網上關於spring循環依賴的介紹有不少，寫的也很是好，文末的參考文章中列舉了我看到的認爲寫的很不錯的文章，此篇文章主要是一個總結性說明。本身也寫了一個測試demo，詳見 https://github.com/gsonkeno/spring-training/tree/master/springbean-circular-dependencyjava
先看一段構造器循環依賴的demo日誌git
// 開始建立bean staffA .support.DefaultSingletonBeanRegistry 213 getSingleton - Creating shared instance of singleton bean 'studentA' .support.AbstractAutowireCapableBeanFactory 460 createBean - Creating instance of bean 'studentA' // 由於staffA的構造器方法參數依賴一個staffB，因此開始建立 staffB .support.DefaultSingletonBeanRegistry 213 getSingleton - Creating shared instance of singleton bean 'studentB' .support.AbstractAutowireCapableBeanFactory 460 createBean - Creating instance of bean 'studentB' // 由於staffB的構造器方法參數依賴一個staffC，因此開始建立 staffC .support.DefaultSingletonBeanRegistry 213 getSingleton - Creating shared instance of singleton bean 'studentC' .support.AbstractAutowireCapableBeanFactory 460 createBean - Creating instance of bean 'studentC' // 由於staffC的構造器方法參數依賴一個staffA，因此開始建立 staffA .support.DefaultSingletonBeanRegistry 213 getSingleton - Creating shared instance of singleton bean 'studentA' .support.AbstractApplicationContext 556 refresh - Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.UnsatisfiedDependencyException: Error creating bean with name 'studentA' defined in file ...
由於建立staffA—>staffB—>staffC，再從staffC—>staffA的時候，spring檢測到staffA正在建立中，直接拋出異常。關鍵代碼github
public class DefaultSingletonBeanRegistry protected void beforeSingletonCreation(String beanName) { // 若是beanName當前正在處理中，說明遇到了環，直接拋異常 if (!this.inCreationCheckExclusions.contains(beanName) && !this.singletonsCurrentlyInCreation.add(beanName)) { throw new BeanCurrentlyInCreationException(beanName); } } }
先看一段setter循環依賴(singleton)的demo日誌，每行日誌中頭部不重要的信息已刪除:web
// 開始建立單例bean(staffA) .support.DefaultSingletonBeanRegistry 213 getSingleton - Creating shared instance of singleton bean 'staffA' .support.AbstractAutowireCapableBeanFactory 460 createBean - Creating instance of bean 'staffA' // 檢測到staffA上的注入方法setStaffB .annotation.InjectionMetadata 74 checkConfigMembers - Registered injected element on class [com.gsonkeno.circular.dependency.staff.StaffA]: AutowiredMethodElement for public void com.gsonkeno.circular.dependency.staff.StaffA.setStaffB(com.gsonkeno.circular.dependency.staff.StaffB) // 過早地把staffA緩存起來，爲可能出現的循環依賴提供解決辦法 .support.AbstractAutowireCapableBeanFactory 563 doCreateBean - Eagerly caching bean 'staffA' to allow for resolving potential circular references // 開始處理staffA上的注入方法setStaffB .annotation.InjectionMetadata 88 inject - Processing injected element of bean 'staffA': AutowiredMethodElement for public void com.gsonkeno.circular.dependency.staff.StaffA.setStaffB(com.gsonkeno.circular.dependency.staff.StaffB) // 開始建立單例staffB .support.DefaultSingletonBeanRegistry 213 getSingleton - Creating shared instance of singleton bean 'staffB' .support.AbstractAutowireCapableBeanFactory 460 createBean - Creating instance of bean 'staffB' // 檢測到staffB上的注入方法setStaffC .annotation.InjectionMetadata 74 checkConfigMembers - Registered injected element on class [com.gsonkeno.circular.dependency.staff.StaffB]: AutowiredMethodElement for public void com.gsonkeno.circular.dependency.staff.StaffB.setStaffC(com.gsonkeno.circular.dependency.staff.StaffC) // 過早地把staffB緩存起來，爲可能出現的循環依賴提供解決辦法 .support.AbstractAutowireCapableBeanFactory 563 doCreateBean - Eagerly caching bean 'staffB' to allow for resolving potential circular references // 開始處理staffB上的注入方法setStaffC .annotation.InjectionMetadata 88 inject - Processing injected element of bean 'staffB': AutowiredMethodElement for public void com.gsonkeno.circular.dependency.staff.StaffB.setStaffC(com.gsonkeno.circular.dependency.staff.StaffC) // 開始建立單例staffC .support.DefaultSingletonBeanRegistry 213 getSingleton - Creating shared instance of singleton bean 'staffC' .support.AbstractAutowireCapableBeanFactory 460 createBean - Creating instance of bean 'staffC' // 檢測到staffC上的注入方法setStaffA .annotation.InjectionMetadata 74 checkConfigMembers - Registered injected element on class [com.gsonkeno.circular.dependency.staff.StaffC]: AutowiredMethodElement for public void com.gsonkeno.circular.dependency.staff.StaffC.setStaffA(com.gsonkeno.circular.dependency.staff.StaffA) // 過早地把staffC緩存起來，爲可能出現的循環依賴提供解決辦法 .support.AbstractAutowireCapableBeanFactory 563 doCreateBean - Eagerly caching bean 'staffC' to allow for resolving potential circular references // 開始處理staffC上的注入方法setStaffA .annotation.InjectionMetadata 88 inject - Processing injected element of bean 'staffC': AutowiredMethodElement for public void com.gsonkeno.circular.dependency.staff.StaffC.setStaffA(com.gsonkeno.circular.dependency.staff.StaffA) // 從循環依賴環中，獲取到了早早緩存的了單例staffA，儘管它尚未徹底初始化 .support.AbstractBeanFactory 250 doGetBean - Returning eagerly cached instance of singleton bean 'staffA' that is not fully initialized yet - a consequence of a circular reference // 裝配staffC所依賴的staffA到staffC上去 .annotation.AutowiredAnnotationBeanPostProcessor 527 registerDependentBeans - Autowiring by type from bean name 'staffC' to bean named 'staffA' // 結束建立staffC .support.AbstractAutowireCapableBeanFactory 497 createBean - Finished creating instance of bean 'staffC' // 裝配staffB所依賴的StaffC到staffB上去 .annotation.AutowiredAnnotationBeanPostProcessor 527 registerDependentBeans - Autowiring by type from bean name 'staffB' to bean named 'staffC' // 結束建立staffB .support.AbstractAutowireCapableBeanFactory 497 createBean - Finished creating instance of bean 'staffB' // 裝配staffA所依賴的staffB到staffA上去 .annotation.AutowiredAnnotationBeanPostProcessor 527 registerDependentBeans - Autowiring by type from bean name 'staffA' to bean named 'staffB' // 結束建立staffA .support.AbstractAutowireCapableBeanFactory 497 createBean - Finished creating instance of bean 'staffA' // 上面的日誌是建立staffA時打出來的，能夠認爲staffB、staffC這兩個bean是在建立staffA的過程當中 // 被迫建立的，因此等到建立staffB、staffC時，因爲已經建立過，直接從緩存中拿去便可 .support.AbstractBeanFactory 254 doGetBean - Returning cached instance of singleton bean 'staffB' .support.AbstractBeanFactory 254 doGetBean - Returning cached instance of singleton bean 'staffC'
如何判斷一個單例bean正在建立中呢？ 主要在於singletonsCurrentlyInCreation,以下：面試
public class DefaultSingletonBeanRegistry{ /** Names of beans that are currently in creation */ private final Set<String> singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16)); }
在bean建立前(確定在執行構造器方法前)，會把它放進去,建立bean完成後，會放出來。主要邏輯已經在上面的日誌中體現了出來，若是有興趣，深刻debug一遍理解的會更加清楚。spring
關鍵詞:
解決bean的依賴前提早暴露bean的獲取方式，建立中的bean進行緩存，遞歸深刻緩存
spring不支持原型bean屬性注入循環依賴，不一樣於構造器注入循環依賴會在建立spring容器context時報錯，它會在用戶執行代碼如context.getBean()時拋出異常。由於對於原型bean，spring容器只有在須要時纔會實例化，初始化它。svg
由於spring容器不緩存prototype類型的bean，使得沒法提早暴露出一個建立中的bean。spring容器在獲取prototype類型的bean時，若是由於環的存在，檢測到當前線程已經正在處理該bean時，就會拋出異常。核心代碼學習
public abstract class AbstractBeanFactory{ /** Names of beans that are currently in creation */ private final ThreadLocal<Object> prototypesCurrentlyInCreation = new NamedThreadLocal<>("Prototype beans currently in creation"); protected boolean isPrototypeCurrentlyInCreation(String beanName) { Object curVal = this.prototypesCurrentlyInCreation.get(); // 若是beanName已經存在於正在處理中的prototype類型的bean集中，後面會拋出異常 return (curVal != null && (curVal.equals(beanName) || (curVal instanceof Set && ((Set<?>) curVal).contains(beanName)))); } }
另外補充一句，三個bean中，只要有一個bean是singleton模式，就不會拋出異常。測試