它们的定义PropertyPlaceHolder无法完成更换任务

Spring默认PropertyPlaceholderConfigurer只能加载properties格风格简介，现在，我们需要能够从类的完整支持允许似hadoop格风格xml配置文件读取配置信息，并更换相关bean占位符，对其进行了扩展，详细扩展例如以下：

public class CustomPropertyPlaceholderConfigurer extends PropertyPlaceholderConfigurer {
    private Resource[] locations;

    public void loadProperties(Properties props) throws IOException {
        if (this.locations != null) {
            PropertiesPersister propertiesPersister = new DefaultPropertiesPersister();
            for (int i = 0; i < this.locations.length; i++) {
                Resource location = this.locations[i];
                if (logger.isInfoEnabled()) {
                    logger.info("Loading properties file from " + location);
                }
                InputStream is = null;
                try {
                    is = location.getInputStream();
                    propertiesPersister.load(props, is);

                    Configuration conf = SquirrelConfiguration.create();
                    Map<String, String> map = conf.listAllConfEntry();  // 从squirrel-site.xml中读取配置信息
                    for (Map.Entry<String, String> entry : map.entrySet()) {
                        props.put(entry.getKey(), entry.getValue());
                    }
                } finally {
                    if (is != null) is.close();
                }
            }
        }
    }

    public void setLocations(Resource[] locations) {
        super.setLocations(locations);
        this.locations = locations;
    }

}

可是执行却一直报错，${jdbc_driver_name}没有被替换。通过查询发现。原来是在spring里使用org.mybatis.spring.mapper.MapperScannerConfigurer 进行自己主动扫描的时候。设置了sqlSessionFactory 的话。可能会导致PropertyPlaceholderConfigurer失效，也就是用${jdbc_driver_name}这样之类的表达式，将无法获取到properties文件中的内容。导致这一原因是由于。MapperScannerConigurer实际是在解析载入bean定义阶段的，这个时候要是设置sqlSessionFactory的话，会导致提前初始化一些类。这个时候，
PropertyPlaceholderConfigurer还没来得及替换定义中的变量。导致把表达式当作字符串复制了。但假设不设置sqlSessionFactory 属性的话，就必需要保证sessionFactory在spring中名称一定要是sqlSessionFactory 。否则就无法自己主动注入。又或者直接定义MapperFactoryBean 。再或者放弃自己主动代理接口方式。

能够例如以下方式改动：

<bean id="propertyConfigurer"  class="com.yowu.common.CustomPropertyPlaceholderConfigurer">
        <property name="locations">
            <list>
                <value>classpath:important.properties</value>
            </list>
        </property>
     </bean>

	<!-- 配置线程池 -->
	<bean id="taskExecutor"
		class="org.springframework.scheduling.concurrent.ThreadPoolTaskExecutor">
		<!-- 线程池维护线程的最少数量 -->
		<property name="corePoolSize" value="10" />
		<!-- 线程池维护线程所同意的空暇时间 -->
		<property name="keepAliveSeconds" value="0" />
		<!-- 线程池维护线程的最大数量 -->
		<property name="maxPoolSize" value="10" />
		<!-- 线程池所使用的缓冲队列 -->
		<property name="queueCapacity" value="0" />
	</bean>

	<bean id="dataSource" class="org.apache.commons.dbcp.BasicDataSource">
		<property name="driverClassName" value="${squirrel_jdbc_driver}" />
		<property name="url" value="${squirrel_jdbc_url}" />
		<property name="username" value="${squirrel_jdbc_username}" />
		<property name="password" value="${squirrel_jdbc_password}" />
		<property name="validationQuery" value="select 1" />
		<property name="initialSize" value="5" />
		<property name="testWhileIdle" value="true" />
		<property name="maxIdle" value="20" />
		<property name="minIdle" value="5" />
		<property name="maxActive" value="50" />
		<property name="removeAbandonedTimeout" value="180" />
		<property name="maxWait" value="30000" />
	</bean>

    <bean id="ysqlSessionFactory" class="org.mybatis.spring.SqlSessionFactoryBean">
        <property name="dataSource" ref="dataSource"/>
        <property name="mapperLocations" >
            <list>
                <value>classpath*:mybatis/*.xml</value>
            </list>
        </property>
    </bean>
    <bean id="sqlSession" class="org.mybatis.spring.SqlSessionTemplate">
        <constructor-arg index="0" ref="ysqlSessionFactory"></constructor-arg>
        <constructor-arg index="1" value="BATCH"></constructor-arg>
    </bean>

    <bean class="org.mybatis.spring.mapper.MapperScannerConfigurer">
        <property name="basePackage" value="com.yowu.dao, com.yowu.app.repository.dao"/>
        <!--核心就是加入以下一句。
后面那个属性是value。不是ref，切记-->
        <property name="sqlSessionFactoryBeanName" value="ysqlSessionFactory" />
    </bean>

通过查看MapperScannerConfigurer源代码发现，事实上有这么一段代码：

public void postProcessBeanDefinitionRegistry(BeanDefinitionRegistry beanDefinitionRegistry) throws BeansException {
    if (this.processPropertyPlaceHolders) {
      processPropertyPlaceHolders();
    }

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

    scanner.scan(StringUtils.tokenizeToStringArray(this.basePackage, ConfigurableApplicationContext.CONFIG_LOCATION_DELIMITERS));
  }

  /*
   * BeanDefinitionRegistries are called early in application startup, before
   * BeanFactoryPostProcessors. This means that PropertyResourceConfigurers will not have been
   * loaded and any property substitution of this class' properties will fail. To avoid this, find
   * any PropertyResourceConfigurers defined in the context and run them on this class' bean
   * definition. Then update the values.
   */
  private void processPropertyPlaceHolders() {

大概意思是能够设置processPropertyPlaceHolders为true，强制让PropertyResourceConfigure运行下替换工作。大家最好还是试一下，不一个优雅的解决方案亏损。