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本篇文章給大家分享的是有關在Spring中如何實現加載Bean的，小編覺得挺實用的，因此分享給大家學習，希望大家閱讀完這篇文章后可以有所收獲，話不多說，跟著小編一起來看看吧。
之前寫過bean的解析，這篇來講講bean的加載，加載要比bean的解析復雜些，從之前的例子開始.
Spring中加載一個bean的方式：
TestBean bean = factory.getBean("testBean");來看看getBean(String name)方法源碼，
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}該getBean(String name)方法位于AbstractBeanFactory抽象類中，AbstractBeanFactory與XmlBeanFactory類關系可以看下圖：
接下去跟進doGetBean()方法源碼：
protected <T> T doGetBean(
final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
throws BeansException {
// 提取beanName
final String beanName = transformedBeanName(name);
Object bean;
// 試著從緩存或實例工廠中獲取
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 + "'");
}
}
// 返回實例，有時存在如BeanFactory這樣情況時并不是直接返回實例本身而是返回指定方法返回的實例
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.
BeanFactory parentBeanFactory = getParentBeanFactory();
// 如果在所有已經加載的類中沒有beanName則會嘗試從parentBeanFactory中檢測
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
// 到BeanFactory查找
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);
}
}
// 如果不是只做類型檢查則是創建bean
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
// 將存儲XML配置文件的GernericBeanDefinition轉換成RootBeanDefinition，如果BeanName是子Bean的話會合并父類的相關屬性
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
// 如果存在依賴的話要遞歸實例化依賴的bean
if (dependsOn != null) {
for (String dependsOnBean : dependsOn) {
if (isDependent(beanName, dependsOnBean)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");
}
registerDependentBean(dependsOnBean, beanName);
getBean(dependsOnBean);
}
}
// Create bean instance.
// 依賴的bean實例化完后就可以實例化mbd了
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);
}
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 {
// 在對應的scope上實例化bean
String scopeName = mbd.getScope();
final Scope scope = this.scopes.get(scopeName);
if (scope == null) {
throw new IllegalStateException("No Scope registered for scope '" + 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;
}
}
// 檢查需要的類型是否符合實例bean的實際類型
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;
}整個bean加載的過程步驟相對繁瑣，主要步驟有以下幾點：
1、轉換beanName
要知道平時開發中傳入的參數name可能只是別名，也可能是FactoryBean，所以需要進行解析轉換，一般會進行以下解析：
（1）消除修飾符，比如name="&test"，會去除&使name="test"；
（2）取alias表示的最后的beanName，比如別名test01指向名稱為test02的bean則返回test02。
2、從緩存中加載實例
實例在Spring的同一個容器中只會被創建一次，后面再想獲取該bean時，就會嘗試從緩存中獲取；如果獲取不到的話再從singletonFactories中加載。
3、實例化bean
緩存中記錄的bean一般只是最原始的bean狀態，這時就需要對bean進行實例化。如果得到的是bean的原始狀態，但又要對bean進行處理，這時真正需要的是工廠bean中定義的factory-method方法中返回的bean，上面源碼中的getObjectForBeanInstance就是來完成這個工作的。
4、檢測parentBeanFacotory
從源碼可以看出如果緩存中沒有數據會轉到父類工廠去加載，源碼中的!containsBeanDefinition(beanName)就是檢測如果當前加載的xml配置文件中不包含beanName所對應的配置，就只能到parentBeanFacotory去嘗試加載bean。
5、存儲XML配置文件的GernericBeanDefinition轉換成RootBeanDefinition之前的文章介紹過XML配置文件中讀取到的bean信息是存儲在GernericBeanDefinition中的，但Bean的后續處理是針對于RootBeanDefinition的，所以需要轉換后才能進行后續操作。
6、初始化依賴的bean
這里應該比較好理解，就是bean中可能依賴了其他bean屬性，在初始化bean之前會先初始化這個bean所依賴的bean屬性。
7、創建bean
Spring容器根據不同scope創建bean實例。
整個流程就是如此，下面會講解一些重要步驟的源碼。
上面有提到，單例在Spring中的同一容器中只會被創建一次，后面再獲取bean的話會直接從緩存中獲取，這里是嘗試加載，先從緩存中加載，再次就是從singletonFactories中加載；因為在bean中可能會在依賴注入，要避免循環依賴，Spring創建bean時會不等bean創建完成就會將創建該bean的ObjectFactory提前曝光加入到緩存中，但下一個bean創建時要依賴上個bean的話，就直接使用ObjectFacotry。
@Override
public Object getSingleton(String beanName) {
return getSingleton(beanName, true); // true表示允許早期依賴
}
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 嘗試從緩存獲取實例
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
// 若該bean正在加載則不處理
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
// 存入到緩存中
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
/* 這兩個方法在DefaultSingletonBeanRegistry類中 */從源碼可以看出這個方法先嘗試從singletonObjects中獲取實例，如果獲取不到值就從earlySingletonObject中去獲取，如果再獲取不到的話則到singletonFactories里獲取beanName對應的ObjectFactory，再調用這個ObjectFactory的getObject來創建bean，并放到earlySingletonObject中，并且從singletonFactories里remove掉這個ObjectFactory。這里有幾個存儲bean的不同map：
如果上面緩存中不存在已經加載的單例bean就要重新開始bean的加載過程了，Spring中使用getSingleton重載方法實現bean的加載過程。
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "'beanName' must not be null");
synchronized (this.singletonObjects) {
// 先檢查bean是否已經加載
Object singletonObject = this.singletonObjects.get(beanName);
// 如果空才進行singleton的bean的初始化
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while the singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<Exception>();
}
try {
// 初始化bean
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
afterSingletonCreation(beanName);
}
if (newSingleton) {
// 存入緩存
addSingleton(beanName, singletonObject);
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
}這段代碼使用了回調方法，使程序可以在單例創建的前后做一些準備及處理操作，真正的獲取單例bean的方法其實并不是在這個方法實現的，而是在ObjectFactory類型的實例singletonFactory中實現的。
下面準備創建bean
看看createBean()方法源碼（該方法在AbstractAutowireCapableBeanFactory類中）：
protected Object createBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
// 鎖定class，根據設置的class屬性或根據className來解析Class
resolveBeanClass(mbd, beanName);
// 驗證和準備覆蓋的方法
try {
mbd.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbd.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// 用BeanPostProcessors返回代理來替代真正的實例
Object bean = resolveBeforeInstantiation(beanName, mbd);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
Object beanInstance = doCreateBean(beanName, mbd, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}從createBean()方法源碼可以看出主要做了以下操作：
在createBean()方法里執行完resolveBeforeInstantiation方法后，如果創建了代理且不為空的話就直接返回，否則需要進行常規bean的創建，這個創建過程是在doCreateBean中完成的，跟進源碼：
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
// 根據指定bean使用相應策略創建實例
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;
}
}
// 檢測循環依賴，是否需要提早曝光
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");
}
// 避免循環依賴，可以在bean初始化完成前將創建實例的ObjectFactory加入工廠
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
// 對bean再次依賴引用
// AOP也是在這里將advice動態織入bean中，若沒有則直接返回bean，不做處理
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
// 填充bean，注入屬性值，如果存在依賴于其他bean的屬性，會遞歸初始化
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
// 調用初始化方法
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只有在檢測到有循環依賴的情況下才會不為空
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) {
// 檢測依賴
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 {
// 依據scopse注冊bean
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}上面源碼完成的操作可以概括為以下幾點：
接下來創建bean實例，看createBeanInstance()方法：
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, Object[] args) {
// 解析class
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());
}
// 若工廠方法不為空則使用工廠方法初始化
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) {
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方法不需要再次鎖定
if (resolved) {
if (autowireNecessary) {
// 構造方法自動注入
return autowireConstructor(beanName, mbd, null, null);
}
else {
// 使用默認構造方法
return instantiateBean(beanName, mbd);
}
}
// 根據參數解析構造方法
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.
return instantiateBean(beanName, mbd);
}可以看出如果在RootBeanDefinition中存在factoryMethodName屬性，或者說配置文件中配置了factory-method，那么Spring會嘗試使用instantiateUsingFactoryMethod(beanName, mbd, args)方法根據RootBeanDefinition中的配置生成bean實例。
再解析構造方法并進行實例化，Spring會根據參數及類型判斷使用哪個構造方法再進行實例化。判斷調用哪個構造方法的過程會采用緩存機制，如果已經解析過則不需要重復解析而是從RootBeanDefinition中的屬性resolvedConstructorOrFactoryMethod緩存的值去取，不然則需要再次解析。
創建bean后接下來就進行屬性注入，屬性注入的操作在populateBean()方法中，跟進源碼：
protected void populateBean(String beanName, RootBeanDefinition mbd, BeanWrapper bw) {
PropertyValues pvs = mbd.getPropertyValues();
if (bw == null) {
if (!pvs.isEmpty()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
}
else {
// Skip property population phase for null instance.
return;
}
}
// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
// state of the bean before properties are set. This can be used, for example,
// to support styles of field injection.
boolean continueWithPropertyPopulation = true;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
continueWithPropertyPopulation = false;
break;
}
}
}
}
if (!continueWithPropertyPopulation) {
return;
}
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME ||
mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// Add property values based on autowire by name if applicable.
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
if (mbd.getResolvedAutowireMode() == RootBeanDefinition.AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != RootBeanDefinition.DEPENDENCY_CHECK_NONE);
if (hasInstAwareBpps || needsDepCheck) {
PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
if (hasInstAwareBpps) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof InstantiationAwareBeanPostProcessor) {
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
pvs = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvs == null) {
return;
}
}
}
}
if (needsDepCheck) {
checkDependencies(beanName, mbd, filteredPds, pvs);
}
}
applyPropertyValues(beanName, mbd, bw, pvs);
}在populateBean方法的中的主要處理流程：
最后初始化bean
學過Spring的都知道bean配置時有一個init-method屬性，這個屬性的作用是在bean實例化前調用init-method指定的方法進行需要的實例化操作，現在就進入這個方法了；Spring中程序已經執行過bean的實例化，并且進行了屬性的填充，而就在這時將會調用用戶設定的初始化方法。
protected Object initializeBean(final String beanName, final Object bean, RootBeanDefinition mbd) {
if (System.getSecurityManager() != null) {
AccessController.doPrivileged(new PrivilegedAction<Object>() {
@Override
public Object run() {
invokeAwareMethods(beanName, bean);
return null;
}
}, getAccessControlContext());
}
else {
// 特殊bean處理
invokeAwareMethods(beanName, bean);
}
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
}
try {
invokeInitMethods(beanName, wrappedBean, mbd);
}
catch (Throwable ex) {
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
}
if (mbd == null || !mbd.isSynthetic()) {
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
}
return wrappedBean;
}最后加載完Bean并執行完初始化操作后，一個bean的加載基本就結束了。
以上就是在Spring中如何實現加載Bean的，小編相信有部分知識點可能是我們日常工作會見到或用到的。希望你能通過這篇文章學到更多知識。更多詳情敬請關注億速云行業資訊頻道。
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