MyBatis中一个sqlSessionFactory代表一个数据源，那么配置多个数据源只需要注入多个sqlSessionFactory即可。

首先需要说明的是，用mybatis-spring-1.1.0貌似无法配置多个数据源（这里说的不对，应该是无法在配置数据源中使用${..}占位符），这里大概折腾了我一整天的时间。后来才想到可能是版本问题，于是换了最新的1.2.2版，问题就迎刃而解了。

下面记录一下分析这个问题的过程：

首先我在Spring的配置文件中配置了org.springframework.beans.factory.config.PropertyPlaceholderConfigurer，这个Bean中的作为一个BeanFactoryPostProcessor会在载入所有BeanDefinition后运行，然后利用指定的properties文件来替换BeanDefinition中定义的${...}占位符，Spring的配置文件如下：

<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns="http://www.springframework.org/schema/beans"
	xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:p="http://www.springframework.org/schema/p"
	xsi:schemaLocation="http://www.springframework.org/schema/beans 
		http://www.springframework.org/schema/beans/spring-beans-3.0.xsd">

	<bean id="pmsDatasource" class="org.apache.ibatis.datasource.pooled.PooledDataSource">
		<property name="driver" value=http://www.mamicode.com/"${pms.driver}" />>
但是启动容器后，却报错：Cannot find class: ${kft.driver}
因为记得几个月前也弄过这个，但是没成功，后来我就改成不读properties文件，而是直接写在xml中了，算是一种逃避吧
不过这次，我不打算放过它。问了度娘，上面铺天盖地的都是说不要注入MapperScannerConfigurer的sqlSessionFactory属性，而应该使用sqlSessionFactoryBeanName。可问题是我这里已经使用sqlSessionFactoryBeanName，显然不是这个问题。可是度娘上只有这么一个答案，真是略坑啊~难道没人用1.0.0的版本？好吧，那么我做第一人，若以后有人碰到诸如此类的问题，就不用像我搞这么久了~
虽然度娘上没给出解决答案，但是分析的还是有道理的：说是因为MapperScannerConfigurer初始化的时候同时也初始化了SqlSessionFactoryBean，并且由于MapperScannerConfigurer要先于PropertyPlaceholderConfigurer初始化，那么此时对于datasource中配置的占位符无法被替换，所以也就导致出现了上面的错误。
根据上面的思路，跟踪MapperScannerConfigurer的源码看看：
  public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry beanDefinitionRegistry) throws BeansException {
    processPropertyPlaceHolders();

    Scanner scanner = new Scanner(beanDefinitionRegistry);
    scanner.setResourceLoader(this.applicationContext);

    scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
  }
发现MapperScannerConfigurer实现了BeanDefinitionRegistryPostProcessor接口，这类Bean会在BeanDefinition被全部载入后，BeanPostProcessor前初始化。并且在初始化时，会执行postProcessBeanDefinitionRegistry(BeanDefinitionRegistry beanDefinitionRegistry)方法。
其中后面几行我们看的明白，是自动扫描Mapper的。而第一行执行了processPropertyPlaceHolders()方法，看了代码注释，这个方法就是设计出来防止MapperScannerConfigurer先于PropertyPlaceholderConfigurer初始化而无法转换${..}的。但是为什么没起作用呢？我们来看看源码：
  private void processPropertyPlaceHolders() {
    Map<String, PropertyResourceConfigurer> prcs = applicationContext.getBeansOfType(PropertyResourceConfigurer.class);

    if (!prcs.isEmpty() && applicationContext instanceof GenericApplicationContext) {
      BeanDefinition mapperScannerBean = ((GenericApplicationContext) applicationContext)
          .getBeanFactory().getBeanDefinition(beanName);

      // PropertyResourceConfigurer does not expose any methods to explicitly perform
      // property placeholder substitution. Instead, create a BeanFactory that just
      // contains this mapper scanner and post process the factory.
      DefaultListableBeanFactory factory = new DefaultListableBeanFactory();
      factory.registerBeanDefinition(beanName, mapperScannerBean);

      for (PropertyResourceConfigurer prc : prcs.values()) {
        prc.postProcessBeanFactory(factory);
      }

      PropertyValues values = mapperScannerBean.getPropertyValues();

      this.basePackage = updatePropertyValue("basePackage", values);
      this.sqlSessionFactoryBeanName = updatePropertyValue("sqlSessionFactoryBeanName", values);
      this.sqlSessionTemplateBeanName = updatePropertyValue("sqlSessionTemplateBeanName", values);
    }
  }
首先这个方法不管从名字或者意义上来看，我们都不难才想到他是用来得到所有定义的PropetyResourceConfigurer类。
其次，注意到上面的条件了吗？这里需要applicationContext是GenericApplicationContext才起作用，而我们这里的applicationContext的实现类是XmlWebApplicationContext，所以这一步完全不起作用（不对，应该说还起了副作用，因为上面的报错就是和这个方法有关）可以找到这个方法最终其实是交给DefaultListableBeanFactory的getBeansOfType(...)来实现的：
	public <T> Map<String, T> getBeansOfType(Class<T> type, boolean includeNonSingletons, boolean allowEagerInit)
			throws BeansException {

		String[] beanNames = getBeanNamesForType(type, includeNonSingletons, allowEagerInit);
		Map<String, T> result = new LinkedHashMap<String, T>(beanNames.length);
		for (String beanName : beanNames) {
			try {
				result.put(beanName, getBean(beanName, type));
			}
			catch (BeanCreationException ex) {
				Throwable rootCause = ex.getMostSpecificCause();
				if (rootCause instanceof BeanCurrentlyInCreationException) {
					BeanCreationException bce = (BeanCreationException) rootCause;
					if (isCurrentlyInCreation(bce.getBeanName())) {
						if (this.logger.isDebugEnabled()) {
							this.logger.debug("Ignoring match to currently created bean '" + beanName + "': " +
									ex.getMessage());
						}
						onSuppressedException(ex);
						// Ignore: indicates a circular reference when autowiring constructors.
						// We want to find matches other than the currently created bean itself.
						continue;
					}
				}
				throw ex;
			}
		}
		return result;
	}
这里先去寻找相应type的beanName，继续跟踪getBeanNameForType(...)：
	public String[] getBeanNamesForType(Class<?> type, boolean includeNonSingletons, boolean allowEagerInit) {
		if (type == null || !allowEagerInit) {
			return this.doGetBeanNamesForType(type, includeNonSingletons, allowEagerInit);
		}
		Map<Class<?>, String[]> cache = includeNonSingletons ?
				this.nonSingletonBeanNamesByType : this.singletonBeanNamesByType;
		String[] resolvedBeanNames = cache.get(type);
		if (resolvedBeanNames != null) {
			return resolvedBeanNames;
		}
		resolvedBeanNames = this.doGetBeanNamesForType(type, includeNonSingletons, allowEagerInit);
		cache.put(type, resolvedBeanNames);
		return resolvedBeanNames;
	}
后两个boolean参数在这里都是true，且在cache中没有该bean，继续跟踪doGetBeanNamesForType(...)
	private String[] doGetBeanNamesForType(Class<?> type, boolean includeNonSingletons, boolean allowEagerInit) {
		List<String> result = new ArrayList<String>();

		// Check all bean definitions.
		String[] beanDefinitionNames = getBeanDefinitionNames();
		for (String beanName : beanDefinitionNames) {
			// Only consider bean as eligible if the bean name
			// is not defined as alias for some other bean.
			if (!isAlias(beanName)) {
				try {
					RootBeanDefinition mbd = getMergedLocalBeanDefinition(beanName);
					// Only check bean definition if it is complete.
					if (!mbd.isAbstract() && (allowEagerInit ||
							((mbd.hasBeanClass() || !mbd.isLazyInit() || this.allowEagerClassLoading)) &&
									!requiresEagerInitForType(mbd.getFactoryBeanName()))) {
						// In case of FactoryBean, match object created by FactoryBean.
						boolean isFactoryBean = isFactoryBean(beanName, mbd);
						boolean matchFound = (allowEagerInit || !isFactoryBean || containsSingleton(beanName)) &&
								(includeNonSingletons || isSingleton(beanName)) && isTypeMatch(beanName, type);
						if (!matchFound && isFactoryBean) {
							// In case of FactoryBean, try to match FactoryBean instance itself next.
							beanName = FACTORY_BEAN_PREFIX + beanName;
							matchFound = (includeNonSingletons || mbd.isSingleton()) && isTypeMatch(beanName, type);
						}
						if (matchFound) {
							result.add(beanName);
						}
					}
				}
				catch (CannotLoadBeanClassException ex) {
					if (allowEagerInit) {
						throw ex;
					}
					// Probably contains a placeholder: let's ignore it for type matching purposes.
					if (this.logger.isDebugEnabled()) {
						this.logger.debug("Ignoring bean class loading failure for bean '" + beanName + "'", ex);
					}
					onSuppressedException(ex);
				}
				catch (BeanDefinitionStoreException ex) {
					if (allowEagerInit) {
						throw ex;
					}
					// Probably contains a placeholder: let's ignore it for type matching purposes.
					if (this.logger.isDebugEnabled()) {
						this.logger.debug("Ignoring unresolvable metadata in bean definition '" + beanName + "'", ex);
					}
					onSuppressedException(ex);
				}
			}
		}

		// Check singletons too, to catch manually registered singletons.
		String[] singletonNames = getSingletonNames();
		for (String beanName : singletonNames) {
			// Only check if manually registered.
			if (!containsBeanDefinition(beanName)) {
				// In case of FactoryBean, match object created by FactoryBean.
				if (isFactoryBean(beanName)) {
					if ((includeNonSingletons || isSingleton(beanName)) && isTypeMatch(beanName, type)) {
						result.add(beanName);
						// Match found for this bean: do not match FactoryBean itself anymore.
						continue;
					}
					// In case of FactoryBean, try to match FactoryBean itself next.
					beanName = FACTORY_BEAN_PREFIX + beanName;
				}
				// Match raw bean instance (might be raw FactoryBean).
				if (isTypeMatch(beanName, type)) {
					result.add(beanName);
				}
			}
		}

		return StringUtils.toStringArray(result);
	}
可以看到要得到 指定类型的类，其实最后是通过遍历所有的beanDefinition来的。其中判断type是否一致是用isTypeMatch(...)
	public boolean isTypeMatch(String name, Class<?> targetType) throws NoSuchBeanDefinitionException {
		String beanName = transformedBeanName(name);
		Class<?> typeToMatch = (targetType != null ? targetType : Object.class);

		// Check manually registered singletons.
		Object beanInstance = getSingleton(beanName, false);
		if (beanInstance != null) {
<pre name="code" class="java">			<span style="font-family: Arial, Helvetica, sans-serif;">//省略部分代码.....</span>
 else {
			<span style="font-family: Arial, Helvetica, sans-serif;">//省略部分代码.....</span>
// Check bean class whether we‘re dealing with a FactoryBean.if (FactoryBean.class.isAssignableFrom(beanClass)) {if (!BeanFactoryUtils.isFactoryDereference(name)) {// If it‘s a FactoryBean, we want to look at what it creates, not the factory class.Class<?> type = getTypeForFactoryBean(beanName, mbd);return (type != null && typeToMatch.isAssignableFrom(type));}else {return typeToMatch.isAssignableFrom(beanClass);}}else {return !BeanFactoryUtils.isFactoryDereference(name) &&typeToMatch.isAssignableFrom(beanClass);}}}
在我模拟的时候发现在mapper类进来的时候会造成sqlSessionFactory的初始化，这是为什么呢？
由于mapper被MyBatis代理了，mapper定义的时候是用下面这样的形式：
<bean id="userMapper" class="org.mybatis.spring.mapper.MapperFactoryBean">
  <property name="mapperInterface" value=http://www.mamicode.com/"org.mybatis.spring.sample.mapper.UserMapper" />>
所以当要获取userMapper的类型时，由于判断其是FactoryBean，且不是Dereference(不是以‘&‘开头去取FactoryBean的类型)，就去执行getTypeForFactoryBean(beanName, mbd)这个方法：
	protected Class<?> getTypeForFactoryBean(String beanName, RootBeanDefinition mbd) {
		if (!mbd.isSingleton()) {
			return null;
		}
		try {
			FactoryBean<?> factoryBean = doGetBean(FACTORY_BEAN_PREFIX + beanName, FactoryBean.class, null, true);
			return getTypeForFactoryBean(factoryBean);
		}
		catch (BeanCreationException ex) {
			// Can only happen when getting a FactoryBean.
			if (logger.isDebugEnabled()) {
				logger.debug("Ignoring bean creation exception on FactoryBean type check: " + ex);
			}
			onSuppressedException(ex);
			return null;
		}
	}
继续跟进doGetBean(...)方法查看：
	protected <T> T doGetBean(
			final String name, final Class<T> requiredType, final Object[] args, boolean typeCheckOnly)
			throws BeansException {

<pre name="code" class="java">			<span style="font-family: Arial, Helvetica, sans-serif;">//省略部分代码.....</span>
 }else { //省略部分代码.....// Create bean instance.if (mbd.isSingleton()) {// 总算找到了，这里是会去创建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);}//省略部分代码.....}
			<span style="font-family: Arial, Helvetica, sans-serif;">//省略部分代码.....</span>
 return (T) bean;}
所以关键原因是因为要得到FactoryBean的类型（其实是得到创建的Object的类型），那么先要去初始化它再通过getObjectType方法来得到其类型。于是就造成了sqlSessionFactory提前初始化了！另外，分析过程应该是倒推的，而不是正推~
而看了下mybatis-spring-1.2.2的源码，发现MapperScannerConfigurer初始化中的processPropertyPlaceHolders()方法加上了一个条件。也就是说，默认是不执行的：
 public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) throws BeansException {
    if (this.processPropertyPlaceHolders) {
      processPropertyPlaceHolders();
    }

    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
    scanner.setAddToConfig(this.addToConfig);
    scanner.setAnnotationClass(this.annotationClass);
    scanner.setMarkerInterface(this.markerInterface);
    scanner.setSqlSessionFactory(this.sqlSessionFactory);
    scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
    scanner.setSqlSessionFactoryBeanName(this.sqlSessionFactoryBeanName);
    scanner.setSqlSessionTemplateBeanName(this.sqlSessionTemplateBeanName);
    scanner.setResourceLoader(this.applicationContext);
    scanner.setBeanNameGenerator(this.nameGenerator);
    scanner.registerFilters();
    scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
  }
并且可以看到，它提供了更多配置项用于自动发现Mapper。


Cannot find class: ${jdbc.driver}——配置了sqlSessionFactoryBeanName也报错之问题分析