spring中獲取bean的常見的方式，一般從BeanFactory中或者ApplicationContext中獲取。因為ApplicationContext接口繼承了BeanFactory接口，所以直接講BeanFactory.getBean(String beanName)
AbstractBeanFactory的getBean調用的是該類的doGetBean方法

protected <T> T doGetBean(final String name, @Nullable final Class<T> requiredType,@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {final String beanName = transformedBeanName(name);Object bean;// 從spring的三級緩存中獲取bean---解決循環依賴的關鍵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 + "'");}}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);}//忽略部分代碼.....try {final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args);//忽略部分代碼,getDependsOn依賴bean先加載.....// Create bean instance.if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, () -> {try {return createBean(beanName, mbd, args);}catch (BeansException ex) {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 {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;}}//忽略部分代碼--類型的校驗.....}return (T) bean;} 復制代碼

上述的代碼中大致的流程如下：

從Spring維護的三級緩存中獲取一個singleton bean，獲取成功直接返回

判斷該bean是否處于創建中，如果是說明出現了循環依賴，拋異常

根據bean的作用域，分別用不同的方式獲取bean，如果獲取不到最終調用的都是同一個createBean方式

getObjectForBeanInstance 如果上述方法獲取到的是一個FactoryBean的情況下，需要從里面獲取對應的我們需要的Bean

下面分析下singleton bean的獲取

// Create bean instance. if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, () -> {try {return createBean(beanName, mbd, args);}catch (BeansException ex) {destroySingleton(beanName);throw ex;}});bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd); } 復制代碼public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(beanName, "Bean name must not be null");synchronized (this.singletonObjects) {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 + "'");}beforeSingletonCreation(beanName);//定義異常...try {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) {//封裝異常...}finally {...afterSingletonCreation(beanName);}if (newSingleton) {addSingleton(beanName, singletonObject);}}return singletonObject;}} 復制代碼

因為singleton的bean只能存在一個，所以對singletonObjects這個緩存做加鎖獲取

做循環依賴判斷

beforeSingletonCreation(beanName);//也就是把這個beanName標記成正在創建中

通過singletonFactory的方法獲取bean --> 也就是調用createBean方法

afterSingletonCreation(beanName);//去除該beanName正在創建中的標記

如果是新創建的singleton的bean的話，把他加入singletonObjects緩存中

這個singletonObjects緩存也就是上面所說的三級緩存的中的第一級（具體后續再說）

不管是singleton還是Prototype還是其他的scope，在從作用域中獲取不到bean的情況下，都是調用的createBean

• 知識點梳理：

FactoryBean：www.cnblogs.com/davidwang45…

bean的作用域：

singleton:整個容器中保留一份

prototype:每次使用的時候，創建新的

其他scope

SimpleThreadScope

RequestScope

SessionScope

轉載于:https://juejin.im/post/5d272c57f265da1b6d405031

總結

以上是生活随笔為你收集整理的spring 源码分析01的全部內容，希望文章能夠幫你解決所遇到的問題。

如果覺得生活随笔網站內容還不錯，歡迎將生活随笔推薦給好友。