set<String>,設置到ThreadLocal中
else if (curVal instanceof String) {
Set<String> beanNameSet = new HashSet<String>(2);
beanNameSet.add((String) curVal);
beanNameSet.add(beanName);
this.prototypesCurrentlyInCreation.set(beanNameSet);
}
// 4.如果不爲空,並且不是String,則必然是Set<String>類型,將當前的beanName加到Set中去
else {
Set<String> beanNameSet = (Set<String>) curVal;
beanNameSet.add(beanName);
}
}
protected void afterPrototypeCreation(String beanName) {
// 1.拿到當前線程中正在創建的prototype的bean的beanName集合
Object curVal = this.prototypesCurrentlyInCreation.get();
// 2.如果是String類型,則代表目前只有一個beanName,則直接移除
if (curVal instanceof String) {
this.prototypesCurrentlyInCreation.remove();
}
else if (curVal instanceof Set) {
// 3.如果是Set類型,則從Set從移除beanName
Set<String> beanNameSet = (Set<String>) curVal;
beanNameSet.remove(beanName);
if (beanNameSet.isEmpty()) {
this.prototypesCurrentlyInCreation.remove();
}
}
}
該方法和代碼塊9的兩個方法類似。主要是在進行 bean 實例的創建前,將 beanName 添加到 prototypesCurrentlyInCreation 緩存;bean 實例創建後,將 beanName 從 prototypesCurrentlyInCreation 緩存中移除。這邊 prototypesCurrentlyInCreation 存放的類型爲 Object,在只有一個 beanName 的時候,直接存該 beanName,也就是 String 類型;當有多個 beanName 時,轉成 Set 來存放。
總結
本文介紹了獲取 bean 實例的大部分內容,包括先從緩存中檢查、 FactoryBean 的 bean 創建、實例化自己的依賴(depend-on 屬性)、創建 bean 實例的前後一些標記等,在下篇文章中,將解析創建 bean 的內容。