[TOC]
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![](https://i.loli.net/2019/03/13/5c88eb1c4cc26.png)
> `getBean()`创建单例bean实例
* `getBean`调用`getSingleton(beanName, ObjectFactory)`,匿名工厂类中的`getObject()`方法调用了`creatBean()`。
* 对于已实例化好的单例`bean`,`getBean(String)`方法并不会再一次去创建,而是从缓存中获取。如果缓存中没有则根据`bean`的配置信息去创建这个`bean`。
```
public T doGetBean(...) {
// ...
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
try {
//调用creatBean方法
return createBean(beanName, mbd, args);
}
catch (BeansException ex) {
destroySingleton(beanName);
throw ex;
}
}
});
bean = getObjectForBeanInstance(sharedInstance, name, beanName, mbd);
}
//...
```
<br/>
> ### `getSingleton(String, ObjectFactory)`方法
```
public Object getSingleton(String beanName, ObjectFactory<?> singletonFactory) {
Assert.notNull(beanName, "'beanName' must not be null");
synchronized (this.singletonObjects) {
// 从缓存中获取单例 bean,若不为空,则直接返回,不用再初始化
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 + "'");
}
/*
* 将 beanName 添加到 singletonsCurrentlyInCreation 集合中,
* 用于表明 beanName 对应的 bean 正在创建中
*/
beforeSingletonCreation(beanName);
boolean newSingleton = false;
boolean recordSuppressedExceptions = (this.suppressedExceptions == null);
if (recordSuppressedExceptions) {
this.suppressedExceptions = new LinkedHashSet<Exception>();
}
try {
// 通过 getObject 方法调用 createBean 方法创建 bean 实例
singletonObject = singletonFactory.getObject();
newSingleton = true;
}
catch (IllegalStateException ex) {
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;
}
// 将 beanName 从 singletonsCurrentlyInCreation 移除
afterSingletonCreation(beanName);
}
if (newSingleton) {
/*
* 将 <beanName, singletonObject> 键值对添加到 singletonObjects 集合中,
* 并从其他集合(比如 earlySingletonObjects)中移除 singletonObject 记录
*/
addSingleton(beanName, singletonObject);
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
}
```
<br/>
> ### `createBean `方法
```
protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {
if (logger.isDebugEnabled()) {
logger.debug("Creating instance of bean '" + beanName + "'");
}
RootBeanDefinition mbdToUse = mbd;
// 解析 bean 的类型
Class<?> resolvedClass = resolveBeanClass(mbd, beanName);
if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {
mbdToUse = new RootBeanDefinition(mbd);
mbdToUse.setBeanClass(resolvedClass);
}
try {
// 处理 lookup-method 和 replace-method 配置,Spring 将这两个配置统称为 override method
mbdToUse.prepareMethodOverrides();
}
catch (BeanDefinitionValidationException ex) {
throw new BeanDefinitionStoreException(mbdToUse.getResourceDescription(),
beanName, "Validation of method overrides failed", ex);
}
try {
// 在 bean 初始化前应用后置处理,如果后置处理返回的 bean 不为空,则直接返回
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);
}
// 调用 doCreateBean 创建 bean
Object beanInstance = doCreateBean(beanName, mbdToUse, args);
if (logger.isDebugEnabled()) {
logger.debug("Finished creating instance of bean '" + beanName + "'");
}
return beanInstance;
}
```
<br/>
> ### `doCreateBean `方法
```
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args)
throws BeanCreationException {
// BeanWrapper 包装bean实例
//通过 BeanWrapper 的实现类可以方便的设置/获取 bean 实例的属性
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
// 从缓存中获取 BeanWrapper,并清理相关记录
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
/*
* 创建 bean 实例,并将实例包裹在 BeanWrapper 实现类对象中返回。createBeanInstance
* 中包含三种创建 bean 实例的方式:
* 1. 通过工厂方法创建 bean 实例
* 2. 通过构造方法自动注入(autowire by constructor)的方式创建 bean 实例
* 3. 通过无参构造方法方法创建 bean 实例
*
* 若 bean 的配置信息中配置了 lookup-method 和 replace-method,则会使用 CGLIB
* 增强 bean 实例。
*/
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
// 此处的 bean 可以认为是一个原始的 bean 实例,暂未填充属性
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class<?> beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
mbd.resolvedTargetType = beanType;
// 这里又遇到后置处理了,此处的后置处理是用于处理已“合并的 BeanDefinition”。关于这种后置处理器具体的实现细节就不深入理解了,大家有兴趣可以自己去看
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;
}
}
boolean earlySingletonExposure = (mbd.isSingleton() && this.allowCircularReferences &&
isSingletonCurrentlyInCreation(beanName));
if (earlySingletonExposure) {
if (logger.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
// 添加工厂对象到 singletonFactories 缓存中
addSingletonFactory(beanName, new ObjectFactory<Object>() {
@Override
public Object getObject() throws BeansException {
// 获取早期 bean 的引用,如果 bean 中的方法被 AOP 切点所匹配到,此时 AOP 相关逻辑会介入
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
Object exposedObject = bean;
try {
// 向 bean 实例中填充属性,populateBean 方法也是一个很重要的方法,后面会专门写文章分析
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
/*
* 进行余下的初始化工作,详细如下:
* 1. 判断 bean 是否实现了 BeanNameAware、BeanFactoryAware、
* BeanClassLoaderAware 等接口,并执行接口方法
* 2. 应用 bean 初始化前置操作
* 3. 如果 bean 实现了 InitializingBean 接口,则执行 afterPropertiesSet
* 方法。如果用户配置了 init-method,则调用相关方法执行自定义初始化逻辑
* 4. 应用 bean 初始化后置操作
*
* 另外,AOP 相关逻辑也会在该方法中织入切面逻辑,此时的 exposedObject 就变成了
* 一个代理对象了
*/
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) {
Object earlySingletonReference = getSingleton(beanName, false);
if (earlySingletonReference != null) {
// 若 initializeBean 方法未改变 exposedObject 的引用,则此处的条件为 true。
if (exposedObject == bean) {
exposedObject = earlySingletonReference;
}
else if (!this.allowRawInjectionDespiteWrapping && hasDependentBean(beanName)) {
String[] dependentBeans = getDependentBeans(beanName);
Set<String> actualDependentBeans = new LinkedHashSet<String>(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 " +
"'getBeanNamesOfType' with the 'allowEagerInit' flag turned off, for example.");
}
}
}
}
try {
// 注册销毁逻辑
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(
mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
```
<br/>
<br/>
***
> ### 参考
#### [Spring-IOC-容器源码分析-创建单例-bean](http://www.tianxiaobo.com/2018/06/04/Spring-IOC-%E5%AE%B9%E5%99%A8%E6%BA%90%E7%A0%81%E5%88%86%E6%9E%90-%E5%88%9B%E5%BB%BA%E5%8D%95%E4%BE%8B-bean/)
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