最近在弄Spring3 + Mybatis的配置时，遇到了一些以前一直没有发现的问题：在配置数据源的时候使用了动态配置参数，这些参数是在外部的属性文件里定义的。但是在启动Web容器的时候出现了问题，说是无法识别那些动态参数。以下是异常原文：

org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'org.mybatis.spring.mapper.MapperScannerConfigurer#0' defined in file [E:\sinoiov-workspace\.metadata\.plugins\org.eclipse.wst.server.core\tmp1\wtpwebapps\goods-service\WEB-INF\classes\applicationContext.xml]: Initialization of bean failed; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'sqlSessionFactory' defined in file [E:\sinoiov-workspace\.metadata\.plugins\org.eclipse.wst.server.core\tmp1\wtpwebapps\goods-service\WEB-INF\classes\applicationContext.xml]: Cannot resolve reference to bean 'dataSource' while setting bean property 'dataSource'; nested exception is org.springframework.beans.factory.BeanCreationException: Error creating bean with name 'dataSource' defined in file [E:\sinoiov-workspace\.metadata\.plugins\org.eclipse.wst.server.core\tmp1\wtpwebapps\goods-service\WEB-INF\classes\applicationContext.xml]: Initialization of bean failed; nested exception is org.springframework.beans.TypeMismatchException: Failed to convert property value of type 'java.lang.String' to required type 'int' for property 'initialPoolSize'; nested exception is java.lang.NumberFormatException: For input string: "${jdbc.initialPoolSize}"

以下是我们配置外部参数定义文件的地方：

<context:property-placeholder
        location="file:///${catalina.home}/etc/jdbc.properties"/>

以下是我们配置MybatisMapperScanner的地方：

<!-- DataSource -->
    <bean id="dataSource" class="com.mchange.v2.c3p0.ComboPooledDataSource" destroy-method="close">
        <property name="driverClass" value="${jdbc.driverClassName}" />
        <property name="jdbcUrl" value="${jdbc.url}" />
        <property name="user" value="${jdbc.username}" />
        <property name="password" value="${jdbc.password}" />
        <property name="initialPoolSize" value="${jdbc.initialPoolSize}" />
        <property name="minPoolSize" value="${jdbc.minPoolSize}" />
        <property name="maxPoolSize" value="${jdbc.maxPoolSize}" />
        <property name="maxIdleTime" value="${jdbc.maxIdleTime}" />
        <property name="maxStatements" value="${jdbc.maxStatements}" />
    </bean>
    <bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <property name="dataSource" ref="dataSource" />
        <property name="mapperLocations">
            <value>classpath*:com/***/***/dao/mapper/xml/**/*.xml</value>
        </property>
    </bean>
    <!-- 扫描包下的Mapper ,生成 beans -->
    <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
        <property name="basePackage" value="com.***.***.dao.mapper" />
    </bean>

从异常中的信息判断，是因为无法识别"${jdbc.initialPoolSize}"这个动态参数。可是我们利用其它方法是可以打印出这个参数值的，所以唯一的判断是：MapperScannerConfigurer的加载顺序要早于PropertyPlaceholderConfigurer，并且我们通过跟踪源码发现，在MapperScannerConfigurer初始化后，立即去初始化dataSource，而在这时${}里面的变量值还没有来得及替换成真实值呢。这就是导致问题的最主要原因。

后来我们又对比了Spring3与Spring4的源码，发现了问题的根本所在，下面的Spring3的源码：

// Invoke BeanDefinitionRegistryPostProcessors first, if any.
Set<String> processedBeans = new HashSet<String>();
if (beanFactory instanceof BeanDefinitionRegistry) {
    BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
    List<BeanFactoryPostProcessor> regularPostProcessors = new LinkedList<BeanFactoryPostProcessor>();
    List<BeanDefinitionRegistryPostProcessor> registryPostProcessors =
            new LinkedList<BeanDefinitionRegistryPostProcessor>();
    for (BeanFactoryPostProcessor postProcessor : getBeanFactoryPostProcessors()) {
        if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
            BeanDefinitionRegistryPostProcessor registryPostProcessor =
                    (BeanDefinitionRegistryPostProcessor) postProcessor;
            registryPostProcessor.postProcessBeanDefinitionRegistry(registry);
            registryPostProcessors.add(registryPostProcessor);
        }
        else {
            regularPostProcessors.add(postProcessor);
        }
    }
    Map<String, BeanDefinitionRegistryPostProcessor> beanMap =
            beanFactory.getBeansOfType(BeanDefinitionRegistryPostProcessor.class, true, false);
    List<BeanDefinitionRegistryPostProcessor> registryPostProcessorBeans =
            new ArrayList<BeanDefinitionRegistryPostProcessor>(beanMap.values());
    OrderComparator.sort(registryPostProcessorBeans);
    for (BeanDefinitionRegistryPostProcessor postProcessor : registryPostProcessorBeans) {
        postProcessor.postProcessBeanDefinitionRegistry(registry);
    }
    invokeBeanFactoryPostProcessors(registryPostProcessors, beanFactory);
    invokeBeanFactoryPostProcessors(registryPostProcessorBeans, beanFactory);
    invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
    processedBeans.addAll(beanMap.keySet());
}
else {
    // Invoke factory processors registered with the context instance.
    invokeBeanFactoryPostProcessors(getBeanFactoryPostProcessors(), beanFactory);
}
// Do not initialize FactoryBeans here: We need to leave all regular beans
// uninitialized to let the bean factory post-processors apply to them!
String[] postProcessorNames =
        beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);
// Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
// Ordered, and the rest.
List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
List<String> orderedPostProcessorNames = new ArrayList<String>();
List<String> nonOrderedPostProcessorNames = new ArrayList<String>();
for (String ppName : postProcessorNames) {
    if (processedBeans.contains(ppName)) {
        // skip - already processed in first phase above
    }
    else if (isTypeMatch(ppName, PriorityOrdered.class)) {
        priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
    }
    else if (isTypeMatch(ppName, Ordered.class)) {
        orderedPostProcessorNames.add(ppName);
    }
    else {
        nonOrderedPostProcessorNames.add(ppName);
    }
}
// First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
OrderComparator.sort(priorityOrderedPostProcessors);
invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);
// Next, invoke the BeanFactoryPostProcessors that implement Ordered.
List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
for (String postProcessorName : orderedPostProcessorNames) {
    orderedPostProcessors.add(getBean(postProcessorName, BeanFactoryPostProcessor.class));
}
OrderComparator.sort(orderedPostProcessors);
invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);
// Finally, invoke all other BeanFactoryPostProcessors.
List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
for (String postProcessorName : nonOrderedPostProcessorNames) {
    nonOrderedPostProcessors.add(getBean(postProcessorName, BeanFactoryPostProcessor.class));
}
invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

下面是Spring4的源码：

// Invoke BeanDefinitionRegistryPostProcessors first, if any.
Set<String> processedBeans = new HashSet<String>();
if (beanFactory instanceof BeanDefinitionRegistry) {
    BeanDefinitionRegistry registry = (BeanDefinitionRegistry) beanFactory;
    List<BeanFactoryPostProcessor> regularPostProcessors = new LinkedList<BeanFactoryPostProcessor>();
    List<BeanDefinitionRegistryPostProcessor> registryPostProcessors =
            new LinkedList<BeanDefinitionRegistryPostProcessor>();
    for (BeanFactoryPostProcessor postProcessor : beanFactoryPostProcessors) {
        if (postProcessor instanceof BeanDefinitionRegistryPostProcessor) {
            BeanDefinitionRegistryPostProcessor registryPostProcessor =
                    (BeanDefinitionRegistryPostProcessor) postProcessor;
            registryPostProcessor.postProcessBeanDefinitionRegistry(registry);
            registryPostProcessors.add(registryPostProcessor);
        }
        else {
            regularPostProcessors.add(postProcessor);
        }
    }
    // Do not initialize FactoryBeans here: We need to leave all regular beans
    // uninitialized to let the bean factory post-processors apply to them!
    // Separate between BeanDefinitionRegistryPostProcessors that implement
    // PriorityOrdered, Ordered, and the rest.
    String[] postProcessorNames =
            beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
    // First, invoke the BeanDefinitionRegistryPostProcessors that implement PriorityOrdered.
    List<BeanDefinitionRegistryPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanDefinitionRegistryPostProcessor>();
    for (String ppName : postProcessorNames) {
        if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
            priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
            processedBeans.add(ppName);
        }
    }
    OrderComparator.sort(priorityOrderedPostProcessors);
    registryPostProcessors.addAll(priorityOrderedPostProcessors);
    invokeBeanDefinitionRegistryPostProcessors(priorityOrderedPostProcessors, registry);
    // Next, invoke the BeanDefinitionRegistryPostProcessors that implement Ordered.
    postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
    List<BeanDefinitionRegistryPostProcessor> orderedPostProcessors = new ArrayList<BeanDefinitionRegistryPostProcessor>();
    for (String ppName : postProcessorNames) {
        if (!processedBeans.contains(ppName) && beanFactory.isTypeMatch(ppName, Ordered.class)) {
            orderedPostProcessors.add(beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class));
            processedBeans.add(ppName);
        }
    }
    OrderComparator.sort(orderedPostProcessors);
    registryPostProcessors.addAll(orderedPostProcessors);
    invokeBeanDefinitionRegistryPostProcessors(orderedPostProcessors, registry);
    // Finally, invoke all other BeanDefinitionRegistryPostProcessors until no further ones appear.
    boolean reiterate = true;
    while (reiterate) {
        reiterate = false;
        postProcessorNames = beanFactory.getBeanNamesForType(BeanDefinitionRegistryPostProcessor.class, true, false);
        for (String ppName : postProcessorNames) {
            if (!processedBeans.contains(ppName)) {
                BeanDefinitionRegistryPostProcessor pp = beanFactory.getBean(ppName, BeanDefinitionRegistryPostProcessor.class);
                registryPostProcessors.add(pp);
                processedBeans.add(ppName);
                pp.postProcessBeanDefinitionRegistry(registry);
                reiterate = true;
            }
        }
    }
    // Now, invoke the postProcessBeanFactory callback of all processors handled so far.
    invokeBeanFactoryPostProcessors(registryPostProcessors, beanFactory);
    invokeBeanFactoryPostProcessors(regularPostProcessors, beanFactory);
}
else {
    // Invoke factory processors registered with the context instance.
    invokeBeanFactoryPostProcessors(beanFactoryPostProcessors, beanFactory);
}
// Do not initialize FactoryBeans here: We need to leave all regular beans
// uninitialized to let the bean factory post-processors apply to them!
String[] postProcessorNames =
        beanFactory.getBeanNamesForType(BeanFactoryPostProcessor.class, true, false);
// Separate between BeanFactoryPostProcessors that implement PriorityOrdered,
// Ordered, and the rest.
List<BeanFactoryPostProcessor> priorityOrderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
List<String> orderedPostProcessorNames = new ArrayList<String>();
List<String> nonOrderedPostProcessorNames = new ArrayList<String>();
for (String ppName : postProcessorNames) {
    if (processedBeans.contains(ppName)) {
        // skip - already processed in first phase above
    }
    else if (beanFactory.isTypeMatch(ppName, PriorityOrdered.class)) {
        priorityOrderedPostProcessors.add(beanFactory.getBean(ppName, BeanFactoryPostProcessor.class));
    }
    else if (beanFactory.isTypeMatch(ppName, Ordered.class)) {
        orderedPostProcessorNames.add(ppName);
    }
    else {
        nonOrderedPostProcessorNames.add(ppName);
    }
}
// First, invoke the BeanFactoryPostProcessors that implement PriorityOrdered.
OrderComparator.sort(priorityOrderedPostProcessors);
invokeBeanFactoryPostProcessors(priorityOrderedPostProcessors, beanFactory);
// Next, invoke the BeanFactoryPostProcessors that implement Ordered.
List<BeanFactoryPostProcessor> orderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
for (String postProcessorName : orderedPostProcessorNames) {
    orderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
}
OrderComparator.sort(orderedPostProcessors);
invokeBeanFactoryPostProcessors(orderedPostProcessors, beanFactory);
// Finally, invoke all other BeanFactoryPostProcessors.
List<BeanFactoryPostProcessor> nonOrderedPostProcessors = new ArrayList<BeanFactoryPostProcessor>();
for (String postProcessorName : nonOrderedPostProcessorNames) {
    nonOrderedPostProcessors.add(beanFactory.getBean(postProcessorName, BeanFactoryPostProcessor.class));
}
invokeBeanFactoryPostProcessors(nonOrderedPostProcessors, beanFactory);

通过以上的源码对比就可以看出，从第19行开始就是在Spring4中对于这种加载顺序混乱问题的处理。因此，在使用Spring + Mybatis时，建议大家尽量使用Spring4以上的版本。