spring是如何解析xml配置文件中的占位符

前言

我们在配置Spring Xml配置文件的时候，可以在文件路径字符串中加入 ${} 占位符，Spring会自动帮我们解析占位符，这么神奇的操作Spring是怎么帮我们完成的呢？这篇文章我们就来一步步揭秘。

1.示例

ClassPathXmlApplicationContext applicationContext = new ClassPathXmlApplicationContext();
applicationContext.setConfigLocation("${java.version}.xml");
applicationContext.refresh();
String[] beanNames = applicationContext.getBeanDefinitionNames();
for (String beanName : beanNames) {
 System.out.println(beanName);
}

这段代码在我工程里是会报错的，如下：

Caused by: java.io.FileNotFoundException: class path resource [1.8.0_144.xml] cannot be opened because it does not exist
 at org.springframework.core.io.ClassPathResource.getInputStream(ClassPathResource.java:190)
 at org.springframework.beans.factory.xml.XmlBeanDefinitionReader.loadBeanDefinitions(XmlBeanDefinitionReader.java:336)
 ... 11 more

可以看到报错里面的文件路径变成了1.8.0_144.xml，也就是说Spring帮我们把${java.version}解析成了实际值。

2.原理

AbstractRefreshableConfigApplicationContext
我们在之前的文章里提到过这个类的resolve方法，我们再来瞧一眼：

/**
  * Resolve the given path, replacing placeholders with corresponding
  * environment property values if necessary. Applied to config locations.
  * @param path the original file path
  * @return the resolved file path
  * @see org.springframework.core.env.Environment#resolveRequiredPlaceholders(String)
  */
 protected String resolvePath(String path) {
  //通过当前环境去 解析 必要的占位符
  return getEnvironment().resolveRequiredPlaceholders(path);
 }

获取当前环境，这个环境在示例代码中就是 StandardEnvironment ,并且根据当前环境去解析占位符，这个占位符解析不到还会报错。

resolveRequiredPlaceHolders由StandardEnvironment的父类AbstractEnvironment实现。

AbstractEnvironment

//把propertySources放入 Resolver中
private final ConfigurablePropertyResolver propertyResolver =
   new PropertySourcesPropertyResolver(this.propertySources);
@Override
public String resolveRequiredPlaceholders(String text) throws IllegalArgumentException {
 return this.propertyResolver.resolveRequiredPlaceholders(text);
}

这里的propertySources很重要了，从命名也可以看出我们解析占位符的来源就是从这个集合中来的。这个集合是在我们StandardEnvironment实例化的时候去自定义的。

StandardEnvironment

/**
  * Create a new {@code Environment} instance, calling back to
  * {@link #customizePropertySources(MutablePropertySources)} during construction to
  * allow subclasses to contribute or manipulate(操作) {@link PropertySource} instances as
  * appropriate.
  * @see #customizePropertySources(MutablePropertySources)
  */
 //StandardEnvironment 实例化调用
 public AbstractEnvironment() {
  customizePropertySources(this.propertySources);
 }
@Override
protected void customizePropertySources(MutablePropertySources propertySources) {

 //todo Java提供了System类的静态方法getenv()和getProperty()用于返回系统相关的变量与属性，
 //todo getenv方法返回的变量大多于系统相关，
 //todo getProperty方法返回的变量大多与java程序有关。
 //https://www.cnblogs.com/Baronboy/p/6030443.html
 propertySources.addLast(new MapPropertySource(SYSTEM_PROPERTIES_PROPERTY_SOURCE_NAME, getSystemProperties()));

 //SystemEnvironmentPropertySource 是System.getenv()
 propertySources.addLast(new SystemEnvironmentPropertySource(SYSTEM_ENVIRONMENT_PROPERTY_SOURCE_NAME, getSystemEnvironment()));
}

最重要的肯定是我们的 propertyResolver.resolveRequiredPlaceholders 方法了，propertyResolver.resolveRequiredPlaceholders其实是PropertySourcesPropertyResolver的父类AbstractPropertyResolver来实现。

AbstractPropertyResolver

//创建一个占位符的helper去解析
@Override
public String resolveRequiredPlaceholders(String text) throws IllegalArgumentException {
 if (this.strictHelper == null) {
  //不忽略
  this.strictHelper = createPlaceholderHelper(false);
 }
 return doResolvePlaceholders(text, this.strictHelper);
}
 //私有方法
 //是否忽略 无法解决的占位符
private PropertyPlaceholderHelper createPlaceholderHelper(boolean ignoreUnresolvablePlaceholders) {

  //默认使用${ placeholderPrefix
  return new PropertyPlaceholderHelper(this.placeholderPrefix, this.placeholderSuffix,
    this.valueSeparator, ignoreUnresolvablePlaceholders);
}
private String doResolvePlaceholders(String text, PropertyPlaceholderHelper helper) {

  //PlaceholderResolver function interface
  //todo important 重要的是这个getPropertyAsRawString
  return helper.replacePlaceholders(text, this::getPropertyAsRawString);
 }

这里的 this::getPropertyAsRawString 很重要，利用了java8的函数式接口来实现。它的定义在AbstractPropertyResolver里

/**
  * Retrieve the specified property as a raw String,
  * i.e. without resolution of nested placeholders.
  * @param key the property name to resolve
  * @return the property value or {@code null} if none found
  */
 @Nullable
 protected abstract String getPropertyAsRawString(String key);

但是我们在doResolvePlaceholders里指向的this,所以还得看PropertySourcesPropertyResolver类。

PropertySourcesPropertyResolver

//提供给函数接口 PlaceholderResolver
 //todo 解析 xml配置文件路径占位符的时候调用的是这个 2020-09-11
 @Override
 @Nullable
 protected String getPropertyAsRawString(String key) {
  return getProperty(key, String.class, false);
 }
@Nullable
 protected <T> T getProperty(String key, Class<T> targetValueType, boolean resolveNestedPlaceholders) {
  if (this.propertySources != null) {

   //例如遍历的是MutablePropertySources 的propertySourceList
   for (PropertySource<?> propertySource : this.propertySources) {
    if (logger.isTraceEnabled()) {
     logger.trace("Searching for key '" + key + "' in PropertySource '" +
       propertySource.getName() + "'");
    }
    Object value = propertySource.getProperty(key);
    if (value != null) {
     //todo 解析 profile变量的时候 会去 解析 变量中的占位符 2020-09-11
     //TODO 解析xml配置文件路径字符串的时候 如果占位符 变量 的值 包含占位符 在这里 不会去解析 通过Helper 去解析 PropertyPlaceholderHelper
     if (resolveNestedPlaceholders && value instanceof String) {
      value = resolveNestedPlaceholders((String) value);
     }
     logKeyFound(key, propertySource, value);
     //跳出for 循环
     return convertValueIfNecessary(value, targetValueType);
    }
   }
  }
  if (logger.isTraceEnabled()) {
   logger.trace("Could not find key '" + key + "' in any property source");
  }
  return null;
 }

看到没有，我们是遍历this.propertySources集合，然后根据key调用它的getProperty方法获取value。我们从上面的StandardEnvrionment中看到我们定义的是 MapPropertySource 和 SystemEnvironmentPropertySource .

MapPropertySource

//从source中取得属性
@Override
@Nullable
public Object getProperty(String name) {
 return this.source.get(name);
}

这里的source就是getSystemProperties()，也就是 AbstractEnvironment中的方法：

@Override
@SuppressWarnings({"unchecked", "rawtypes"})
public Map<String, Object> getSystemProperties() {
 try {
  //Hashtable
  return (Map) System.getProperties();
 }
 catch (AccessControlException ex) {
  return (Map) new ReadOnlySystemAttributesMap() {
   @Override
   @Nullable
   protected String getSystemAttribute(String attributeName) {
   try {
    return System.getProperty(attributeName);
   }
   catch (AccessControlException ex) {
    if (logger.isInfoEnabled()) {
     logger.info("Caught AccessControlException when accessing system property '" +
      attributeName + "'; its value will be returned [null]. Reason: " + ex.getMessage());
    }
    return null;
   }
   }
  };
 }
}

我们还忘了很重要的一步，就是PropertyPlaceholderHelper的replacePlaceholders方法。

PropertyPlaceholderHelper

//protected 范围
protected String parseStringValue(
  String value, PlaceholderResolver placeholderResolver, Set<String> visitedPlaceholders) {

 StringBuilder result = new StringBuilder(value);

 //如果value中没有占位符前缀 那直接返回result
 int startIndex = value.indexOf(this.placeholderPrefix);
 while (startIndex != -1) {
  //找到占位符的最后一个索引
  int endIndex = findPlaceholderEndIndex(result, startIndex);
  if (endIndex != -1) {
   String placeholder = result.substring(startIndex + this.placeholderPrefix.length(), endIndex);
   String originalPlaceholder = placeholder;
   if (!visitedPlaceholders.add(originalPlaceholder)) {
   throw new IllegalArgumentException(
     "Circular placeholder reference '" + originalPlaceholder + "' in property definitions");
   }

   //1. todo 2020-09-01 解析出来占位符，比如java.version
   //解析内嵌占位符
   // Recursive invocation, parsing placeholders contained in the placeholder key.
   placeholder = parseStringValue(placeholder, placeholderResolver, visitedPlaceholders);
   // Now obtain the value for the fully resolved key...
   //2.todo 2020-09-01 获取实际值
   String propVal = placeholderResolver.resolvePlaceholder(placeholder);
   if (propVal == null && this.valueSeparator != null) {
   int separatorIndex = placeholder.indexOf(this.valueSeparator);
   if (separatorIndex != -1) {
    String actualPlaceholder = placeholder.substring(0, separatorIndex);
    String defaultValue = placeholder.substring(separatorIndex + this.valueSeparator.length());
    //这里就是实际获取占位符中值得地方。
    propVal = placeholderResolver.resolvePlaceholder(actualPlaceholder);

   }
   }
  if (propVal != null) {
     //从占位符里获取的值也有可能包含占位符 这里可能会报 Circular placeholder reference
     propVal = parseStringValue(propVal, placeholderResolver, visitedPlaceholders);
     //替换占位符 为 实际值
      result.replace(startIndex, endIndex + this.placeholderSuffix.length(), propVal);
     if (logger.isTraceEnabled()) {
      logger.trace("Resolved placeholder '" + placeholder + "'");
     }
     startIndex = result.indexOf(this.placeholderPrefix, startIndex + propVal.length());
    }
   //省略部分代码
  }
  else {
   startIndex = -1;
  }
 }
 return result.toString();
}

到这里我们就可以看到Spring在处理一个小小的占位符就做了这么多设计。可见这个架构是如此严谨。下篇文章我们就来探讨下Spring是如何加载这个Xml文件的。

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