通過之前spring容器啓動過程分析,以ClassPathXmlApplicationContext爲例說明,我們看到在最後的
finishBeanFactoryInitialization完成了對單例bean的實例化步驟。
其最終調用了：
DefaultListableBeanFactory.preInstantiateSingletons
其代碼邏輯如下：
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
// 轉換bean的名稱，就是將看是否是alias名稱，是否有工廠bean前綴，最後轉換爲原始的beanName
final String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 這個時候獲取循環依賴的單例，某個單例可能這時候實例化結束開始填充屬性，需要獲取相關屬性，
// 比如A依賴B，B也依賴A，A實例化之後需要填充屬性，填充屬性需要B的實例，但是B在實例化之後填充屬性需要A，
// 這時候在getSingleton的時候能夠獲取到A對應的實例的引用，即解決了循環依賴
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isTraceEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
}
}
// 如果sharedInstance是factorybean的話，通過工廠bean獲取實際的bean實例
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
// 緩存中沒有，實例化bean
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 多例正在創建
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 查看當前beanDefinition是否在父容器中，如果有父容器的話
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
return (T) parentBeanFactory.getBean(nameToLookup);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
// 將GenericBeanDefinition轉換爲RootBeanDefinition，
// 如果有parent屬性，則會將parent的BeanDefinition和
// 當前的BeanDefinition整合成一個BeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
// 不能形成循環依賴，下面我們能看到，會先實例化當前beanName依賴的depBean，
// 如果depBean又依賴當前beanName，則在實例化依賴bean時會形成死循環
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 註冊依賴關係，將當前beanName和depBeanName相互註冊，
// 形成 beanName -> depBeanNames 和 depBeanName->beanNames雙重映射
registerDependentBean(dep, beanName);
try {
// 實例化 depBean
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
// 先判斷bean是否是singleton，如果是通過ObjectFactory獲取單例實例
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
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;
}
});
// 這裏判斷如果是FactoryBean，通過FactoryBean.getObject獲取bean實例，
// 如果不是FactoryBean，直接返回Bean本身
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 如果是prototype類型
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
// prototypesCurrentlyInCreation是一個ThreadLocal類型，將beanName添加到裏面
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, () -> {
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.
// 如果調用方法時傳入了requiredType在這裏進行類型的轉換
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
if (logger.isTraceEnabled()) {
logger.trace("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
我們發現上述過程主要如下：
- 從單例的緩存中獲取bean，如果有的話，判斷是否是工程bean，如果是工廠bean，獲取實際的bean的實例
- 判斷當前線程是否有對應的prototype的bean正在創建，如果正在創建，拋出異常
- 如果有父容器，查看是否能夠從父容器中獲取bean的實例
- 將GenericBeanDefinition轉換爲RootBeanDefinition，如果有parent屬性，則會將parent的BeanDefinition和當前的BeanDefinition整合成一個BeanDefinition
- 查看當前beanName的depend-on，通過getBean實例化依賴的bean
- 如果是單例，通過工廠方法獲取單例bean的實例（工廠方法通過createBean放回bean實例）
- 如果是prototype原型bean，直接通過createBean獲取實例
- 其他scope，通過對應的scope策略來獲取
- 如果傳入了類型限制，將上述步驟中獲得的bean的實例進行類型轉換
- 返回最終的bean的實例
可以發現，在上述過程中，創建bean的主要方法最後集中在createBean方法中，最終邏輯在doCreateBean中,
AbstractAutowireCapableBeanFactory.doCreateBean
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
// 從緩存中獲取BeanWrapper
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
// 調用MergedBeanDefinitionPostProcessor的postProcessMergedBeanDefinition方法
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.isTraceEnabled()) {
logger.trace("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
//
try {
// 對bean的屬性進行填充，對並調用InstantiationAwareBeanPostProcessor的相關方法
populateBean(beanName, mbd, instanceWrapper);
// 如果實現了Aware相關接口，調用相關接口方法，主要是：
// 1. ((BeanNameAware) bean).setBeanName
// 2. ((BeanClassLoaderAware) bean).setBeanClassLoader
// 3. ((BeanFactoryAware) bean).setBeanFactory(
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<>(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;
}
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
// 將bean的class類型確定下來
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());
}
// 如果beanDefinition中定義了Supplier，通過supplier來生成bean
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
// 如果設置了factory-method工廠方法，通過工廠方法獲取
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;
}
}
}
// 如果構造器或工廠方法存在，此處進入if執行
if (resolved) {
if (autowireNecessary) {
return autowireConstructor(beanName, mbd, null, null);
}
else {
return instantiateBean(beanName, mbd);
}
}
// Candidate constructors for autowiring?
// 選出bean的構造函數候選集
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
// Preferred constructors for default construction?
// 獲取優先構造器
ctors = mbd.getPreferredConstructors();
if (ctors != null) {
return autowireConstructor(beanName, mbd, ctors, null);
}
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}
從上述代碼跟蹤發現主要有如下幾個步驟：
- 獲取instanceWrapper，首先從緩存獲取，沒有的話通過createBeanInstance創建獲取
- 調用MergedBeanDefinitionPostProcessor的postProcessMergedBeanDefinition方法
- 判斷是否需要提前暴露bean實例引用（解決循環依賴），如果需要提前暴露，通過
addSingletonFactory暴露單例的引用，這個Factory返回的就是上面已經創建好的beanInstance - 填充bean的屬性，通過
populateBean實現bean實例屬性填充 - 調用Aware接口相關實現（如果實現Aware接口）
- 註冊disposeBean（如果bean實現了DisposableBean或者有destroy-method方法）