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本文主要梳理了Spring框架Bean创建过程中应对循环依赖问题的相关源码。我在手写super-mini-webpackhttps://blog.csdn.net/q1424966670/article/details/135628531?spm1001.2014.3001.5502的时候也介绍过解决循环依赖的算法#xff1a;Map记忆化搜索。可以猜测这段源码…引言
本文主要梳理了Spring框架Bean创建过程中应对循环依赖问题的相关源码。我在手写super-mini-webpackhttps://blog.csdn.net/q1424966670/article/details/135628531?spm1001.2014.3001.5502的时候也介绍过解决循环依赖的算法Map记忆化搜索。可以猜测这段源码也实现了这个算法所以在看这段源码的时候我们可以先找到递归点再去分析调用栈涉及的那些函数顺便找出其用到的Map数据结构。
本文所用工程https://github.com/Hans774882968/spring-source-code-learn工程创建方式用VSCode插件Spring Initializr Java Supportctrl shift PSpring Initializr: Create a Maven Project。具体看参考链接1https://www.cnblogs.com/miskis/p/9816135.html。 JDK Version17 SpringBoot Version3.1.2 Spring Version6.0.11
三级缓存数据结构简介
三级缓存数据结构定义和操作三级缓存的函数都位于spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistry.java /** Cache of singleton objects: bean name to bean instance. */private final MapString, Object singletonObjects new ConcurrentHashMap(256);/** Cache of singleton factories: bean name to ObjectFactory. */private final MapString, ObjectFactory? singletonFactories new HashMap(16);/** Cache of early singleton objects: bean name to bean instance. */private final MapString, Object earlySingletonObjects new ConcurrentHashMap(16);
缓存先找到第1级第1级没有才查第2级依此类推。singletonObjects, earlySingletonObjects, singletonFactories分别是1到3级。第1级缓存是bean名到成品bean的映射第2级缓存是bean名到半成品bean的映射第3级缓存是bean名到函数式接口的映射作用为延迟调用函数。
FunctionalInterface
public interface ObjectFactoryT {/*** 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3级缓存的Value里的函数的调用方式就是调用.getObject()。
为什么需要2级缓存因为要体现一个分层的思想半成品bean原则上是不能暴露到外部的。TODO外部是指这里我没研究清楚就先引用了52pojie特别的你大佬的解释。“防止线程切换时其他线程取到半成品bean”我觉得很有道理。 外部可能是调用spring容器中BEAN的其他参与者有些应用启动时如果懒加载将会是使用时极其复杂如果多线程调用将会出现问题2级缓存加锁是防止多线程创建这里指该BEAN如果没有创建好而且只有一个容器那么其他线程就会使用到认为是完整的BEAN。 为什么需要第3级缓存如果所有bean都没有代理对象就不需要第3级缓存。TODO补充说明。
源码阅读
如何启动调试
新建一个普通的SpringBoot项目。然后可以尝试通过xml配置文件和注解等方式构造循环引用。后文称为场景1和场景2。
场景1通过xml配置文件来构造循环引用
入口src\main\java\com\hans\bean_dependency_cycle\HansApplication.java
package com.hans.bean_dependency_cycle;import org.springframework.boot.autoconfigure.SpringBootApplication;
import org.springframework.context.support.AbstractApplicationContext;
import org.springframework.context.support.ClassPathXmlApplicationContext;SpringBootApplication
public class HansApplication {public static void main(String[] args) {AbstractApplicationContext 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());ac.close();}}
src\main\resources\cycle.xml
?xml version1.0 encodingUTF-8?
beansxmlnshttp://www.springframework.org/schema/beansxmlns:xsihttp://www.w3.org/2001/XMLSchema-instancexmlns:contexthttp://www.springframework.org/schema/contextxmlns:mvchttp://www.springframework.org/schema/mvcxmlns:phttp://www.springframework.org/schema/pxsi:schemaLocationhttp://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsdhttp://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsdhttp://www.springframework.org/schema/mvc http://www.springframework.org/schema/mvc/spring-mvc.xsd
bean ida classcom.hans.bean_dependency_cycle.Aproperty nameb refb/property/beanbean idb classcom.hans.bean_dependency_cycle.Bproperty namea refa/property/bean
/beans
A.java和B.java
// A 和 B 不能都使用 lombok否则无法打印。这里选择了 B 不使用 lombok
package com.hans.bean_dependency_cycle;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;public class B {private A a;public A getA() {return this.a;}public void setA(A a) {this.a a;}}
期望输出
A(bcom.hans.bean_dependency_cycle.B750e2b97)
com.hans.bean_dependency_cycle.B750e2b97
com.hans.bean_dependency_cycle.B750e2b97
A(bcom.hans.bean_dependency_cycle.B750e2b97)
场景1不需要配置spring.main.allow-circular-references为true也能得到期望输出TODO原因未知。
【Optional】顺便补充一下场景1的单测src\test\java\com\hans\bean_dependency_cycleApplicationTests.java
package com.hans.bean_dependency_cycle;import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;
import org.springframework.context.ApplicationContext;
import org.springframework.test.context.ContextConfiguration;// 自动收集 cycle.xml 的 bean
ContextConfiguration(locations { classpath:cycle.xml })
SpringBootTest(classes HansApplication.class)
class HansApplicationTests {AutowiredApplicationContext ac;Testvoid contextLoads() {A beanA ac.getBean(A.class);Assertions.assertNotNull(beanA);B beanB ac.getBean(B.class);Assertions.assertNotNull(beanB);Assertions.assertEquals(beanA, beanB.getA());Assertions.assertEquals(beanB, beanA.getB());}}
场景2通过注解自动装配属性来构造循环引用以Controller为例
入口src\main\java\com\hans\bean_dependency_cycle\AnotherEntry.java
package com.hans.bean_dependency_cycle;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());}
}
根据参考链接2https://juejin.cn/post/7096798740593246222接下来一定要记得修改application.yml
spring:main:allow-circular-references: true
否则会报错
***************************
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.ControllerB com.hansn_dependency_cycle.hans.ControllerA.cb)
↑ ↓
| controllerB (field private com.hans.bean_dependency_cycle.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.
两个普通的Controller
// src\main\java\com\hans\bean_dependency_cycle\ControllerA.java
package com.hans.bean_dependency_cycle;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 {Autowiredprivate ControllerB cb;public ControllerB getCb() {return cb;}RequestMapping(/controllerA)public String index() {return hello controllerA!;}
}// src\main\java\com\hans\bean_dependency_cycle\ControllerB.java
package com.hans.bean_dependency_cycle;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 {Autowiredprivate ControllerA ca;public ControllerA getCa() {return ca;}RequestMapping(/controllerB)public String index() {return hello controllerB!;}
}
期望输出
com.hans.bean_dependency_cycle.ControllerA39f82681
com.hans.bean_dependency_cycle.ControllerB4bd9e7fd
com.hans.bean_dependency_cycle.ControllerB4bd9e7fd
com.hans.bean_dependency_cycle.ControllerA39f82681
【Optional】顺便补充一下场景2的单测src\test\java\com\hans\bean_dependency_cycle\AnotherEntryTests.java
package com.hans.bean_dependency_cycle;import org.junit.jupiter.api.Assertions;
import org.junit.jupiter.api.Test;
import org.springframework.beans.factory.annotation.Autowired;
import org.springframework.boot.test.context.SpringBootTest;SpringBootTest(classes AnotherEntry.class)
class AnotherEntryTests {AutowiredControllerA ca;AutowiredControllerB cb;Testvoid contextLoads() {Assertions.assertNotNull(ca);Assertions.assertNotNull(cb);Assertions.assertEquals(ca, cb.getCa());Assertions.assertEquals(cb, ca.getCb());}}
脉络
根据参考链接3有一个经验“do”开头的方法名是真正含有大量逻辑的方法。参考链接3Java之父精选的脉络函数如下
getBeandoGetBeancreateBeandoCreateBeancreateBeanInstancepopulateBean
场景1到递归点为止的调用链preInstantiateSingletons getBean doGetBean getSingleton函数有多个其中调用了beforeSingletonCreation的函数调用singletonFactory.getObject()时才真正调用了createBean createBean doCreateBean createBeanInstance, populateBean 都在 doCreateBean 的实现里 populateBean调用了applyPropertyValues resolveValueIfNecessary resolveReference getBean。
场景2的调用链到populateBean开始和场景1的调用链岔开两个场景的差异放在后文《Controller和普通Bean解决循环依赖过程的相同点与不同点》分析。
场景1递归到A - B - A时的调用链getBean doGetBean 未调用beforeSingletonCreation的getSingleton操作第2级缓存后离开。
操作三级缓存的函数都位于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级缓存的函数doCreateBean调用的addSingletonFactory。放入第2级缓存的函数首先要知道getSingleton在DefaultSingletonBeanRegistry.java里本质上的实现有两个一个public Object getSingleton(String beanName, ObjectFactory? singletonFactory)调用了beforeSingletonCreation并间接调用了createBean另一个protected Object getSingleton(String beanName, boolean allowEarlyReference)则是操作了第2级缓存。放入第1级缓存的函数addSingleton。在间接调用了createBean函数的getSingleton处调用。
场景1的执行过程
真正意义上的入口spring-context/src/main/java/org/springframework/context/support/AbstractApplicationContext.javahttps://github.com/spring-projects/spring-framework/blob/3a481a7d7f7ee485e25003b2855b4379ab5cce3a/spring-context/src/main/java/org/springframework/context/support/AbstractApplicationContext.java的beanFactory.preInstantiateSingletons();。这里的beanFactory就有singletonObjects那3级缓存的对象。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 Overridepublic 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.ListString beanNames new ArrayList(this.beanDefinitionNames);// 触发所有非延迟加载的non-lazy单例 bean 的初始化// Trigger initialization of all non-lazy singleton beans...for (String beanName : beanNames) {// 假如先遍历 A 再遍历 B 那么遍历到 B 的时候因为循环引用解决的关系B 已经放到了第1级缓存所以 doGetBean 的 getSingleton 可以直接从第1级缓存取到值不用再走一遍 createBean 方法// 合并父类 BeanDefinitionRootBeanDefinition bd getMergedLocalBeanDefinition(beanName);// 非抽象、是单例、非懒加载if (!bd.isAbstract() bd.isSingleton() !bd.isLazyInit()) {// 如果实现了 FactoryBean 接口则是 FactoryBeanif (isFactoryBean(beanName)) {Object bean getBean(FACTORY_BEAN_PREFIX beanName); // 比如FACTORY_BEAN_PREFIX beanName Aif (bean instanceof SmartFactoryBean? smartFactoryBean smartFactoryBean.isEagerInit()) {getBean(beanName);}}else {// 不是 FactoryBean只是普通 Bean则走这个分支getBean(beanName);}}}// 触发所有 SmartInitializingSingleton 的后初始化回调// 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();}}}
顺便看下isFactoryBean的实现https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractBeanFactory.java Overridepublic 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.javagetBean只有1行 Overridepublic Object getBean(String name) throws BeansException {return doGetBean(name, null, null, false);}
doGetBean和getBean, isFactoryBean都在AbstractBeanFactory.java /*** 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 ClassT requiredType, Nullable Object[] args, boolean typeCheckOnly)throws BeansException {String beanName transformedBeanName(name);Object beanInstance;// 提前检查单例缓存中是否有手动注册的单例对象跟循环依赖有关。如果是初次进这里比如 controllerA 初次进这个方法肯定是拿不到值的就会是 null// 对于最简单的循环依赖controllerA - controllerB - controllerA 之后需要进入 getSingleton 了这里的逻辑就是要从第3级缓存里拿到工厂函数调用后得到 controllerA 半成品从而可以直接 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 were already creating this bean instance:// Were 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()) {// 返回 beanName 的原始单例对象。如果尚未注册则使用 singletonFactory 创建并注册一个对象sharedInstance getSingleton(beanName, () - {try {// 为给定的合并后的 BeanDefinition 和参数创建一个 bean 实例// createBean 真实执行时机是调用了 beforeSingletonCreation 方法的 getSingleton 方法执行 singletonObject singletonFactory.getObject() 时// 首次运行到 beanName controllerA 时 args nullreturn 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.// 显式地从单例缓存中删除bean实例因为这个实例可能是由创建过程急切地放在那里的放在那里的目的是允许循环引用解析。// 还要删除所有被这个bean临时引用的所有bean。如果找到相应的一次性bean实例则委托给 destroyBeandestroySingleton(beanName);throw ex;}});beanInstance getObjectForBeanInstance(sharedInstance, name, beanName, mbd);}else if (mbd.isPrototype()) {// Its 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);}
我们先只关注createBean所以需要关注getSingleton。注意 getSingleton的函数在DefaultSingletonBeanRegistry.java里本质上的实现有两个一个调用了beforeSingletonCreation并间接调用了createBean另一个则是操作了第2级缓存。 所以目前我们需要关注的是调用了beforeSingletonCreation的getSingleton方法。路径https://github.com/spring-projects/spring-framework/blob/502997d8e986dcfde1f49b2b2f443a32b5488b13/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistry.java /*** 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 );}// 创建单例前的回调默认实现为将单例注册为当前正在创建中实现只有3行可以看下。beforeSingletonCreation(beanName);// flag 表示是否生成了新的单例对象boolean newSingleton false;// flag 表示是否有抑制异常的记录true表示没有boolean recordSuppressedExceptions (this.suppressedExceptions null);if (recordSuppressedExceptions) {// 若没有抑制异常记录则对抑制异常列表进行初始化this.suppressedExceptions new LinkedHashSet();}try {// 从单例工厂获取对象。注意 singletonFactory 是本方法第二个参数之前也介绍了// ObjectFactory 对象通过调 getObject 来正式执行函数所以 createBean 在此时才真正执行singletonObject singletonFactory.getObject();// 获取后就已经生成了新的单例对象标记为 truenewSingleton 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级缓存addSingleton(beanName, singletonObject);}}return singletonObject;}}
在调用singletonObject singletonFactory.getObject();以间接调用createBean后会调用addSingleton将bean加入第1级缓存这标志着bean变为成品。接下来我们看下addSingleton的代码它和getSingleton定义于同一个文件。 /*** Add the given singleton object to the singleton cache of this factory.* pTo 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 在调用了 createBean 的 getSingleton 方法中被调用标志着 bean 变为成品对象synchronized (this.singletonObjects) {// 第1级缓存添加第2、3级缓存移除this.singletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);this.earlySingletonObjects.remove(beanName);// 添加到已注册的单例集合。 registeredSingletons 为 SetStringthis.registeredSingletons.add(beanName);}}
接下来看createBean。https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java /*** Central method of this class: creates a bean instance,* populates the bean instance, applies post-processors, etc.* see #doCreateBean*/Overrideprotected Object createBean(String beanName, RootBeanDefinition mbd, Nullable Object[] args)throws BeanCreationException {// 首次看源码直接看 doCreateBean 调用处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 {// 实际创建 beanObject 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目前唯一值得关注的点就是调用了doCreateBean。doCreateBean做了几件值得本文关注的事
调用了createBeanInstance完成bean的实例化。调用了addSingletonFactory即加入了第3级缓存。调用了populateBean完成bean的属性赋值操作。
doCreateBean和createBean都定义在https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java。 /*** Actually create the specified bean. Pre-creation processing has already happened* at this point, e.g. checking {code postProcessBeforeInstantiation} callbacks.* pDifferentiates 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 持有创建出的 bean 对象BeanWrapper instanceWrapper null;// 获取 factoryBean 实例缓存if (mbd.isSingleton()) {// 如果是单例对象从 factoryBean 实例缓存中移除当前 bean 的信息// 首次 controllerA 进来会进行移除操作并且会调用 createBeanInstanceinstanceWrapper this.factoryBeanInstanceCache.remove(beanName);}// 没有就创建实例if (instanceWrapper null) {// 根据执行 bean 使用对应的策略创建新的工厂实例如工厂方法、构造函数主动注入、简单初始化// 第一次读源码时不需要点进去看 createBeanInstance 。下一个主干方法 populateBean 是在本函数下文调用的instanceWrapper createBeanInstance(beanName, mbd, args);}// 从包装类中获取原始 bean 。首次执行 controllerA 的时候, bean 就是普通的 controllerA 对象 ControllerA100{cbnull}Object bean instanceWrapper.getWrappedInstance();// 获取具体 bean 对象的 Class 属性Class? beanType instanceWrapper.getWrappedClass();if (beanType ! NullBean.class) {// 不等于 NullBean 类型时就修改目标类型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.// 判断当前 bean 是否需要提前曝光条件为是单例 允许循环依赖 spring.main.allow-circular-references 配置为 true 当前 bean 正在创建中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);}// 为避免后期循环依赖可以在 bean 初始化完成前将创建实例的 ObjectFactory 加入工厂// 注意这个方法会操作三级缓存的数据结构尤其是第3级缓存。在 controllerA - controllerB - controllerA 的时候未调用 createBean 的 getSingleton 方法会真正调用这个匿名函数从而调用 getEarlyBeanReference 进而操作第2级缓存addSingletonFactory(beanName, () - getEarlyBeanReference(beanName, mbd, bean));}// Initialize the bean instance.// controllerA 首次执行到这里时exposedObject ControllerA100{cbnull}Object exposedObject bean;try {// populateBean 是主干方法给刚刚实例化的 bean 半成品填充属性populateBean(beanName, mbd, instanceWrapper);// controllerA - controllerB - controllerA 之后getSingleton 返回 controllerA 后回到这里 controllerB 的 ca 属性就有值了// 随后递归返回到这句话且调用栈只有 controllerA 的时候发现两者都有值了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);SetString 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的细节与本文主题无关不关注。接下来我们看下addSingletonFactory的实现。addSingletonFactory位于https://github.com/spring-projects/spring-framework/blob/502997d8e986dcfde1f49b2b2f443a32b5488b13/spring-beans/src/main/java/org/springframework/beans/factory/support/DefaultSingletonBeanRegistry.java主要动作是操作第3级缓存。 /*** Add the given singleton factory for building the specified singleton* if necessary.* pTo 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 首次执行到这里第1级缓存肯定是查不到的if (!this.singletonObjects.containsKey(beanName)) {// 第3级缓存放入。再回顾一下 singletonFactory存的是 beanName 到一个延迟执行的函数的映射// controllerA 首次执行到这里的时候 singletonFactory () - getEarlyBeanReference(beanName, mbd, bean)this.singletonFactories.put(beanName, singletonFactory);// 从早期单例对象的高速缓存即第2级缓存移除当前 beanName 对应的缓存对象this.earlySingletonObjects.remove(beanName);// 添加到已注册的单例集合里和三级缓存无关。值得注意的是 A 首次加入第3级缓存时就是首次加入已注册的单例集合this.registeredSingletons.add(beanName);}}}
接下来关注位于https://github.com/spring-projects/spring-framework/blob/4786e2bf53a3f882c10e25d7ff79a18ff47b5e51/spring-beans/src/main/java/org/springframework/beans/factory/support/AbstractAutowireCapableBeanFactory.java的populateBean。populateBean方法有一个功能是给bean的属性赋值包含了递归点。在创建Bean的源码中递归点指的是递归调用getBean方法。 /*** 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) {// 对于 Controller Bean 自动装配属性产生循环依赖的场景遍历到 AutowiredAnnotationBeanPostProcessor 时这句话包含递归点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) {// 对于普通 Bean 的场景这句话包含递归点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主要功能为完成bean初始化。其中Object resolvedValue valueResolver.resolveValueIfNecessary(pv, originalValue)包含了递归点通过动调可以验证bw.setPropertyValues(new MutablePropertyValues(deepCopy))真正完成了属性赋值工作。 /*** Apply the given property values, resolving any runtime references* to other beans in this bean factory. Must use deep copy, so we* dont 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 是真正完成赋值操作的函数// 如果 pvs 没有 PropertyValue就直接结束if (pvs.isEmpty()) {return;}MutablePropertyValues mpvs null;ListPropertyValue 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);}}// 获取 mpvs 的 PropertyValue 列表original mpvs.getPropertyValueList();}else {// 获取 pvs 的 PropertyValue 对象数组并转为列表original Arrays.asList(pvs.getPropertyValues());}// 用户自定义的类型转换器默认转换器为 bean 的包装类对象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.ListPropertyValue deepCopy new ArrayList(original.size());// resolveNecessary flag 含义为是否还需要解析boolean resolveNecessary false;for (PropertyValue pv : original) {// 属性已解析过则加入 deepCopyif (pv.isConverted()) {deepCopy.add(pv);}else {String propertyName pv.getName();// 获取未经类型转换的值Object originalValue pv.getValue();if (originalValue AutowiredPropertyMarker.INSTANCE) {Method writeMethod bw.getPropertyDescriptor(propertyName).getWriteMethod();// 如果 setter 方法为空if (writeMethod null) {// 异常自动装配标记属性没有写方法throw new IllegalArgumentException(Autowire marker for property without write method: pv);}originalValue new DependencyDescriptor(new MethodParameter(writeMethod, 0), true);}// 交由 valueResolver 根据 pv 解析出 originalValue 所封装的对象。注意这个函数包含递归点Object resolvedValue valueResolver.resolveValueIfNecessary(pv, originalValue);Object convertedValue resolvedValue;// flag 含义为是否可转换 propertyName 是 bw 中的可写属性 propertyName 不是索引属性或嵌套属性boolean convertible bw.isWritableProperty(propertyName) !PropertyAccessorUtils.isNestedOrIndexedProperty(propertyName);if (convertible) {// 可转换则将 resolvedValue 转换为指定的目标属性对象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 {// 完成属性赋值工作。咱们做个简单的实验动调执行此句前后各点击调用栈看一次 populateBean 调用处的下一句的 exposedObject 或者 bean 变量这就证实了 bean 的属性赋值确实是在这句话完成的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。其实现也很长但目前只需要关注return resolveReference(argName, ref)因为resolveReference包含递归点。 /*** Given a PropertyValue, return a value, resolving any references to other* beans in the factory if necessary. The value could be:* liA 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.* liA RuntimeBeanReference, which must be resolved.* liA ManagedList. This is a special collection that may contain* RuntimeBeanReferences or Collections that will need to be resolved.* liA ManagedSet. May also contain RuntimeBeanReferences or* Collections that will need to be resolved.* liA ManagedMap. In this case the value may be a RuntimeBeanReference* or Collection that will need to be resolved.* liAn ordinary object or {code null}, in which case its 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*/Nullablepublic 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当属性值对象是工厂中另一个 bean 的引用时使用不可变的占位符类在运行时进行解析if (value instanceof RuntimeBeanReference ref) {// 解析出对应 ref 所封装的 Bean 的元信息的 Bean 对象。Bean 的元信息Bean 名Bean类型。注意这个函数里包含递归点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) {SetString 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。其实现也很长但目前只需要关注bean this.beanFactory.getBean(resolvedName)因为这就是递归点。 /*** Resolve a reference to another bean in the factory.*/Nullableprivate Object resolveReference(Object argName, RuntimeBeanReference ref) {try {Object bean;Class? beanType ref.getBeanType();// 如果引用来自父工厂if (ref.isToParent()) {// 获取父工厂BeanFactory parent this.beanFactory.getParentBeanFactory();if (parent null) {// 没有父工厂则报错在父工厂中无法解析对 Bean 的引用因为父工厂就不存在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) {// 解析与 beanType 唯一匹配的 bean 实例包括其 bean 名NamedBeanHolder? namedBean this.beanFactory.resolveNamedBean(beanType);// 让 bean 引用 namedBean 所封装的 bean 对象bean namedBean.getBeanInstance();resolvedName namedBean.getBeanName();}else {resolvedName String.valueOf(doEvaluate(ref.getBeanName()));// 获取 ref 所包装的 Bean 名对应的 Bean 对象// 注意这就是递归点了bean this.beanFactory.getBean(resolvedName);}// 注册依赖关系到 Bean 工厂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创建过程调用栈是完全一样的接下来我们假设现在走到了A - B - A回到了doGetBean的Object sharedInstance getSingleton(beanName);处。此时我们需要关注其实现了因为这个函数要调用第3级缓存的函数获取半成品bean并放入第2级缓存。 /*** Return the (raw) singleton object registered under the given name.* pChecks 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*/Nullableprotected Object getSingleton(String beanName, boolean allowEarlyReference) {// Quick check for existing instance without full singleton lock// 该函数主要是调用放入第3级缓存的 getEarlyBeanReference 并放入第2级缓存// 首先从第1级缓存获取 beanObject singletonObject this.singletonObjects.get(beanName);// 第1级缓存没有并且已标记为创建中if (singletonObject null isSingletonCurrentlyInCreation(beanName)) {// 从第2级缓存获取 bean.因为controllerA - controllerB - controllerA 的时候是创建中的 bean,只放到了第3级缓存,所以是查不到的singletonObject this.earlySingletonObjects.get(beanName);// controllerA - controllerB - controllerA 的时候进来, allowEarlyReference 肯定是 trueif (singletonObject null allowEarlyReference) {synchronized (this.singletonObjects) {// Consistent creation of early reference within full singleton lock// 这段做二次确认的代码让我联想到线程安全的单例模式的写法// 从第1级缓存取singletonObject this.singletonObjects.get(beanName);if (singletonObject null) {// 从第2级缓存取singletonObject this.earlySingletonObjects.get(beanName);if (singletonObject null) {// 从第3级缓存取。如果是 controllerA 初次进来因为 not in creation 所以不会进这里就算进了这里因为第3级缓存取不到所以还是会直接 return。如果是 controllerA - controllerB - controllerA 则会操作第2级缓存ObjectFactory? singletonFactory this.singletonFactories.get(beanName);if (singletonFactory ! null) {// 回顾一下DefaultSingletonBeanRegistry.java 的 addSingletonFactory 函数在操作第3级缓存的时候放入的匿名函数就是// singletonFactory () - getEarlyBeanReference(beanName, mbd, bean) 所以 getEarlyBeanReference 返回值会在此被放入第2级缓存singletonObject singletonFactory.getObject();// 为什么是放第2级缓存因为 getBean 的递归还没返回// 放入第2级缓存后第3级缓存的就可以移除了this.earlySingletonObjects.put(beanName, singletonObject);this.singletonFactories.remove(beanName);}}}}}}return singletonObject;}
再回忆一遍什么时候加入第1级缓存调用了beforeSingletonCreation的getSingleton方法在间接调用createBean后会调用addSingleton方法将成品bean加入第1级缓存。
A - B - A的递归返回后A, B两个单例bean都已经是成品beanName遍历到B的时候进入getSingleton就能命中第1级缓存了不用再走一遍createBean方法。 for (String beanName : beanNames) {// 假如先遍历 A 再遍历 B 那么遍历到 B 的时候因为循环引用解决的关系B 已经放到了第1级缓存所以 doGetBean 的 getSingleton 可以直接从第1级缓存取到值不用再走一遍 createBean 方法// 合并父类 BeanDefinitionRootBeanDefinition bd getMergedLocalBeanDefinition(beanName);// 非抽象、是单例、非懒加载if (!bd.isAbstract() bd.isSingleton() !bd.isLazyInit()) {// 如果实现了 FactoryBean 接口则是 FactoryBeanif (isFactoryBean(beanName)) {Object bean getBean(FACTORY_BEAN_PREFIX beanName); // 比如FACTORY_BEAN_PREFIX beanName Aif (bean instanceof SmartFactoryBean? smartFactoryBean smartFactoryBean.isEagerInit()) {getBean(beanName);}}else {// 不是 FactoryBean只是普通 Bean则走这个分支getBean(beanName);}}}
经过上面的分析我们来看更简单的情况如果没有循环依赖比如只有A依赖B对三级缓存数据结构的操作是怎样的梳理出调用栈如下A doGetBean - A doCreateBean - A addSingletonFactory - A applyPropertyValues - B doGetBean - B doCreateBean - B addSingletonFactory - B applyPropertyValues不走到递归点 - B回到getSingleton调用addSingleton - A addSingleton。可见如果没有循环依赖就不会操作第2级缓存但仍然会操作第1级缓存。第3级缓存虽然被操作但没有影响。
至此场景1的递归点和三级缓存的操作时机都已经清楚了。
Controller自动装配属性和普通Bean解决循环依赖过程的相同点与不同点
场景2的Controller使用了Autowired注解来构造循环依赖。动调可知这个场景并不是在populateBean的applyPropertyValues(beanName, mbd, bw, pvs);完成修改的而是在populateBean的 for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {PropertyValues pvsToUse bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);if (pvsToUse null) {return;}pvs pvsToUse;}
这个循环里完成属性赋值的。动调发现getBeanPostProcessorCache().instantiationAware有一个元素是AutowiredAnnotationBeanPostProcessor133所有元素分别是ConfigurationClassPostProcessor$ImportAwareBeanPostProcessor129, InfrastructureAdvisorAutoProxyCreator130, PersistenceExceptionTranslationPostProcessor131, CommonAnnotationBeanPostProcessor132, AutowiredAnnotationBeanPostProcessor133遍历到这个元素时执行的操作完成了自动装配属性的赋值。那我们跟进去看下。
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 Overridepublic 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;}
显然是在metadata.inject处完成自动装配的。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 public void inject(Object target, Nullable String beanName, Nullable PropertyValues pvs) throws Throwable {CollectionInjectedElement checkedElements this.checkedElements;CollectionInjectedElement elementsToIterate (checkedElements ! null ? checkedElements : this.injectedElements);if (!elementsToIterate.isEmpty()) {for (InjectedElement element : elementsToIterate) {element.inject(target, beanName, pvs);}}}
element.inject最终跳入的是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方法显然这个case里我们调用的是AutowiredFieldElement的inject方法。 Overrideprotected 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-resolvethis.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);}}
显然field.set(bean, value);最终完成了属性的自动装配。值得注意的是动调看到resolveFieldValue获取到的cb是已经装配好的这里一定存在递归调用。我们用一个简单的动态调试技巧来找到递归点在执行到value resolveFieldValue(field, bean, beanName)时给doGetBean函数下一个临时的断点。得到的调用栈如下
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);这就是递归点了beanName controllerB。
总而言之Controller的bean自动装配属性的场景和普通的bean的递归点不一样但对三级缓存的操作逻辑是完全一致的。谜底已揭晓~
一些扩展结论
【1】为什么Spring不能解决构造器的循环依赖
在doCreateBean调用createBeanInstance时一二三级缓存都没有Bean的相关信息在实例化之后才调用addSingletonFactory放入第3级缓存中因此当getBean的时候缓存不会命中所以会抛出循环依赖的异常。
【2】为什么多实例Bean不能解决循环依赖
多实例Bean是每次创建都会调用doGetBean方法mbd.isSingleton()是false不走sharedInstance getSingleton(beanName, () - {这个分支所以也不会使用三级缓存不能解决循环依赖。
总结
根据常识猜测Spring创建Bean过程解决循环依赖的算法也是Map记忆化搜索。所以我们可以先找到递归点再去分析调用栈涉及的那些函数顺便找出其用到的Map数据结构。在不了解Spring框架的情况下可以用一个动态调试技巧快速找到递归点在执行到某条顺序靠后的语句时给顺序靠前的语句下一个断点若下断成功则说明找到了递归点。剩下的工作就是关注调用栈涉及的函数。
参考资料
VSCode搭建Java开发环境SpringBoot项目创建、运行、调试https://www.cnblogs.com/miskis/p/9816135.htmlhttps://www.cnblogs.com/miskis/p/9816135.html痛快SpringBoot终于禁掉了循环依赖痛快SpringBoot终于禁掉了循环依赖 - 掘金Spring的Bean管理一直是整个体系中津津乐道的东西。尤其是Bean的循环依赖更是很多面试官最喜欢考察的2B知识点之一。 但事实上项目中存在Bean的循环依赖是代码质量低下的表现。多数人寄https://juejin.cn/post/7096798740593246222循环依赖的详细debug过程_哔哩哔哩_bilibili循环依赖的详细debug过程是2023金三银四Java面试突击班短期跳槽、长期系统学习、面试题、马士兵系统课程都在这里的第82集视频该合集共计153集视频收藏或关注UP主及时了解更多相关视频内容。https://www.bilibili.com/video/BV1J14y1A7eE/?p82 震惊这位零基础前端同学在学习Java的第7天就可以研究Spring框架源码了这是怎么做到的呢噢原来是站在了Java之父士兵的肩膀上
