Spring源码之bean的加载(一)

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从本节开始,我们将对bean的加载进行探索。对于其的探索,我们主要是围绕着这句话进行的:MyTestBean bean = (MyTestBean)bf.getBean(“myTestBean”)。相信大家对这句并不要陌生。那么Spring的代码是如何实现这个功能的呢?这里主要的类是AbstractBeanFactory,这个类包含了主要的调用方法,然后我们再进行展开来讲解。(相关资源可到这里下载:http://pan.baidu.com/s/1sjSo9a9)

@Overridepublic Object getBean(String name) throws BeansException {return doGetBean(name, null, null, false);}

其实当读者点开AbstractBeanFactory的源码,就可以看到多个的getBean()。我们是调用MyTestBeanbean = (MyTestBean)bf.getBean(“myTestBean”)这个的,所以我们只用上面那个getBean函数就行了,而这个函数又去调用doGetBean的函数,这时候我们跟踪代码,就可以看到主要方法的调用。

@SuppressWarnings("unchecked")protected <T> T doGetBean(final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)throws BeansException {//提取对应的beanNamefinal String beanName = transformedBeanName(name);Object bean;// Eagerly check singleton cache for manually registered singletons./** * 检查缓存中或者实例工厂中是否有对应的实例 * 为什么首先会使用这段代码呢? * 因为在创建单例bean的时候会存在依赖注入的情况,而在创建依赖的时候为了避免循环依赖, * Spring创建bean的原则是不等bean创建完成就会创建bean的ObjectFactory提供曝光 * 也就是将Object加入到缓存中,一旦下个bean创建时候需要依赖上个bean则直接使用ObjectFactory *///直接尝试从缓存获取或者singlestonFactories中的ObjetFactory中获取Object sharedInstance = getSingleton(beanName);if (sharedInstance != null && args == null) {if (logger.isDebugEnabled()) {if (isSingletonCurrentlyInCreation(beanName)) {logger.debug("Returning eagerly cached instance of singleton bean '" + beanName +"' that is not fully initialized yet - a consequence of a circular reference");}else {logger.debug("Returning cached instance of singleton bean '" + beanName + "'");}}//返回对应的实例,有时候存在诸如BeanFactory的情况并不是直接返回实例本身,而是返回指定方法返回的实例bean = getObjectForBeanInstance(sharedInstance, name, beanName, null);}else {// Fail if we're already creating this bean instance:// We're assumably within a circular reference.//只有在单例情况下才会尝试解决循环依赖,原型模式情况下,如果存在A中有B的属性,B中有A的属性,那么依赖注入//的时候,就会产生当A还未创建完的时候因为//对B的创建再次返回创建A,造成循环依赖,也就是下面的情况if (isPrototypeCurrentlyInCreation(beanName)) {throw new BeanCurrentlyInCreationException(beanName);}// Check if bean definition exists in this factory.BeanFactory parentBeanFactory = getParentBeanFactory();//如果beanDefinitionMap中也就是在所在已经加载的类中不包括beanName则尝试从parentBeanFactory中检测if (parentBeanFactory != null && !containsBeanDefinition(beanName)) {// Not found -> check parent.//递归到BeanFactory中寻找String nameToLookup = originalBeanName(name);if (args != null) {// Delegation to parent with explicit args.return (T) parentBeanFactory.getBean(nameToLookup, args);}else {// No args -> delegate to standard getBean method.return parentBeanFactory.getBean(nameToLookup, requiredType);}}          //如果不是仅仅做类型检查则是创建bean,这里要进行记录if (!typeCheckOnly) {markBeanAsCreated(beanName);}try {//将存储XML配置文件的GernericBeanDefinition转换为RootBeanDefinition,如果指定BeanName是子//Bean的话同时会合并父类相关属性final RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);checkMergedBeanDefinition(mbd, beanName, args);// Guarantee initialization of beans that the current bean depends on.String[] dependsOn = mbd.getDependsOn();if (dependsOn != null) {for (String dependsOnBean : dependsOn) {if (isDependent(beanName, dependsOnBean)) {throw new BeanCreationException(mbd.getResourceDescription(), beanName,"Circular depends-on relationship between '" + beanName + "' and '" + dependsOnBean + "'");}//缓存依赖调用registerDependentBean(dependsOnBean, beanName);getBean(dependsOnBean);}}// Create bean instance.//实例化依赖的bean后便可以实例化mdb本身//singleton模式的创建if (mbd.isSingleton()) {sharedInstance = getSingleton(beanName, new ObjectFactory<Object>() {@Overridepublic 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);}else if (mbd.isPrototype()) {// It's a prototype -> create a new instance.//prototype模式的创建(new)Object prototypeInstance = null;try {beforePrototypeCreation(beanName);prototypeInstance = createBean(beanName, mbd, args);}finally {afterPrototypeCreation(beanName);}bean = getObjectForBeanInstance(prototypeInstance, name, beanName, mbd);}else {//指定scope上实例化beanString scopeName = mbd.getScope();final 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, new ObjectFactory<Object>() {@Overridepublic Object getObject() throws BeansException {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.//检查需要的类型是否符合bean的实际类型if (requiredType != null && bean != null && !requiredType.isAssignableFrom(bean.getClass())) {try {return getTypeConverter().convertIfNecessary(bean, requiredType);}catch (TypeMismatchException ex) {if (logger.isDebugEnabled()) {logger.debug("Failed to convert bean '" + name + "' to required type [" +ClassUtils.getQualifiedName(requiredType) + "]", ex);}throw new BeanNotOfRequiredTypeException(name, requiredType, bean.getClass());}}return (T) bean;}

上面的代码比较多。对于加载过程中涉及的步骤大致可以分为以下几步:

1)        转换对应beanName。这里beanName的获取有这几个要求。首先,如果beanName含有特殊的修饰符,那么就要转换掉,如“&owe”就要转换为“owen”;其次,获取最终的beanName,如A指向B,B指向C,那么最终应该得到C。

2)        尝试从缓存中加载单例。单例在Spring的同一个容器中只会被加载创建一次,后续再获取bean,就直接从单例缓存中获取。

3)        bean的实例化。从缓存中得到的bean并不一定是我们相要的,那只是原始的bean状态,则需要对其进行实例化。

4)        原型模式的依赖检查。

5)        检测parentBeanFactory。加载有XMNL配置文件中不包含beanName所对应的配置,就只能到parentBeanFactory去尝试下了,然后再去递归的调用getBean方法。

6)        将存储XML配置文件的GernericBeanDefinition转换为RootBeanDefinition.

7)        寻找依赖。

8)        针对不同的scope进行bean的创建。在Spring中存在着不同的scope,其中默认提singleton,当然还有protope、request。

9)        类型转换。




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