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這篇文章主要介紹了spring如何獲取bean的源碼解析，具有一定借鑒價值，感興趣的朋友可以參考下，希望大家閱讀完這篇文章之后大有收獲，下面讓小編帶著大家一起了解一下。
直接看源碼吧
@Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
@Override
public <T> T getBean(String name, @Nullable 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, @Nullable Class<T> requiredType, @Nullable Object... args)
throws BeansException {
return doGetBean(name, requiredType, args, false);
}
@SuppressWarnings("unchecked")
protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
// 把name轉化成beanName，也就是把FactoryBean的名稱轉化成beanName如果有別名則用別名
final String beanName = transformedBeanName(name);
Object bean;
// 從緩存中獲取實例
// 可能是需要的Bean實例，也可能是FactoryBean
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 + "'");
}
}
// 獲取需要的bean或者FactoryBean
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// 判斷prototype類型的bean是否存在循環引用
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// 校驗父類BeanFactory
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// 父類去獲取bean
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 {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
// 標記成已創建
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
try {
// 把原來BeanDefinition轉換成RootBeanDefinition
final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// 獲取依賴的bean，也就是通過@DependsOn注入進來的bean
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
// 校驗dependsOn的bean是否存在循環應用
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 加入到引用的緩存中，由于校驗dependsOn循環引用
registerDependentBean(dep, beanName);
// 獲取@dependsOn的bean
getBean(dep);
}
}
// 創建單例的bean
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;
}
});
// 獲取需要的bean或者FactoryBean
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 創建prototype的bean
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);
}
// 創建其他的bean，比如session，request等
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;
}
}
// 如果要求的類型不是這個bean的實例類型，則進行轉換
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.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;
}獲取bean的整體流程就像上面源碼所示，這里再梳理下spring獲取bean的整個流程
1.先轉換bean的名稱，轉換成beanName。這里意思就是，我們在獲取bean的時候，可能是FactoryBean的名稱（&開頭），這里轉成不帶&開頭的名稱，如果有別名，再獲取別名。
2.從緩存中獲取bean，這里的緩存分為一二三級緩存，也就是我們常常被問到了spring三級緩存，具體邏輯下面再說。
3.根據獲取的到對象再去獲取我們想要的bean，因為這里獲取到的對象可能是我們需要的bean，也可能是FactoryBean。
4.如果緩存中沒有，那么我們就要自己去創建bean了。
5.查看有沒有父類的BeanFactory，如果有，那么就父類去創建bean。
6.獲取要創建的bean對象的@DependsOn注解上的名稱，先去創建DependsOn的bean，并且校驗是否存在循環引用
7.創建bean，根據類型創建不同的bean，比如singleton，prototype，request，session等。
8.如果需要轉換類型，則進行類型轉換。
整體的獲取bean的流程就是這樣了，其中有些具體流程接著分析。
從緩存中獲取bean對象
public Object getSingleton(String beanName) {
return getSingleton(beanName, true);
}
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 從一級緩存中獲取
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
// 從二級緩存中獲取
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
// 從三級緩存中獲取
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}spring通過三級緩存來解決循環依賴的問題。簡單來介紹下三級緩存。
1. singletonObjects為一級緩存，我們實例化的bean都在這個map里，俠義的說singletonObjects才是我們真正的spring容器，存放bean的地方。
2. earlySingletonObjects為二級緩存，是存放未完成的bean的緩存，如果有代理的話，存放的是代理對象。
3. singletonFactories為三級緩存，存放的是一個ObjectFactory，數據通過getObject方法獲得。
接下來看getObjectForBeanInstance方法。
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, @Nullable RootBeanDefinition mbd) {
// name是不是factoryBean的name（&開頭的）
if (BeanFactoryUtils.isFactoryDereference(name)) {
if (beanInstance instanceof NullBean) {
return beanInstance;
}
// 如果是FactoryBeanName,但是獲取到的bean不是FactoryBean，則拋異常
if (!(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
}
// 如果bean不是FactoryBean，或者名稱是FactoryBeanName，直接返回BeanInstace
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
Object object = null;
if (mbd == null) {
// 從緩存中獲取
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// 這里可以確定beanInstance是FactoryBean了
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
// 通過FactoryFBean中獲取需要的beanInstance
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}這里是通過BeanInstance獲取我們想要的bean，這里也簡單說下流程
1. 首先判斷name是不是FactoryBean的name，也就是&開頭的name，如果是去判斷beanInstance是不是FactoryBean，如果beanInstance不是FactoryBean則拋異常。
2. 由于上面已經判斷過，如果name是FactoryBeanName，但是BeanInstance不是FactoryBean的話，就會拋出異常。所以如果BeanInstance如果不是FactoryBean的話，那么name一定不是FactoryBeanName。那么就直接返回BeanInstance就是我們需要的了。
如果name是FactoryBeanName，那么我們需要獲取的就是FactoryBean，也直接返回就可以了。
3. 如果都沒有返回，那么已經可以確定我們此時的已經可以確定BeanInstance是FactoryBean了，因為如果不是FactoryBean的話，在!(beanInstance instanceof FactoryBean)就已經返回了。
4. 通過FactoryBean的getObject方法獲取我們需要的bean實例。
根據@dependsOn查找依賴的bean并且加到依賴里面去沒有什么好說的，代碼邏輯也很簡單，接下來看創建單例bean。其他類型的bean的創建也都差別不大。看源碼
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
// 直接從一級緩存中取
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while 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 + "'");
}
// 在沒創建bean之前的處理
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
// 獲取創建的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;
}
// 創建結束之后的工作
afterSingletonCreation(beanName);
}
if (newSingleton) {
// 加到一級緩存中，其實也就是真正的容器中了
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}對于創建單例bean的主要流程就是如此，傳入一個beanName，和一個ObjectFactory。ObjectFactory中具體實現了創建bean的邏輯。在看具體創建bean的邏輯之前，我們還需要去看下getSingleton中的創建bean之前的工作和創建bean之后的工作。這里面就是查找bean的循環依賴的方法（和dependsOn不同）。主要是查找根據filed，set，構造器方法的循環依賴。
protected void beforeSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
}
protected void afterSingletonCreation(String beanName) {
if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.remove(beanName)) {
throw new IllegalStateException("Singleton '" + beanName + "' isn't currently in creation");
}
}可以看到代碼非常的簡單，就是創建bean之前，如果沒有排除依賴檢查，那么就加入到正在創建的Set中，如果加入不進去，說明之前已經加過，這就產生了循環依賴，從而拋出異常。
如果在創建bean之后，沒有排除檢查依賴，并且移除失敗，說明已經不在Set中，也會拋出異常。
好了，既然明白了spring是如何校驗循環依賴的，也看到了三級緩存，后面再說為什么不能解決構造器依賴就很好說了。接著看創建bean的方法。
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;
// 獲取要創建bean的class
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
// 如果沒有beanclass，設置beanclass
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// 配置方法重載
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// 那些beanPostProcessor如果能產生代理，則直接返回bean
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
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);
}
}這里其實還是沒有到創建bean的過程，還是在創建bean的一些準備工作。其實我們可以發現，spring中，真正做事的都是do開頭的方法。
這邊的流程就是設置beanClass，后面需要根據反射來創建bean。然后會根據spring里面的beanPostProcessor，看看有沒有能產生代理bean的，如果有就返回，沒有就去創建bean。
看真正的doCreateBean方法
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
// 裝飾Bean的對象
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
// 通過緩存獲取
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
// 創建bean
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;
}
}
// 加入到三級緩存中去
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, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
// 填充依賴的bean，field注入，和方法注入的bean
populateBean(beanName, mbd, instanceWrapper);
// 調用初始化的方法
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) {
// 獲取二級緩存的值
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
// 如果一致，則直接使用二級緩存的對象
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
// 如果調用初始化后的bean和之前的bean不一致，并且有依賴
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
// 查找循環依賴
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
// 如果有循環依賴并且在創建中，則拋出異常
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.");
}
}
}
}
// 注冊disposableBean
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}真正創建bean這里還是有點復雜的。這里再進行一個簡單梳理。
1. 根據class還有bean以及參數創建bean。
2. 調用beanPostprocessor的方法，調用屬于MergedBeanDefinitionPostProcessor的方法。對bean進行一些處理，比如找到那些依賴的bean的field和method。
3. 將bean加入到三級緩存中去。
4. 填充bean需要注入的其他bean。
5. 調用初始化方法，先去調用@PostConstruct注解方法，然后調用InitializingBean的afterPropertiesSet，以及自定義的init-method方法。在bean調用初始化方法之后，再去調用后置接口看看是否需要生成Aop代理。
6. 接著進行校驗。這里稍微比較復雜一點。如果從二級緩存能取到，那就說明之前被別人從三級緩存拿出來過了。可能是因為循環依賴，也可能是因為別的地方調用了getBean方法了。從三級緩存拿出來的時候有個getEarlyBeanReference的方法，就是查看是否要生成代理的bean。如果生成過了，那么在調用第五步的時候，就不會在生成代理了。這樣exposedObject ==bean，直接只用代理返回。
如果不相等：這里的情況就是如果是spring自己的@Async，在從二級緩存生成代理之后，再去調用第五步時候一樣會生成代理。所以exposedObject !=bean，所以在再往下發現有循環調用，并且bean還在創建時，就會拋出異常了。所以一般慎用spring的@Async。但是一般也可以使用@Lazy進行處理。至于原理后面再說。
到這里spring的創建bean就結束了。然后返回時候就到了入口方法getBean的getObjectForBeanInstance的方法，到底需要的bean還是FactoryBean。
最后就是如果requiredType和實例不一樣就要進行類型轉換了。
感謝你能夠認真閱讀完這篇文章，希望小編分享的“spring如何獲取bean的源碼解析”這篇文章對大家有幫助，同時也希望大家多多支持億速云，關注億速云行業資訊頻道，更多相關知識等著你來學習!
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