Encoding Individual Strings with XML entities to replace "&", " ", etc.

Question

We use templates to create XML files. The tags are in the templates and just the values get substituted in. Sometimes the variable value contains an ampersand or other XML-banned character.
1) Is there a simple method in the Java or Xerces class libraries that takes a string and returns one with the suspect characters replaced by XML entities?
2) Sometimes, though rarely (I have no control over this!) the variables arrive with the forbidden characters already replaced by entities. Is there a method which will tell me if the string already contains entities, or at least is already a valid XML string? Otherwise I'm afraid the ampersand in an already-converted entity name will itself be converted to another entity.
Thanks!

Answer 1

I'm not aware of any built-in mechanism for doing that kind of escaping short of a full XML generator. Here's a lightweight solution that replaces ampersands only if they're not part of an XML entity. Just grab a copy of Elliott Hughes' Rewriter and implement it like this: public class Test
  /* Rewriter can be found here:
   * http://elliotth.blogspot.com/2004/07/java-implementation-of-rubys-gsub.html
  public static void main(String... args) throws Exception
    Rewriter xmlEscaper = new Rewriter("[<>'\"]|&(?!(?:lt|gt|apos|quot|amp);)")
      public String replacement()
        char ch = group(0).charAt(0);
        switch (ch)
          case '<'  : return "&#38;lt;";
          case '>'  : return "&#38;gt;";
          case '\'' : return "&#38;apos;";
          case '"'  : return "&#38quot;";
          case '&'  : return "&#38amp;";
          default :
            throw new IllegalStateException("this can't happen");
    String str = "this & that & the <other&#38gt;";
    System.out.println(xmlEscaper.rewrite(str));
}