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