Spring原始碼分析(三)(IoC容器的依賴注入)(1)
依賴注入的過程是使用者第一次向IoC容器索要Bean時才觸發的,當然也有例外,可以在BeanDefinition資訊中通過控制lazy-init屬性來讓容器完成對Bean的預例項化。這個預例項化實際上也是一個完成依賴注入的過程,但它是在初始化的過程中完成的。當用戶向IoC容器索要Bean時。基本的loC容器介面BeanFactory中,有一個getBean的介面定義,這個介面的實現就是觸發依賴注入發生的地方。 下面從DefaultListableBeanFactory的基類AbstractBeanFactory入手去看看getBean()的實現,如程式碼所示:
@Override public Object getBean(String name) throws BeansException { return doGetBean(name, null, null, false); } @Override public <T> T getBean(String name, Class<T> requiredType) throws BeansException { return doGetBean(name, requiredType, null, false); } @Override public Object getBean(String name, Object... args) throws BeansException { return doGetBean(name, null, args, false); } public <T> T getBean(String name, Class<T> requiredType, Object... args) throws BeansException { return doGetBean(name, requiredType, args, false); } //實際取得Bean的地方,也是觸發依賴注入發生的地方 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; // Eagerly check singleton cache for manually registered singletons. //從快取中取得Bean,處理那些已經被建立過得單例模式的Bean,對這種Bean的請求不需要重複建立 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的生產結果,BeanFactory和FactoryBean的區別: // BeanFactory是個Factory,也就是IOC容器或物件工廠,FactoryBean是個Bean。在Spring中,所有的Bean都是由BeanFactory(也就是IOC容器)來進行管理的。 // 但對FactoryBean而言,這個Bean是一個能生產或者修飾物件生成的工廠Bean 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); } // Check if bean definition exists in this factory. //這裡對IoC容器中的BeanDefintion是否存在進行檢查,檢查是否能在當前BeanFactory中取得需要的Bean。 // 如果當前的工廠中取不到,則到雙親BeanFactory中取,如果當前的雙親工廠取不到,那就順著雙親BeanFactory鏈一直向上找 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. //獲取當前Bean的所有依賴Bean,這樣會觸發getBean的遞迴呼叫,直到取到一個沒有任何依賴的Bean位置 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); try { getBean(dep); } catch (NoSuchBeanDefinitionException ex) { throw new BeanCreationException(mbd.getResourceDescription(), beanName, "'" + beanName + "' depends on missing bean '" + dep + "'", ex); } } } // Create bean instance. //這裡會通過呼叫crateBean方法建立Singleton bean的例項,這裡有一個回撥函式 //下面會進入createBean進行詳細分析 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); } //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; } } //這裡對建立的Bean進行型別檢查,如果沒問題就返回這個新建立Bean,這個Bean已經是包含了依賴關係的Bean // Check if required type matches the type of the actual bean instance. if (requiredType != null && bean != null && !requiredType.isInstance(bean)) { 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; }
這個就是依賴注入的入口,在這裡觸發了依賴注入,而依賴注人的發生是在容器中的BeanDefinition資料已經建立好的前提下進行的。“ 程式=資料+演算法 ”很經典的一句話,前面的BeanDefinition就是資料,下面看看這些資料是怎樣為依賴注入服務的。雖然依賴注入的過程不涉及複雜的演算法問題,但這個過程也不簡單,因為對於IoC容器的使用,Spring提供了許多的引數配置,每一個引數配置實際上代表了一個IoC容器的實現特性,這些特性的實現很多都需要在依賴注入的過程中或者對Bean進行生命週期管理的過程中完成。儘管可以用最簡單的方式來描述loC容器,將它視為一個hashMap ,但只能說這個hashMap是容器的最基本的資料結構,而不是IoC容器的全部。Spring IoC容器作為一個產品,其價值體現在一系列相關的產品特性上,這些產品特性以依賴反轉模式的實現為核心,為使用者更好地使用依賴反轉提供便利,從而實現了一個完整的loC容器產品。這些產品特性的實現並不是一個簡單的過程,它提供了一個成熟的IoC容器產品供使用者使用。所以,儘管Spring IoC容器沒有什麼獨特的演算法, 但卻可以看成是一個成功的軟體工程產品。
AbstractAutowireCapableBeanFactory
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.
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
//判斷需要建立的Bean是否可以例項化,這個類是否可以通過類裝載器來載入
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,那麼這裡返回的是一個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);
}
//建立Bean的呼叫
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
//然後到doCreteBean中看看Bean是如何生成的
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
// Instantiate the bean.
//BeanWrapper是用來持有創建出來的Bean物件的
BeanWrapper instanceWrapper = null;
//如果是Singleton先把快取中同名的清除
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
//這是建立Bean的地方由createBeanInstance來完成
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
mbd.resolvedTargetType = beanType;
// Allow post-processors to modify the merged bean definition.
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;
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
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");
}
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
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在初始化處理完後會返回作為依賴注入完成後的bean
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
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);
if (earlySingletonReference != null) {
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<String>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
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 {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
在這裡我們看到,與依賴注入關係特別密切的方法有createBeanlnstance和populateBean,下面分別介紹這兩個方法。在createBeanlnstance中生成了Bean所包含的Java物件,這個物件的生成有很多種不同的方式,可以通過工廣方法生成,也可以通過容器的autowire特性生成,這些生成方式都是自相關的BeanDefinition來指定的。如以下程式碼所示,可以看到不同生成方式對應的實現:
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
// Make sure bean class is actually resolved at this point.
//確認需要建立的Bean例項的類可以例項化
Class<?> beanClass = resolveBeanClass(mbd, beanName);
//這裡使用工廠對Bean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());
}
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) {
if (mbd.resolvedConstructorOrFactoryMethod != null) {
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
}
}
}
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
//使用建構函式進行例項化
// Need to determine the constructor...
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
//使用預設的建構函式對Bean進行例項化
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}
//最常見的例項化過程instantiateBean
protected BeanWrapper instantiateBean(final String beanName, final RootBeanDefinition mbd) {
try {
//使用預設的例項化策略對Bean進行例項化,預設例項策略是
// CglibSubclassingInstantiationStrategy,CGLib來對Bean進行例項化
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);
}
這裡用CGLIB對Bean進行例項化。CGLIB是一個常用的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loC容器中,要了解怎樣使用CGLIB來生成Bean物件,需要看一下SimpleInstantiationStrategy類。這個Strategy是Spring用來生成Bean物件的預設類,它提供了兩種例項化Java物件的方法,一種是通過BeanUtils ,它使用了JVM的反射功能, 一種是通過前面提到的CGLIB來生成,如程式碼所示:
SimpleInstantiationStrategy
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物件的工廠方法來對Bean進行例項化
Constructor<?> constructorToUse;
synchronized (bd.constructorArgumentLock) {
//這裡取得指定的構造器或者生成物件的工廠方法來對Bean進行例項化
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進行例項化,這個BeanUtils的例項化通過Constructor來例項化Bean,
// 在BeanUtils中可以看到具體的呼叫ctor.newInstance(args);
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
//使用CGLIB來例項化物件
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
在CglibSubclassinglnstantiationStrategy 中可以看到具體的例項化過程和CGLIB的使用方法這裡的Enhancer類, 是CGLIB的類,通過這個Enhancer生成Java物件,使用的是Enhancer的create方法,如程式碼所示。
CglibSubclassingInstantiationStrategy
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, String beanName, BeanFactory owner) {
return instantiateWithMethodInjection(bd, beanName, owner, null);
}
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, String beanName, BeanFactory owner,
Constructor<?> ctor, Object... args) {
// Must generate CGLIB subclass...
return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);
}
public Object instantiate(Constructor<?> ctor, Object... args) {
//生成Enhancer物件,併為Enhancer物件設定生成Java物件的引數,比如基類、回撥方法等
Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
Object instance;
if (ctor == null) {
// 通過BeanUtils進行例項化,這個BeanUtils的例項化通過Constructor來例項化Bean
instance = BeanUtils.instantiateClass(subclass);
}
else {
try {
Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());
instance = enhancedSubclassConstructor.newInstance(args);
}
catch (Exception ex) {
throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
"Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);
}
}
// SPR-10785: set callbacks directly on the instance instead of in the
// enhanced class (via the Enhancer) in order to avoid memory leaks.
//設定回撥
Factory factory = (Factory) instance;
factory.setCallbacks(new Callback[] {NoOp.INSTANCE,
new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
return instance;
}
到這裡已經分析了例項化Bean物件的整個過程。在例項化Bean物件生成的基礎上,再介紹一下Spring是怎樣對這些物件進行處理的,也就是Bean物件生成以後,怎樣把這些Bean物件的依賴關係設定好,完成整個依賴注入過程。這個過程涉及對各種Bean物件的屬性的處理過程(即依賴關係處理的過程),這些依賴關係處理的依據就是已經解析得到的BeanDefinition。要詳細瞭解這個過程,需要回到前面的populateBean方法,這個方法在AbstractAutowireCapableBeanFactory中的實現如程式碼所示。
參考《SPRING技術內幕》