初始化的過程,主要完成的工作是在容器中建立 BeanDefinition 資料映射,并沒有看到容器對Bean依賴關系進行注入
假設目前IoC容器已經載入使用者定義的Bean資訊,依賴注入主要發生在兩個階段
- 正常情況下,由使用者第一次向IoC容器索要Bean時觸發
- 但我們可以在 BeanDefinition 資訊中通過控制
lazy-init
1 getBean觸發的依賴注入
BeanFactory 是最原始的 ioc 容器，有以下方法 1.getBean2.判斷是否有 Bean，containsBean3.判斷是否單例 isSingleton。
BeanFactory 隻是對 ioc 容器最基本行為作了定義，而不關心 Bean 是怎樣定義和加載的。如果我們想要知道一個工廠具體産生對象的過程，則要看這個接口的實作類。
在基本的容器接口
BeanFactory
中,有一個
getBean
接口,這個接口的實作就是觸發依賴注入發生的地方.
為了進一步了解這個依賴注入的過程,我們從
DefaultListableBeanFactory
的基類
AbstractBeanFactory
入手去看看
getBean
的實作
// 這裡是對 BeanFactory 接口的實作，比如getBean接口方法
//這些getBean接口方法最終是通過調用doGetBean來實作的
public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException {
return doGetBean(name, requiredType, args, false);
}
//這裡是實際取得Bean的地方,也就是觸發依賴注入發生的地方
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
final String beanName = transformedBeanName(name);
Object bean;
//急切地檢查單例模式緩存手動注冊的單例
//先從緩存中取得Bean,處理那些已經被建立過的單例Bean,這種Bean不要重複建立
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 + "'");
}
}
//這裡的getObjectForBeanInstance完成的是FactoryBean的相關處理，以取得FactoryBean的相關處理,以取得FactoryBean的生産結果,BeanFactory和FactoryBean的差別已在前面講過,這個過程在後面還會詳細地分析
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 檢查IoC容器中的BeanDefinition是否存在,若在目前工廠不存在則去順着雙親BeanFactory鍊一直向上找
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);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
//根據Bean的名字取得BeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
//遞歸獲得目前Bean依賴的所有Bean（如果有的話）
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
getBean(dep);
}
}
//通過調用createBean方法建立Singleton bean執行個體
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
try {
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;
}
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
//這裡是建立prototype bean的地方
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
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 name '" + scopeName + "'");
}
try {
Object scopedInstance = scope.get(beanName, new ObjectFactory<Object>() {
@Override
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;
}
}
// Check if required type matches the type of the actual bean instance.
// 這裡對建立的Bean進行類型檢查,如果沒有問題,就傳回這個新建立的Bean,這個Bean已經是包含了依賴關系的Bean
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;
}
依賴注入就是在這裡被觸發的.
而依賴注入的發生是在容器中的
BeanDefinition
資料已經建立好的前提下進行的.
雖然我們可以用最簡單的方式來描述容器,那就是視其為一個HashMap,但隻能說這個HashMap是容器的最基本的資料結構,而不是IoC容器的全部
關于依賴注入過程會在下面詳解
圖1.1 依賴注入的過程
getBean是依賴注入的起點,之後會調用
AbstractAutowireCapableBeanFactory
中的
createBean
來生産需要的Bean,還對Bean初始化進行了處理,比如實作了在BeanDefinition中的init-method屬性定義,Bean後置處理器等.
下面通過createBean代碼了解這個過程
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
//這裡判斷需要建立的Bean是否可以被執行個體化,這個類是否可以通過類加載器來載入
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
//如果Bean配置了PostProcessor,那麼這裡傳回的是一個Proxy
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);
}
try {
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException ex) {
// A previously detected exception with proper bean creation context already...
throw ex;
}
catch (ImplicitlyAppearedSingletonException ex) {
// An IllegalStateException to be communicated up to DefaultSingletonBeanRegistry...
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
//接着到doCreate中去看看Bean是怎樣生成的
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
// Instantiate the bean.
//用來持有建立出來的Bean對象
BeanWrapper instanceWrapper = null;
//如果是單例,則先把緩存中的同名Bean清除
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
//這裡是建立Bean的地方,由createBeanInstance來完成
if (instanceWrapper == null) {
//根據指定bean使用對應的政策建立新的執行個體,如:工廠方法,構造函數自動注入,簡單初始化
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
mbd.postProcessed = true;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
//是否需要提前曝光:單例&允許循環依賴&目前bean正在建立中,檢測循環依賴
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");
}
//為避免後期循環依賴,可以在bean初始化完成前将建立執行個體的ObjectFactory加入工廠
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
//對bean再次依賴引用,主要應用SMartInstantialiationAware BeanPostProcessor,
//其中我們熟知的AOP就是在這裡将advice動态織入bean中,若無則直接傳回bean,不做任何處理
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// Initialize the bean instance.
//這裡是對Bean的初始化,依賴注入往往在這裡發生,這個exposedObject在初始化處理完後悔傳回作為依賴注入完成後的Bean
Object exposedObject = bean;
try {
//對bean進行填充,将各個屬性值注入,其中可能存在依賴于其他bean的屬性,則會遞歸初始化依賴bean
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
//調用初始化方法,比如init-method
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);
}
}
if (earlySingletonExposure) {
Object earlySingletonReference = getSingleton(beanName, false);
// earlySingletonReference 隻有在檢測到有循環依賴的情況下才會非空
if (earlySingletonReference != null) {
if (exposedObject == bean) {
//如果exposedObject 沒有在初始化方法中被改變,也就是沒有被增強
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
//檢測依賴
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
//因為bean建立後其所依賴的bean一定是已經建立的,actualDependentBeans非空則表示目前bean建立後其依賴的bean卻沒有全部建立完,也就是說存在循環依賴
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] in its raw version as part of a circular reference, but has eventually been " +
"wrapped. This means that said other beans do not use the final version of the " +
"bean. This is often the result of over-eager type matching - consider using " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
try {
//根據scope注冊bean
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
依賴注入其實包括兩個主要過程
- 生産Bean所包含的Java對象
- Bean對象生成之後,把這些Bean對象的依賴關系設定好
我們從上可以看到與依賴注入關系特别密切的方法有
-
createBeanInstance
-
populateBean
.
處理對各種Bean對象的屬性進行處理的過程（即依賴關系處理的過程）
先來看
createBeanInstance
源碼
/**
* Create a new instance for the specified bean, using an appropriate instantiation strategy:
* factory method, constructor autowiring, or simple instantiation.
*/
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
// 确認需要建立的Bean執行個體的類可以執行個體化
Class<?> beanClass = resolveBeanClass(mbd, beanName);
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);
}
//若工廠方法非空,則使用工廠方法政策對Bean進行執行個體化
if (mbd.getFactoryMethodName() != null) {
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
// Shortcut when re-creating the same bean...
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null) {
synchronized (mbd.constructorArgumentLock) {
//一個類有多個構造函數,每個構造函數都有不同的參數,是以調用前需要先根據參數鎖定構造函數或對應的工廠方法
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
//如果已經解析過則使用解析好的構造函數方法不需要再次鎖定
if (resolved) {
if (autowireNecessary) {
//構造函數自動注入
return autowireConstructor(beanName, mbd, null, null);
}
else {
//使用預設構造函數構造
return instantiateBean(beanName, mbd);
}
}
// 使用構造函數對Bean進行執行個體化
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
// No special handling: simply use no-arg constructor.
//使用預設的構造函數對Bean進行執行個體化
return instantiateBean(beanName, mbd);
}
//最常見的執行個體化過程instantiateBean
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
//使用預設的執行個體化政策對Bean進行執行個體化,預設的執行個體化政策是
//CglibSubclassingInstantiationStrategy,也就是使用CGLIB執行個體化Bean
try {
Object beanInstance;
final BeanFactory parent = this;
if (System.getSecurityManager() != null) {
beanInstance = AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
return getInstantiationStrategy().instantiate(mbd, beanName, parent);
}
}, getAccessControlContext());
}
else {
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);
}
}
這裡使用了CGLIB對Bean進行執行個體化
CGLIB是一個位元組碼生成器的類庫,它提供了一系列的API來提供生成和轉換Java的位元組碼的功能.
在Spring AOP中也使用CGLIB對Java的位元組碼進行增強.
在IoC容器中,要了解怎樣使用CGLIB來生成Bean對象,需要看一下
SimpleInstantiationStrategy
類.
它是Spring用來生成Bean對象的預設類,它提供了兩種執行個體化Bean對象的方法
- 通過BeanUtils,使用Java原生的反射功能
- 通過CGLIB
public class SimpleInstantiationStrategy implements InstantiationStrategy {
@Override
public Object instantiate(RootBeanDefinition bd, String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
if (bd.getMethodOverrides().isEmpty()) {
//這裡取得指定的構造器或者生成對象的工廠方法來對Bean進行執行個體化
Constructor<?> constructorToUse;
synchronized (bd.constructorArgumentLock) {
constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null) {
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(new PrivilegedExceptionAction<Constructor<?>>() {
@Override
public Constructor<?> run() throws Exception {
return clazz.getDeclaredConstructor((Class[]) null);
}
});
}
else {
constructorToUse = clazz.getDeclaredConstructor((Class[]) null);
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
//通過BeanUtils進行執行個體化，這個BeanUtils的執行個體化通過Constructor來執行個體化Bean,在BeanUtils中可以看到具體的調用ctor.newInstance(args)
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// 使用CGLIB來執行個體化對象
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
}
2 Bean之間依賴關系的處理
依賴關系處理的入口是前面提到的populateBean方法.
由于其中涉及的面太多,在這裡就不貼代碼了.
簡要介紹一下依賴關系處理的流程:
在populateBean方法中,
- 首先取得在BeanDefinition中設定的property值,然後開始依賴注入的過程
- 首先處理autowire的注入，可以by Name/Type，之後對屬性進行注入
- 接着需要對Bean Reference進行解析，在對ManageList、ManageSet、ManageMap等進行解析完之後，就已經為依賴注入準備好了條件，這是真正把Bean對象設定到它所依賴的另一個Bean屬性中去的地方，其中處理的屬性是各種各樣的
- 依賴注入發生在BeanWrapper的setPropertyValues中，具體的完成卻是在BeanWrapper的子類BeanWrapperImpl中實作的，它會完成Bean的屬性值的注入，其中包括對Array的注入、對List等集合類以及對非集合類的域進行注入
經過一系列的注入，這樣就完成了對各種Bean屬性的依賴注入過程
在Bean的建立和對象依賴注入的過程中，需要依據
BeanDefinition
中的資訊來遞歸地完成依賴注入。
從前面的幾個遞歸過程中可以看到，這些遞歸都是以
getBean
為入口
- 一個遞歸是在上下文中查找需要的Bean和建立Bean的遞歸調用
- 另一個遞歸是在依賴注入時，通過遞歸調用容器的getBean方法，得到目前Bean的依賴Bean，同時也觸發對依賴Bean的建立和注入。
在對Bean的屬性進行依賴注入時，解析的過程也是一個遞歸的過程
這樣，根據依賴關系，一層層地完成Bean的建立和注入，直到最後完成目前Bean的建立
有了這個頂層Bean的建立和對它屬性依賴注入的完成，意味着和目前Bean相關的整個依賴鍊的注入也就完成了
在Bean建立和依賴注入完成後,在容器中建立起一系列依靠依賴關系聯系起來的Bean，這個Bean已經不再是簡單的Java對象了。該Bean系列以及Bean之間的依賴關系建立完成之後，通過IoC的相關接口方法，就可以非常友善地供上層應用使用了。
2. lazy-init屬性和預執行個體化
在前面的
refresh
中，可看到調用了
finishBeanFactoryInitialization
來對配置了
lazy-init
的Bean進行處理
其實在這個方法中,封裝了對
lazy-init
屬性的處理,實際的處理是在
DefaultListableBeanFactory
這個基本容器的
preInstantiateSingleton
方法中完成的
該方法對單例Bean完成預執行個體化，這個預執行個體化的完成巧妙地委托給了容器來實作
如果需要預執行個體化,那麼就直接在這裡采用
getBean
去觸發依賴注入
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