要弄清楚循環(huán)依賴
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1、需要知道Bean初始化的兩個(gè)階段
① Bean實(shí)例化創(chuàng)建實(shí)例對(duì)象（new Bean()）
② Bean實(shí)例對(duì)象初始化（DI:注解自動(dòng)注入）
2、DefaultSingletonBeanRegistry類中的5個(gè)容器
/** 記錄已將創(chuàng)建的單例 */
private final Map singletonObjects = new ConcurrentHashMap<>(256);
/** 記錄singletonFactory singeletonFactory中存放beanName和上面的①階段的bean：Bean實(shí)例化實(shí)例對(duì)象（還未初始化DI）*/
private final Map> singletonFactories = new HashMap<>(16);
/** 記錄早期的singletonBean 存放的也是① */
private final Map earlySingletonObjects = new HashMap<>(16);
/** 存放已經(jīng)初始化后的beanName，有序 */
private final Set registeredSingletons = new LinkedHashSet<>(256);
/** 記錄正在初始化的bean的beanName */
private final Set singletonsCurrentlyInCreation = Collections.newSetFromMap(new ConcurrentHashMap<>(16));
其中跟循環(huán)依賴相關(guān)的是singletonFactories、singeletonsCurrentlyInCreation、earlysingletonObjects.
3、循環(huán)依賴實(shí)現(xiàn)
①bean初始化前后會(huì)打標(biāo)，加入到singletonsCurrentlyInCreation容器中，這個(gè)打標(biāo)會(huì)在核心方法getSingleton()中起作用
/* org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton(java.lang.String, org.springframework.beans.factory.ObjectFactory<?>) */
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
...
//循環(huán)依賴相關(guān)：初始化前先singletonsCurrentlyInCreation.add(beanName)
beforeSingletonCreation(beanName);
...
//lamda表達(dá)式：其實(shí)是調(diào)用createBean(beanName, mbd, args):Bean初始化
singletonObject = singletonFactory.getObject();
...
//循環(huán)依賴相關(guān)：初始化后singletonsCurrentlyInCreation.remove(beanName)
afterSingletonCreation(beanName);
...//初始化完后
//this.singletonObjects.put(beanName, singletonObject);放入到單例容器中
//this.singletonFactories.remove(beanName);清空循環(huán)依賴的兩個(gè)打標(biāo)
//this.earlySingletonObjects.remove(beanName);
//this.registeredSingletons.add(beanName);放入單例beanName容器中
addSingleton(beanName, singletonObject);
...
}
}② 上面singletonObject = singletonFactory.getObject()時(shí)向singletonFactories中記錄了(new Bean())，singletonFactories也會(huì)在核心方法getSingleton()中起作用
/* org.springframework.beans.factory.support.AbstractAutowireCapableBeanFactory#doCreateBean */
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final @Nullable Object[] args)
throws BeanCreationException {
...
//循環(huán)依賴相關(guān)邏輯：
//this.singletonFactories.put(beanName, singletonFactory);
//將實(shí)例化bean（①階段）、beanName組裝成singletonFactory裝入singletonFactories容器
//this.earlySingletonObjects.remove(beanName);
//刪除earlySingletonObjects中beanName
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
...
//實(shí)例初始化 就是在這里面實(shí)現(xiàn)依賴注入DI的:反射實(shí)現(xiàn)
//調(diào)用AutowiredAnnotationBeanPostProcessor.postProcessProperties
populateBean(beanName, mbd, instanceWrapper);
...
}③ 核心方法getSingleton
/* org.springframework.beans.factory.support.DefaultSingletonBeanRegistry#getSingleton(java.lang.String, boolean) */
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
Object singletonObject = this.singletonObjects.get(beanName);
//循環(huán)依賴核心就在于這個(gè)判斷，由于打標(biāo)+記錄了①階段的bean，
//循環(huán)依賴第二次調(diào)用getBean("a")時(shí)，這里會(huì)直接返回第一次調(diào)用getBean("a")創(chuàng)建的①階段的bean
//而不會(huì)調(diào)用createBean("a")再次bean初始化（造成兩個(gè)bean的循環(huán)創(chuàng)建）
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}④ 循環(huán)依賴流程
/* org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean */
protected T doGetBean(final String name, @Nullable final Class requiredType,
@Nullable final Object[] args, boolean typeCheckOnly) throws BeansException {
...
//假設(shè)A、B互相依賴
//第一次getBean(A)，sharedInstance == null,走else,createBean
//A正在創(chuàng)建打標(biāo)，①中beforeSingletonCreation()
//A實(shí)例化后保存到singletonFactories中②中addSingletonFactory(beanName,singletonFactory)
//DI依賴注入：②中populateBean(beanName,mbd,instanceWrapper),發(fā)現(xiàn)依賴B，調(diào)用getBean(B)初始化B的單例
//調(diào)用getBean(B)重復(fù)上面步驟，DI依賴注入發(fā)現(xiàn)依賴A，調(diào)用getBean(A)
//第二次getBean(A),③中if(singletonObject == null && isSingletonCurrentlyInCreation(A))由于打標(biāo)了所以返回singleFactory.getObject()
//下面if條件直接返回bean,沒有走else破壞了循環(huán)
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
//
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
...
//
createBean(beanName, mbd, args);
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
return bean;
}
四、總結(jié)
未看源碼之前，其實(shí)對(duì)循環(huán)依賴有一個(gè)想法：循環(huán)依賴可以看做是一個(gè)死鎖。
預(yù)防死鎖的方法：打破死鎖的四個(gè)必要條件（互斥、請(qǐng)求并等待、不可剝奪、循環(huán)等待），由于循環(huán)依賴的資源是對(duì)象自身，所以常用破壞循環(huán)等待條件方法：編號(hào)順序執(zhí)行，不適用
選擇破壞請(qǐng)求并等待條件：先創(chuàng)建對(duì)象，再賦值,模型
A a = new A();
B b = new B();
A.b = b;
B.a = a;
研究源碼之后發(fā)現(xiàn)：想法差不多，但是代碼實(shí)現(xiàn)非常精彩。模型（打標(biāo)沒想到過）
A a = new A();
B b = new B();
b.a = a;
a.b = b;
以上就是本文的全部?jī)?nèi)容，希望對(duì)大家的學(xué)習(xí)有所幫助，也希望大家多多支持創(chuàng)新互聯(lián)。
本文標(biāo)題：SpringIOCDI循環(huán)依賴實(shí)例詳解
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