由於Spring的IOC特性,Bean都是由Spring容器生成的,那麼如果Bean是單例的,存在兩個Bean,分別爲beanA、beanB,beanA依賴beanB,同時beanB也依賴beanA,那麼可以想象假如容器不做特殊處理的話,就會發生循環依賴,產生死鎖,Bean構造就進行不下去了。但是我們在使用時,其實並沒有關注循環依賴的問題,Spring是可以解決這種循環依賴的情況的,本篇文章我們來看一下Spring是如何解決循環依賴的。
1. Spring循環依賴示例
首先定義兩個Bean,BeanA和BeanB,兩個Bean分別由一個對方類型的成員變量,如下:
public class BeanA {
private BeanB beanB;
public BeanB getBeanB() {
return beanB;
}
public void setBeanB(BeanB beanB) {
this.beanB = beanB;
}
}
public class BeanB {
private BeanA beanA;
public BeanA getBeanA() {
return beanA;
}
public void setBeanA(BeanA beanA) {
this.beanA = beanA;
}
}
spring xml配置文件,配置屬性依賴:
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="
http://www.springframework.org/schema/beans
http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">
<bean id="beanA" class="com.zhuoli.service.spring.explore.circular.dependence.BeanA">
<property name="beanB" ref="beanB"/>
</bean>
<bean id="beanB" class="com.zhuoli.service.spring.explore.circular.dependence.BeanB">
<property name="beanA" ref="beanA"/>
</bean>
</beans>
測試代碼:
public class CircularDependenceTest {
public static void main(String[] args) {
AbstractApplicationContext abstractApplicationContext = new ClassPathXmlApplicationContext("classpath:spring-circular-dependence.xml");
BeanA beanA = (BeanA) abstractApplicationContext.getBean("beanA");
BeanB beanB = (BeanB) abstractApplicationContext.getBean("beanB");
System.out.println(beanA.getBeanB() == beanB);
System.out.println(beanA == beanB.getBeanA());
}
}
運行結果:
true
true
也就是講,beanA拿到了beanB的引用,beanB同時也拿到了beanA的引用。雖然BeanA和BeanB之間存在循環依賴,但是Spring容器並沒有發生死鎖,成功解決了循環依賴問題,並構造了BeanA和BeanB對象。
2. Spring解決循環依賴
其實Spring是如何解決循環依賴問題,在上篇文章介紹Spring初始化流程的時候已經簡單提過,我們這裏突出再來理一下。同時要注意的是,我們所說的Spring解決循環依賴,只限於單例Bean,對於非單例Bean,是不支持的。
簡單來講,Spring解決循依賴,其實是通過提早緩存未實例結束的bean來實現的。首先在doGetBean()方法中,該方法獲取bean,首先會嘗試從緩存中獲取,如下:
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;
// 嘗試從緩存中獲取bean實例
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
//
}
else {
//
}
// Check if required type matches the type of the actual bean instance.
if (requiredType != null && !requiredType.isInstance(bean)) {
//
}
return (T) bean;
}
public Object getSingleton(String beanName) {
return getSingleton(beanName, true);
}
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
//1. 查詢緩存中是否有創建好的單例
Object singletonObject = this.singletonObjects.get(beanName);
//2. 如果緩存不存在,判斷是否正在創建中
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
//加鎖防止併發
synchronized (this.singletonObjects) {
// 從earlySingletonObjects中查詢是否有early緩存
singletonObject = this.earlySingletonObjects.get(beanName);
// early緩存也不存在,且允許early引用
if (singletonObject == null && allowEarlyReference) {
// 從單例工廠Map裏查詢beanName對應的ObjectFactory
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
// 如果beanName對應的ObjectFactory存在,則調用getObject方法拿到單例對象
singletonObject = singletonFactory.getObject();
// 將單例對象添加到early緩存中
this.earlySingletonObjects.put(beanName, singletonObject);
// 移除單例工廠中對應的singletonFactory
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
從以上的代碼可以看出:
- 緩存機制,只針對單例的bean
- 默認的singletonObjects緩存不存在要get的beanName時,判斷beanName是否正在創建中
- 從early緩存earlySingletonObjects中再查詢,early緩存是用來緩存已實例化但未組裝完成的bean
- 如果early緩存也不存在,從singletonFactories中查找是否有beanName對應的ObjectFactory對象工廠
- 如果對象工廠存在,則調用getObject方法拿到bean對象
- 將bean對象加入early緩存,並移除singletonFactories的對象工廠
上面最重要的就是singletonFactories何時放入了可以通過getObject獲得bean對象的ObjectFactory。考慮到循環依賴的場景,應該會是bean對象實例化後,而屬性注入之前。仔細尋找後發現,在AbstractAutowireCapableBeanFactory類的doCreateBean方法,執行完createBeanInstance實例化bean之後,populateBean屬性注入之前,有這樣一段代碼:
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
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));
}
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
if (!this.singletonObjects.containsKey(beanName)) {
this.singletonFactories.put(beanName, singletonFactory);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
}
當判斷bean爲單例且正在創建中,而Spring允許循環引用時,將能獲得bean對象的引用的ObjectFactory添加到singletonFactories中,此時就與之前的getSingleton方法相呼應。而allowCircularReferences標識在spring中默認爲true,但是也可以通過setAllowCircularReferences方法對AbstractAutowireCapableBeanFactory進行設置。
再來看下getObject方法中的getEarlyBeanReference方法。這裏也設置了一個InstantiationAwareBeanPostProcessor後置處理器的擴展點,允許在對象返回之前修改甚至替換bean。
protected Object getEarlyBeanReference(String beanName, RootBeanDefinition mbd, Object bean) {
Object exposedObject = bean;
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (BeanPostProcessor bp : getBeanPostProcessors()) {
if (bp instanceof SmartInstantiationAwareBeanPostProcessor) {
SmartInstantiationAwareBeanPostProcessor ibp = (SmartInstantiationAwareBeanPostProcessor) bp;
exposedObject = ibp.getEarlyBeanReference(exposedObject, beanName);
}
}
}
return exposedObject;
}
最後,來梳理一下上面abstractApplicationContext.getBean(“beanA”)的執行過程:
- 實例化BeanA
- 將能獲取BeanA對象的ObjectFactory添加到singletonFactories中
- BeanA注入BeanB屬性,調用getBean(“beanB”)方法
- 實例化BeanB
- 將能獲取BeanB對象的ObjectFactory添加到singletonFactories中
- BeanB注入BeanA屬性,調用getBean(“beanA”)
- 從singletonFactories中獲取ObjectFactory並調用getObject方法拿到beanA對象的引用
- BeanB創建完成,注入到BeanA的beanB屬性中
- BeanA創建完成返回
上面我們瞭解了單例的bean循環引用的處理過程,那麼多例的呢?其實我們可以按上面的思路來思考一下,單例bean的循環引用是因爲每個對象都是固定的,只是提前暴露對象的引用,最終這個引用對應的對象是創建完成的。但是多例的情況下,每次getBean都會創建一個新的對象,那麼應該引用哪一個對象呢,這本身就已經是矛盾的了。因而spring中對於多例之間相互引用是會提示錯誤的。在doGetBean protoType處理的邏輯中,第一步就存在下面的判斷:
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
參考鏈接:
1. Spring源碼
2. bean的循環依賴