帶有@Transactional和@Async的迴圈依賴問題的解決

今天我們來探討一個有意思的spring原始碼問題，也是一個學生告訴了我現象我從原始碼裡面找到了這個有意思的問題。
首先我們看service層的程式碼案例，如下：
@Service("transationServiceImpl") public class TransationServiceImpl implements TransationService { @Autowired TransationService transationService; @Transactional @Async @Override public void transation() { } }
在transation方法上面加上了@Transactional和@Async兩個註解，然後在TransationServiceImpl 類中自己把自己的例項注入到transationService屬性中，存在迴圈依賴，理論上單例的迴圈依賴是允許的。但是我們啟動容器會報錯，測試程式碼如下：
public class MyTest { @Test public void test1() { AnnotationConfigApplicationContext applicationContext = new AnnotationConfigApplicationContext(ComponentScanBean.class); } } @Component @ComponentScan(basePackages = {"com.xiangxue"}) public class ComponentScanBean { }
然後右鍵執行test1單元測試載入spring容器就會報錯，報錯資訊如下：
org.springframework.beans.factory.BeanCurrentlyInCreationException: Error creating bean with name 『transationServiceImpl': Bean with name 『transationServiceImpl' has been injected into other beans [transationServiceImpl] 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.
從報錯的字面意思來看，是存在了多版本的迴圈依賴，如果要解決這個問題，我們必須追溯到原始碼中。
首先我們從TransationServiceImpl 例項化開始講起。
例項化從getBean方法看起，前面程式碼我就不貼了，這篇文章是給讀過spring原始碼的人看的，沒讀過也看不懂，哈哈 。
1、首先第一次建立TransationServiceImpl例項的時候會從快取中獲取例項 ，如果快取裡面有例項則直接返回，第一次建立的時候快取中是沒有例項的，所以會走到else程式碼塊中。
這裡是從三個快取中獲取例項化的詳細程式碼。後面會分析
@Nullable protected Object getSingleton(String beanName, boolean allowEarlyReference) { //根據beanName從快取中拿例項 //先從一級快取拿 Object singletonObject = this.singletonObjects.get(beanName); //如果bean還正在建立，還沒建立完成，其實就是堆記憶體有了，屬性還沒有DI依賴注入 if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) { synchronized (this.singletonObjects) { //從二級快取中拿 singletonObject = this.earlySingletonObjects.get(beanName); //如果還拿不到，並且允許bean提前暴露 if (singletonObject == null && allowEarlyReference) { //從三級快取中拿到物件工廠 ObjectFactory

singletonFactory = this.singletonFactories.get(beanName); if (singletonFactory != null) { //從工廠中拿到物件 singletonObject = singletonFactory.getObject(); //升級到二級快取 System.out.println("======get instance from 3 level cache->beanName->" + beanName + "->value->" + singletonObject ); this.earlySingletonObjects.put(beanName, singletonObject); //刪除三級快取 this.singletonFactories.remove(beanName); } } } } return singletonObject; }
2、第一次進來快取中沒有則建立TransationServiceImpl的例項
最終會走到doCreateBean方法中進行例項化，部分程式碼如下
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args) throws BeanCreationException { ............非關鍵程式碼不貼了 // Eagerly cache singletons to be able to resolve circular references // even when triggered by lifecycle interfaces like BeanFactoryAware. //是否 單例bean提前暴露 boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences && isSingletonCurrentlyInCreation(beanName)); if (earlySingletonExposure) { if (logger.isTraceEnabled()) { logger.trace("Eagerly caching bean '" + beanName + "' to allow for resolving potential circular references"); } //這裡著重理解，對理解迴圈依賴幫助非常大，重要程度 5 新增三級快取 addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean)); } // Initialize the bean instance. Object exposedObject = bean; try { //ioc di，依賴注入的核心方法，該方法必須看，重要程度：5 populateBean(beanName, mbd, instanceWrapper); //bean 例項化+ioc依賴注入完以後的呼叫，非常重要，重要程度：5 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); if (earlySingletonReference != null) { if (exposedObject == bean) { exposedObject = earlySingletonReference; } else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) { String[] dependentBeans = getDependentBeans(beanName); Set