Spring5.0原始碼學習系列之淺談迴圈依賴問題
阿新 • • 發佈:2020-11-17
## 前言介紹
附錄:[Spring原始碼學習專欄](https://blog.csdn.net/u014427391/category_10493299.html)
在[上一章](https://blog.csdn.net/u014427391/article/details/109625421)的學習中,我們對Bean的建立有了一個粗略的瞭解,接著本文淺談Spring迴圈依賴問題,這是一個面試比較常見的問題
## 1、什麼是迴圈依賴?
所謂的迴圈依賴就是兩個以及兩個以上的類互相呼叫依賴,形成閉環
```java
// 類A依賴於B
class A{
public B b;
}
// 類B依賴了C
class B{
public C c;
}
// 類C依賴了A
class C{
public A a;
}
```
![在這裡插入圖片描述](https://img-blog.csdnimg.cn/20201112164821955.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTQ0MjczOTE=,size_16,color_FFFFFF,t_70#pic_center)
然後?看起來是很正常的,我們隨便new一個類,迴圈依賴的類都是能正常呼叫的
```java
A a = new A();
System.out.println(a);
```
為什麼?因為這種情況,`A.java->A.class`,我們new就能獲取到例項的物件,這個通過jvm支援的,jdk是能支援這種情況的,不過本文不詳細說明,本文要討論的Spring中的bean,迴圈依賴在Spring中就是一個問題了
為什麼?首先回顧一下之前的知識點,首先在Spring框架中類的建立都是給Spring IOC容器建立的,如圖:
![在這裡插入圖片描述](https://img-blog.csdnimg.cn/20201116161018698.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTQ0MjczOTE=,size_16,color_FFFFFF,t_70#pic_center)
然後?真的出現這種情況,會怎麼樣?
![在這裡插入圖片描述](https://img-blog.csdnimg.cn/20201116161927610.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTQ0MjczOTE=,size_16,color_FFFFFF,t_70#pic_center)
Spring框架檢測到這種場景會拋 BeanCurrentlyInCreationException,提前暴露物件的方法,因為Spring建立bean的過程是一個很複雜的過程,首先是xml解析為document物件,document物件再轉成BeanDefinition,然後進行bean的生命週期,才算得上是一個真正的spring bean,接著進行後置處理器加工,假如出現這種,設想一下會怎麼樣?spring容器就會一直迴圈呼叫,當然是在特定的條件,為什麼說是特定情況?請看下文
## 2、實驗環境準備
實驗環境:
* SpringFramework版本
* Springframework5.0.x
* 開發環境
* JAR管理:gradle 4.9/ Maven3.+
* 開發IDE:IntelliJ IDEA 2018.2.5
* JDK:jdk1.8.0_31
* Git Server:Git fro window 2.8.3
* Git Client:SmartGit18.1.5(可選)
## 3、迴圈依賴問題
我們可以通過例子進行驗證,建立類A:
```java
package com.example.bean;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.stereotype.Component;
/**
*
*
*
*
*
*
* T doGetBean(
String name, @Nullable Class requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
// 處理BeanName,前面說的FactoryBean帶‘&’符號,要在這裡進行轉換
String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 從map(singletonObjects)裡獲取單例bean,確定是否已經存在對應例項
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 + "'");
}
}
// 兩種情況:普通的bean,直接從singletonObjects返回sharedInstance
//如果是FactoryBean,返回其建立的物件例項
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 校驗是否是多例(Prototype)的Bean,多例的bean是不支援迴圈依賴的
// 為了避免迴圈依賴,遇到這種情況,直接丟擲異常
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
// 檢查BeanFactory是否存在這個BeanDefinition
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
// 當前容器找不到BeanDefinition,去parent容器查詢
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
// 返回parent容器的查詢結果
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
}
if (!typeCheckOnly) {
//typeCheckOnly為false的情況,將beanName放在一個alreadyCreated的集合
markBeanAsCreated(beanName);
}
try {
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 校驗是否配置了 depends-on
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
// 存在迴圈引用的情況,要丟擲異常
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
// 正常情況,註冊依賴關係
registerDependentBean(dep, beanName);
try {
// 初始化被依賴項
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
// 單例的Bean
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, () -> {
try {
// 建立單例bean
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
// 多例的Bean,scope = protoType
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
// 多例的情況,建立bean之前新增標記(用於迴圈依賴校驗)
beforePrototypeCreation(beanName);
// 執行多例Bean建立
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
// 建立原型(多例)bean之後擦除標記
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
// 如果不是單例bean也不是多例的bean,委託給對應的實現類
else {
String scopeName = mbd.getScope();
if (!StringUtils.hasLength(scopeName)) {
throw new IllegalStateException("No scope name defined for bean ´" + beanName + "'");
}
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 {
// 執行bean建立
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;
}
}
// Check if required type matches the type of the actual bean instance.
// 檢查一下型別是否正確,不正確丟擲異常,正確返回例項
if (requiredType != null && !requiredType.isInstance(bean)) {
try {
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null) {
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
return convertedBean;
}
catch (TypeMismatchException ex) {
if (logger.isDebugEnabled()) {
logger.debug("Failed to convert bean '" + name + "' to required type '" +
ClassUtils.getQualifiedName(requiredType) + "'", ex);
}
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
}
}
return (T) bean;
}
```
* 原始碼比較複雜,所以可以帶著疑問來跟,首先以單例Bean的情況:#doGetBean.getSingleton
![在這裡插入圖片描述](https://img-blog.csdnimg.cn/20201116170829230.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTQ0MjczOTE=,size_16,color_FFFFFF,t_70#pic_center)
![在這裡插入圖片描述](https://img-blog.csdnimg.cn/20201116171103583.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTQ0MjczOTE=,size_16,color_FFFFFF,t_70#pic_center)
```java
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// 一級快取:singletonObjects (單例池)
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
// 二級快取:earlySingletonObjects(BeanDefinition還沒進行屬性填充)
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
// 三級快取:singletonFactories
ObjectFactory> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return singletonObject;
}
```
在某些情況,迴圈依賴會造成迴圈呼叫,所以需要怎麼解決?
![在這裡插入圖片描述](https://img-blog.csdnimg.cn/20201116161927610.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTQ0MjczOTE=,size_16,color_FFFFFF,t_70#pic_center)
Spring框架的方法是使用了三級快取,其實最關鍵的是earlySingletonObjects
* 一級快取:singletonObjects,這是Spring BeanDefinition的單例池,首先只儲存單例Bean的BeanDefinition,而且這個Bean是一個真正的bean,也就是進行過屬性填充的
* 二級快取:earlySingletonObjects,early從單詞意思來說,這個快取是在singletonObjects之前的,也就是BeanDefinition還沒進行屬性填充等等操作,Spring引入這個快取的目的就是為了處理單例bean的迴圈依賴問題
* 三級快取:singletonFactories,快取的是ObjectFactory,表示物件工廠,為什麼要加上這個快取?原因比較複雜,涉及到AOP等等原因,因為我還沒理解清楚,所以本文不說明
加上了earlySingletonObjects快取之後,Spring就能支援單例bean的迴圈依賴,參考語雀某大佬的筆記,畫圖表示:
![在這裡插入圖片描述](https://img-blog.csdnimg.cn/20201116173032175.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTQ0MjczOTE=,size_16,color_FFFFFF,t_70#pic_center)
* 帶著疑問來跟一下多例Bean的情況:
Spring框架是不支援多例bean的迴圈依賴的,原因跟下程式碼:`#doGetBean`
```java
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
// 校驗是否是多例(Prototype)的Bean,多例的bean是不支援迴圈依賴的
// 為了避免迴圈依賴,遇到這種情況,直接丟擲異常
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
```
多例的情況:看程式碼是通過`prototypesCurrentlyInCreation`裡的資料校驗的,prototypesCurrentlyInCreation是一個`ThreadLocal`物件
```java
protected boolean isPrototypeCurrentlyInCreation(String beanName) {
Object curVal = this.prototypesCurrentlyInCreation.get();
return (curVal != null &&
(curVal.equals(beanName) || (curVal instanceof Set && ((Set>) curVal).contains(beanName))));
}
```
繼續找程式碼,找到`beforePrototypeCreation`:
```java
protected void beforePrototypeCreation(String beanName) {
Object curVal = this.prototypesCurrentlyInCreation.get();
if (curVal == null) {
this.prototypesCurrentlyInCreation.set(beanName);
}
else if (curVal instanceof String) {
Set beanNameSet = new HashSet<>(2);
beanNameSet.add((String) curVal);
beanNameSet.add(beanName);
this.prototypesCurrentlyInCreation.set(beanNameSet);
}
else {
Set beanNameSet = (Set) curVal;
beanNameSet.add(beanName);
}
}
```
Ctrl+Alt+H,檢視這個方法的呼叫棧:其實就是在`#doGetBean`就呼叫了,也就是bean建立之前
![在這裡插入圖片描述](https://img-blog.csdnimg.cn/20201116174501665.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTQ0MjczOTE=,size_16,color_FFFFFF,t_70#pic_center)
```java
try {
// 多例的情況,建立bean之前新增標記(用於迴圈依賴校驗)
beforePrototypeCreation(beanName);
// 執行多例Bean建立
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
// 建立原型(多例)bean之後擦除標記
afterPrototypeCreation(beanName);
}
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
```
## 知識點歸納
* Spring中的迴圈依賴場景
* 構造器的迴圈依賴,通過建構函式
* Field屬性的迴圈依賴,通過set方法
* Spring的迴圈依賴是預設開啟的(setAllowCircularReferences)
* Spring對單例和多例Bean的支援
* 單例Bean(singleton) :只能通過`@Autowired`和set方法支援
* 多例Bean(prototype):預設不支援,直接拋異常`BeanCurrentlyInCreationException`
* Spring支援單例bean的迴圈依賴原因:使用了三級快取
* A class *
* @author mazq * 修改記錄 * 修改後版本: 修改人: 修改日期: 2020/11/05 10:31 修改內容: **/ @Component public class A { //@Autowired B b; public A() { b = new B(); System.out.println("A class is create"); } } ``` 類B: ```java package com.example.bean; import org.springframework.beans.factory.annotation.Autowired; import org.springframework.stereotype.Component; /** *
* B class *
* @author mazq * 修改記錄 * 修改後版本: 修改人: 修改日期: 2020/11/16 14:03 修改內容: **/ @Component public class B { //@Autowired A a; public B() { a = new A(); System.out.println("B class is create"); } } ``` 註冊類A、B ```java package com.example.config; import com.example.bean.B; import org.springframework.context.annotation.Bean; import org.springframework.context.annotation.Configuration; import com.example.bean.A; /** *
* AppConfiguration *
* @author mazq * 修改記錄 * 修改後版本: 修改人: 修改日期: 2020/11/05 10:26 修改內容: **/ @Configuration public class AppConfiguration { @Bean public A a(){ return new A(); } @Bean public B b() { return new B(); } } ``` ```java package com.example; import com.example.config.AppConfiguration; import org.springframework.context.annotation.AnnotationConfigApplicationContext; import com.example.bean.A; /** *
* TestController ** *
* @author mazq * 修改記錄 * 修改後版本: 修改人: 修改日期: 2020/11/05 10:22 修改內容: **/ public class TestApplication { public static void testCircularReferences() { AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(); context.register(AppConfiguration.class); //context.setAllowCircularReferences(false); context.refresh(); A bean = context.getBean(A.class); System.out.println(bean); } public static void main(String[] args) { // 測試Sprin迴圈依賴 testCircularReferences(); } } ``` 經過測試,一直在迴圈呼叫: ![在這裡插入圖片描述](https://img-blog.csdnimg.cn/20201116144846992.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTQ0MjczOTE=,size_16,color_FFFFFF,t_70#pic_center) ## 4、迴圈依賴解決方法 對於這種情況,Spring有處理方法?答案是有的,方法就是通過`@Autowired`註解,當然bean要是單例的,多例的情況不支援,原因後面分析 ```java @Component public class A { @Autowired B b; public A() { System.out.println("A class is create"); } } ``` ![在這裡插入圖片描述](https://img-blog.csdnimg.cn/2020111616465576.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTQ0MjczOTE=,size_16,color_FFFFFF,t_70#pic_center) > 補充:除了`@Autowired`方法,我們還可以通過set方法處理迴圈依賴問題,當然也是僅支援單例bean,多例的情況不支援 ## 5、關閉Spring迴圈依賴 有個疑問?Spring的迴圈依賴支援,預設情況是開啟?是否有什麼開關控制?通過原始碼學習,可以通過`setAllowCircularReferences`設定 ```java AnnotationConfigApplicationContext context = new AnnotationConfigApplicationContext(); context.register(AppConfiguration.class); // 關閉Spring迴圈依賴支援 context.setAllowCircularReferences(false); context.refresh(); ``` 通過測試,設定不開啟這個屬性的時候,即使加上`@Autowired`,程式碼還是拋異常了 ## 6、prototype(多例)迴圈依賴 在多例的情況,Spring能支援迴圈依賴?加上`@Scope("prototype")`,將bean變成多例的 ![在這裡插入圖片描述](https://img-blog.csdnimg.cn/20201116164145181.png?x-oss-process=image/watermark,type_ZmFuZ3poZW5naGVpdGk,shadow_10,text_aHR0cHM6Ly9ibG9nLmNzZG4ubmV0L3UwMTQ0MjczOTE=,size_16,color_FFFFFF,t_70#pic_center) 經過測試:多例的情況會丟擲異常,即使加上了`@Autowired`,原因請看下文 ## 7、Spring迴圈依賴特徵 ok,經過前面例子的驗證,到這來,可以對Spring的迴圈依賴特點進行歸納: * Spring中的迴圈依賴場景 * 構造器的迴圈依賴,通過建構函式 * Field屬性的迴圈依賴,通過set方法 * Spring的迴圈依賴是預設開啟的(setAllowCircularReferences) * Spring對單例和多例Bean的支援 * 單例Bean(singleton) :只能通過`@Autowired`和set方法支援 * 多例Bean(prototype):預設不支援,直接拋異常`BeanCurrentlyInCreationException` ## 8、Spring迴圈依賴原理 我們通過實驗進行了驗證,也歸納出了Spring迴圈依賴的特點,然後具體原因是什麼?我們只能通過原始碼學習得到答案 在[上一章](https://blog.csdn.net/u014427391/article/details/109625421)的學習中,我們對Bean的建立有了一個粗略的瞭解,所以,順著這條路線,跟下原始碼: 在前面的學習,我們知道了`{@link org.springframework.beans.factory.support.AbstractBeanFactory#doGetBean}`這個方法就是Spring Bean建立的真正執行方法 ```java protected