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這篇文章主要講解了“Spring Ioc中Bean的加載實現方法”，文中的講解內容簡單清晰，易于學習與理解，下面請大家跟著小編的思路慢慢深入，一起來研究和學習“Spring Ioc中Bean的加載實現方法”吧！
代碼：
//AbstractAutowireCapableBeanFactory.java
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable 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.
//判斷需要創建的Bean是否可以實例化，即是否可以通過當前的類加載器加載
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
//校驗和準備Bean中的方法覆蓋
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配置了初始化前和初始化后的處理器，則試圖返回一個需要創建Bean的代理對象
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 {
//創建Bean的入口
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);
}
}這段代碼分為以下幾個步驟：
判斷需要創建的Bean是否可以實例化，即是否可以通過當前的類加載器加載
校驗和準備Bean中的方法注入
如果Bean配置了初始化前和初始化后的處理器，則試圖返回一個需要創建Bean的代理對象
創建Bean
主要是獲取bean的class，并設置到BeanDefinition中
主要是處理方法注入
代碼：
public void prepareMethodOverrides() throws BeanDefinitionValidationException {
// Check that lookup methods exists.
//檢測是否存在方法注入,并循環預處理方法注入
if (hasMethodOverrides()) {
Set<MethodOverride> overrides = getMethodOverrides().getOverrides();
synchronized (overrides) {
//遍歷處理
for (MethodOverride mo : overrides) {
prepareMethodOverride(mo);
}
}
}
}prepareMethodOverride(mo)：
protected void prepareMethodOverride(MethodOverride mo) throws BeanDefinitionValidationException {
// 統計注入的方法個數
int count = ClassUtils.getMethodCountForName(getBeanClass(), mo.getMethodName());
if (count == 0) {
throw new BeanDefinitionValidationException(
"Invalid method override: no method with name '" + mo.getMethodName() +
"' on class [" + getBeanClassName() + "]");
}
// 如果為1,則將注入方法標記為未重載
// 注意:當有多個重載方法時,為了確定調用哪個具體的方法,Spring對重載方法的參數解析是很復雜的
// 所以,如果注入方法沒有被重載這里就將其標記,省去了對方法參數的解析過程,直接調用即可
else if (count == 1) {
// Mark override as not overloaded, to avoid the overhead of arg type checking.
mo.setOverloaded(false);
}
}代碼讀到這里，大家可能有疑問，從代碼上看明明是處理的方法重載，但是為什么處理的是方法注入呢？而且如果我們在bean里設置幾個方法重載的話，hasMethodOverrides()方法返回的是false。如果我們打開 AbstractBeanDefinition 類的 hasMethodOverrides() 方法，就能打消我們之前的疑問。
public boolean hasMethodOverrides() {
return (this.methodOverrides != null && !this.methodOverrides.isEmpty());
}其中methodOverrides是做什么的呢？通過類名AbstractBeanDefinition我們可以發現，該類是BeanDefinition的一個子類，那么它保存的應該是我們解析到的beanDefinition，spring在解析配置文件的時候，如果發現配置了replace-method或者lookup-method那么，就會對應的標簽解析，并存入到 AbstractBeanDefinition 的 methodOverrides 屬性中，那么當bean實例化的時候，如果檢測到了methodOverrides屬性不為空，則動態的為當前bean生成代理并使用相應的攔截器對bean做處理，這里大家只要把概念搞清楚即可。
主要是對bean前置后置處理器的處理，給 BeanPostProcessors 后置處理器一個返回代理對象的機會
詳細代碼：
protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {
Object bean = null;
if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {
// Make sure bean class is actually resolved at this point.
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
Class<?> targetType = determineTargetType(beanName, mbd);
if (targetType != null) {
bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);
if (bean != null) {
bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);
}
}
}
mbd.beforeInstantiationResolved = (bean != null);
}
return bean;
}如果代理對象不為空，則直接返回代理對象，這一步驟有非常重要的作用，Spring 后續實現 AOP 就是基于這個地方判斷的。
這個方法核心就在于 applyBeanPostProcessorsBeforeInstantiation()和 applyBeanPostProcessorsAfterInitialization() 兩個方法，before 為實例化前的后處理器應用，after 為實例化后的后處理器應用。
doCreateBean()
創建Bean
//真正創建Bean的方法
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
//BeanWrapper是對Bean的包裝，其接口中所定義的功能很簡單包括設置獲取被包裝的對象，獲取被包裝bean的屬性描述器
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
//單例模式，刪除factoryBean緩存
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//使用合適的實例化策略來創建Bean：工廠方法、構造函數自動注入、簡單初始化
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
//從包裝類中獲取實例化的Bean
final Object bean = instanceWrapper.getWrappedInstance();
//獲取實例化對象的類型
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
//檢查是否有后置處理
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
//調用PostProcessor后置處理器，修改 BeanDefinition
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.
// 解決單例模式的循環依賴
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");
}
//這里是一個匿名內部類，為了防止循環引用，盡早持有對象的引用
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
//Bean對象的初始化，依賴注入在此觸發
//這個exposedObject在初始化完成之后返回作為依賴注入完成后的Bean
Object exposedObject = bean;
try {
//將Bean實例對象封裝，并且Bean定義中配置的屬性值賦值給實例對象
populateBean(beanName, mbd, instanceWrapper);
//初始化Bean對象
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) {
//獲取指定名稱的已注冊的單例模式Bean對象
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
//根據名稱獲取的已注冊的Bean和正在實例化的Bean是同一個
if (exposedObject == bean) {
//當前實例化的Bean初始化完成
exposedObject = earlySingletonReference;
}
//當前Bean依賴其他Bean，并且當發生循環引用時不允許新創建實例對象
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
//獲取當前Bean所依賴的其他Bean
for (String dependentBean : dependentBeans) {
//對依賴Bean進行類型檢查
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.
//注冊完成依賴注入的Bean
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}代碼很長，不過別慌，我們來按步驟分析一波
如果是單例模式，從factoryBeanInstanceCache 緩存中獲取BeanWrapper 實例對象并刪除緩存
調用 createBeanInstance() 實例化 bean
（主要是將 BeanDefinition 轉換為 BeanWrapper）
后置處理
單例模式的循環依賴處理
初始化 bean 實例對象
（屬性填充）
依賴檢查
注冊 DisposableBean
doCreateBean() 完成 bean 的創建和初始化工作，內容太多，比較復雜，這里只列出大致流程，接下來我們將分幾篇文章來分別闡述相關內容。
感謝各位的閱讀，以上就是“Spring Ioc中Bean的加載實現方法”的內容了，經過本文的學習后，相信大家對Spring Ioc中Bean的加載實現方法這一問題有了更深刻的體會，具體使用情況還需要大家實踐驗證。這里是億速云，小編將為大家推送更多相關知識點的文章，歡迎關注！
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