不难知道， property-placeholder 的解析是 PropertyPlaceholderBeanDefinitionParser 完成的， 但是 它仅仅是个parser ， 它仅仅是读取了 location 等配置属性， 并没有完成真正的解析，及 注册。
<context:property-placeholder location="appa.properties"/>
我们把 location 设置为一个错误的 路径，就会报错， 从错误堆栈中可以看出， 实际的解析是 PropertySourcesPlaceholderConfigurer 完成的：
警告: Exception encountered during context initialization - cancelling refresh attempt: org.springframework.beans.factory.BeanInitializationException: Could not load properties; nested exception is java.io.FileNotFoundException: class path resource [appa.properties] cannot be opened because it does not exist
Exception in thread "main" org.springframework.beans.factory.BeanInitializationException: Could not load properties; nested exception is java.io.FileNotFoundException: class path resource [appa.properties] cannot be opened because it does not exist
at org.springframework.context.support.PropertySourcesPlaceholderConfigurer.postProcessBeanFactory(PropertySourcesPlaceholderConfigurer.java:148)
at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:281)
at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:161)
at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:686)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:524)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
at LKtest.main(LKtest.java:11)
Caused by: java.io.FileNotFoundException: class path resource [appa.properties] cannot be opened because it does not exist
at org.springframework.core.io.ClassPathResource.getInputStream(ClassPathResource.java:172)
at org.springframework.core.io.support.EncodedResource.getInputStream(EncodedResource.java:154)
at org.springframework.core.io.support.PropertiesLoaderUtils.fillProperties(PropertiesLoaderUtils.java:98)
at org.springframework.core.io.support.PropertiesLoaderSupport.loadProperties(PropertiesLoaderSupport.java:177)
at org.springframework.core.io.support.PropertiesLoaderSupport.mergeProperties(PropertiesLoaderSupport.java:158)
at org.springframework.context.support.PropertySourcesPlaceholderConfigurer.postProcessBeanFactory(PropertySourcesPlaceholderConfigurer.java:139)
... 7 more
但是， PropertyPlaceholderBeanDefinitionParser具体是 何时被注册的呢？ 我纠结了很久， 仔细调试代码，终于发现了奥秘：
at org.springframework.context.config.PropertyPlaceholderBeanDefinitionParser.getBeanClass(PropertyPlaceholderBeanDefinitionParser.java:48)
at org.springframework.beans.factory.xml.AbstractSingleBeanDefinitionParser.parseInternal(AbstractSingleBeanDefinitionParser.java:66) // 又交给了实际的子类去parse
at org.springframework.beans.factory.xml.AbstractBeanDefinitionParser.parse(AbstractBeanDefinitionParser.java:61)
at org.springframework.beans.factory.xml.NamespaceHandlerSupport.parse(NamespaceHandlerSupport.java:74)
at org.springframework.beans.factory.xml.BeanDefinitionParserDelegate.parseCustomElement(BeanDefinitionParserDelegate.java:1411)
at org.springframework.beans.factory.xml.BeanDefinitionParserDelegate.parseCustomElement(BeanDefinitionParserDelegate.java:1401)
at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.parseBeanDefinitions(DefaultBeanDefinitionDocumentReader.java:172)
at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.doRegisterBeanDefinitions(DefaultBeanDefinitionDocumentReader.java:142)
at org.springframework.beans.factory.xml.DefaultBeanDefinitionDocumentReader.registerBeanDefinitions(DefaultBeanDefinitionDocumentReader.java:94)
at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.registerBeanDefinitions(XmlBeanDefinitionReader.java:508)
at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.doLoadBeanDefinitions(XmlBeanDefinitionReader.java:392)
at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:336)
at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:304)
at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:181)
at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:217)
at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:188)
at org.springframework.beans.factory.support.AbstractBeanDefinitionReader.loadBeanDefinitions(AbstractBeanDefinitionReader.java:252)
at org.springframework.context.support.AbstractXmlApplicationContext.loadBeanDefinitions(AbstractXmlApplicationContext.java:127)
at org.springframework.context.support.AbstractXmlApplicationContext.loadBeanDefinitions(AbstractXmlApplicationContext.java:93)
at org.springframework.context.support.AbstractRefreshableApplicationContext.refreshBeanFactory(AbstractRefreshableApplicationContext.java:129)
at org.springframework.context.support.AbstractApplicationContext.obtainFreshBeanFactory(AbstractApplicationContext.java:613)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:514)
- locked <0x5d2> (a java.lang.Object)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
at LKtest.main(LKtest.java:11)
原来是在解析 property-placeholder 这个xml元素的时候完成的， parseCustomElement 。
解析 property-placeholder的时候，找到了 PropertyPlaceholderBeanDefinitionParser 。 然后把 配置中的location 交给了它处理。
AbstractSingleBeanDefinitionParser：
protected final AbstractBeanDefinition parseInternal(Element element, ParserContext parserContext) {
BeanDefinitionBuilder builder = BeanDefinitionBuilder.genericBeanDefinition();
String parentName = this.getParentName(element);
if(parentName != null) {
builder.getRawBeanDefinition().setParentName(parentName);
} Class<?> beanClass = this.getBeanClass(element); 根据context:property-placeholder， 找到class。返回的正是 PropertySourcesPlaceholderConfigurer .. } PropertyPlaceholderBeanDefinitionParser：
protected Class<?> getBeanClass(Element element) {
return "ENVIRONMENT".equals(element.getAttribute("system-properties-mode"))?PropertySourcesPlaceholderConfigurer.class:PropertyPlaceholderConfigurer.class;
}
但是我还是不知道 properties 文件是如何被读取和解析的， 直到我注意到 PropertySourcesPlaceholderConfigurer 实现了 BeanFactoryPostProcessor ：
at org.springframework.core.io.support.PropertiesLoaderUtils.fillProperties(PropertiesLoaderUtils.java:85)
at org.springframework.core.io.support.PropertiesLoaderSupport.loadProperties(PropertiesLoaderSupport.java:177) // 这里完成了properties 的读取
at org.springframework.core.io.support.PropertiesLoaderSupport.mergeProperties(PropertiesLoaderSupport.java:158)
at org.springframework.context.support.PropertySourcesPlaceholderConfigurer.postProcessBeanFactory(PropertySourcesPlaceholderConfigurer.java:139) // 因为PropertySourcesPlaceholderConfigurer 实现了 BeanFactoryPostProcessor
at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:281)
at org.springframework.context.support.PostProcessorRegistrationDelegate.invokeBeanFactoryPostProcessors(PostProcessorRegistrationDelegate.java:161)
at org.springframework.context.support.AbstractApplicationContext.invokeBeanFactoryPostProcessors(AbstractApplicationContext.java:686)
at org.springframework.context.support.AbstractApplicationContext.refresh(AbstractApplicationContext.java:524)
- locked <0x6c6> (a java.lang.Object)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:139)
at org.springframework.context.support.ClassPathXmlApplicationContext.<init>(ClassPathXmlApplicationContext.java:83)
at LKtest.main(LKtest.java:11)
具体来说：
public void postProcessBeanFactory(ConfigurableListableBeanFactory beanFactory) throws BeansException {
if(this.propertySources == null) {
this.propertySources = new MutablePropertySources();
if(this.environment != null) {
this.propertySources.addLast(new PropertySource<Environment>("environmentProperties", this.environment) {
public String getProperty(String key) {
return ((Environment)this.source).getProperty(key);
}
});
}
try {
PropertySource<?> localPropertySource = new PropertiesPropertySource("localProperties", this.mergeProperties()); // BeanFactory 创建完成后， 读取所有的 properties 文件
if(this.localOverride) {
this.propertySources.addFirst(localPropertySource);
} else {
this.propertySources.addLast(localPropertySource);
}
} catch (IOException var3) {
throw new BeanInitializationException("Could not load properties", var3);
}
}
this.processProperties(beanFactory, (ConfigurablePropertyResolver)(new PropertySourcesPropertyResolver(this.propertySources)));// 将properties 文件内容应用到所有的配置中去，也就是替换 ${} 部分
this.appliedPropertySources = this.propertySources;
}
对 ${} 的替换：
at org.springframework.beans.factory.config.BeanDefinitionVisitor.visitPropertyValues(BeanDefinitionVisitor.java:142)
at org.springframework.beans.factory.config.BeanDefinitionVisitor.visitBeanDefinition(BeanDefinitionVisitor.java:82)
at org.springframework.beans.factory.config.PlaceholderConfigurerSupport.doProcessProperties(PlaceholderConfigurerSupport.java:220) // 完成了所有的beanDefinition 的预处理之后， 开始替换 Placeholder
at org.springframework.context.support.PropertySourcesPlaceholderConfigurer.processProperties(PropertySourcesPlaceholderConfigurer.java:180)
at org.springframework.context.support.PropertySourcesPlaceholderConfigurer.postProcessBeanFactory(PropertySourcesPlaceholderConfigurer.java:152)
...
protected void doProcessProperties(ConfigurableListableBeanFactory beanFactoryToProcess, StringValueResolver valueResolver) {
BeanDefinitionVisitor visitor = new BeanDefinitionVisitor(valueResolver);
String[] beanNames = beanFactoryToProcess.getBeanDefinitionNames(); // 获取所有已经注册的 bean 定义
String[] var5 = beanNames;
int var6 = beanNames.length;
for(int var7 = 0; var7 < var6; ++var7) {
String curName = var5[var7];
if(!curName.equals(this.beanName) || !beanFactoryToProcess.equals(this.beanFactory)) {
BeanDefinition bd = beanFactoryToProcess.getBeanDefinition(curName);
try {
visitor.visitBeanDefinition(bd);
} catch (Exception var11) {
throw new BeanDefinitionStoreException(bd.getResourceDescription(), curName, var11.getMessage(), var11);
}
}
}
beanFactoryToProcess.resolveAliases(valueResolver);
beanFactoryToProcess.addEmbeddedValueResolver(valueResolver);
}
PropertySourcesPlaceholderConfigurer的父类定义了 如何设置 placeholder， 可见前缀和后缀：
public abstract class PlaceholderConfigurerSupport extends PropertyResourceConfigurer implements BeanNameAware, BeanFactoryAware {
public static final String DEFAULT_PLACEHOLDER_PREFIX = "${";
public static final String DEFAULT_PLACEHOLDER_SUFFIX = "}";
public static final String DEFAULT_VALUE_SEPARATOR = ":";
protected String placeholderPrefix = "${";
protected String placeholderSuffix = "}";
protected String valueSeparator = ":";
PropertySource 就是一个kv 配置项。
其中位于 BeanDefinitionVisitor ：
public void visitBeanDefinition(BeanDefinition beanDefinition) {
this.visitParentName(beanDefinition);
this.visitBeanClassName(beanDefinition);
this.visitFactoryBeanName(beanDefinition);
this.visitFactoryMethodName(beanDefinition);
this.visitScope(beanDefinition);
this.visitPropertyValues(beanDefinition.getPropertyValues());
ConstructorArgumentValues cas = beanDefinition.getConstructorArgumentValues();
this.visitIndexedArgumentValues(cas.getIndexedArgumentValues());
this.visitGenericArgumentValues(cas.getGenericArgumentValues());
}