Spring源码之bean的加载(五)准备创建bean

我们继续跟踪代码到createBean，也就是创建创建bean，一同来看一下createBean函数中做了哪些准备工作。首先，我们先来看一张时序图。（相关资源可到这里下载：http://pan.baidu.com/s/1sjSo9a9）

protected Object createBean(String beanName, RootBeanDefinition mbd, Object[] args) throws BeanCreationException {if (logger.isDebugEnabled()) {logger.debug("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.Class<?> resolvedClass = resolveBeanClass(mbd, beanName);if (resolvedClass != null && !mbd.hasBeanClass() && mbd.getBeanClassName() != null) {mbdToUse = new RootBeanDefinition(mbd);mbdToUse.setBeanClass(resolvedClass);}// Prepare method overrides.try {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.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);}Object beanInstance = doCreateBean(beanName, mbdToUse, args);if (logger.isDebugEnabled()) {logger.debug("Finished creating instance of bean '" + beanName + "'");}return beanInstance;}

从代码中，我们可以做以下的总结：

1)        根据设置的class属性或者根据className来解析Class。

2)        对override属性进行标记及验证

public void prepareMethodOverrides() throws BeanDefinitionValidationException {// Check that lookup methods exists.MethodOverrides methodOverrides = getMethodOverrides();if (!methodOverrides.isEmpty()) {for (MethodOverride mo : methodOverrides.getOverrides()) {prepareMethodOverride(mo);}}}

上面的方法中，我们要留意的是prepareMethodOverride函数，所以下面是这个函数的实现。

protected void prepareMethodOverride(MethodOverride mo) throws BeanDefinitionValidationException {//获取对应类中对应方法的个数int count = ClassUtils.getMethodCountForName(getBeanClass(), mo.getMethodName());if (count == 0) {throw new BeanDefinitionValidationException("Invalid method override: no method with name '" + mo.getMethodName() +"' on class [" + getBeanClassName() + "]");}else if (count == 1) {// Mark override as not overloaded, to avoid the overhead of arg type checking.mo.setOverloaded(false);}}

3)        应用初始化前的后处理器，解析指定bean是否存在初始化前的短路操作

protected Object resolveBeforeInstantiation(String beanName, RootBeanDefinition mbd) {Object bean = null;if (!Boolean.FALSE.equals(mbd.beforeInstantiationResolved)) {// Make sure bean class is actually resolved at this point.if (!mbd.isSynthetic() && hasInstantiationAwareBeanPostProcessors()) {Class<?> targetType = determineTargetType(beanName, mbd);if (targetType != null) {bean = applyBeanPostProcessorsBeforeInstantiation(targetType, beanName);if (bean != null) {bean = applyBeanPostProcessorsAfterInitialization(bean, beanName);}}}mbd.beforeInstantiationResolved = (bean != null);}return bean;}

上述方法，我们关心的是applyBeanPostProcessorsAfterInitialization的方法，下面给出其代码。

public Object applyBeanPostProcessorsAfterInitialization(Object existingBean, String beanName)throws BeansException {Object result = existingBean;for (BeanPostProcessor beanProcessor : getBeanPostProcessors()) {result = beanProcessor.postProcessAfterInitialization(result, beanName);if (result == null) {return result;}}return result;}

4)        创建bean