PropertySourcesPlaceholderConfigurer 是 Spring 框架中用于解析属性占位符 (如 ${property.name}) 的类, 它通过读取配置文件 (如 .properties 文件) 并将这些属性值注入到 Spring 的 BeanDefinition 中;
PropertySourcesPlaceholderConfigurer 代替了之前的 PropertyPlaceholderConfigurer,

继承关系

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BeanFactoryPostProcessor
            ^
            |
PropertyResourceConfigurer
            ^
            |
PlaceholderConfigurerSupport
            ^
            |
PropertySourcesPlaceholderConfigurer

加载时机

springboot 场景

当应用标记了 @EnableAutoConfiguration, springboot 便会激活 AutoConfigurationImportSelector, 通过 SpringFactoriesLoader 扫描到与占位符替换相关的 PropertyPlaceholderAutoConfiguration, 从而自动注入 PropertySourcesPlaceholderConfigurer 类:

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@Configuration(proxyBeanMethods = false)
@AutoConfigureOrder(Ordered.HIGHEST_PRECEDENCE)
public class PropertyPlaceholderAutoConfiguration {

	@Bean
	@ConditionalOnMissingBean(search = SearchStrategy.CURRENT)
	public static PropertySourcesPlaceholderConfigurer propertySourcesPlaceholderConfigurer() {
		return new PropertySourcesPlaceholderConfigurer();
	}

}

PropertySourcesPlaceholderConfigurer 实现了 EnvironmentAware, 其能够感知到 Environment, 并最终获取所有 properties 的真实值: 通过 environment.getPropertySources() 拿到 propertySources:

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public class PropertySourcesPlaceholderConfigurer extends PlaceholderConfigurerSupport implements EnvironmentAware {
    private PropertySources propertySources;

    @Override
    public void setEnvironment(Environment environment) {
        this.propertySources = ((ConfigurableEnvironment) environment).getPropertySources();
    }
}

而 Environment 是基于 EnvironmentPostProcessor 通过 resourceLoader 读取所有 properties 的:

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public class ConfigFileApplicationListener implements EnvironmentPostProcessor, SmartApplicationListener {

    @Override
    public void postProcessEnvironment(ConfigurableEnvironment environment, SpringApplication application) {
        // 加载 application.properties 或 application.yml 文件
        addPropertySources(environment, application.getResourceLoader());
    }

    private void addPropertySources(ConfigurableEnvironment environment, ResourceLoader resourceLoader) {
        // 加载默认的 application.properties 文件
        PropertySource<?> propertySource = new ResourcePropertySource("applicationProperties", resourceLoader.getResource("classpath:application.properties"));
        environment.getPropertySources().addLast(propertySource);
    }
}

传统 xml 场景

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PropertyPlaceholderBeanDefinitionParser -> AbstractPropertyLoadingBeanDefinitionParser -> AbstractSingleBeanDefinitionParser -> AbstractBeanDefinitionParser -> BeanDefinitionParser

执行过程

PropertySourcesPlaceholderConfigurer 需要在 bean 被实例化之前执行, 也就是说它操纵的不是 spring bean, 而是 BeanDefinition; 如果 PropertySourcesPlaceholderConfigurer 不在 bean 实例化之前处理 BeanDefinition, 那么在 bean 实例化时属性占位符将无法被正确解析, 导致 bean 的属性值不正确;
为了在 Spring 容器实例化 bean 之前完成这些操作，PropertySourcesPlaceholderConfigurer 实现了 BeanFactoryPostProcessor 接口:

  • BeanFactoryPostProcessor 允许在 spring 容器实例化 bean 之前对 BeanDefinition 进行修改;
    • 在 spring 容器加载 BeanDefinition 之后 & 实例化 bean 之前, BeanFactoryPostProcessor 的 postProcessBeanFactory 方法会被调用;
  • PropertyPlaceholderConfigurer 实现了这个方法, 其会遍历所有的 BeanDefinition, 查找其中的属性占位符 (如 ${property.name}), 并从配置文件中读取相应的属性值, 替换这些占位符;

 
简要过程如下:

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// BeanFactoryPostProcessor#postProcessBeanFactory
@Override
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
    ......
    processProperties(beanFactory, new PropertySourcesPropertyResolver(this.propertySources));
    ......
}

protected void processProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
			final ConfigurablePropertyResolver propertyResolver) throws BeansException {
    ......
	doProcessProperties(beanFactoryToProcess, valueResolver);
}

在 PlaceholderConfigurerSupport#doProcessProperties 方法中, 会对注册到 beanFactory 中的每一个 beanDefinition 执行扫描:

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   protected void doProcessProperties(ConfigurableListableBeanFactory beanFactoryToProcess,
		StringValueResolver valueResolver) {
	BeanDefinitionVisitor visitor = new BeanDefinitionVisitor(valueResolver);

	String[] beanNames = beanFactoryToProcess.getBeanDefinitionNames();
	for (String curName : beanNames) {
		// Check that we're not parsing our own bean definition,
		// to avoid failing on unresolvable placeholders in properties file locations.
		if (!(curName.equals(this.beanName) && beanFactoryToProcess.equals(this.beanFactory))) {
			BeanDefinition bd = beanFactoryToProcess.getBeanDefinition(curName);
			try {
			    // 扫描 beanDefinition 并替换
				visitor.visitBeanDefinition(bd);
			}
			catch (Exception ex) {
				throw new BeanDefinitionStoreException(bd.getResourceDescription(), curName, ex.getMessage(), ex);
			}
		}
	}
       ......
}

在 BeanDefinitionVisitor#visitBeanDefinition 方法里, 会对目标 beanDefinition 做一次全身大体检, 所有的 bd 属性均要 resolve 一遍:

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   public void visitBeanDefinition(BeanDefinition beanDefinition) {
	visitParentName(beanDefinition);
	visitBeanClassName(beanDefinition);
	visitFactoryBeanName(beanDefinition);
	visitFactoryMethodName(beanDefinition);
	visitScope(beanDefinition);
	if (beanDefinition.hasPropertyValues()) {
		visitPropertyValues(beanDefinition.getPropertyValues());
	}
	if (beanDefinition.hasConstructorArgumentValues()) {
		ConstructorArgumentValues cas = beanDefinition.getConstructorArgumentValues();
		visitIndexedArgumentValues(cas.getIndexedArgumentValues());
		visitGenericArgumentValues(cas.getGenericArgumentValues());
	}
}
  • 对于 String 类型的属性使用 resolveStringValue 方法解析占位符, 比如: parentClassName、factoryBeanName、scope 等;
  • 对于非字符串类型的属性使用 resolveValue 方法解析占位符, 比如: propertyValues、constructorArgumentValues 等; 
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       // visitBeanDefinition 方法片段
       if (beanDefinition.hasPropertyValues()) {
    		visitPropertyValues(beanDefinition.getPropertyValues());
    }

       protected void visitPropertyValues(MutablePropertyValues pvs) {
    	PropertyValue[] pvArray = pvs.getPropertyValues();
    	for (PropertyValue pv : pvArray) {
    		Object newVal = resolveValue(pv.getValue());
    		if (!ObjectUtils.nullSafeEquals(newVal, pv.getValue())) {
    			pvs.add(pv.getName(), newVal);
    		}
    	}
    }

职责划分

PropertySourcesPlaceholderConfigurer 只负责将占位符解析后填充到 beanDefinition, 而不负责填充到最终 spring bean 的这 “最后一公里”; 将 spring bean 中的占位符最终替换为真实值的逻辑其实另有其人:

  • @Value: 通过 AutowiredAnnotationBeanPostProcessor 实现注入: 
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       // pvs 是 beanDefinition 的 propertyValues 字段
       @Override
    public PropertyValues postProcessProperties(PropertyValues pvs, Object bean, String beanName) {
    	InjectionMetadata metadata = findAutowiringMetadata(beanName, bean.getClass(), pvs);
    	try {
    		metadata.inject(bean, beanName, pvs);
    	}
    	catch (BeanCreationException ex) {
    		throw ex;
    	}
    	catch (Throwable ex) {
    		throw new BeanCreationException(beanName, "Injection of autowired dependencies failed", ex);
    	}
    	return pvs;
    }
    AutowiredAnnotationBeanPostProcessor 类实现了 SmartInstantiationAwareBeanPostProcessor 接口, 在 AbstractAutowireCapableBeanFactory#populateBean 方法中, 会将所有已注册的 InstantiationAwareBeanPostProcessor 执行一遍, 从而完成对占位符的增强替换: 
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       PropertyValues pvs = (mbd.hasPropertyValues() ? mbd.getPropertyValues() : null);
       
       for (InstantiationAwareBeanPostProcessor bp : getBeanPostProcessorCache().instantiationAware) {
    	PropertyValues pvsToUse = bp.postProcessProperties(pvs, bw.getWrappedInstance(), beanName);
    	if (pvsToUse == null) {
    		if (filteredPds == null) {
    			filteredPds = filterPropertyDescriptorsForDependencyCheck(bw, mbd.allowCaching);
    		}
    		pvsToUse = bp.postProcessPropertyValues(pvs, filteredPds, bw.getWrappedInstance(), beanName);
    		if (pvsToUse == null) {
    			return;
    		}
    	}
    	pvs = pvsToUse;
    }