spring扩展点之PropertyPlaceholderConfigurer

背景

在使用Spring框架的开发过程中肯定少不了项目的配置文件，通常是Java的Properties文件。并且会通过类PropertyPlaceholderConfigurer在运行时动态替换xml配置文件中的Bean配置项。

先看例子

<bean id="propertiesConfig" class="org.springframework.beans.factory.config.PropertyPlaceholderConfigurer">
   <property name="ignoreUnresolvablePlaceholders" value="true"/>
   <property name="locations">
       <list>
           <value>classpath*:config.properties</value>
       </list>
   </property>
</bean>

原理解析

PropertyPlaceholderConfigurer类是如何做到xml配置的bean的配置熟悉的运行时替换的呢？这个离不开Spring框架提供的许多扩展点。这个类依赖的扩展点就是BeanFactoryPostProcessors见下面的类继承图：

BeanFactoryPostProcessors类有一个方法，提供了在Bean创建前对Bean的Definition进行处理的钩子机制。

//在Bean初始化前被调用（例如在InitializingBean的afterPropertiesSet前）
void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException;

从上面的继承图我们知道PropertyPlaceholderConfigurer继承自类PropertyResourceConfigurer, 而该类实现了接口BeanFactoryPostProcessors

public abstract class PropertyResourceConfigurer extends PropertiesLoaderSupport
		implements BeanFactoryPostProcessor, PriorityOrdered {
@Override
truepublic void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
truetruetry {
truetruetrueProperties mergedProps = mergeProperties();

truetruetrue// Convert the merged properties, if necessary.
truetruetrueconvertProperties(mergedProps);

truetruetrue// Let the subclass process the properties.
truetruetrueprocessProperties(beanFactory, mergedProps);
truetrue}
truetruecatch (IOException ex) {
truetruetruethrow new BeanInitializationException("Could not load properties", ex);
truetrue}
true}
trueprotected abstract void processProperties(ConfigurableListableBeanFactory beanFactory, Properties props)
			throws BeansException;
}

我们的主角PropertyPlaceholderConfigurer实现了父类中的方法processProperties, 在该类中对xml配置文件中的展位符进行替换处理

/**
 * Visit each bean definition in the given bean factory and attempt to replace ${...} property
 * placeholders with values from the given properties.
 */
@Override
protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess, Properties props)
        throws BeansException {

    StringValueResolver valueResolver = new PlaceholderResolvingStringValueResolver(props);
    doProcessProperties(beanFactoryToProcess, valueResolver);
}

protected void doProcessProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
		StringValueResolver valueResolver) {

trueBeanDefinitionVisitor visitor = new BeanDefinitionVisitor(valueResolver);

trueString[] beanNames = beanFactoryToProcess.getBeanDefinitionNames();
truefor (String curName : beanNames) {
truetrue// Check that we're not parsing our own bean definition,
truetrue// to avoid failing on unresolvable placeholders in properties file locations.
truetrueif (!(curName.equals(this.beanName) && beanFactoryToProcess.equals(this.beanFactory))) {
truetruetrueBeanDefinition bd = beanFactoryToProcess.getBeanDefinition(curName);
truetruetruetry {
truetruetruetruevisitor.visitBeanDefinition(bd);
truetruetrue}
truetruetruecatch (Exception ex) {
truetruetruetruethrow new BeanDefinitionStoreException(bd.getResourceDescription(), curName, ex.getMessage(), ex);
truetruetrue}
truetrue}
true}

true// New in Spring 2.5: resolve placeholders in alias target names and aliases as well.
truebeanFactoryToProcess.resolveAliases(valueResolver);

true// New in Spring 3.0: resolve placeholders in embedded values such as annotation attributes.
truebeanFactoryToProcess.addEmbeddedValueResolver(valueResolver);
}

这样就解开了Spring中xml配置文件中占位符的替换魔法功能。