知识的价值不在于占有,而在于使用。这篇文章主要讲述Spring源码解析之八finishBeanFactoryInitialization方法即初始化单例bean相关的知识,希望能为你提供帮助。
Spring源码解析之八finishBeanFactoryInitialization方法即初始化单例bean
七千字长文深刻解读,Spirng中是如何初始化单例bean的,和面试中最常问的Spring是如何解决循环依赖?
今天解读Spring核心方法refresh()中最最重要的一个方法finishBeanFactoryInitialization()方法,该方法负责初始化所有的单例bean。
finishBeanFactoryInitialization()方法位于refresh()中下标为8的位置。
到目前为止,应该说 BeanFactory 已经创建完成,并且所有的实现了 BeanFactoryPostProcessor 接口的 Bean 都已经初始化并且其中的 postProcessBeanFactory(factory) 方法已经得到回调执行了。而且 Spring 已经“手动”注册了一些特殊的 Bean,如 environment
、systemProperties
等。
剩下的就是初始化 singleton beans 了,大都数我们的业务中都是单例bean,就像我们写的@Controller、@Service的类(没有设置懒加载的)都是在这个地方初始化,以供我们使用,如果没有设置懒加载,那么 Spring 会在接下来初始化所有的 singleton beans。
我们先看一下refresh()的源码,大概看下finishBeanFactoryInitialization(beanFactory)所处的位置。
@Override
public void refresh() throws BeansException, IllegalStateException
synchronized (this.startupShutdownMonitor)
// Prepare this context for refreshing.
//1、刷新前的准备
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
//2、将会初始化 BeanFactory、加载 Bean、注册 Bean
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
//3、设置 BeanFactory 的类加载器,添加几个 BeanPostProcessor,手动注册几个特殊的 bean
prepareBeanFactory(beanFactory);
try
//4、模板方法
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
//执行BeanFactory后置处理器
invokeBeanFactoryPostProcessors(beanFactory);
// 5、Register bean processors that intercept bean creation.
//注册bean后置处理器
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
//国际化
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
//6、模板方法--springboot实现了这个方法
onRefresh();
// Check for listener beans and register them.
//7、注册监听器
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
//8、完成bean工厂的初始化**方法重要**********************************************
finishBeanFactoryInitialization(beanFactory);
//9、 Last step: publish corresponding event.
finishRefresh();
我们深入finishBeanFactoryInitialization(beanFactory)中,里面的调用线路错综复杂,还望读者可以做好心理准备。
/**
* 负责单例bean的初始化
* Finish the initialization of this contexts bean factory,
* initializing all remaining singleton beans.
*/
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory)
// Initialize conversion service for this context.
//最先初始化名字为 conversionService的类,conversionService类 它用来将前端传过来的参数和后端的 controller 方法上的参数进行绑定的时候用
//尤其是用于非基础类型的转换
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &
&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class))
beanFactory.setConversionService(
//初始化在getBean()方法中实现
beanFactory.getBean(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class));
// Register a default embedded value resolver if no bean post-processor
// (such as a PropertyPlaceholderConfigurer bean) registered any before:
// at this point, primarily for resolution in annotation attribute values.
if (!beanFactory.hasEmbeddedValueResolver())
beanFactory.addEmbeddedValueResolver(strVal ->
getEnvironment().resolvePlaceholders(strVal));
// Initialize LoadTimeWeaverAware beans early to allow for registering their transformers early.
// 先初始化 LoadTimeWeaverAware 类型的 Bean aop相关注:大概有个印象,以后解析aop会和它串起来。
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames)
getBean(weaverAwareName);
// Stop using the temporary ClassLoader for type matching.
beanFactory.setTempClassLoader(null);
// Allow for caching all bean definition metadata, not expecting further changes.
//freeze的单词意思是冻结,这个时候已经开始预初始化, bean 定义解析、加载、注册先停止
beanFactory.freezeConfiguration();
// Instantiate all remaining (non-lazy-init) singletons.
//开始初始化
beanFactory.preInstantiateSingletons();
该方法是判断bean的一系列是不是属于某个类型的bean,如果是就调用getBean()方法,如果不是,就调用beanFactory.preInstantiateSingletons()进行初始化,我们先把getBean()放一放,重点看一看beanFactory.preInstantiateSingletons()方法。
@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.
// this.beanDefinitionNames 保存了所有的 beanNamesList<
String>
beanNames = new ArrayList<
>
(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
//// 下面这个循环,触发所有的非懒加载的 singleton beans 的初始化操作
for (String beanName : beanNames)
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
// 非抽象、非懒加载的 singletons。如果配置了 abstract = true,那是不需要初始化的
if (!bd.isAbstract() &
&
bd.isSingleton() &
&
!bd.isLazyInit())
// 处理 FactoryBean (负责初始化工厂的bean)
if (isFactoryBean(beanName))
// FactoryBean 的话,在 beanName 前面加上 ‘&
’ 符号
//此处调用getBean()方法
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
if (bean instanceof FactoryBean)
FactoryBean<
?>
factory = (FactoryBean<
?>
) bean;
// 判断当前 FactoryBean 是否是 SmartFactoryBean 的实现
boolean isEagerInit;
if (System.getSecurityManager() != null &
&
factory instanceof SmartFactoryBean)
isEagerInit = AccessController.doPrivileged(
(PrivilegedAction<
Boolean>
) ((SmartFactoryBean<
?>
) factory)::isEagerInit,
getAccessControlContext());
else
isEagerInit = (factory instanceof SmartFactoryBean &
&
((SmartFactoryBean<
?>
) factory).isEagerInit());
if (isEagerInit)
getBean(beanName);
else
// 对于普通的 Bean,只要调用 getBean(beanName) 这个方法就可以进行初始化了
getBean(beanName);
// Trigger post-initialization callback for all applicable beans...
// 到这里说明所有的非懒加载的 singleton beans 已经完成了初始化
// 如果我们定义的 bean 是实现了 SmartInitializingSingleton 接口的,那么在这里得到回调
//如果你想在单例bean初始化后做一些事 那就实现该接口
for (String beanName : beanNames)
Object singletonInstance = getSingleton(beanName);
if (singletonInstance instanceof SmartInitializingSingleton)
SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null)
AccessController.doPrivileged((PrivilegedAction<
Object>
) () ->
smartSingleton.afterSingletonsInstantiated();
return null;
, getAccessControlContext());
else
smartSingleton.afterSingletonsInstantiated();
preInstantiateSingletons()方法的主要任务是进行初始化的,在初始化前同样是一系列判断,如,是否是懒加载的,是否是一个factorybean(一个特别的bean,负责工厂创建的bean),最后调用getBean()方法。
其中有个插曲是否实现了SmartInitializingSingleton接口,将接口让你可以在bean初始化后做一些事,我们写一个简单的实例测试一下。
文章图片
其他地方读者看注释了解一下即可,我们开始继续深入getBean()方法。
getBean()方法内部调用了doGetBean()我们直接看doGetBean方法。
// 我们在剖析初始化 Bean 的过程,但是 getBean 方法我们经常是用来从容器中获取 Bean 用的,注意切换思路,
// 已经初始化过了就从容器中直接返回,否则就先初始化再返回
protected <
T>
T doGetBean(
String name, @Nullable Class<
T>
requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException // 获取一个 “正统的” beanName,处理两种情况,一个是前面说的 FactoryBean(前面带 ‘&
’),
// 一个是别名问题,因为这个方法是 getBean,获取 Bean 用的,你要是传一个别名进来,是完全可以的
String beanName = transformedBeanName(name);
// 返回值
Object bean;
// Eagerly check singleton cache for manually registered singletons.
// 检查下是不是已经创建过了
Object sharedInstance = getSingleton(beanName);
// 这里说下 args ,虽然看上去一点不重要。前面我们一路进来的时候都是 getBean(beanName),
// 所以 args 传参其实是 null 的,但是如果 args 不为空的时候,那么意味着调用方不是希望获取 Bean,而是创建 Bean
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 + "");
// 下面这个方法:如果是普通 Bean 的话,直接返回 sharedInstance,
// 如果是 FactoryBean 的话,返回它创建的那个实例对象
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
else
// Fail if were already creating this bean instance:
// Were assumably within a circular reference.
// 创建过了此 beanName 的 prototype 类型的 bean,那么抛异常,
// 往往是因为陷入了循环引用 哦,原来之前的循环依赖都是在这抛的异常,再有问题就不是无头苍蝇了
if (isPrototypeCurrentlyInCreation(beanName))
throw new BeanCurrentlyInCreationException(beanName);
// Check if bean definition exists in this factory.
// 检查一下这个 BeanDefinition 在容器中是否存在 BeanDefinition既是包含了bean的一系列信息
BeanFactory parentBeanFactory = getParentBeanFactory();
if (parentBeanFactory != null &
&
!containsBeanDefinition(beanName))
// Not found ->
check parent.
// 如果当前容器不存在这个 BeanDefinition,试试父容器中有没有
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory)
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
else if (args != null)
// Delegation to parent with explicit args.
// 返回父容器的查询结果
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)
// typeCheckOnly 为 false,将当前 beanName 放入一个 alreadyCreated 的 Set 集合中。
markBeanAsCreated(beanName);
/*
* 稍稍总结一下:
* 到这里的话,要准备创建 Bean 了,对于 singleton 的 Bean 来说,容器中还没创建过此 Bean;
* 对于 prototype 的 Bean 来说,本来就是要创建一个新的 Bean。
*/
try
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
// 先初始化依赖的所有 Bean,这个很好理解。
// 注意,这里的依赖指的是 depends-on 中定义的依赖
String[] dependsOn = mbd.getDependsOn();
if (dependsOn != null)
for (String dep : dependsOn)
// 检查是不是有循环依赖,这里的循环依赖和我们前面说的循环依赖又不一样
if (isDependent(beanName, dep))
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Circular depends-on relationship between " + beanName + " and " + dep + "");
// 注册一下依赖关系
registerDependentBean(dep, beanName);
try
// 先初始化被依赖项
getBean(dep);
catch (NoSuchBeanDefinitionException ex)
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"" + beanName + " depends on missing bean " + dep + "", ex);
// Create bean instance.
// 如果是 singleton scope 的,创建 singleton 的实例
if (mbd.isSingleton())
sharedInstance = getSingleton(beanName, () ->
try
// 执行创建 Bean,详情继续深入
// 第三个参数 args 数组代表创建实例需要的参数,不就是给构造方法用的参数,或者是工厂 Bean 的参数嘛,不过要注意,在我们的初始化阶段,args 是 null。
// 这回我们要到一个新的类了 AbstractAutowireCapableBeanFactory,看类名,AutowireCapable?类名是不是也说明了点问题了。
// 主要是为了以下场景,采用 @Autowired 注解注入属性值:
return createBean(beanName, mbd, args);
catch (BeansException ex)
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
destroySingleton(beanName);
throw ex;
);
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
// 如果是 prototype scope 的,创建 prototype 的实例
else if (mbd.isPrototype())
// Its a prototype ->
create a new instance.
Object prototypeInstance = null;
try
beforePrototypeCreation(beanName);
// 执行创建 Bean
prototypeInstance = createBean(beanName, mbd, args);
finally
afterPrototypeCreation(beanName);
bean = 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);
);
bean = getObjectForBeanInstance(scopedInstance, name, beanName, mbd);
catch (IllegalStateException ex)
throw new BeanCreationException(beanName,
"Scope " + scopeName + " is not active for the current thread;
consider " +
"defining a scoped proxy for this bean if you intend to refer to it from a singleton",
ex);
catch (BeansException ex)
cleanupAfterBeanCreationFailure(beanName);
throw ex;
// Check if required type matches the type of the actual bean instance.
// 最后,检查一下类型对不对,不对的话就抛异常,对的话就返回了
if (requiredType != null &
&
!requiredType.isInstance(bean))
try
T convertedBean = getTypeConverter().convertIfNecessary(bean, requiredType);
if (convertedBean == null)
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
return convertedBean;
catch (TypeMismatchException ex)
if (logger.isTraceEnabled())
logger.trace("Failed to convert bean " + name + " to required type " +
ClassUtils.getQualifiedName(requiredType) + "", ex);
throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());
return (T) bean;
具体的实例化过程在createBean()方法中,我们继续深入createBean()方法。
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException 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.
// 确保 BeanDefinition 中的 Class 被加载
Class<
?>
resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null &
&
!mbd.hasBeanClass() &
&
mbd.getBeanClassName() != null)
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
// Prepare method overrides.
// 准备方法覆写,这里又涉及到一个概念:MethodOverrides,它来自于 bean 定义中的 <
lookup-method />
// 和 <
replaced-method />
,如果读者感兴趣,回到 bean 解析的地方看看对这两个标签的解析。
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.
// 让 InstantiationAwareBeanPostProcessor 在这一步有机会返回代理,
// 在 《Spring AOP 源码分析》那篇文章中有解释,这里先跳过
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
// 重头戏,创建 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);
我们继续往里看 doCreateBean 这个方法,这个调用过程是真的深。
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException // Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton())
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
if (instanceWrapper == null)
// 说明不是 FactoryBean,这里实例化 Bean,这里非常关键,细节之后再说**********
instanceWrapper = createBeanInstance(beanName, mbd, args);
Object bean = instanceWrapper.getWrappedInstance();
Class<
?>
beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class)
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.postProcessed = true;
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
// 下面这块代码是为了解决循环依赖的问题,这是个重头戏,解决循环依赖问题
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");
addSingletonFactory(beanName, () ->
getEarlyBeanReference(beanName, mbd, bean));
// Initialize the bean instance.
Object exposedObject = bean;
try
// 这一步也是非常关键的,这一步负责属性装配,因为前面的实例只是实例化了,并没有设值,这里就是设值***************
populateBean(beanName, mbd, instanceWrapper);
// 还记得 init-method 吗?还有 InitializingBean 接口?还有 BeanPostProcessor 接口?
// 这里就是处理 bean 初始化完成后的各种回调**************
exposedObject = initializeBean(beanName, exposedObject, mbd);
catch (Throwable ex)
if (ex instanceof BeanCreationException &
&
beanName.equals(((BeanCreationException) ex).getBeanName()))
throw (BeanCreationException) ex;
else
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Initialization of bean failed", ex);
// 下面这块代码是为了解决循环依赖的问题,这是个重头戏,解决循环依赖问题
if (earlySingletonExposure)
//循环依赖的核心方法调用
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;
到这里,我们已经分析完了 doCreateBean 方法,总的来说,我们已经说完了整个初始化流程。
在实例化bean后有一个特别重要的知识点,也是面试中最常问的,Spring怎么解决循环依赖问题?核心代码就在这个方法里面。
循环依赖其实就是循环引用,也就是两个或则两个以上的bean互相持有对方,最终形成闭环。比如A依赖于B,B依赖于C,C又依赖于A。如下图:
文章图片
doCreateBean 方法有三个核心流程。
文章图片
(1)createBeanInstance:实例化,其实也就是调用对象的构造方法实例化对象
(2)populateBean:填充属性,这一步主要是多bean的依赖属性进行填充
(3)initializeBean:调用spring xml中的init 方法。
从上面讲述的单例bean初始化步骤我们可以知道,循环依赖主要发生在第一、第二步。也就是构造器循环依赖和field循环依赖。
那么我们要解决循环引用也应该从初始化过程着手,对于单例来说,在Spring容器整个生命周期内,有且只有一个对象,所以很容易想到这个对象应该存在Cache中,Spring为了解决单例的循环依赖问题,使用了三级缓存。
我们看一下getSingleton方法。
该方法还依赖于三个map,这三个map就是三级缓存。
/** Cache of singleton objects: bean name to bean instance. */
//单例对象的cache
private final Map<
String, Object>
singletonObjects = new ConcurrentHashMap<
>
(256);
/** Cache of singleton factories: bean name to ObjectFactory. */
// 单例对象工厂的cache
private final Map<
String, ObjectFactory<
?>
>
singletonFactories = new HashMap<
>
(16);
/** Cache of early singleton objects: bean name to bean instance. */
//提前曝光的单例对象的Cache
private final Map<
String, Object>
earlySingletonObjects = new ConcurrentHashMap<
>
(16);
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference)
// Quick check for existing instance without full singleton lock
Object singletonObject = this.singletonObjects.get(beanName);
//判断当前单例bean是否正在创建中,也就是没有初始化完成(比如A的构造器依赖了B对象所以得先去创建B对象
// 或则在A的populateBean过程中依赖了B对象,得先去创建B对象,这时的A就是处于创建中的状态。
if (singletonObject == null &
&
isSingletonCurrentlyInCreation(beanName))
singletonObject = this.earlySingletonObjects.get(beanName);
// 是否允许从singletonFactories中通过getObject拿到对象
if (singletonObject == null &
&
allowEarlyReference)
synchronized (this.singletonObjects)
// Consistent creation of early reference within full singleton lock
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null)
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null)
ObjectFactory<
?>
singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null)
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
return singletonObject;
分析getSingleton()的整个过程,Spring首先从一级缓存singletonObjects中获取。如果获取不到,并且对象正在创建中,就再从二级缓存earlySingletonObjects中获取。
如果还是获取不到且允许singletonFactories通过getObject()获取,就从三级缓存singletonFactory.getObject()(三级缓存)获取,如果获取到了则:
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
从singletonFactories中移除,并放入earlySingletonObjects中。其实也就是从三级缓存移动到了二级缓存。
从上面三级缓存的分析,我们可以知道,Spring解决循环依赖的诀窍就在于singletonFactories这个三级cache。
里就是解决循环依赖的关键,这段代码发生在createBeanInstance之后,也就是说单例对象此时已经被创建出来(调用了构造器)。这个对象已经被生产出来了,虽然还不完美(还没有进行初始化的第二步和第三步),但是已经能被人认出来了(根据对象引用能定位到堆中的对象),所以Spring此时将这个对象提前曝光出来让大家认识,让大家使用。
这样做有什么好处呢?
让我们来分析一下“A的某个field或者setter依赖了B的实例对象,同时B的某个field或者setter依赖了A的实例对象”这种循环依赖的情况。
A首先完成了初始化的第一步,并且将自己提前曝光到singletonFactories中,此时进行初始化的第二步,发现自己依赖对象B,此时就尝试去get(B),发现B还没有被create,所以走create流程,B在初始化第一步的时候发现自己依赖了对象A,于是尝试get(A),尝试一级缓存singletonObjects(肯定没有,因为A还没初始化完全),尝试二级缓存earlySingletonObjects(也没有),尝试三级缓存singletonFactories,由于A通过ObjectFactory将自己提前曝光了,所以B能够通过ObjectFactory.getObject拿到A对象(虽然A还没有初始化完全,但是总比没有好呀),B拿到A对象后顺利完成了初始化阶段1、2、3,完全初始化之后将自己放入到一级缓存singletonObjects中。
此时返回A中,A此时能拿到B的对象顺利完成自己的初始化阶段2、3,最终A也完成了初始化,进去了一级缓存singletonObjects中,而且更加幸运的是,由于B拿到了A的对象引用,所以B现在hold住的A对象完成了初始化。
知道了这个原理时候,肯定就知道为啥Spring不能解决“A的构造方法中依赖了B的实例对象,同时B的构造方法中依赖了A的实例对象”这类问题了!因为加入singletonFactories三级缓存的前提是执行了构造器,所以构造器的循环依赖没法解决。
接下来我们挑 doCreateBean 中的三个细节出来说说。一个是创建 Bean 实例的 createBeanInstance 方法,一个是依赖注入的 populateBean 方法,还有就是回调方法 initializeBean。
这三个方法也是极其复杂的,读者有兴趣可以继续的深入进去。
1、 createBeanInstance 方法
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
// Make sure bean class is actually resolved at this point.
// 确保已经加载了此 class
Class<
?>
beanClass = resolveBeanClass(mbd, beanName);
// 校验一下这个类的访问权限
if (beanClass != null &
&
!Modifier.isPublic(beanClass.getModifiers()) &
&
!mbd.isNonPublicAccessAllowed())
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Bean class isnt public, and non-public access not allowed: " + beanClass.getName());
Supplier<
?>
instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null)
return obtainFromSupplier(instanceSupplier, beanName);
if (mbd.getFactoryMethodName() != null)
// 采用工厂方法实例化,不熟悉这个概念的读者请看附录,注意,不是 FactoryBean
return instantiateUsingFactoryMethod(beanName, mbd, args);
// Shortcut when re-creating the same bean...
// 如果不是第一次创建,比如第二次创建 prototype bean。
// 这种情况下,我们可以从第一次创建知道,采用无参构造函数,还是构造函数依赖注入 来完成实例化
boolean resolved = false;
boolean autowireNecessary = false;
if (args == null)
synchronized (mbd.constructorArgumentLock)
if (mbd.resolvedConstructorOrFactoryMethod != null)
resolved = true;
autowireNecessary = mbd.constructorArgumentsResolved;
if (resolved)
if (autowireNecessary)
// 构造函数依赖注入
return autowireConstructor(beanName, mbd, null, null);
else
// 无参构造函数
return instantiateBean(beanName, mbd);
// Candidate constructors for autowiring?
// 判断是否采用有参构造函数
Constructor<
?>
[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args))
// 构造函数依赖注入
return autowireConstructor(beanName, mbd, ctors, args);
// Preferred constructors for default construction?
ctors = mbd.getPreferredConstructors();
if (ctors != null)
// 构造函数依赖注入
return autowireConstructor(beanName, mbd, ctors, null);
// No special handling: simply use no-arg constructor.
// 调用无参构造函数
return instantiateBean(beanName, mbd);
看一下instantiateBean方法是怎么做的。
protected BeanWrapper instantiateBean(String beanName, RootBeanDefinition mbd)
try
Object beanInstance;
if (System.getSecurityManager() != null)
beanInstance = AccessController.doPrivileged(
(PrivilegedAction<
Object>
) () ->
getInstantiationStrategy().instantiate(mbd, beanName, this),
getAccessControlContext());
else
// 实例化
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
// 包装一下,返回
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
catch (Throwable ex)
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
我们可以看到,关键的地方在于:beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, parent);
里面是具体是实例化过程,我们进去看看。
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner)
// Dont override the class with CGLIB if no overrides.
// 如果不存在方法覆写,那就使用 java 反射进行实例化,否则使用 CGLIB,
// 方法覆写 请参见附录"方法注入"中对 lookup-method 和 replaced-method 的介绍
if (!bd.hasMethodOverrides())
Constructor<
?>
constructorToUse;
synchronized (bd.constructorArgumentLock)
constructorToUse = (Constructor<
?>
) bd.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null)
final Class<
?>
clazz = bd.getBeanClass();
if (clazz.isInterface())
throw new BeanInstantiationException(clazz, "Specified class is an interface");
try
if (System.getSecurityManager() != null)
constructorToUse = AccessController.doPrivileged(
(PrivilegedExceptionAction<
Constructor<
?>
>
) clazz::getDeclaredConstructor);
else
constructorToUse = clazz.getDeclaredConstructor();
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
catch (Throwable ex)
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
// 利用构造方法进行实例化
return BeanUtils.instantiateClass(constructorToUse);
else
// Must generate CGLIB subclass.
// 存在方法覆写,利用 CGLIB 来完成实例化,需要依赖于 CGLIB 生成子类,这里就不展开了。
// tips: 因为如果不使用 CGLIB 的话,存在 override 的情况 JDK 并没有提供相应的实例化支持
return instantiateWithMethodInjection(bd, beanName, owner);
到这里,我们就算实例化完成了。我们开始说怎么进行属性注入。
2、populateBean 方法
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;
// 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 (BeanPostProcessor bp : getBeanPostProcessors())
if (bp instanceof InstantiationAwareBeanPostProcessor)
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
if (!ibp.postProcessAfterInstantiation(bw.getWrappedInstance(), beanName))
return;
// bean 实例的所有属性都在这里了
PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
int resolvedAutowireMode = mbd.getResolvedAutowireMode();
// 通过名字找到所有属性值,如果是 bean 依赖,先初始化依赖的 bean。记录依赖关系
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;
boolean hasInstAwareBpps = hasInstantiationAwareBeanPostProcessors();
boolean needsDepCheck = (mbd.getDependencyCheck() != AbstractBeanDefinition.DEPENDENCY_CHECK_NONE);
PropertyDescriptor[] filteredPds = null;
if (hasInstAwareBpps)
if (pvs == null)
pvs = mbd.getPropertyValues();
for (BeanPostProcessor bp : getBeanPostProcessors())
if (bp instanceof InstantiationAwareBeanPostProcessor)
InstantiationAwareBeanPostProcessor ibp = (InstantiationAwareBeanPostProcessor) bp;
PropertyValues pvsToUse = ibp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
if (pvsToUse == null)
if (filteredPds == null)
filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
// 这里有个非常有用的 BeanPostProcessor 进到这里: AutowiredAnnotationBeanPostProcessor
// 对采用 @Autowired、@Value 注解的依赖进行设值,这里的内容也是非常丰富的
pvsToUse = ibp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
if (pvsToUse == null)
return;
pvs = pvsToUse;
if (needsDepCheck)
if (filteredPds == null)
filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
checkDependencies(beanName, mbd, filteredPds, pvs);
if (pvs != null)
// 设置 bean 实例的属性值
applyPropertyValues(beanName, mbd, bw, pvs);
属性注入完成后,这一步其实就是处理各种回调了,这块代码比较简单。
3、 initializeBean方法
protected Object initializeBean(String beanName, Object bean, @Nullable RootBeanDefinition mbd)
if (System.getSecurityManager() != null)
AccessController.doPrivileged((PrivilegedAction<
Object>
) () ->
invokeAwareMethods(beanName, bean);
return null;
, getAccessControlContext());
else
// 如果 bean 实现了 BeanNameAware、BeanClassLoaderAware 或 BeanFactoryAware 接口,回调
invokeAwareMethods(beanName, bean);
Object wrappedBean = bean;
if (mbd == null || !mbd.isSynthetic())
// BeanPostProcessor 的 postProcessBeforeInitialization 回调
wrappedBean = applyBeanPostProcessorsBeforeInitialization(wrappedBean, beanName);
try
// 处理 bean 中定义的 init-method,
// 或者如果 bean 实现了 InitializingBean 接口,调用 afterPropertiesSet() 方法
invokeInitMethods(beanName, wrappedBean, mbd);
catch (Throwable ex)
throw new BeanCreationException(
(mbd != null ? mbd.getResourceDescription() : null),
beanName, "Invocation of init method failed", ex);
if (mbd == null || !mbd.isSynthetic())
// BeanPostProcessor 的 postProcessAfterInitialization 回调
//BeanPostProcessor 的两个回调都发生在这边,只不过中间处理了 init-method
wrappedBean = applyBeanPostProcessorsAfterInitialization(wrappedBean, beanName);
return wrappedBean;
自此,Spring实例化单例非懒加载bean的过程也就完成了,这也是Spirng最最重要的方法了。在我们的日常使用Spring中,定义好各个类,然后在上面加上,@Controller,@Service,Autowired等注解,这些注解是怎么起作用的呢?
【Spring源码解析之八finishBeanFactoryInitialization方法即初始化单例bean】想必大部分同学都是知其然,不知其所以然,想必通过本文,读者心中能有一个清楚的认识。
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