Quick People Blog

Making the most of Scala's XML templating

07 Nov 2011

The Problem

On one of my current projects, the system receives a web request from a consumer, makes further requests to upstream services and collates a response. For the sake of illustration, let’s say that one of these upstream systems was IMDB and its response looked like this:

<?xml version="1.0" encoding="UTF-8" standalone="yes"?>  
<film id="12345">  
 <name>Avatar 5</name>  
 <actors/>  
 <actor/>Tom Cruise</actor>  
 <actor>Angelina Jolie</actor>  
 <actor>Johnny Depp</actor>  
 </actors>  
 <releasedates>  
 <releasedate country="fr-FR">18th December 2015</releasedate>  
 <releasedate country="de-DE">29th December 2015</releasedate>  
 <releasedate country="en-UK">1st December 2015</releasedate>  
 <releasedate country="en-US">4th July 2015</releasedate>  
 </releasedates>  
</film>

Let’s assume that we want to verify that our system handles the above XML correctly. Because we want to avoid external dependencies in our integration tests, we fake IMDB out and build the fake’s response inline within the test:

fakes.imdb().toReturn(  
 film()  
 .withId("12345")  
 .withName("Avatar 5")  
 .withActors("Tom Cruise", "Angelina Jolie", "Johnny Depp")  
 .withAReleaseDate("fr-FR", "18th December 2015")  
 .withAReleaseDate("de-DE", "29th December 2015")  
 .withAReleaseDate("en-UK", "1st December 2015")  
 .withAReleaseDate("en-US", "4th July 2015")  
)

(here film() is just a method that invokes new FilmBuilder())

We need some code which satisfies this API and produces the appropriate XML.

The Java Version

The original, (Java) implementation looks like this:

public class FilmBuilder extends Builder {
  private String id = "";
  private String[] actors;
  private Map < String,
  String > releaseDates = new LinkedHashMap < String,
  String > ();
  private String name;

  public String build() {
    String xml = "<?xml version=\"1.0\" encoding=\"UTF-8\" standalone=\"yes\"?>\n" + "<film id=\"" + id + "\">\n";
    xml += " <name>" + name + "</name>\n";
    if (actors != null) {
      xml += "<actors>";
      for (String actor: actors) {
        xml += "<actor>" + actor + "</actor>";
      }
      xml += "</actors>";
    }
    xml += " <releasedates>\n";
    xml = renderTextThing(xml, releaseDates, "releasedate");
    xml += " </releasedates>\n" + "</film>";
    return xml;
  }

  private String renderTextThing(String primePlaceXml, Map < String, String > textMap, String elementName) {
    for (Map.Entry < String, String > entry: textMap.entrySet()) {
      primePlaceXml += "<" + elementName + " country='" + entry.getKey() + "'>" + entry.getValue() + "</" + elementName + ">";
    }
    return primePlaceXml;
  }

  public FilmBuilder withId(String id) {
    this.id = id;
    return this;
  }

  public FilmBuilder withActors(String...actors) {
    this.actors = actors;
    return this;
  }

  public FilmBuilder withAReleaseDate(String country, String releaseDate) {
    this.releaseDates.put(country, releaseDate);
    return this;
  }

  public FilmBuilder withName(String name) {
    this.name = name;
    return this;
  }
}

This satisfies the requirements but is very meat and two veg (and ain’t really much of a looker). Perhaps we can improve on it.

Enter Scala’s XML Support

Scala supports XML natively (when trying to figure out the ins and outs, Daniel Spiewak’s blog post was the most useful documentation I could find online).

The Scala port of the FilmBuilder looks like this:

import java.util.LinkedHashMap
import java.util.Map
import xml.Unparsed

class FilmBuilder extends Builder {
  import scala.collection.JavaConversions._

  def build: String = {
    """<?xml version="1.0" encoding="UTF-8" standalone="yes"?>""" +
      <film id={id}>
        <name>{name}</name>  
        {
          if (actors != null) {
            <actors>{actors.map(name => <actor>{Unparsed(name)}</actor>)}</actors>
          }
        }  
       <releasedates>  
         {releaseDates.map(kv => <releasedate country={kv.\ _1}>{kv.\ _2}</releasedate>)}  
       </releasedates>  
     </film>
  }

  def withId(id: String): FilmBuilder = { this.id = id; this }
  def withName(name: String): FilmBuilder = { this.name = name; this }
  def withActors(actors: String*): FilmBuilder = { this.actors = actors; this }
  def withAReleaseDate(language: String, dateString: String): FilmBuilder = {
    this.releaseDates.put(language, dateString); this
  }

  private var id: String = null
  private var name: String = null
  private var actors: Seq[String] = null
  private val releaseDates: Map[String, String] =
    new LinkedHashMap[String, String]
}

Interesting features and gotchas to mention:

  • Your XML needs to be valid markup. Your IDE’s compiler will be an angry red until you put that closing tag in. This is a nice contrast to the Java solution’s string building approach, which is very susceptible to leaving a dangling tag open somewhere accidentally.
  • You can use conditionals in your template. This is particularly useful when you have optional tags in your XML.
  • Collections get flattened correctly inline (which is pretty awesome). This means that you only really have to generate an enumeration of XML nodes and let Scala do the rest.
  • Inline strings get helpfully escaped. This does mean though that if you have an inline string <tag>...</tag>, it will in fact be rendered as & lt;tag & gt;...& lt;tag & gt;, which may not be exactly what you wanted. To prevent this from happening, you need to wrap your string in an Unparsed (I’ve done this above for the actor tag).
  • I like how you can recognise the XML structure in the Scala version. This isn’t really possible (with all that noise) in the Java variant.

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