前言
前面系列章节我们分析了Spring的IOC的启动流程，包括：容器创建，配置加载，配置解析，Bean注册等几个阶段，所以Bean注册其实就是把Bean的相关属性,依赖关系等封装成BeanDeafinition对象，然后注册到一个ConcurrentHashMap中。要注意的是这个BeanDeafinition只是对Bean的一个定义封装而已，并不是真正的Bean的实例，那Bean的实例是在什么时候创建的？有三种情况
- 如果是单利Bean，且lazy-init=false 急切初始(即时，立即，迫切，饥饿都是一个意思)的情况，在IOC容器启动之后就会根据BeanDeafinition对Bean进行实例化。
- 如果某个Bean还没被实例化，通过BeanFacotry.getBean 获取Bean的时候,就会触发Bean的实例化。
- 通过自动注入的时候，会触发Bean的实例化
虽然有多种情况会触Bean的发实例化，但是实例化的过程都是一样的，这篇文章就讲一下IOC启动之后，单利Bean的实例化过程吧。
refresh() 单利Bean实例化
代码还得回到 AbstractApplicationContext#refresh 方法中，在《IOC启动流程》一文中我们对容器刷新refresh方法已经有过研究
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
//准备刷新工作 ,记录开始时间，初始化属性，校验配置文件，准备事件的存储Set
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
//告诉子类，刷新Bean工厂，销毁旧beanFactory，创建新beanFactory，默认DefaultListableBeanFactory
//从子容器的refreshBeanFactory方法中载入Bean的资源文件
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
...省略...
// Instantiate all remaining (non-lazy-init) singletons.
//实例化所有剩余的（非延迟初始化）单例的Bean
finishBeanFactoryInitialization(beanFactory);
...省略...
}
在 obtainFreshBeanFactory方法中完成了Bean的资源文件加载，Bean的解析，BeanDefinition的注册等一些列过程。最终把BeanDefinition保存到了DefaultListableBeanFactory管理的一个ConcurrentHashMap中。
我们接下来要探讨的是finishBeanFactoryInitialization; 方法，它里面实现了急切初始化的单例的Bean的创建流程。
/**
* Finish the initialization of this context's bean factory,
* initializing all remaining singleton beans.
* 完成此上下文的 bean 工厂的初始化，初始化所有剩余的单例 bean。
*/
protected void finishBeanFactoryInitialization(ConfigurableListableBeanFactory beanFactory) {
// Initialize conversion service for this context.
//用于类型转换的服务接口
if (beanFactory.containsBean(CONVERSION_SERVICE_BEAN_NAME) &&
beanFactory.isTypeMatch(CONVERSION_SERVICE_BEAN_NAME, ConversionService.class)) {
beanFactory.setConversionService(
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.
String[] weaverAwareNames = beanFactory.getBeanNamesForType(LoadTimeWeaverAware.class, false, false);
for (String weaverAwareName : weaverAwareNames) {
getBean(weaverAwareName);
}
// Stop using the temporary ClassLoader for type matching.
//停止使用临时类加载器进行类型匹配
beanFactory.setTempClassLoader(null);
// Allow for caching all bean definition metadata, not expecting further changes.
//允许缓存所有 beandefinition 元数据,防止更改
beanFactory.freezeConfiguration();
// Instantiate all remaining (non-lazy-init) singletons.
//[重要]入口在这 ，实例化 lazy-init= false的单利Bean
beanFactory.preInstantiateSingletons();
}
这里初始化Bean Factory后，调用 beanFactory.preInstantiateSingletons 方法实例化 lazy-init= false的单利Bean, DefaultListableBeanFactory#preInstantiateSingletons源码如下：
@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.
//获取到所有Bean的名字
List<String> beanNames = new ArrayList<>(this.beanDefinitionNames);
// Trigger initialization of all non-lazy singleton beans...
for (String beanName : beanNames) {
//获取Bean的BeanDefinition 定义对象
RootBeanDefinition bd = getMergedLocalBeanDefinition(beanName);
//如果不是抽象类，如果是单利，如果lazyInit为false,触发实例化逻辑
if (!bd.isAbstract() && bd.isSingleton() && !bd.isLazyInit()) {
//判断是否是FactoryBean
if (isFactoryBean(beanName)) {
//调用 getBean 方法得到FactoryBean工厂类实例。
//【重要】getBean方法是实例化Bean的核心，后面我们要重点分析这个方法
Object bean = getBean(FACTORY_BEAN_PREFIX + beanName);
//如果是FactoryBean
if (bean instanceof FactoryBean) {
FactoryBean<?> factory = (FactoryBean<?>) bean;
//是否要马上初始化
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) {
//【重要】 如果需要马上实例化，就调用getBean方法进行Bean的实例化
getBean(beanName);
}
}
}
else {
//【重要】 如果是普通的单利Bean，就调用getBean方法进行Bean的实例化
getBean(beanName);
}
}
}
// Trigger post-initialization callback for all applicable beans...
// 触发所有的Bean的生命周期方法的 init初始化方法.
for (String beanName : beanNames) {
//获取到单利Bean的实例
Object singletonInstance = getSingleton(beanName);
//判断Bean有没有实现 SmartInitializingSingleton接口，SmartInitializingSingleton是针对单利Bean的初始化接口
if (singletonInstance instanceof SmartInitializingSingleton) {
SmartInitializingSingleton smartSingleton = (SmartInitializingSingleton) singletonInstance;
if (System.getSecurityManager() != null) {
AccessController.doPrivileged((PrivilegedAction<Object>) () -> {
smartSingleton.afterSingletonsInstantiated();
return null;
}, getAccessControlContext());
}
else {
//触发afterSingletonsInstantiated方法的调用
smartSingleton.afterSingletonsInstantiated();
}
}
}
}
该方法中先是获取到IOC容器注册的所有的Bean的名字，然后循环拿到每个Bean的BeanDefinition定义对象，然后做了三个判断：不能是抽象，必须是单利，必须是急切初始化， 如果满足条件再看是普通的Bean还是FactoryBean，最终都会调用 getBean(beanName); 方法，该方法调用 AbstractBeanFactory#doGetBean 该方法创建Bean。
创建好Bean之后，会判断Bean是否实现了 SmartInitializingSingleton 初始化接口,这个是针对单利Bean的初始化接口，然后调用afterSingletonsInstantiated进行Bean的初始化。
Bean实例化流程图
先上一个流程图，待会儿你可以根据这个图来看代码。
AbstractBeanFactory#doGetBean
protected <T> T doGetBean(
String name, @Nullable Class<T> requiredType, @Nullable Object[] args, boolean typeCheckOnly)
throws BeansException {
//得到Bean的名字,如果指定的是别名，将别名转换为规范的 Bean 名称
String beanName = transformedBeanName(name);
Object bean;
// Eagerly check singleton cache for manually registered singletons.
//从缓存中获取单利的Bean，对于单利只会创建一次，创建好之后就会缓存到map中
//默认情况下Bean都是迫切加载的，在容器启动的过程中就实例化好缓存到Map中
Object sharedInstance = getSingleton(beanName);
if (sharedInstance != null && args == null) {
if (logger.isTraceEnabled()) {
//判断bean是否在 singletonsCurrentlyInCreation set集合中，该集合中存储的是正在创建的单利Bean
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 + "'");
}
}
//获取 Bean 实例的对象，主要是完成 FactoryBean 的相关处理
//即：naem是是以 $ 开头 ，就返回FactoryBean工厂
bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);
}
else {
//代码走到这里，说明单利缓存的map中是没有正在创建的bean。
// Fail if we're already creating this bean instance:
// We're assumably within a circular reference.
//如果缓存中没有单利Bean,但是缓存中有一个多利的Bean，目前正在创建，由于循环依赖问题，抛出异常
if (isPrototypeCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(beanName);
}
// Check if bean definition exists in this factory.
//得到容器IOC工厂
BeanFactory parentBeanFactory = getParentBeanFactory();
//通过beanName检查，如果容器中不包含这个Bean
//如果容器中没有Bean,沿着继承体系交给父级去找
if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {
// Not found -> check parent.
//得到Bean的原始名字，将别名解析为规范名称
String nameToLookup = originalBeanName(name);
if (parentBeanFactory instanceof AbstractBeanFactory) {
//根据名字，类型，参数找Bean
return ((AbstractBeanFactory) parentBeanFactory).doGetBean(
nameToLookup, requiredType, args, typeCheckOnly);
}
else if (args != null) {
//委托父工厂去找Bean ， 根据名字和参数
// Delegation to parent with explicit args.
return (T) parentBeanFactory.getBean(nameToLookup, args);
}
else if (requiredType != null) {
// 无参数 -> 委托给标准的 getBean 方法，根据名字和类型
// No args -> delegate to standard getBean method.
return parentBeanFactory.getBean(nameToLookup, requiredType);
}
else {
//根据名字找Bean
return (T) parentBeanFactory.getBean(nameToLookup);
}
}
//创建Bean是否要类型检查
if (!typeCheckOnly) {
//标记Bean被创建
markBeanAsCreated(beanName);
}
try {
//合并Bean，主要解决Bean继承时子类和父类的公共属性
RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
checkMergedBeanDefinition(mbd, beanName, args);
// Guarantee initialization of beans that the current bean depends on.
//保证当前 bean 所依赖的 bean 的初始化。
String[] dependsOn = mbd.getDependsOn();
//如果当前Bean依赖了其他Bean
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 + "'");
}
//为给定的 bean 注册一个依赖 bean
registerDependentBean(dep, beanName);
try {
//递归，从容器中获取依赖的Bean
getBean(dep);
}
catch (NoSuchBeanDefinitionException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"'" + beanName + "' depends on missing bean '" + dep + "'", ex);
}
}
}
// Create bean instance.
//创建单利Bean
if (mbd.isSingleton()) {
//根据Bean名字获取单利Bean，这里是真正创建Bean的地方，通过ObjectFactory创建
//这里是匿名内部类
sharedInstance = getSingleton(beanName, () -> {
try {
//创建Bean
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
// Explicitly remove instance from singleton cache: It might have been put there
// eagerly by the creation process, to allow for circular reference resolution.
// Also remove any beans that received a temporary reference to the bean.
//销毁实例，从缓存的map中清除单利Bean
destroySingleton(beanName);
throw ex;
}
});
//返回bean实例，主要处理FactoryBean相关
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
//如果是多利
else if (mbd.isPrototype()) {
// It's a prototype -> create a new instance.
//如果是prototype模式，每次都会创建新的实例
Object prototypeInstance = null;
try {
beforePrototypeCreation(beanName);
//创建指定 Bean 对象实例 ,如果是prototype模式，每次都会创建新的实例
prototypeInstance = createBean(beanName, mbd, args);
}
finally {
//默认实现将原型标记为不再创建
afterPrototypeCreation(beanName);
}
//返回bean实例，主要处理FactoryBean相关
bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);
}
else {
String scopeName = mbd.getScope();
//如果不是单利，不是多利，Bean没有 scope属性，则不合法
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;
}
}
//检查所需类型是否与实际 bean 实例的类型匹配。
// Check if required type matches the type of the actual bean instance.
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;
}
上面这个方法中代码量有点多，这里做一下总结
- transformedBeanName(name)：转换Bean的名字，如果传入的name是别名需要找到真实的Bean的名字，如果传入的name是FactoryBean，比如“&bean” ,要去掉 “&”。
- getSingleton(name)：从缓存中查找Bean（ConcurrentHashMap中），单利Bean只会在容器中创建一次，后续都从缓存中加载,如果缓存中没有就就会进行创建。默认情况下Bean都是迫切创建的，即在容器启动的过程中就会根据BeaDefinition创建好Bean的实例存储到缓存中。
在创建单利Bean的时候会存在依赖注入的情况，为了解决循环依赖（A依赖B，B依赖A），在创建Bean的时候会提前将ObjectFactory 加入缓存，如果下个Bean创建时需要依赖上一个Bean就会直接使用ObejctFactory(后面讲循环依赖) - createBean ：Bean的实例化，从BeanDefinition的缓存中获取当前Bean的BeanDefinition定义对象，根据BeanDefinition进行Bean的创建。如果是通过工厂Bea 方式实例化，则需要得到工厂Bean中的factory-method方法返回的的Bean的实例，通过getObjectForBeanInstance 处理的。
isPrototypeCurrentlyInCreation：Prototype依赖检查，单利情况下才会解决循环依赖，如果A中依赖了B,B中依赖了A，在创建依赖的时候就会产生循环依赖。
如果是多利Bean直接走createBean创建一个新的Bean
DefaultSingletonBeanRegistry#getSingleton(String, ObjectFactory)
对于 getSingleton从缓存中获取单利Bean的方法，这里有两个重载方法第一个方法是在 Object sharedInstance = getSingleton(beanName);
时候调用，源码如下：
//allowEarlyReference ：是否创建早期应用，主要用来解决循环依赖
@Nullable
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
// Quick check for existing instance without full singleton lock
//从Map中 singletonObjects = new ConcurrentHashMap<>(256); 获取单利Bean
//singletonObject缓存中是否有Bean
Object singletonObject = this.singletonObjects.get(beanName);
//如果singletonObjects中没有Bean，但是 Set<String> singletonsCurrentlyInCreation中有Bean，代表Bean正在创建
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
//从早期单例对象的缓存 earlySingletonObjects 中获取
singletonObject = this.earlySingletonObjects.get(beanName);
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) {
//获取ObjectFactory ， 对象创建工厂
ObjectFactory<?> singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
//获取对象实例
singletonObject = singletonFactory.getObject();
//把对象存储到缓存中
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
}
}
return singletonObject;
}
这个代码看着就晕，先理一下这几个缓存
- singletonObjects:保存beanName和Bean的关系
- singletonFactories：保存beanName和Bean的工厂之间的关系，ObjectFactory
- earlySingletonObjects：存放beanName和Bean组件的关系，这个Map中的Bean代表正在创建中，用来检测循环依赖用的
- registeredSingletons：保存所有已经注册成功的Bean
这个方法首先从 singletonObjects 获取Bean，如果没有就从earlySingletonObjects中获取正在创建的Bean，如果也没有就从singletonFactories中得到ObjectFactory，通过ObjectFactory.getObject 来创建Bean,并且放到earlySingletonObjects中。且把ObjectFactory从 singletonFactories移除。
如果上面 getSingleton 返回实例为空，就会走到 if (mbd.isSingleton()) 然后调用 另外一个 getSingleton方法 ，这个方法需要传入ObjectFactory，源码如下：
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "Bean name must not be null");
synchronized (this.singletonObjects) {
//从singletonObjects获取Bean
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
...省略...
//把 beanName 添加到 singletonsCurrentlyInCreation 中，用来做循环依赖检查
beforeSingletonCreation(beanName);
try {
//通过工厂获取Bean
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
...省略...
if (newSingleton) {
//把Bean添加到缓存
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}
//注册到registeredSingletons
protected void addSingleton(String beanName, Object singletonObject) {
synchronized (this.singletonObjects) {
this.singletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
this.registeredSingletons.add(beanName);
}
}
这里先从singletonObjects获取Bean，如果没有，就调用ObjectFactory.getObject方法创建Bean的实例，然后调用addSingletion 把Bean添加到 singletonObjects ，和registeredSingletons中。且从 singletonFactories和 earlySingletonObjects移除该Bean。
上面讲的都是走缓存获取Bean，当缓存没有Bean就会走 ObjectFactory.getObject 创建Bean。
AbstractAutowireCapableBeanFactory#createBean
创建Bean的工作是交给 ObjectFactory#createBean方法完成，代码中采用了匿名内部类的方式实现。而又把具体的实例化过程委派给 AbstractAutowireCapableBeanFactory#createBean 方法来完成
下面是 AbstractAutowireCapableBeanFactory#createBean 创建Bean方法源码
@Override
protected Object createBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
...省略...
try {
//创建Bean【重要】
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isTraceEnabled()) {
logger.trace("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
...省略...
}
下面是 AbstractAutowireCapableBeanFactory#doCreateBean 方法源码
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
//对Bean的装饰
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
//如果是单利，清除缓存
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
//创建Bean方法【重要】
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
Object bean = instanceWrapper.getWrappedInstance();
Class<?> beanType = instanceWrapper.getWrappedClass();
if (beanType != NullBean.class) {
mbd.resolvedTargetType = beanType;
}
//允许后处理器修改合并的 bean 定义。
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
try {
//调用后置处理器
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName,
"Post-processing of merged bean definition failed", ex);
}
mbd.postProcessed = true;
}
}
//急切地缓存单例，以便能够解析循环引用
//是否允许提前曝光：单利&&允许循环依赖&&Bean在创建中
// Eagerly cache singletons to be able to resolve circular references
// even when triggered by lifecycle interfaces like BeanFactoryAware.
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");
}
//匿名内部类，防止循环引用，尽早持有对象的引用
//为了解决循环依赖，在Bean初始化完成前把ObjectFactory放入工厂
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
//初始化Bean，处理依赖注入
// Initialize the bean instance.
Object exposedObject = bean;
try {
//填充Bean ， 主要是赋值属性，使用 bean 定义中的属性值填充给定 BeanWrapper 中的 bean 实例。
//如果依赖其他Bean，会进行递归初始化依赖的Bean
populateBean(beanName, mbd, instanceWrapper);
//初始化Bean，如果Bean实现了InitializingBean接口，就会在这里调用afterPropertiesSet方法
//同时会调用init-method方法
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);
}
}
if (earlySingletonExposure) {
//获取已经注册的单利的Bean
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 {
//注册一个 DisposableBean 实现销毁回调
//注册其 DisposableBean 接口和/或在工厂关闭时调用的给定销毁方法
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
总结一下方法的步骤
- 如果是单利，清除缓存
- createBeanInstance： 调用了createBeanInstance方法创建实例，转换成BeanWrapper
- createBeanInstance ，创建BeanDean的实例
- addSingletonFactory：添加ObjectFactory到缓存中
依赖处理 ：当A中有属性B,B中有属性A的时候，实例化A时发现依赖了B,此时会实例化B，而实例化B时发现依赖了A，此时不会直接实例化A，而是通过缓存中的ObjectFactory来创建，目的是解决循环依赖。 - populateBean，对Bean的属性进行依赖注入.
- initializeBean，对Bean进行初始化，如果Bean实现了
InitializingBean接口，就会在这里调用afterPropertiesSet方法 - registerDisposableBeanIfNecessary ：销毁方法支持，如果Bean实现了
DisposableBean接口，通过DefaultSingletonBeanRegistry把Bean包装成一个DisposableBeanAdapter，注册到一个disposableBeans = new LinkedHashMap中。当容器关闭如：applicaiton.close() ,就会触发AbstractApplicationContext#destroyBeans方法遍历该集合中的所有Bean，执行Bean的destroy方法。
下面是 AbstractAutowireCapableBeanFactory#createBeanInstance 创建实例的代码 ，该方法中使用了策略模式，
protected BeanWrapper createBeanInstance(String beanName, RootBeanDefinition mbd, @Nullable Object[] args) {
// Make sure bean class is actually resolved at this point.
//获取到Bean的class
Class<?> beanClass = resolveBeanClass(mbd, beanName);
Supplier<?> instanceSupplier = mbd.getInstanceSupplier();
if (instanceSupplier != null) {
return obtainFromSupplier(instanceSupplier, beanName);
}
//如果BeanDefinition中有 工厂方法，通过工厂方式实例化
if (mbd.getFactoryMethodName() != null) {
//通过工厂方法实例化Bean
return instantiateUsingFactoryMethod(beanName, mbd, args);
}
//使用自动装配功能实例化Bean
// Shortcut when re-creating the same bean...
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) {
//如果配置了自动装配属性，通过自动装配方式实例化
//构造器函数主动注入
return autowireConstructor(beanName, mbd, null, null);
}
else {
//使用默认的无参构造方法实例化
return instantiateBean(beanName, mbd);
}
}
...省略...
//确定构造器，用来创建实例
//需要根据参数解析构造函数
// Candidate constructors for autowiring?
Constructor<?>[] ctors = determineConstructorsFromBeanPostProcessors(beanClass, beanName);
if (ctors != null || mbd.getResolvedAutowireMode() == AUTOWIRE_CONSTRUCTOR ||
mbd.hasConstructorArgumentValues() || !ObjectUtils.isEmpty(args)) {
return autowireConstructor(beanName, mbd, ctors, args);
}
...省略...
//实例化Bean[重要]
//使用默认的无参构造方法实例化
// No special handling: simply use no-arg constructor.
return instantiateBean(beanName, mbd);
}
上面代码体现出了几种实例化方式，通过工厂方法实例化，或者通过自动装配特性实例化Bean,这需要调用对应的构造器完成，但是我们通常使用的是默认无参构造，这就要用到了反射机制来实例化了，见： instantiateBean 方法。下面是 AbstractAutowireCapableBeanFactory#instantiateBean 方法源码如下：
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 {
//重要的代码在这里，通过 InstantiationStrategy 实例化Bean
beanInstance = getInstantiationStrategy().instantiate(mbd, beanName, this);
}
BeanWrapper bw = new BeanWrapperImpl(beanInstance);
initBeanWrapper(bw);
return bw;
}
catch (Throwable ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Instantiation of bean failed", ex);
}
}
方法中通过 InstantiationStrategy 实例化策略来实例化Bean,单利的Bean使用的是 SimpleInstantiationStrategy策略来实例化。得到实例对象后包装到BeanWrapper对象中。
SimpleInstantiationStrategy#instantiate
下面是 SimpleInstantiationStrategy#instantiate() 方法源码
@Override
public Object instantiate(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner) {
// Don't override the class with CGLIB if no overrides.
//如果Bean定义没有覆盖方法，不要用 CGLIB 覆盖类。
if (!bd.hasMethodOverrides()) {
Constructor<?> constructorToUse;
synchronized (bd.constructorArgumentLock) {
constructorToUse = (Constructor<?>) bd.resolvedConstructorOrFactoryMethod;
if (constructorToUse == null) {
//反射，得到Bean的类型
final Class<?> clazz = bd.getBeanClass();
//接口没法实例化
if (clazz.isInterface()) {
throw new BeanInstantiationException(clazz, "Specified class is an interface");
}
try {
if (System.getSecurityManager() != null) {
//匿名内部类，得到构造器
constructorToUse = AccessController.doPrivileged(
(PrivilegedExceptionAction<Constructor<?>>) clazz::getDeclaredConstructor);
}
else {
//反射，获取到构造器
constructorToUse = clazz.getDeclaredConstructor();
}
bd.resolvedConstructorOrFactoryMethod = constructorToUse;
}
catch (Throwable ex) {
throw new BeanInstantiationException(clazz, "No default constructor found", ex);
}
}
}
//使用Bean工具,通过反射创建Bean，通过构造方法.newInstance(arg)创建Bean
return BeanUtils.instantiateClass(constructorToUse);
}
else {
// Must generate CGLIB subclass.
//使用CGLIB 实例化Bean
//调用子类 CglibSubclassingInstantiationStrategy.instantiateWithMethodInjection 来实例化
return instantiateWithMethodInjection(bd, beanName, owner);
}
}
public static <T> T instantiateClass(Constructor<T> ctor, Object... args) throws BeanInstantiationException {
Assert.notNull(ctor, "Constructor must not be null");
try {
ReflectionUtils.makeAccessible(ctor);
//通过 ctor.newInstance 构造器实例化Bean
return (KotlinDetector.isKotlinType(ctor.getDeclaringClass()) ?
KotlinDelegate.instantiateClass(ctor, args) : ctor.newInstance(args));
}
方法中做了一个判断如果Bean有方法被覆盖，就使用JDK反射来创建Bean的实例，否则使用CGLIB来实例化。 CGLIB调用的是子类 CglibSubclassingInstantiationStrategy 来完成的 ， 如果一个类没有接口，就只能使用CGLIB
CglibSubclassingInstantiationStrategy#instantiateWithMethodInjection
下面是 CglibSubclassingInstantiationStrategy#instantiateWithMethodInjection CGLIB创建对象的代码
@Override
protected Object instantiateWithMethodInjection(RootBeanDefinition bd, @Nullable String beanName, BeanFactory owner,
@Nullable Constructor<?> ctor, Object... args) {
// Must generate CGLIB subclass...
return new CglibSubclassCreator(bd, owner).instantiate(ctor, args);
}
public Object instantiate(@Nullable Constructor<?> ctor, Object... args) {
//【重要】创建代理子类，返回class
Class<?> subclass = createEnhancedSubclass(this.beanDefinition);
Object instance;
if (ctor == null) {
//通过反射把class变成对象
instance = BeanUtils.instantiateClass(subclass);
}
else {
try {
//代理子类构造器
Constructor<?> enhancedSubclassConstructor = subclass.getConstructor(ctor.getParameterTypes());
//通过构造器创建实例
instance = enhancedSubclassConstructor.newInstance(args);
}
catch (Exception ex) {
throw new BeanInstantiationException(this.beanDefinition.getBeanClass(),
"Failed to invoke constructor for CGLIB enhanced subclass [" + subclass.getName() + "]", ex);
}
}
// SPR-10785: set callbacks directly on the instance instead of in the
// enhanced class (via the Enhancer) in order to avoid memory leaks.
Factory factory = (Factory) instance;
factory.setCallbacks(new Callback[] {
NoOp.INSTANCE,
//查找覆盖方法拦截器
//CGLIB MethodInterceptor 覆盖方法，将它们替换为返回在容器中查找的 bean 的实现。
new LookupOverrideMethodInterceptor(this.beanDefinition, this.owner),
//CGLIB MethodInterceptor 来覆盖方法，将它们替换为对通用 MethodReplacer 的调用
new ReplaceOverrideMethodInterceptor(this.beanDefinition, this.owner)});
return instance;
}
private Class<?> createEnhancedSubclass(RootBeanDefinition beanDefinition) {
Enhancer enhancer = new Enhancer();
//增强类(代理类)将会把当前类作为父类
enhancer.setSuperclass(beanDefinition.getBeanClass());
enhancer.setNamingPolicy(SpringNamingPolicy.INSTANCE);
if (this.owner instanceof ConfigurableBeanFactory) {
ClassLoader cl = ((ConfigurableBeanFactory) this.owner).getBeanClassLoader();
enhancer.setStrategy(new ClassLoaderAwareGeneratorStrategy(cl));
}
enhancer.setCallbackFilter(new MethodOverrideCallbackFilter(beanDefinition));
enhancer.setCallbackTypes(CALLBACK_TYPES);
return enhancer.createClass();
}
上面就是CGLIB创建代理的代码，对于JDK动态代理而言被代理类必须有接口，JDK动态代理会为被代理类生成一个代理类和被代理类实现相同的接口，从而实现相同的方法，只不过在代理类的方法中可以持有并调用被代理类，从而实现对被代理类的方法做了增强的目的。但是如果被代理类么有接口，那就必须切换成CGLIB代理方式。
这种方式生成的代理类会把被代理类作为父类，对被代理类的方法进行复写和增强。如果没有接触过CGLIB动态代理这一块的可能看起来就比较晕菜了。
总结
到这里，我已经把IOC容器创建Bean的具体过程分析完毕，下面做个小结
- 我们讨论了Bean的实例化时机，IOC启动之后对单利Bean实例化，手动getBean时触发实例化，自动注入时触发实例化
- Bean的实例化是通过AbstractBeanFactory#doGetBean方法完成的。如果是单利的Bean,从单利Bean缓存的map中查找（ConcurrentHashMap中），找到直接返回，保证了单利，如果缓存中没有就就会进行创建。如果是多利Bean则每次都会创建一个新的Bean
- 创建Bean的过程主要是通过Bean的BeanDeifnition对象，使用JDK反射，或者CGLIB方式创建Bean。
- 当Bean创建成功之后会触发Bean的依赖注入，以及Bean的初始化方法。
- 最后注册了DisposableBean 来支持Bean的销毁方法调用
好了，本篇文章就到这吧，喜欢的话就给个好评吧，如果好评上500，我就是头发掉光，也出下章!!!
