MyBatis集成到Spring时配置MapperScannerConfigurer出错

问题描述

在web项目中同时集成了spring mvc和mybatis。
将jdbc配置参数独立在外部配置文件中，然后通过<context:property-placeholder>引入。
此时在Spring中注入org.mybatis.spring.mapper.MapperScannerConfigurer，如下所示：

<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
    <property name="basePackage" value="org.chench.test.springmvc.dao" />
    <property name="sqlSessionFactory" ref="sqlSessionFactory" />
</bean>

如果直接配置属性sqlSessionFactory，并设置为指定的sqlSessionFactory对象，那么在启动spring时会报错：

Caused by: java.lang.NumberFormatException: For input string: "${master.acquireIncrement}"

数据源配置中无法正确引用外部文件中配置的jdbc参数。

必须修改为配置属性sqlSessionFactoryBeanName，才能正确引用到对应的jdbc配置参数。

<bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
    <property name="basePackage" value="org.chench.test.springmvc.dao" />
    <property name="sqlSessionFactoryBeanName" value="sqlSessionFactory" />
</bean>

这是什么原因呢？

原因分析及解决方案

实际上，这是MyBatis早期版本的一个BUG，详见：https://github.com/mybatis/old-google-code-issues/issues/414 。
而且，在最新版本的MyBatis中，同样不再推荐使用设置Bean属性的方式，而是通过设置Value属性。

  /**
   * Specifies which {@code SqlSessionFactory} to use in the case that there is
   * more than one in the spring context. Usually this is only needed when you
   * have more than one datasource.
   * <p>
   * @deprecated Use {@link #setSqlSessionFactoryBeanName(String)} instead.
   *
   * @param sqlSessionFactory
   */
   // 这是一个声明为废弃的方法，不推荐使用
  @Deprecated
  public void setSqlSessionFactory(SqlSessionFactory sqlSessionFactory) {
    this.sqlSessionFactory = sqlSessionFactory;
  }
  
  /**
   * Specifies which {@code SqlSessionFactory} to use in the case that there is
   * more than one in the spring context. Usually this is only needed when you
   * have more than one datasource.
   * <p>
   * Note bean names are used, not bean references. This is because the scanner
   * loads early during the start process and it is too early to build mybatis
   * object instances.
   *
   * @since 1.1.0
   *
   * @param sqlSessionFactoryName Bean name of the {@code SqlSessionFactory}
   */
  // 解释了为什么应该使用设置Value的方式而不是引用Bean的原因
  public void setSqlSessionFactoryBeanName(String sqlSessionFactoryName) {
    this.sqlSessionFactoryBeanName = sqlSessionFactoryName;
  }

实际上，通常情况下无需手动为MapperScannerConfigurer注册SqlSessionFactory，这个工作由Spring框架完成，因为MapperScannerConfigurer已经实现了接口BeanDefinitionRegistryPostProcessor。

public class MapperScannerConfigurer implements BeanDefinitionRegistryPostProcessor, InitializingBean, ApplicationContextAware, BeanNameAware {
	@Override
  public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry registry) {
    if (this.processPropertyPlaceHolders) {
      processPropertyPlaceHolders();
    }

    ClassPathMapperScanner scanner = new ClassPathMapperScanner(registry);
    scanner.setAddToConfig(this.addToConfig);
    scanner.setAnnotationClass(this.annotationClass);
    scanner.setMarkerInterface(this.markerInterface);
    // 由Spring框架自动注入SqlSessionFactory
    scanner.setSqlSessionFactory(this.sqlSessionFactory);
    scanner.setSqlSessionTemplate(this.sqlSessionTemplate);
    // 由Spring框架自注入sqlSessionFactoryBeanName
    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));
  }
}