引言
本文主要梳理了Spring架構Bean建立過程中應對循環依賴問題的相關源碼。我在手寫super-mini-webpack的時候也介紹過解決循環依賴的算法：Map+記憶化搜尋。可以猜測這段源碼也實作了這個算法，是以在看這段源碼的時候，我們可以先找到遞歸點，再去分析調用棧涉及的那些函數，順便找出其用到的Map資料結構。
三級緩存資料結構簡介
三級緩存資料結構定義和操作三級緩存的函數都位于：spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistry.java
java複制代碼 /** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name to ObjectFactory. */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name to bean instance. */
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);
緩存先找到第一級，第一級沒有才查第二級，依此類推。singletonObjects, earlySingletonObjects, singletonFactories分别是1到3級。第一級緩存是bean名到成品bean的映射；第二級緩存是bean名到半成品bean的映射；第三級緩存是bean名到函數式接口的映射，作用為延遲調用函數。
java複制代碼@FunctionalInterface
public interface ObjectFactory<T> {
/**
* Return an instance (possibly shared or independent)
* of the object managed by this factory.
* @return the resulting instance
* @throws BeansException in case of creation errors
*/
T getObject() throws BeansException;
}
第三級緩存的Value裡的函數的調用方式就是調用.getObject()。
為什麼需要2級緩存？因為要展現一個分層的思想，半成品bean原則上是不能暴露到外部的。TODO：外部是指？
為什麼需要3級緩存？如果所有bean都沒有代理對象就不需要第3級緩存。TODO：補充說明。
源碼閱讀
如何啟動調試？
建立一個普通的SpringBoot項目。然後可以嘗試通過xml配置檔案和注解等方式構造循環引用。後文稱為場景1和場景2。
場景1：通過xml配置檔案來構造循環引用
入口src\main\java\com\hans\bean_dependency_cycle\hans\HansApplication.java：
java複制代碼package com.hans.bean_dependency_cycle.hans;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.ApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;
@SpringBootApplication
public class HansApplication {
public static void main(String[] args) {
ApplicationContext ac = new ClassPathXmlApplicationContext("cycle.xml");
A beanA = ac.getBean(A.class);
System.out.println(beanA);
System.out.println(beanA.getB());
B beanB = ac.getBean(B.class);
System.out.println(beanB);
System.out.println(beanB.getA());
}
}
src\main\resources\cycle.xml：
xml複制代碼<?xml version="1.0" encoding="UTF-8"?>
<beans
xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xmlns:context="http://www.springframework.org/schema/context"
xmlns:mvc="http://www.springframework.org/schema/mvc"
xmlns:p="http://www.springframework.org/schema/p"
xsi:schemaLocation="http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
http://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd
"
>
<bean id="a" class="com.hans.bean_dependency_cycle.hans.A">
<property name="b" ref="b"></property>
</bean>
<bean id="b" class="com.hans.bean_dependency_cycle.hans.B">
<property name="a" ref="a"></property>
</bean>
</beans>
A.java和B.java：
java複制代碼// A 和 B 不能都使用 lombok，否則無法列印。這裡選擇了 B 不使用 lombok
package com.hans.bean_dependency_cycle.hans;
import lombok.Data;
@Data
public class A {
private B b;
public String toString(A o) {
return o.getClass().getName() + "@" +
Integer.toHexString(System.identityHashCode(o));
}
}
package com.hans.bean_dependency_cycle.hans;
public class B {
private A a;
public A getA() {
return this.a;
}
public void setA(A a) {
this.a = a;
}
}
期望輸出：
less複制代碼A(b=com.hans.bean_dependency_cycle.hans.B@750e2b97)
com.hans.bean_dependency_cycle.hans.B@750e2b97
com.hans.bean_dependency_cycle.hans.B@750e2b97
A(b=com.hans.bean_dependency_cycle.hans.B@750e2b97)
場景1不需要配置spring.main.allow-circular-references為true也能得到期望輸出，TODO：原因未知。
場景2：通過注解+自動裝配屬性來構造循環引用：以@Controller為例
入口src\main\java\com\hans\bean_dependency_cycle\hans\AnotherEntry.java：
java複制代碼package com.hans.bean_dependency_cycle.hans;
import org.springframework.boot.SpringApplication;
import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.boot.autoconfigure.jdbc.DataSourceAutoConfiguration;
import org.springframework.context.ConfigurableApplicationContext;
@SpringBootApplication(exclude = { DataSourceAutoConfiguration.class })
public class AnotherEntry {
public static void main(String[] args) {
ConfigurableApplicationContext cac = SpringApplication.run(AnotherEntry.class, args);
ControllerA beanA = cac.getBean(ControllerA.class);
System.out.println(beanA);
System.out.println(beanA.getCb());
ControllerB beanB = cac.getBean(ControllerB.class);
System.out.println(beanB);
System.out.println(beanB.getCa());
}
}
根據參考連結1，接下來一定要記得修改application.yml：
yaml複制代碼spring:
main:
allow-circular-references: true
否則會報錯：
vbnet複制代碼***************************
APPLICATION FAILED TO START
***************************
Description:
The dependencies of some of the beans in the application context form a cycle:
┌─────┐
| controllerA (field private com.hans.bean_dependency_cycle.hans.ControllerB com.hansn_dependency_cycle.hans.ControllerA.cb)
↑ ↓
| controllerB (field private com.hans.bean_dependency_cycle.hans.ControllerA com.hansn_dependency_cycle.hans.ControllerB.ca)
└─────┘
Action:
Relying upon circular references is discouraged and they are prohibited by default. Up your application to remove the dependency cycle between beans. As a last resort, it me possible to break the cycle automatically by setting spring.main.allow-circular-refees to true.
兩個普通的Controller：
java複制代碼// src\main\java\com\hans\bean_dependency_cycle\hans\ControllerA.java
package com.hans.bean_dependency_cycle.hans;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class ControllerA {
@Autowired
private ControllerB cb;
public ControllerB getCb() {
return cb;
}
@RequestMapping("/controllerA")
public String index() {
return "hello controllerA!";
}
}
// src\main\java\com\hans\bean_dependency_cycle\hans\ControllerB.java
package com.hans.bean_dependency_cycle.hans;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.web.bind.annotation.RequestMapping;
import org.springframework.web.bind.annotation.RestController;
@RestController
public class ControllerB {
@Autowired
private ControllerA ca;
public ControllerA getCa() {
return ca;
}
@RequestMapping("/controllerB")
public String index() {
return "hello controllerB!";
}
}
期望輸出：
kotlin複制代碼com.hans.bean_dependency_cycle.hans.ControllerA@39f82681
com.hans.bean_dependency_cycle.hans.ControllerB@4bd9e7fd
com.hans.bean_dependency_cycle.hans.ControllerB@4bd9e7fd
com.hans.bean_dependency_cycle.hans.ControllerA@39f82681
脈絡
根據參考連結2，有一個經驗：“do”開頭的方法名是真正含有大量邏輯的方法。參考連結2Java之父精選的脈絡函數如下：
- getBean
- doGetBean
- createBean
- doCreateBean
- createBeanInstance
- populateBean
場景1到遞歸點為止的調用鍊：preInstantiateSingletons > getBean > doGetBean > getSingleton函數有多個，其中調用了beforeSingletonCreation的函數調用singletonFactory.getObject()時才真正調用了createBean > createBean > doCreateBean > createBeanInstance, populateBean 都在 doCreateBean 的實作裡 > populateBean調用了applyPropertyValues > resolveValueIfNecessary > resolveReference > getBean。
場景2的調用鍊到populateBean開始和場景1的調用鍊岔開，兩個場景的差異放在後文《Controller和普通Bean解決循環依賴過程的相同點與不同點》分析。
場景1遞歸到A -> B -> A時的調用鍊：getBean > doGetBean > 未調用beforeSingletonCreation的getSingleton，操作第2級緩存後離開。
操作三級緩存的函數都位于https://github.com/spring-projects/spring-framework/blob/502997d8e986dcfde1f49b2b2f443a32b5488b13/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistry.java：
- 放入第3級緩存的函數：doCreateBean調用的addSingletonFactory。
- 放入第2級緩存的函數：首先要知道getSingleton在DefaultSingletonBeanRegistry.java裡本質上的實作有兩個，一個public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory)調用了beforeSingletonCreation并間接調用了createBean；另一個protected Object getSingleton(String beanName, boolean allowEarlyReference)則是操作了第2級緩存。
- 放入第1級緩存的函數：addSingleton。在間接調用了createBean函數的getSingleton處調用。
場景1的執行過程
真正意義上的入口：spring-context/src/main/java/org/springframework/context/support/AbstractApplicationContext.java的beanFactory.preInstantiateSingletons();。這裡的beanFactory就有singletonObjects那3級緩存的對象。于是跳到https://github.com/spring-projects/spring-framework/blob/bbde68c49e66c3c531920cb80a55742262507be7/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultListableBeanFactory.java：
java複制代碼 @Override
public void preInstantiateSingletons() throws BeansException {
if (logger.isTraceEnabled()) {
logger.trace("Pre-instantiating singletons in " + this);
}
// Iterate over a copy to allow for init methods which in turn register new bean definitions.
// While this may not be part of the regular factory bootstrap, it does otherwise work fine.
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// 觸發所有非延遲加載的（non-lazy）單例 bean 的初始化
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
// 假如先周遊 A 再周遊 B 那麼周遊到 B 的時候，因為循環引用解決的關系，B 已經放到了第1級緩存，是以 doGetBean 的 getSingleton 可以直接從第1級緩存取到值，不用再走一遍 createBean 方法
// 合并父類 BeanDefinition
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
// 非抽象、是單例、非懶加載
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
// 如果實作了 FactoryBean 接口則是 FactoryBean
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName); // 比如：FACTORY_BEAN_PREFIX + beanName = "&A"
if (bean instanceof SmartFactoryBean<?> smartFactoryBean && smartFactoryBean.isEagerInit()) {
getBean(beanName);
}
}
else {
// 不是 FactoryBean，隻是普通 Bean，則走這個分支
getBean(beanName);
}
}
}
// 觸發所有 SmartInitializingSingleton 的後初始化回調
// Trigger post-initialization callback for all applicable beans...
for (String beanName : beanNames) {
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton smartSingleton) {
StartupStep smartInitialize = getApplicationStartup().start("spring.beans.smart-initialize")
.tag("beanName", beanName);
smartSingleton.afterSingletonsInstantiated();
smartInitialize.end();
}
}
}
順便看下isFactoryBean的實作https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractBeanFactory.java：
java複制代碼 @Override
public boolean isFactoryBean(String name) throws NoSuchBeanDefinitionException {
String beanName = transformedBeanName(name);
Object beanInstance = getSingleton(beanName, false);
if (beanInstance != null) {
return (beanInstance instanceof FactoryBean);
}
// No singleton instance found -> check bean definition.
if (!containsBeanDefinition(beanName) && getParentBeanFactory() instanceof ConfigurableBeanFactory cbf) {
// No bean definition found in this factory -> delegate to parent.
return cbf.isFactoryBean(name);
}
return isFactoryBean(beanName, getMergedLocalBeanDefinition(beanName));
}
getBean和isFactoryBean都位于https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractBeanFactory.java，getBean隻有1行
java複制代碼 @Override
public Object getBean(String name) throws BeansException {
return doGetBean(name, null, null, false);
}
doGetBean和getBean, isFactoryBean都在AbstractBeanFactory.java：
java複制代碼 /**
* Return an instance, which may be shared or independent, of the specified bean.
* @param name the name of the bean to retrieve
* @param requiredType the required type of the bean to retrieve
* @param args arguments to use when creating a bean instance using explicit arguments
* (only applied when creating a new instance as opposed to retrieving an existing one)
* @param typeCheckOnly whether the instance is obtained for a type check,
* not for actual use
* @return an instance of the bean
* @throws BeansException if the bean could not be created
*/
@SuppressWarnings("unchecked")
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
String beanName = transformedBeanName(name);
Object beanInstance;
// 提前檢查單例緩存中是否有手動注冊的單例對象，跟循環依賴有關。如果是初次進這裡，比如 controllerA 初次進這個方法，肯定是拿不到值的，就會是 null
// 對于最簡單的循環依賴，controllerA -> controllerB -> controllerA 之後，需要進入 getSingleton 了，這裡的邏輯就是要從第3級緩存裡拿到工廠函數，調用後得到 controllerA 半成品，進而可以直接 return
// Eagerly check singleton cache for manually registered singletons.
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isTraceEnabled()) {
if (isSingletonCurrentlyInCreation(beanName)) {
logger.trace("Returning eagerly cached instance of singleton bean '" + beanName +
"' that is not fully initialized yet - a consequence of a circular reference");
}
else {
logger.trace("Returning cached instance of singleton bean '" + beanName + "'");
}
}
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory abf) {
return abf.doGetBean(nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
return (T) parentBeanFactory.getBean(nameToLookup);
}
}
if (!typeCheckOnly) {
markBeanAsCreated(beanName);
}
StartupStep beanCreation = this.applicationStartup.start("spring.beans.instantiate")
.tag("beanName", name);
try {
if (requiredType != null) {
beanCreation.tag("beanType", requiredType::toString);
}
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null) {
for (String dep : dependsOn) {
if (isDependent(beanName, dep)) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between '" + beanName + "' and '" + dep + "'");
}
registerDependentBean(dep, beanName);
try {
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
if (mbd.isSingleton()) {
// 傳回 beanName 的原始單例對象。如果尚未注冊，則使用 singletonFactory 建立并注冊一個對象
sharedInstance = getSingleton(beanName, () -> {
try {
// 為給定的合并後的 BeanDefinition 和參數建立一個 bean 執行個體
// createBean 真實執行時機是調用了 beforeSingletonCreation 方法的 getSingleton 方法執行 singletonObject = singletonFactory.getObject() 時
// 首次運作到 beanName = "controllerA" 時 args = null
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.
// 顯式地從單例緩存中删除bean執行個體：因為這個執行個體可能是由建立過程急切地放在那裡的，放在那裡的目的是允許循環引用解析。
// 還要删除所有被這個bean臨時引用的所有bean。如果找到相應的一次性bean執行個體，則委托給 destroyBean
destroySingleton(beanName);
throw ex;
}
});
beanInstance = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
beanInstance = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
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 {
return createBean(beanName, mbd, args);
}
finally {
afterPrototypeCreation(beanName);
}
});
beanInstance = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
}
catch (IllegalStateException ex) {
throw new ScopeNotActiveException(beanName, scopeName, ex);
}
}
}
catch (BeansException ex) {
beanCreation.tag("exception", ex.getClass().toString());
beanCreation.tag("message", String.valueOf(ex.getMessage()));
cleanupAfterBeanCreationFailure(beanName);
throw ex;
}
finally {
beanCreation.end();
}
}
return adaptBeanInstance(name, beanInstance, requiredType);
}
我們先隻關注createBean，是以需要關注getSingleton。注意：
getSingleton的函數在DefaultSingletonBeanRegistry.java裡本質上的實作有兩個，一個調用了beforeSingletonCreation并間接調用了createBean；另一個則是操作了第2級緩存。
是以目前我們需要關注的是調用了beforeSingletonCreation的getSingleton方法。路徑：https://github.com/spring-projects/spring-framework/blob/502997d8e986dcfde1f49b2b2f443a32b5488b13/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistry.java
java複制代碼 /**
* Return the (raw) singleton object registered under the given name,
* creating and registering a new one if none registered yet.
* @param beanName the name of the bean
* @param singletonFactory the ObjectFactory to lazily create the singleton
* with, if necessary
* @return the registered singleton object
*/
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
if (this.singletonsCurrentlyInDestruction) {
throw new BeanCreationNotAllowedException(beanName,
"Singleton bean creation not allowed while singletons of this factory are in destruction " +
"(Do not request a bean from a BeanFactory in a destroy method implementation!)");
}
if (logger.isDebugEnabled()) {
logger.debug("Creating shared instance of singleton bean '" + beanName + "'");
}
// 建立單例前的回調，預設實作為：将單例注冊為目前正在建立中，實作隻有3行，可以看下。
beforeSingletonCreation(beanName);
// flag 表示是否生成了新的單例對象
boolean newSingleton = false;
// flag 表示是否有抑制異常的記錄，true表示沒有
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
// 若沒有抑制異常記錄，則對抑制異常清單進行初始化
this.suppressedExceptions = new LinkedHashSet<>();
}
try {
// 從單例工廠擷取對象。注意 singletonFactory 是本方法第二個參數，之前也介紹了
// ObjectFactory 對象通過調 getObject 來正式執行函數，是以 createBean 在此時才真正執行
singletonObject = singletonFactory.getObject();
// 擷取後，就已經生成了新的單例對象，标記為 true
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
throw ex;
}
}
catch (BeanCreationException ex) {
if (recordSuppressedExceptions) {
for (Exception suppressedException : this.suppressedExceptions) {
ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {
if (recordSuppressedExceptions) {
this.suppressedExceptions = null;
}
afterSingletonCreation(beanName);
}
if (newSingleton) {
// 操作第1級緩存
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
在調用singletonObject = singletonFactory.getObject();以間接調用createBean後，會調用addSingleton，将bean加入第1級緩存，這标志着bean變為成品。接下來我們看下addSingleton的代碼，它和getSingleton定義于同一個檔案。
java複制代碼 /**
* Add the given singleton object to the singleton cache of this factory.
* <p>To be called for eager registration of singletons.
* @param beanName the name of the bean
* @param singletonObject the singleton object
*/
protected void addSingleton(String beanName, Object singletonObject) {
// addSingleton 在調用了 createBean 的 getSingleton 方法中被調用，标志着 bean 變為成品對象
synchronized (this.singletonObjects) {
// 第1級緩存添加，第2、3級緩存移除
this.singletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
// 添加到已注冊的單例集合。 registeredSingletons 為 Set<String>
this.registeredSingletons.add(beanName);
}
}
接下來看createBean。https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java
java複制代碼 /**
* Central method of this class: creates a bean instance,
* populates the bean instance, applies post-processors, etc.
* @see #doCreateBean
*/
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
// 首次看源碼，直接看 doCreateBean 調用處
if (logger.isTraceEnabled()) {
logger.trace("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
// Make sure bean class is actually resolved at this point, and
// clone the bean definition in case of a dynamically resolved Class
// which cannot be stored in the shared merged bean definition.
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
try {
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// Give BeanPostProcessors a chance to return a proxy instead of the target bean instance.
Object bean = resolveBeforeInstantiation(beanName, mbdToUse);
if (bean != null) {
return bean;
}
}
catch (Throwable ex) {
throw new BeanCreationException(mbdToUse.getResourceDescription(), beanName,
"BeanPostProcessor before instantiation of bean failed", ex);
}
try {
// 實際建立 bean
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
catch (BeanCreationException | ImplicitlyAppearedSingletonException ex) {
// A previously detected exception with proper bean creation context already,
// or illegal singleton state to be communicated up to DefaultSingletonBeanRegistry.
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbdToUse.getResourceDescription(), beanName, "Unexpected exception during bean creation", ex);
}
}
createBean目前唯一值得關注的點就是調用了doCreateBean。doCreateBean做了幾件值得本文關注的事：
- 調用了createBeanInstance完成bean的執行個體化。
- 調用了addSingletonFactory，即加入了第3級緩存。
- 調用了populateBean完成bean的屬性指派操作。
doCreateBean和createBean都定義在https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java。
java複制代碼 /**
* Actually create the specified bean. Pre-creation processing has already happened
* at this point, e.g. checking {@code postProcessBeforeInstantiation} callbacks.
* <p>Differentiates between default bean instantiation, use of a
* factory method, and autowiring a constructor.
* @param beanName the name of the bean
* @param mbd the merged bean definition for the bean
* @param args explicit arguments to use for constructor or factory method invocation
* @return a new instance of the bean
* @throws BeanCreationException if the bean could not be created
* @see #instantiateBean
* @see #instantiateUsingFactoryMethod
* @see #autowireConstructor
*/
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
// instanceWrapper 持有建立出的 bean 對象
BeanWrapper instanceWrapper = null;
// 擷取 factoryBean 執行個體緩存
if (mbd.isSingleton()) {
// 如果是單例對象，從 factoryBean 執行個體緩存中移除目前 bean 的資訊
// 首次 controllerA 進來會進行移除操作，并且會調用 createBeanInstance
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
// 沒有就建立執行個體
if (instanceWrapper == null) {
// 根據執行 bean 使用對應的政策建立新的工廠執行個體，如：工廠方法、構造函數主動注入、簡單初始化
// 第一次讀源碼時不需要點進去看 createBeanInstance 。下一個主幹方法 populateBean 是在本函數下文調用的
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
// 從包裝類中擷取原始 bean 。首次執行 controllerA 的時候, bean 就是普通的 controllerA 對象 ControllerA@100{cb=null}
Object bean = instanceWrapper.getWrappedInstance();
// 擷取具體 bean 對象的 Class 屬性
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
// 不等于 NullBean 類型時就修改目标類型
mbd.resolvedTargetType = beanType;
}
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.markAsPostProcessed();
}
}
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
// 判斷目前 bean 是否需要提前曝光，條件為：是單例 && 允許循環依賴 spring.main.allow-circular-references 配置為 true && 目前 bean 正在建立中
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");
}
// 為避免後期循環依賴，可以在 bean 初始化完成前将建立執行個體的 ObjectFactory 加入工廠
// 注意，這個方法會操作3級緩存的資料結構，尤其是第3級緩存。在 controllerA -> controllerB -> controllerA 的時候未調用 createBean 的 getSingleton 方法會真正調用這個匿名函數，進而調用 getEarlyBeanReference 進而操作第2級緩存
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
// controllerA 首次執行到這裡時，exposedObject = ControllerA@100{cb=null}
Object exposedObject = bean;
try {
// populateBean 是主幹方法，給剛剛執行個體化的 bean （半成品）填充屬性
populateBean(beanName, mbd, instanceWrapper);
// controllerA -> controllerB -> controllerA 之後，getSingleton 傳回 controllerA 後回到這裡， controllerB 的 ca 屬性就有值了
// 随後遞歸傳回到這句話且調用棧隻有 controllerA 的時候，發現兩者都有值了
exposedObject = initializeBean(beanName, exposedObject, mbd);
}
catch (Throwable ex) {
if (ex instanceof BeanCreationException bce && beanName.equals(bce.getBeanName())) {
throw bce;
}
else {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, ex.getMessage(), 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<String> actualDependentBeans = new LinkedHashSet<>(dependentBeans.length);
for (String dependentBean : dependentBeans) {
if (!removeSingletonIfCreatedForTypeCheckOnly(dependentBean)) {
actualDependentBeans.add(dependentBean);
}
}
if (!actualDependentBeans.isEmpty()) {
throw new BeanCurrentlyInCreationException(beanName,
"Bean with name '" + beanName + "' has been injected into other beans [" +
StringUtils.collectionToCommaDelimitedString(actualDependentBeans) +
"] 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 " +
"'getBeanNamesForType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
createBeanInstance的細節與本文主題無關，不關注。接下來我們看下addSingletonFactory的實作。addSingletonFactory位于https://github.com/spring-projects/spring-framework/blob/502997d8e986dcfde1f49b2b2f443a32b5488b13/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistry.java，主要動作是操作第3級緩存。
java複制代碼 /**
* Add the given singleton factory for building the specified singleton
* if necessary.
* <p>To be called for eager registration of singletons, e.g. to be able to
* resolve circular references.
* @param beanName the name of the bean
* @param singletonFactory the factory for the singleton object
*/
protected void addSingletonFactory(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(singletonFactory, "Singleton factory must not be null");
synchronized (this.singletonObjects) {
// controllerA 首次執行到這裡，第一級緩存肯定是查不到的
if (!this.singletonObjects.containsKey(beanName)) {
// 第3級緩存放入。再回顧一下， singletonFactory存的是 beanName 到一個延遲執行的函數的映射
// controllerA 首次執行到這裡的時候， singletonFactory = () -> getEarlyBeanReference(beanName, mbd, bean)
this.singletonFactories.put(beanName, singletonFactory);
// 從早期單例對象的高速緩存（即第2級緩存）移除目前 beanName 對應的緩存對象
this.earlySingletonObjects.remove(beanName);
// 添加到已注冊的單例集合裡，和三級緩存無關。值得注意的是， A 首次加入三級緩存時，就是首次加入已注冊的單例集合
this.registeredSingletons.add(beanName);
}
}
}
接下來關注位于https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java的populateBean。populateBean方法有一個功能是給bean的屬性指派，包含了遞歸點。在建立Bean的源碼中，遞歸點指的是遞歸調用getBean方法。
java複制代碼 /**
* Populate the bean instance in the given BeanWrapper with the property values
* from the bean definition.
* @param beanName the name of the bean
* @param mbd the bean definition for the bean
* @param bw the BeanWrapper with bean instance
*/
protected void populateBean(String beanName, RootBeanDefinition mbd, @Nullable BeanWrapper bw) {
if (bw == null) {
if (mbd.hasPropertyValues()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to null instance");
}
else {
// Skip property population phase for null instance.
return;
}
}
if (bw.getWrappedClass().isRecord()) {
if (mbd.hasPropertyValues()) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Cannot apply property values to a record");
}
else {
// Skip property population phase for records since they are immutable.
return;
}
}
// Give any InstantiationAwareBeanPostProcessors the opportunity to modify the
// state of the bean before properties are set. This can be used, for example,
// to support styles of field injection.
if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {
for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
if (!bp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName)) {
return;
}
}
}
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
int resolvedAutowireMode = mbd.getResolvedAutowireMode();
if (resolvedAutowireMode == AUTOWIRE_BY_NAME || resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
MutablePropertyValues newPvs = new MutablePropertyValues(pvs);
// Add property values based on autowire by name if applicable.
if (resolvedAutowireMode == AUTOWIRE_BY_NAME) {
autowireByName(beanName, mbd, bw, newPvs);
}
// Add property values based on autowire by type if applicable.
if (resolvedAutowireMode == AUTOWIRE_BY_TYPE) {
autowireByType(beanName, mbd, bw, newPvs);
}
pvs = newPvs;
}
if (hasInstantiationAwareBeanPostProcessors()) {
if (pvs == null) {
pvs = mbd.getPropertyValues();
}
for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
// 對于 Controller Bean 自動裝配屬性産生循環依賴的場景，周遊到 AutowiredAnnotationBeanPostProcessor 時，這句話包含遞歸點
PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
if (pvsToUse == null) {
return;
}
pvs = pvsToUse;
}
}
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
if (needsDepCheck) {
PropertyDescriptor[] filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
checkDependencies(beanName, mbd, filteredPds, pvs);
}
if (pvs != null) {
// 對于普通 Bean 的場景，這句話包含遞歸點
applyPropertyValues(beanName, mbd, bw, pvs);
}
}
applyPropertyValues也位于https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java，主要功能為完成bean初始化。其中，Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue)包含了遞歸點，通過動調可以驗證bw.setPropertyValues(new MutablePropertyValues(deepCopy))真正完成了屬性指派工作。
java複制代碼 /**
* Apply the given property values, resolving any runtime references
* to other beans in this bean factory. Must use deep copy, so we
* don't permanently modify this property.
* @param beanName the bean name passed for better exception information
* @param mbd the merged bean definition
* @param bw the BeanWrapper wrapping the target object
* @param pvs the new property values
*/
protected void applyPropertyValues(String beanName, BeanDefinition mbd, BeanWrapper bw, PropertyValues pvs) {
// applyPropertyValues 是真正完成指派操作的函數
// 如果 pvs 沒有 PropertyValue，就直接結束
if (pvs.isEmpty()) {
return;
}
MutablePropertyValues mpvs = null;
List<PropertyValue> original;
if (pvs instanceof MutablePropertyValues _mpvs) {
mpvs = _mpvs;
if (mpvs.isConverted()) {
// Shortcut: use the pre-converted values as-is.
try {
bw.setPropertyValues(mpvs);
return;
}
catch (BeansException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Error setting property values", ex);
}
}
// 擷取 mpvs 的 PropertyValue 清單
original = mpvs.getPropertyValueList();
}
else {
// 擷取 pvs 的 PropertyValue 對象數組并轉為清單
original = Arrays.asList(pvs.getPropertyValues());
}
// 使用者自定義的類型轉換器，預設轉換器為 bean 的包裝類對象
TypeConverter converter = getCustomTypeConverter();
if (converter == null) {
converter = bw;
}
BeanDefinitionValueResolver valueResolver = new BeanDefinitionValueResolver(this, beanName, mbd, converter);
// Create a deep copy, resolving any references for values.
List<PropertyValue> deepCopy = new ArrayList<>(original.size());
// resolveNecessary flag 含義為是否還需要解析
boolean resolveNecessary = false;
for (PropertyValue pv : original) {
// 屬性已解析過則加入 deepCopy
if (pv.isConverted()) {
deepCopy.add(pv);
}
else {
String propertyName = pv.getName();
// 擷取未經類型轉換的值
Object originalValue = pv.getValue();
if (originalValue == AutowiredPropertyMarker.INSTANCE) {
Method writeMethod = bw.getPropertyDescriptor(propertyName).getWriteMethod();
// 如果 setter 方法為空
if (writeMethod == null) {
// 異常：自動裝配标記屬性沒有寫方法
throw new IllegalArgumentException("Autowire marker for property without write method: " + pv);
}
originalValue = new DependencyDescriptor(new MethodParameter(writeMethod, 0), true);
}
// 交由 valueResolver 根據 pv 解析出 originalValue 所封裝的對象。注意：這個函數包含遞歸點
Object resolvedValue = valueResolver.resolveValueIfNecessary(pv, originalValue);
Object convertedValue = resolvedValue;
// flag 含義為是否可轉換： propertyName 是 bw 中的可寫屬性 && propertyName 不是索引屬性或嵌套屬性
boolean convertible = bw.isWritableProperty(propertyName) &&
!PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);
if (convertible) {
// 可轉換則将 resolvedValue 轉換為指定的目标屬性對象
convertedValue = convertForProperty(resolvedValue, propertyName, bw, converter);
}
// Possibly store converted value in merged bean definition,
// in order to avoid re-conversion for every created bean instance.
if (resolvedValue == originalValue) {
if (convertible) {
pv.setConvertedValue(convertedValue);
}
deepCopy.add(pv);
}
else if (convertible && originalValue instanceof TypedStringValue typedStringValue &&
!typedStringValue.isDynamic() &&
!(convertedValue instanceof Collection || ObjectUtils.isArray(convertedValue))) {
pv.setConvertedValue(convertedValue);
deepCopy.add(pv);
}
else {
resolveNecessary = true;
deepCopy.add(new PropertyValue(pv, convertedValue));
}
}
}
if (mpvs != null && !resolveNecessary) {
mpvs.setConverted();
}
// Set our (possibly massaged) deep copy.
try {
// 完成屬性指派工作。咱們做個簡單的實驗，動調執行此句前後各點選調用棧看一次 populateBean 調用處的下一句的 exposedObject 或者 bean 變量，這就證明了 bean 的屬性指派确實是在這句話完成的
bw.setPropertyValues(new MutablePropertyValues(deepCopy));
}
catch (BeansException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, ex.getMessage(), ex);
}
}
resolveValueIfNecessary位于https://github.com/spring-projects/spring-framework/blob/30d6ec3398ce41add7bc44d360b8fb86ac0264b1/spring-beans/src/main/java/org/springframework/beans/factory/support/BeanDefinitionValueResolver.java。其實作也很長，但目前隻需要關注return resolveReference(argName, ref)，因為resolveReference包含遞歸點。
java複制代碼 /**
* Given a PropertyValue, return a value, resolving any references to other
* beans in the factory if necessary. The value could be:
* <li>A BeanDefinition, which leads to the creation of a corresponding
* new bean instance. Singleton flags and names of such "inner beans"
* are always ignored: Inner beans are anonymous prototypes.
* <li>A RuntimeBeanReference, which must be resolved.
* <li>A ManagedList. This is a special collection that may contain
* RuntimeBeanReferences or Collections that will need to be resolved.
* <li>A ManagedSet. May also contain RuntimeBeanReferences or
* Collections that will need to be resolved.
* <li>A ManagedMap. In this case the value may be a RuntimeBeanReference
* or Collection that will need to be resolved.
* <li>An ordinary object or {@code null}, in which case it's left alone.
* @param argName the name of the argument that the value is defined for
* @param value the value object to resolve
* @return the resolved object
*/
@Nullable
public Object resolveValueIfNecessary(Object argName, @Nullable Object value) {
// We must check each value to see whether it requires a runtime reference
// to another bean to be resolved.
// RuntimeBeanReference：當屬性值對象是工廠中另一個 bean 的引用時，使用不可變的占位符類，在運作時進行解析
if (value instanceof RuntimeBeanReference ref) {
// 解析出對應 ref 所封裝的 Bean 的元資訊的 Bean 對象。Bean 的元資訊：Bean 名，Bean類型。注意，這個函數裡包含遞歸點
return resolveReference(argName, ref);
}
else if (value instanceof RuntimeBeanNameReference ref) {
String refName = ref.getBeanName();
refName = String.valueOf(doEvaluate(refName));
if (!this.beanFactory.containsBean(refName)) {
throw new BeanDefinitionStoreException(
"Invalid bean name '" + refName + "' in bean reference for " + argName);
}
return refName;
}
else if (value instanceof BeanDefinitionHolder bdHolder) {
// Resolve BeanDefinitionHolder: contains BeanDefinition with name and aliases.
return resolveInnerBean(bdHolder.getBeanName(), bdHolder.getBeanDefinition(),
(name, mbd) -> resolveInnerBeanValue(argName, name, mbd));
}
else if (value instanceof BeanDefinition bd) {
return resolveInnerBean(null, bd,
(name, mbd) -> resolveInnerBeanValue(argName, name, mbd));
}
else if (value instanceof DependencyDescriptor dependencyDescriptor) {
Set<String> autowiredBeanNames = new LinkedHashSet<>(2);
Object result = this.beanFactory.resolveDependency(
dependencyDescriptor, this.beanName, autowiredBeanNames, this.typeConverter);
for (String autowiredBeanName : autowiredBeanNames) {
if (this.beanFactory.containsBean(autowiredBeanName)) {
this.beanFactory.registerDependentBean(autowiredBeanName, this.beanName);
}
}
return result;
}
else if (value instanceof ManagedArray managedArray) {
// May need to resolve contained runtime references.
Class<?> elementType = managedArray.resolvedElementType;
if (elementType == null) {
String elementTypeName = managedArray.getElementTypeName();
if (StringUtils.hasText(elementTypeName)) {
try {
elementType = ClassUtils.forName(elementTypeName, this.beanFactory.getBeanClassLoader());
managedArray.resolvedElementType = elementType;
}
catch (Throwable ex) {
// Improve the message by showing the context.
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Error resolving array type for " + argName, ex);
}
}
else {
elementType = Object.class;
}
}
return resolveManagedArray(argName, (List<?>) value, elementType);
}
else if (value instanceof ManagedList<?> managedList) {
// May need to resolve contained runtime references.
return resolveManagedList(argName, managedList);
}
else if (value instanceof ManagedSet<?> managedSet) {
// May need to resolve contained runtime references.
return resolveManagedSet(argName, managedSet);
}
else if (value instanceof ManagedMap<?, ?> managedMap) {
// May need to resolve contained runtime references.
return resolveManagedMap(argName, managedMap);
}
else if (value instanceof ManagedProperties original) {
// Properties original = managedProperties;
Properties copy = new Properties();
original.forEach((propKey, propValue) -> {
if (propKey instanceof TypedStringValue typedStringValue) {
propKey = evaluate(typedStringValue);
}
if (propValue instanceof TypedStringValue typedStringValue) {
propValue = evaluate(typedStringValue);
}
if (propKey == null || propValue == null) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Error converting Properties key/value pair for " + argName + ": resolved to null");
}
copy.put(propKey, propValue);
});
return copy;
}
else if (value instanceof TypedStringValue typedStringValue) {
// Convert value to target type here.
Object valueObject = evaluate(typedStringValue);
try {
Class<?> resolvedTargetType = resolveTargetType(typedStringValue);
if (resolvedTargetType != null) {
return this.typeConverter.convertIfNecessary(valueObject, resolvedTargetType);
}
else {
return valueObject;
}
}
catch (Throwable ex) {
// Improve the message by showing the context.
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Error converting typed String value for " + argName, ex);
}
}
else if (value instanceof NullBean) {
return null;
}
else {
return evaluate(value);
}
}
resolveReference和resolveValueIfNecessary都位于https://github.com/spring-projects/spring-framework/blob/30d6ec3398ce41add7bc44d360b8fb86ac0264b1/spring-beans/src/main/java/org/springframework/beans/factory/support/BeanDefinitionValueResolver.java。其實作也很長，但目前隻需要關注bean = this.beanFactory.getBean(resolvedName)，因為這就是遞歸點。
java複制代碼 /**
* Resolve a reference to another bean in the factory.
*/
@Nullable
private Object resolveReference(Object argName, RuntimeBeanReference ref) {
try {
Object bean;
Class<?> beanType = ref.getBeanType();
// 如果引用來自父工廠
if (ref.isToParent()) {
// 擷取父工廠
BeanFactory parent = this.beanFactory.getParentBeanFactory();
if (parent == null) {
// 沒有父工廠則報錯：在父工廠中無法解析對 Bean 的引用，因為父工廠就不存在
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Cannot resolve reference to bean " + ref +
" in parent factory: no parent factory available");
}
if (beanType != null) {
bean = parent.getBean(beanType);
}
else {
bean = parent.getBean(String.valueOf(doEvaluate(ref.getBeanName())));
}
}
else {
String resolvedName;
if (beanType != null) {
// 解析與 beanType 唯一比對的 bean 執行個體，包括其 bean 名
NamedBeanHolder<?> namedBean = this.beanFactory.resolveNamedBean(beanType);
// 讓 bean 引用 namedBean 所封裝的 bean 對象
bean = namedBean.getBeanInstance();
resolvedName = namedBean.getBeanName();
}
else {
resolvedName = String.valueOf(doEvaluate(ref.getBeanName()));
// 擷取 ref 所包裝的 Bean 名對應的 Bean 對象
// 注意，這就是遞歸點了
bean = this.beanFactory.getBean(resolvedName);
}
// 注冊依賴關系到 Bean 工廠
this.beanFactory.registerDependentBean(resolvedName, this.beanName);
}
if (bean instanceof NullBean) {
bean = null;
}
return bean;
}
catch (BeansException ex) {
throw new BeanCreationException(
this.beanDefinition.getResourceDescription(), this.beanName,
"Cannot resolve reference to bean '" + ref.getBeanName() + "' while setting " + argName, ex);
}
}
B建立過程調用棧是完全一樣的，接下來我們假設現在走到了A -> B -> A，回到了doGetBean的Object sharedInstance = getSingleton(beanName);處。此時我們需要關注其實作了：
java複制代碼 /**
* Return the (raw) singleton object registered under the given name.
* <p>Checks already instantiated singletons and also allows for an early
* reference to a currently created singleton (resolving a circular reference).
* @param beanName the name of the bean to look for
* @param allowEarlyReference whether early references should be created or not
* @return the registered singleton object, or {@code null} if none found
*/
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// Quick check for existing instance without full singleton lock
// 該函數主要是調用放入第3級緩存的 getEarlyBeanReference 并放入第2級緩存
// 首先從第1級緩存擷取 bean
Object singletonObject = this.singletonObjects.get(beanName);
// 第1級緩存沒有，并且已标記為建立中
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
// 從第2級緩存擷取 bean.因為controllerA -> controllerB -> controllerA 的時候是建立中的 bean,隻放到了第3級緩存,是以是查不到的
singletonObject = this.earlySingletonObjects.get(beanName);
// controllerA -> controllerB -> controllerA 的時候進來, allowEarlyReference 肯定是 true
if (singletonObject == null && allowEarlyReference) {
synchronized (this.singletonObjects) {
// Consistent creation of early reference within full singleton lock
// 這段做二次确認的代碼讓我聯想到線程安全的單例模式的寫法
// 從第1級緩存取
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
// 從第2級緩存取
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null) {
// 從第3級緩存取。如果是 controllerA 初次進來，因為 not in creation 是以不會進這裡，就算進了這裡，因為第3級緩存取不到是以還是會直接 return。如果是 controllerA -> controllerB -> controllerA 則會操作第2級緩存
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
// 回顧一下，DefaultSingletonBeanRegistry.java 的 addSingletonFactory 函數在操作第3級緩存的時候，放入的匿名函數就是：
// singletonFactory = () -> getEarlyBeanReference(beanName, mbd, bean) 是以 getEarlyBeanReference 傳回值會在此被放入第2級緩存
singletonObject = singletonFactory.getObject();
// 為什麼是放第2級緩存？因為 getBean 的遞歸還沒傳回
// 放入第2級緩存後，三級緩存的就可以移除了
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
}
}
return singletonObject;
}
A -> B -> A的遞歸傳回後，A, B兩個單例bean都已經是成品，beanName周遊到B的時候，進入getSingleton就能命中第1級緩存了，不用再走一遍createBean方法。
java複制代碼 for (String beanName : beanNames) {
// 假如先周遊 A 再周遊 B 那麼周遊到 B 的時候，因為循環引用解決的關系，B 已經放到了第1級緩存，是以 doGetBean 的 getSingleton 可以直接從第1級緩存取到值，不用再走一遍 createBean 方法
// 合并父類 BeanDefinition
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
// 非抽象、是單例、非懶加載
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
// 如果實作了 FactoryBean 接口則是 FactoryBean
if (isFactoryBean(beanName)) {
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName); // 比如：FACTORY_BEAN_PREFIX + beanName = "&A"
if (bean instanceof SmartFactoryBean<?> smartFactoryBean && smartFactoryBean.isEagerInit()) {
getBean(beanName);
}
}
else {
// 不是 FactoryBean，隻是普通 Bean，則走這個分支
getBean(beanName);
}
}
}
至此，場景1的遞歸點和三級緩存的操作時機都已經清楚了。
@Controller+自動裝配屬性和普通Bean解決循環依賴過程的相同點與不同點
場景2的Controller使用了@Autowired注解來構造循環依賴。動調可知，這個場景并不是在populateBean的applyPropertyValues(beanName, mbd, bw, pvs);完成修改的，而是在populateBean的：
java複制代碼 for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
if (pvsToUse == null) {
return;
}
pvs = pvsToUse;
}
這個循環裡完成屬性指派的。動調發現getBeanPostProcessorCache().instantiationAware有一個元素是AutowiredAnnotationBeanPostProcessor@133（所有元素分别是ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor@129, InfrastructureAdvisorAutoProxyCreator@130, PersistenceExceptionTranslationPostProcessor@131, CommonAnnotationBeanPostProcessor@132, AutowiredAnnotationBeanPostProcessor@133），周遊到這個元素時執行的操作完成了自動裝配屬性的指派。那我們跟進去看下。
AutowiredAnnotationBeanPostProcessor.postProcessProperties位于https://github.com/spring-projects/spring-framework/blob/6183f0684684912802021556dce916ba26228c26/spring-beans/src/main/java/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.java
java複制代碼 @Override
public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
try {
metadata.inject(bean, beanName, pvs);
}
catch (BeanCreationException ex) {
throw ex;
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
}
return pvs;
}
顯然是在metadata.inject處完成自動裝配的。InjectionMetadata.inject位于https://github.com/spring-projects/spring-framework/blob/2f33e77ab49f136d83b6ebf5eeb72d200fe23c0b/spring-beans/src/main/java/org/springframework/beans/factory/annotation/InjectionMetadata.java
java複制代碼 public void inject(Object target, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Collection<InjectedElement> checkedElements = this.checkedElements;
Collection<InjectedElement> elementsToIterate =
(checkedElements != null ? checkedElements : this.injectedElements);
if (!elementsToIterate.isEmpty()) {
for (InjectedElement element : elementsToIterate) {
element.inject(target, beanName, pvs);
}
}
}
element.inject最終跳入的是https://github.com/spring-projects/spring-framework/blob/6183f0684684912802021556dce916ba26228c26/spring-beans/src/main/java/org/springframework/beans/factory/annotation/AutowiredAnnotationBeanPostProcessor.java的protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable。AutowiredFieldElement, AutowiredMethodElement都有inject方法，顯然這個case裡我們調用的是AutowiredFieldElement的inject方法。
java複制代碼 @Override
protected void inject(Object bean, @Nullable String beanName, @Nullable PropertyValues pvs) throws Throwable {
Field field = (Field) this.member;
Object value;
if (this.cached) {
try {
value = resolveCachedArgument(beanName, this.cachedFieldValue);
}
catch (BeansException ex) {
// Unexpected target bean mismatch for cached argument -> re-resolve
this.cached = false;
logger.debug("Failed to resolve cached argument", ex);
value = resolveFieldValue(field, bean, beanName);
}
}
else {
value = resolveFieldValue(field, bean, beanName);
}
if (value != null) {
ReflectionUtils.makeAccessible(field);
field.set(bean, value);
}
}
顯然field.set(bean, value);最終完成了屬性的自動裝配。值得注意的是，動調看到resolveFieldValue擷取到的cb是已經裝配好的，這裡一定存在遞歸調用。我們用一個簡單的動态調試技巧來找到遞歸點：在執行到value = resolveFieldValue(field, bean, beanName)時，給doGetBean函數下一個臨時的斷點。得到的調用棧如下：
- AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement.inject()
- AutowiredAnnotationBeanPostProcessor.AutowiredFieldElement.resolveFieldValue()的value = beanFactory.resolveDependency(desc, beanName, autowiredBeanNames, typeConverter);
- DefaultListableBeanFactory.java的resolveDependency方法的result = doResolveDependency(descriptor, requestingBeanName, autowiredBeanNames, typeConverter);
- DefaultListableBeanFactory.java的doResolveDependency方法的instanceCandidate = descriptor.resolveCandidate(autowiredBeanName, type, this);
- https://github.com/spring-projects/spring-framework/blob/f1fe16e3cda66b164f77489f82287116477197bc/spring-beans/src/main/java/org/springframework/beans/factory/config/DependencyDescriptor.java的resolveCandidate方法的return beanFactory.getBean(beanName);，這就是遞歸點了，beanName = controllerB。
總而言之，Controller的bean自動裝配屬性的場景和普通的bean的遞歸點不一樣，但對三級緩存的操作邏輯是完全一緻的。謎底已揭曉~
一些擴充結論
【1】為什麼Spring不能解決構造器的循環依賴？
在doCreateBean調用createBeanInstance時，一二三級緩存都沒有Bean的相關資訊，在執行個體化之後才調用addSingletonFactory放入第3級緩存中，是以當getBean的時候緩存不會命中，是以會抛出循環依賴的異常。
【2】為什麼多執行個體Bean不能解決循環依賴？
多執行個體Bean是每次建立都會調用doGetBean方法，mbd.isSingleton()是false，不走sharedInstance = getSingleton(beanName, () -> {這個分支，是以也不會使用三級緩存，不能解決循環依賴。
總結
- 根據常識猜測Spring建立Bean過程解決循環依賴的算法也是Map+記憶化搜尋。是以我們可以先找到遞歸點，再去分析調用棧涉及的那些函數，順便找出其用到的Map資料結構。
- 在不了解Spring架構的情況下可以用一個動态調試技巧快速找到遞歸點：在執行到某條順序靠後的語句時，給順序靠前的語句下一個斷點，若下斷成功，則說明找到了遞歸點。剩下的工作就是關注調用棧涉及的函數。
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