有关Spring加载bean系列，今天这是最后一篇了，主要接上篇对于从Spring容器中获取Bean的一些细节实现的补充。
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《Spring读书笔记——bean加载》——Spring如何加载消化一个xml配置文件
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《Spring读书笔记——bean解析》——Spring如何将xml文件的各种标签转换为BeanDefinition并注册到Spring容器下
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《Spring读书笔记——bean创建（上）》——概述Spring如何从容器中取出需要的那个Bean
从缓存中加载单例
protected Object getSingleton(String beanName, boolean allowEarlyReference) {
Object singletonObject = this.singletonObjects.get(beanName);
if (singletonObject == null && isSingletonCurrentlyInCreation(beanName)) {
synchronized (this.singletonObjects) {
singletonObject = this.earlySingletonObjects.get(beanName);
if (singletonObject == null && allowEarlyReference) {
ObjectFactory singletonFactory = this.singletonFactories.get(beanName);
if (singletonFactory != null) {
singletonObject = singletonFactory.getObject();
this.earlySingletonObjects.put(beanName, singletonObject);
this.singletonFactories.remove(beanName);
}
}
}
}
return (singletonObject != NULL_OBJECT ? singletonObject : null);
}
在看这段代码之前，我们先了解下Spring对于单例bean出现循环依赖的解决方法。如果出现上面我们介绍的A->B->C->A的情况，那是不是说Spring就无能为力了，显然Spring没有那么弱。那么Spring是怎么做的？
鉴于单例bean的循环依赖问题，Spring创建bean的原则是不等bean创建完成就会将bean的ObjectFactory提前曝光加入到缓存中，一旦有某个bean创建时需要依赖这个bean了，那么就可以直接使用ObjectFactory。
简单说，创建bean的时候，就是打包快递发货，主管为了知道你今天要派发多少个包裹，为了节省大家时间以及以免统计漏掉的情况。你可以先拿出一个包裹箱子，上面写上要寄收件人、收货地址、联系方式等等，但是这时候还没有往里面打包真正的快递。
这里曝光的bean就相当于这个快递箱子。
好了，知道了这个原则之后，我们就好理解代码了。
首先从singletonObjects中获取实例，取不到则从earlySingletonObjects中获取，仍然取不到，我们还可以到singletonFactories中获取相应的ObjectFactory，在调用这个ObjectFactory的getObject方法来创建bean。
然后将其加入到earlySingletonObjects中，在将其从singletonFactories中删除。
想必，你已经被这些用来存储和删除的集合搞疯了，没关系，我们来理一下：
- singletonObjects
/** Cache of singleton objects: bean name --> bean instance */
private final Map<String, Object> singletonObjects = new ConcurrentHashMap<String, Object>(64);
用于保存BeanName和创建bean实例之间的关系。
- singletonFactories:
/** Cache of singleton factories: bean name --> ObjectFactory */
private final Map<String, ObjectFactory> singletonFactories = new HashMap<String, ObjectFactory>(16);
用于保存BeanName和创建bean的工厂之间的关系
- earlySingletonObjects:
/** Cache of early singleton objects: bean name --> bean instance */
private final Map<String, Object> earlySingletonObjects = new HashMap<String, Object>(16);
用于保存BeanName和创建bean的工厂之间的关系，与singletonObjects的区别是当一个bean被放入这个集合后，可以用于其他bean做循环依赖检查
bean实例化
我们从缓存中拿到bean之后，就需要根据bean的不同类型做不同的处理，返回相应的bean，实现这个功能的就是getObjectForBeanInstance方法
protected Object getObjectForBeanInstance(
Object beanInstance, String name, String beanName, RootBeanDefinition mbd) {
// Don't let calling code try to dereference the factory if the bean isn't a factory.
if (BeanFactoryUtils.isFactoryDereference(name) && !(beanInstance instanceof FactoryBean)) {
throw new BeanIsNotAFactoryException(transformedBeanName(name), beanInstance.getClass());
}
// Now we have the bean instance, which may be a normal bean or a FactoryBean.
// If it's a FactoryBean, we use it to create a bean instance, unless the
// caller actually wants a reference to the factory.
if (!(beanInstance instanceof FactoryBean) || BeanFactoryUtils.isFactoryDereference(name)) {
return beanInstance;
}
Object object = null;
if (mbd == null) {
object = getCachedObjectForFactoryBean(beanName);
}
if (object == null) {
// Return bean instance from factory.
FactoryBean<?> factory = (FactoryBean<?>) beanInstance;
// Caches object obtained from FactoryBean if it is a singleton.
if (mbd == null && containsBeanDefinition(beanName)) {
mbd = getMergedLocalBeanDefinition(beanName);
}
boolean synthetic = (mbd != null && mbd.isSynthetic());
object = getObjectFromFactoryBean(factory, beanName, !synthetic);
}
return object;
}
- 首先检测指定的beanName是否是工厂bean相关，如果既不是工厂bean又是以"&"开头，则校验失败，抛出异常
- 如果这个bean不是工厂Bean（FactoryBean），那么就直接返回bean实例
- 剩下代码就是处理FactoryBean，我们顺着这样的顺序依次来到getObjectForBeanInstance->getObjectFromFactoryBean->doGetObjectFromFactoryBean
private Object doGetObjectFromFactoryBean(
final FactoryBean factory, final String beanName, final boolean shouldPostProcess)
throws BeanCreationException {
Object object;
try {
if (System.getSecurityManager() != null) {
AccessControlContext acc = getAccessControlContext();
try {
object = AccessController.doPrivileged(new PrivilegedExceptionAction<Object>() {
public Object run() throws Exception {
return factory.getObject();
}
}, acc);
}
catch (PrivilegedActionException pae) {
throw pae.getException();
}
}
else {
object = factory.getObject();
}
}
catch (FactoryBeanNotInitializedException ex) {
throw new BeanCurrentlyInCreationException(beanName, ex.toString());
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "FactoryBean threw exception on object creation", ex);
}
// Do not accept a null value for a FactoryBean that's not fully
// initialized yet: Many FactoryBeans just return null then.
if (object == null && isSingletonCurrentlyInCreation(beanName)) {
throw new BeanCurrentlyInCreationException(
beanName, "FactoryBean which is currently in creation returned null from getObject");
}
if (object != null && shouldPostProcess) {
try {
object = postProcessObjectFromFactoryBean(object, beanName);
}
catch (Throwable ex) {
throw new BeanCreationException(beanName, "Post-processing of the FactoryBean's object failed", ex);
}
}
return object;
}
这么长的代码，如果嫌累，就只看factory.getObject()这行就好，这诠释了FactoryBean的加载时通过factory.getObject的方式获取到对应的bean实例的。
如何创建单例bean
在上篇的doGetBean方法中，如果从缓存中加载不到，那么我们就需要老老实实的从头开始加载bean了，对于单例bean的加载就都在这里实现了
// Create bean instance.
if (mbd.isSingleton()) {
sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {
public Object getObject() throws BeansException {
try {
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);
}
创建bean
我们从AbstractBeanFactory的createBean方法来到了AbstractAutowiredCapableBeanFactory的createbean方法，而真正的创建bean其实在doCreateBean方法中
protected Object doCreateBean(final String beanName, final RootBeanDefinition mbd, final Object[] args) {
// Instantiate the bean.
BeanWrapper instanceWrapper = null;
if (mbd.isSingleton()) {
instanceWrapper = this.factoryBeanInstanceCache.remove(beanName);
}
if (instanceWrapper == null) {
instanceWrapper = createBeanInstance(beanName, mbd, args);
}
final Object bean = (instanceWrapper != null ? instanceWrapper.getWrappedInstance() : null);
Class beanType = (instanceWrapper != null ? instanceWrapper.getWrappedClass() : null);
// Allow post-processors to modify the merged bean definition.
synchronized (mbd.postProcessingLock) {
if (!mbd.postProcessed) {
applyMergedBeanDefinitionPostProcessors(mbd, beanType, beanName);
mbd.postProcessed = true;
}
}
// 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.isDebugEnabled()) {
logger.debug("Eagerly caching bean '" + beanName +
"' to allow for resolving potential circular references");
}
addSingletonFactory(beanName, new ObjectFactory() {
public Object getObject() throws BeansException {
return getEarlyBeanReference(beanName, mbd, bean);
}
});
}
// Initialize the bean instance.
Object exposedObject = bean;
try {
populateBean(beanName, mbd, instanceWrapper);
if (exposedObject != null) {
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) {
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.");
}
}
}
}
// Register bean as disposable.
try {
registerDisposableBeanIfNecessary(beanName, bean, mbd);
}
catch (BeanDefinitionValidationException ex) {
throw new BeanCreationException(mbd.getResourceDescription(), beanName, "Invalid destruction signature", ex);
}
return exposedObject;
}
- 如果是单例Bean，那么首先是从factoryBeanInstanceCache中清除该beanName对应的记录
- 实例化bean，将BeanDefinition转换为BeanWrapper对象
- bean合并后的处理
- 解决循环依赖问题
- 属性填充，将所有属性填充到bean的实例中
这个方法，本身不算长，但是层层深入就会发现其下面包罗了创建bean的诸多繁杂的细节（这块自己看的也是云里雾里，就不往下延伸扩展了）。
虽然对于Spring加载bean，我只写了四篇，但是其内部实现远比我表述的要复杂的多。
看源码确实很煎熬，对于目前看不懂的地方要么多看几遍，要么先跳过。阅读代码的过程中要懂得取舍，对于非重点部分比如日志或者异常处理可以先忽略，沿着一条主线往下看，最主要是先弄懂代码的只要意图。
Spring的bean加载代码量虽然巨大，但是思路还是比较清晰的，我们知道Spring如何加载xml然后解析xml，再到如何把xml的元素转为自己的BeanDefinition，最后又是如何取出对应的beanName然后返回一个bean实例供容器使用的。
网上有一位大神用一张图就把整个过程画出来了
注：图片来源http://blog.csdn.net/zghwaicsdn/article/details/50910384
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