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。

来源：https://www.tuicool.com/articles/JVBFNbe