前言
在 finishBeanFactoryInitialization 中介绍了创立 Bean 的流程大略流程，这里进入单例 Bean 的创立过程。
这里次要分为三个局部创立单例 Bean
- getSingleton
- createBean
- getObjectForBeanInstance
上面进入源码：
getSingleton
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {Assert.notNull(beanName, "Bean name must not be null");
// 加锁
synchronized (this.singletonObjects) {
// 查看 singletonObjects 缓存中是否有
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {
// 查看是否在执行销毁
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 + "'");
}
// 将 Bean 增加到 singletonsCurrentlyInCreation 汇合中, 示意正在创立
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {this.suppressedExceptions = new LinkedHashSet<>();
}
try {
// 调用工厂办法
// 也就是调用 createBean(beanName, mbd, args)
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
// Has the singleton object implicitly appeared in the meantime ->
// if yes, proceed with it since the exception indicates that state.
singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null) {throw ex;}
}
catch (BeanCreationException ex) {if (recordSuppressedExceptions) {for (Exception suppressedException : this.suppressedExceptions) {ex.addRelatedCause(suppressedException);
}
}
throw ex;
}
finally {if (recordSuppressedExceptions) {this.suppressedExceptions = null;}
// 创立胜利, 从 singletonsCurrentlyInCreation 移除
afterSingletonCreation(beanName);
}
if (newSingleton) {
// 将给定的单例对象增加到该工厂的单例缓存中
// this.singletonObjects.put(beanName, singletonObject);
// this.singletonFactories.remove(beanName);
// this.earlySingletonObjects.remove(beanName);
// this.registeredSingletons.add(beanName);
addSingleton(beanName, singletonObject);
}
}
return singletonObject;
}
}返回以给定名称注册的（原始）单例对象，如果尚未注册，则创立并注册一个新对象。
这一块一共能够拆成三局部来了解：
1. 从缓存中获取 singletonObjects
singletonObject 是什么？
/** Cache of singleton objects: bean name to bean instance. */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);singletonObjects 是一个 ConcurrentHashMap，用来缓存单例对象的实例。
2. 创立 singletonObject
在从缓存中没有获取到 singletonObject，创立新的对象
singletonObject = singletonFactory.getObject();
这一步其实就是调用外边的 createBean(beanName, mbd, args) 办法，这是一个工厂办法。通过 createBean 办法，会创立一个新的 singletonObject。
3. 将创立的 singletonObject 增加到缓存中
protected void addSingleton(String beanName, Object singletonObject) {synchronized (this.singletonObjects) {this.singletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
this.earlySingletonObjects.remove(beanName);
// 曾经胜利创立的单例
this.registeredSingletons.add(beanName);
}
}这一步波及到三个缓存，以及一个胜利创立的单例列表。
/** Cache of singleton objects: bean name to bean instance. */
/** 缓存单例对象，K-V -> BeanName - Bean 实例 */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<>(256);
/** Cache of singleton factories: bean name to ObjectFactory. */
/** 缓存 Bean 工厂 */
private final Map<String, ObjectFactory<?>> singletonFactories = new HashMap<>(16);
/** Cache of early singleton objects: bean name to bean instance. */
/** 缓存晚期单例对象 */
private final Map<String, Object> earlySingletonObjects = new ConcurrentHashMap<>(16);
/** Set of registered singletons, containing the bean names in registration order. */
/** 已注册的单例列表，按注册程序保留 BeanName。*/
private final Set<String> registeredSingletons = new LinkedHashSet<>(256);将创立的单例对象，增加到 单例缓存 中，同时将 工厂缓存 以及 晚期单例对象缓存 中的对应对象删除。
createBean
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.
// 获取实在的类型
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
// 创立新的 mbd 避免 其余线程批改
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
// Prepare method overrides.
try {
// 验证并筹备为此 bean 定义的办法代替。查看是否存在具备指定名称的办法。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.
// 利用实例化之前的后处理器，以解决指定的 bean 是否存在实例化快捷方式。// InstantiationAwareBeanPostProcessor 后置处理器
// postProcessBeforeInstantiation 办法可能会曾经实例化 Bean
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 {
// 实例化 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);
}
}这个办法中波及到：创立 Bean 实例 , 填充 Bean , 利用 PostProcessor。
其中实例化 Bean 是在 doCreateBean 中。当初重点看一下 doCreateBean 办法。
doCreateBean
protected Object doCreateBean(String beanName, RootBeanDefinition mbd, @Nullable Object[] args)
throws BeanCreationException {
// Instantiate the bean.
// Bean 的 对象包装
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
// 从缓存中获取
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
// 缓存中获取不到则间接创立, 这里创立的 BeanInstance !!!
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
// 获取 Bean 实例以及类型
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 {
// 如果容许批改 mbd
// 调用 MergedBeanDefinitionPostProcessor 后置处理器的
// postProcessMergedBeanDefinition(mbd, beanType, beanName);
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.
// mbd 是单例 且 容许循环援用, (默认 true) 且在创立
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");
}
// 先获取 之前的 Bean 的援用, 从 beanPostProcessorCache 中 获取 SmartInstantiationAwareBeanPostProcessor
// 而后从 SmartInstantiationAwareBeanPostProcessor#getEarlyBeanReference 获取之前的援用
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
// 属性赋值
populateBean(beanName, mbd, instanceWrapper);
// 执行 init 办法
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);
}
}
// 这里容许循环依赖
if (earlySingletonExposure) {
// 获取晚期的 Bean, 如果没有循环依赖 则获取不到
Object earlySingletonReference = getSingleton(beanName, false);
// 有循环依赖
if (earlySingletonReference != null) {
// 创立的是不是同一个，可能会有代理对象
if (exposedObject == bean) {exposedObject = earlySingletonReference;}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
// 获取依赖的 Bean 并 循环放入到 actualDependentBeans
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 {
// 注册销毁办法
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}同样是代码很长很长！
分步骤浏览：
如果这个 Bean 是单例 Bean 且容许循环援用且在创立中，则阐明在有循环援用。则调用：
addSingletonFactory(beanName, () -> getEarlyBeanReference(beanName, mbd, bean));
这一行代码波及到两个办法，别离是 getEarlyBeanReference 和 addSingletonFactory
- getEarlyBeanReference
- addSingletonFactory
这一块能够看到将创立的一个单例对象的 singletonFactory 增加到了 singletonFactories 缓存中。
同时将 earlySingletonObjects 缓存中的单例对象移除。
那什么时候增加到 earlySingletonObjects 缓存中的呢？
这块能够参考 Spring 源码学习 15：finishBeanFactoryInitialization 在 getSingleton 办法中 put 进去的。
为了不便，我把这一小块代码也贴出来：
在这里将缓存从 singletonFactories 移到了 earlySingletonObjects。
Spring 的 Bean 实例化的时候用到的三级缓存其实是：
singletonObjects： 一级缓存，存储单例对象，Bean 曾经实例化，初始化实现。
earlySingletonObjects： 二级缓存，存储 singletonObject，这个 Bean 实例化了，还没有初始化。
singletonFactories： 三级缓存，存储 singletonFactory
上面会初始化 Bean
这里关注重点关注上面一部分：
- populateBean
对 Bean 的属性进行赋值。
这块须要留神的是，在对属性进行赋值时，发现依赖了其余 Bean，就会去先创立其余 Bean。
我这边应用的注解 @Autowired 就会执行上面一部分：
在这里解析属性的时候，就会去创立外部依赖的 Bean。
- initializeBean
getObjectForBeanInstance
获取给定 bean 实例的对象，如果是 FactoryBean，则为 bean 实例自身或其创立的对象。
这一块逻辑绝对比较简单，就是依据后面你创立的 beanInstance，判断其类型，从而创立 Bean 实例。
总结
本文次要介绍了一个 单例 Bean 的创立，当然都是大块大块的源码，须要急躁的啃。
浏览完源码，基本上对循环依赖能有个具体的理解，晓得 Spring 在初始化 Bean 的时候是应用三级缓存来解决循环依赖的额，而前面则会独自筹备一篇文章对循环依赖做介绍。
相干举荐
- Spring 源码学习 15：finishBeanFactoryInitialization（重点）
- Spring 源码学习 14：initApplicationEventMulticaster
- Spring 源码学习 13：initMessageSource