背景
spring和mybatis集成過程中，我們可以通過MapperFactoryBean的方式配置Mapper接口。但是這樣需要在配置文件中，爲每個mapper配置相同的代碼塊，浪費時間。關鍵對於代碼潔癖的人來說，一點不能忍。
<bean id="userMapper" class="org.mybatis.spring.mapper.MapperFactoryBean">
<property name="mapperInterface" value="org.mybatis.spring.sample.mapper.UserMapper" />
<property name="sqlSessionFactory" ref="sqlSessionFactory" />
</bean>
使用MapperScannerConfigurer進行自動轉換Mapper的方式，讓代碼更加簡潔優雅。
但是，當我在項目中，按照文文檔配置bean後，仍舊出現properties文件中jdbc的參數無法正常解析。
- 相關配置文件信息
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns:jaxws="http://cxf.apache.org/jaxws" xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop"
xmlns:tx="http://www.springframework.org/schema/tx" xmlns:jdbc="http://www.springframework.org/schema/jdbc"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="
http://cxf.apache.org/jaxws http://cxf.apache.org/schemas/jaxws.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/jdbc http://www.springframework.org/schema/jdbc/spring-jdbc.xsd
http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd"
default-autowire="byName">
<context:property-placeholder location="conf/*.properties"/>
<bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource" lazy-init="true">
<property name="driverClassName" value="${jdbc.driver}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.user}"/>
<property name="password" value="${jdbc.pwd}"/>
</bean>
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="mapperLocations" value="mapper/*.xml"/>
<property name="dataSource" ref="dataSource"/>
</bean>
<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
<property name="basePackage" value="com.yanmushi.**.mapper"/>
<property name="sqlSessionFactoryBeanName" value="sqlSessionFactory"/>
</bean>
</beans>
PS：請原諒在spring-boot，註解方式盛行的年代，還糾結xml中解決方法。
依賴版本
參考依賴
<dependency>
<groupId>org.springframework</groupId>
<artifactId>spring-context-support</artifactId>
<version>4.1.1-RELEASE</version>
</dependency>
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis</artifactId>
<version>3.2.8</version>
</dependency>
<dependency>
<groupId>org.mybatis</groupId>
<artifactId>mybatis-spring</artifactId>
<version>1.2.2</version>
</dependency>
分析過程
在搜索引擎中，找到了一種解決方法。
原理：使用sqlSessionFactoryBeanName注入，不會立即初始化sqlSessionFactory, 所以不會引發提前初始化問題。
同時還應注意在配置org.mybatis.spring.SqlSessionFactoryBean這個Bean時，id不能爲sqlSessionFactory，如果爲這樣的話會導致MapperScannerConigurer在bean定義加載時，加載PropertyPlaceholderConfigurer還沒來得及替換定義中的變量
以上方法，能夠正常解決問題，但是爲什麼呢？
爲什麼sqlsessionfactorybeanname注入呢？
打開MapperScannerConfigurer，會發現它實現了spring的四個接口：
- BeanDefinitionRegistryPostProcessor
- InitializingBean
- ApplicationContextAware
- BeanNameAware
問題的關鍵在於第一個接口，它是幹什麼的呢？主要是允許代碼像xml一樣，註冊自定義Bean到BeanFactory中。
通過這個接口，MapperScannerConfigurer在spring初始化過程中，將掃描basePackage中的自定義mapper接口轉換爲MapperFactoryBean，並註冊到BeanFactory中。
/**
* Extension to the standard {@link BeanFactoryPostProcessor} SPI, allowing for
* the registration of further bean definitions <i>before</i> regular
* BeanFactoryPostProcessor detection kicks in. In particular,
* BeanDefinitionRegistryPostProcessor may register further bean definitions
* which in turn define BeanFactoryPostProcessor instances.
*/
spring容器本身已經很好到解決了Bean管理問題。爲什麼會導致提前初始化呢？
在AbstractApplicationContext#refresh()方法中，詳細註明類初始化的流程。
@Override
public void refresh() throws BeansException, IllegalStateException {
synchronized (this.startupShutdownMonitor) {
// Prepare this context for refreshing.
prepareRefresh();
// Tell the subclass to refresh the internal bean factory.
ConfigurableListableBeanFactory beanFactory = obtainFreshBeanFactory();
// Prepare the bean factory for use in this context.
prepareBeanFactory(beanFactory);
try {
// Allows post-processing of the bean factory in context subclasses.
postProcessBeanFactory(beanFactory);
// Invoke factory processors registered as beans in the context.
invokeBeanFactoryPostProcessors(beanFactory);
// Register bean processors that intercept bean creation.
registerBeanPostProcessors(beanFactory);
// Initialize message source for this context.
initMessageSource();
// Initialize event multicaster for this context.
initApplicationEventMulticaster();
// Initialize other special beans in specific context subclasses.
onRefresh();
// Check for listener beans and register them.
registerListeners();
// Instantiate all remaining (non-lazy-init) singletons.
finishBeanFactoryInitialization(beanFactory);
// Last step: publish corresponding event.
finishRefresh();
}
catch (BeansException ex) {
logger.warn("Exception encountered during context initialization - cancelling refresh attempt", ex);
// Destroy already created singletons to avoid dangling resources.
destroyBeans();
// Reset 'active' flag.
cancelRefresh(ex);
// Propagate exception to caller.
throw ex;
}
}
}
MapperScannerConfigurer和PropertySourcesPlaceholderConfigurer兩個類，都是BeanFactoryPostProcessor的實現類。但是MapperScannerConfigurer實現了BeanFactoryPostProcessor的子接口，BeanDefinitionRegistryPostProcessor。
在執行invokeBeanFactorPostProcessors(beanFacotry)方法時，是調用到PostProcessorRegistrationDelegate#invokeBeanFactoryPostProcessors(ConfigurableListableBeanFactory, List<BeanFactoryPostProcessor>)方法，在這個方法中，BeanDefinitionRegistryPostProcessor被優先執行。 所以，在執行MapperScannerConfigurer中不能關聯需要讀取properties文件的Bean，因爲Properties沒有被初始化。
public static void invokeBeanFactoryPostProcessors(
ConfigurableListableBeanFactory beanFactory, List<BeanFactoryPostProcessor> beanFactoryPostProcessors) {
// 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);
// 此時去執行：postProcessor.postProcessBeanFactory(beanFactory)方法
invokeBeanDefinitionRegistryPostProcessors(priorityOrderedPostProcessors, registry);
// ……
}
爲什麼我們使用sqlSessionFacotryBeanName，但是依舊會出現properties沒有被引用的問題呢？
這是因爲在spring.xml文件中，beans節點，我們配置了**default-autowire="byName"**屬性。 開啓後，使得BeanFacotry在實例化bean的時候，會自動根據bean名字和屬性名字進行自動匹配。 從而在實例化MapperScannerConfigurer的時候，自動匹配到sqlSessionFactory屬性，觸發sqlSessionFactory的實例化，引發dataSource進行實例化，而此時properties屬性並沒有進行解析。
總結
- 關閉default-autowire屬性；否則，需要更改SqlSessionFactory這個bean的id，避免自動關聯。
- 如果只有一個數據源，MapperScannerConfigurer會自動關聯的；如果有多個，可以通過sqlSessionFactoryBeanName來配置bean的名字。
<?xml version="1.0" encoding="UTF-8"?>
<beans xmlns:jaxws="http://cxf.apache.org/jaxws" xmlns="http://www.springframework.org/schema/beans"
xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:aop="http://www.springframework.org/schema/aop"
xmlns:tx="http://www.springframework.org/schema/tx" xmlns:jdbc="http://www.springframework.org/schema/jdbc"
xmlns:context="http://www.springframework.org/schema/context"
xsi:schemaLocation="
http://cxf.apache.org/jaxws http://cxf.apache.org/schemas/jaxws.xsd
http://www.springframework.org/schema/context http://www.springframework.org/schema/context/spring-context.xsd
http://www.springframework.org/schema/beans http://www.springframework.org/schema/beans/spring-beans.xsd
http://www.springframework.org/schema/jdbc http://www.springframework.org/schema/jdbc/spring-jdbc.xsd
http://www.springframework.org/schema/tx http://www.springframework.org/schema/tx/spring-tx.xsd
http://www.springframework.org/schema/aop http://www.springframework.org/schema/aop/spring-aop.xsd">
<context:property-placeholder location="conf/*.properties"/>
<bean id="dataSource" class="org.springframework.jdbc.datasource.DriverManagerDataSource" lazy-init="true">
<property name="driverClassName" value="${jdbc.driver}"/>
<property name="url" value="${jdbc.url}"/>
<property name="username" value="${jdbc.user}"/>
<property name="password" value="${jdbc.pwd}"/>
</bean>
<bean id="sqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
<property name="mapperLocations" value="mapper/*.xml"/>
<property name="dataSource" ref="dataSource"/>
</bean>
<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
<property name="basePackage" value="com.yanmushi.**.mapper"/>
<!-- <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory"/>-->
</bean>
</beans>