這篇文章主要講解了“Spring Ioc中Bean的加載實(shí)現(xiàn)方法”，文中的講解內(nèi)容簡(jiǎn)單清晰，易于學(xué)習(xí)與理解，下面請(qǐng)大家跟著小編的思路慢慢深入，一起來(lái)研究和學(xué)習(xí)“Spring Ioc中Bean的加載實(shí)現(xiàn)方法”吧！
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代碼：
重慶分公司
重慶分公司
//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?jiǎng)?chuàng)建的Bean是否可以實(shí)例化，即是否可以通過當(dāng)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(yàn)和準(zhǔn)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(gè)需要?jiǎng)?chuàng)建Bean的代理對(duì)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(chuàng)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(gè)步驟：
判斷需要?jiǎng)?chuàng)建的Bean是否可以實(shí)例化，即是否可以通過當(dāng)前的類加載器加載
校驗(yàn)和準(zhǔn)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(gè)需要?jiǎng)?chuàng)建Bean的代理對(duì)象
創(chuàng)建Bean
主要是獲取bean的class，并設(shè)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(cè)是否存在方法注入,并循環(huán)預(yù)處理方法注入
if (hasMethodOverrides()) {
Set 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(tǒng)計(jì)注入的方法個(gè)數(shù)
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(biāo)記為未重載
// 注意:當(dāng)有多個(gè)重載方法時(shí),為了確定調(diào)用哪個(gè)具體的方法,Spring對(duì)重載方法的參數(shù)解析是很復(fù)雜的
// 所以,如果注入方法沒有被重載這里就將其標(biāo)記,省去了對(duì)方法參數(shù)的解析過程,直接調(diào)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?cè)赽ean里設(shè)置幾個(gè)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(fā)現(xiàn)，該類是BeanDefinition的一個(gè)子類，那么它保存的應(yīng)該是我們解析到的beanDefinition，spring在解析配置文件的時(shí)候，如果發(fā)現(xiàn)配置了replace-method或者lookup-method那么，就會(huì)對(duì)應(yīng)的標(biāo)簽解析，并存入到 AbstractBeanDefinition 的 methodOverrides 屬性中，那么當(dāng)bean實(shí)例化的時(shí)候，如果檢測(cè)到了methodOverrides屬性不為空，則動(dòng)態(tài)的為當(dāng)前bean生成代理并使用相應(yīng)的攔截器對(duì)bean做處理，這里大家只要把概念搞清楚即可。
主要是對(duì)bean前置后置處理器的處理，給 BeanPostProcessors 后置處理器一個(gè)返回代理對(duì)象的機(jī)會(huì)
詳細(xì)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(duì)象不為空，則直接返回代理對(duì)象，這一步驟有非常重要的作用，Spring 后續(xù)實(shí)現(xiàn) AOP 就是基于這個(gè)地方判斷的。
這個(gè)方法核心就在于 applyBeanPostProcessorsBeforeInstantiation()和 applyBeanPostProcessorsAfterInitialization() 兩個(gè)方法，before 為實(shí)例化前的后處理器應(yīng)用，after 為實(shí)例化后的后處理器應(yīng)用。
doCreateBean()
創(chuàng)建Bean
//真正創(chuàng)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(duì)Bean的包裝，其接口中所定義的功能很簡(jiǎn)單包括設(shè)置獲取被包裝的對(duì)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(shí)例化策略來(lái)創(chuàng)建Bean：工廠方法、構(gòu)造函數(shù)自動(dòng)注入、簡(jiǎn)單初始化
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
//從包裝類中獲取實(shí)例化的Bean
final Object bean = instanceWrapper.getWrappedInstance();
//獲取實(shí)例化對(duì)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(diào)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(huán)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(gè)匿名內(nèi)部類，為了防止循環(huán)引用，盡早持有對(duì)象的引用
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
//Bean對(duì)象的初始化，依賴注入在此觸發(fā)
//這個(gè)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(shí)例對(duì)象封裝，并且Bean定義中配置的屬性值賦值給實(shí)例對(duì)象
populateBean(beanName, mbd, instanceWrapper);
//初始化Bean對(duì)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(cè)的單例模式Bean對(duì)象
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
//根據(jù)名稱獲取的已注冊(cè)的Bean和正在實(shí)例化的Bean是同一個(gè)
if (exposedObject == bean) {
//當(dāng)前實(shí)例化的Bean初始化完成
exposedObject = earlySingletonReference;
}
//當(dāng)前Bean依賴其他Bean，并且當(dāng)發(fā)生循環(huán)引用時(shí)不允許新創(chuàng)建實(shí)例對(duì)象
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
//獲取當(dāng)前Bean所依賴的其他Bean
for (String dependentBean : dependentBeans) {
//對(duì)依賴Bean進(jìn)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(cè)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(zhǎng)，不過別慌，我們來(lái)按步驟分析一波
如果是單例模式，從factoryBeanInstanceCache 緩存中獲取BeanWrapper 實(shí)例對(duì)象并刪除緩存
調(diào)用 createBeanInstance() 實(shí)例化 bean
（主要是將 BeanDefinition 轉(zhuǎn)換為 BeanWrapper）
后置處理
單例模式的循環(huán)依賴處理
初始化 bean 實(shí)例對(duì)象
（屬性填充）
依賴檢查
注冊(cè) DisposableBean
doCreateBean() 完成 bean 的創(chuàng)建和初始化工作，內(nèi)容太多，比較復(fù)雜，這里只列出大致流程，接下來(lái)我們將分幾篇文章來(lái)分別闡述相關(guān)內(nèi)容。
感謝各位的閱讀，以上就是“Spring Ioc中Bean的加載實(shí)現(xiàn)方法”的內(nèi)容了，經(jīng)過本文的學(xué)習(xí)后，相信大家對(duì)Spring Ioc中Bean的加載實(shí)現(xiàn)方法這一問題有了更深刻的體會(huì)，具體使用情況還需要大家實(shí)踐驗(yàn)證。這里是創(chuàng)新互聯(lián)，小編將為大家推送更多相關(guān)知識(shí)點(diǎn)的文章，歡迎關(guān)注！
