Spring SystemEnvironmentPropertySource

@Override
@Nullable
public Object getProperty(String name) {
   // 解析属性名称
   String actualName = resolvePropertyName(name);
   if (logger.isDebugEnabled() && !name.equals(actualName)) {
      logger.debug("PropertySource '" + getName() + "' does not contain property '" + name +
            "', but found equivalent '" + actualName + "'");
   }
   // 从 map 中获取
   return super.getProperty(actualName);
}

@Nullable
private String checkPropertyName(String name) {
   // Check name as-is
   // 是否存在
   if (containsKey(name)) {
      return name;
   }
   // Check name with just dots replaced
   // 将名称中 . 替换成 _ 是否存在
   String noDotName = name.replace('.', '_');
   if (!name.equals(noDotName) && containsKey(noDotName)) {
      return noDotName;
   }
   // Check name with just hyphens replaced
   // 将名称中 - 替换成 _ 是否存在
   String noHyphenName = name.replace('-', '_');
   if (!name.equals(noHyphenName) && containsKey(noHyphenName)) {
      return noHyphenName;
   }
   // Check name with dots and hyphens replaced
   // 替换 . 和 - 都转换为 _ 判断是否存在
   String noDotNoHyphenName = noDotName.replace('-', '_');
   if (!noDotName.equals(noDotNoHyphenName) && containsKey(noDotNoHyphenName)) {
      return noDotNoHyphenName;
   }
   // Give up
   return null;
}

private boolean containsKey(String name) {
   return (isSecurityManagerPresent() ? this.source.containsKey(name) : this.source.containsKey(name));
}

protected boolean isSecurityManagerPresent() {
   return (System.getSecurityManager() != null);
}