spring 之 property-placeholder 分析

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());
    }