Parsing XML response from service on Android without using extra libraries

The XML response from REST services is not commonplace, and today it is problematic to parse it using Retrofit or Ktor on Android. While Retrofit has the SimpleXmlConverterFactory, this library is deprecated, and no viable alternatives exist. Ktor on the other hand, has XML Converter only server side.

The solution presented in this code snippet uses built-in Android DocumentBuilder to process the XML. This class is available in Android since API Level 1. In the following sample, the application needs to fetch a list of photos from Flickr service. This service provides the XML formatted response. We are only interested in populating our FlickrPhotoModel data classes with the attributes from <photo> nodes.

	import android.net.Uri
	import com.commonsound.common.data.flickr.FlickrApi.Routes.FLICKR_REST_SERVICE
	import com.commonsound.common.data.flickr.models.FlickrPhotoModel
	import io.ktor.client.*
	import kotlinx.coroutines.Dispatchers
	import kotlinx.coroutines.withContext
	import org.w3c.dom.NodeList
	import java.io.BufferedInputStream
	import java.io.BufferedReader
	import java.net.URL
	import javax.net.ssl.HttpsURLConnection
	import javax.xml.parsers.DocumentBuilder
	import javax.xml.parsers.DocumentBuilderFactory
	data class FlickrPhotoModel(
	var id: String,
	var server: String,
	var secret: String
	)
	interface FlickrApi {
	suspend fun getPhotos(tag: String): List<FlickrPhotoModel>
	companion object Routes {
	private const val BASE_FLICKR_URL = "https://www.flickr.com/"
	const val FLICKR_REST_SERVICE = "${BASE_FLICKR_URL}services/rest"
	}
	}
	class FlickrApiImpl(private val client: HttpClient): FlickrApi {
	override suspend fun getPhotos(tag: String): List<FlickrPhotoModel> {
	return withContext(Dispatchers.IO) {
	val encodedQuery = Uri.Builder()
	.appendQueryParameter("tags", tag)
	.appendQueryParameter("method", "flickr.photos.search")
	.appendQueryParameter("api_key", "some_key")
	.appendQueryParameter("per_page", "5")
	.build().encodedQuery
	val urlConnection = URL("$FLICKR_REST_SERVICE?$encodedQuery")
	.openConnection() as HttpsURLConnection
	try {
	urlConnection.requestMethod = "GET"
	val inStream = BufferedInputStream(urlConnection.getInputStream())
	val content = BufferedReader(inStream.reader()).readText()
	parseFlickrPhotosXml(content)
	} finally {
	urlConnection.disconnect()
	}
	}
	}
	private fun parseFlickrPhotosXml(s: String): List<FlickrPhotoModel> {
	val builder: DocumentBuilder = DocumentBuilderFactory.newInstance().newDocumentBuilder()
	val photoNodes: NodeList = builder.parse(s.byteInputStream()).getElementsByTagName("photo")
	val result = mutableListOf<FlickrPhotoModel>()
	for (p in 0 until photoNodes.length) {
	val photo = photoNodes.item(p)
	result.add(FlickrPhotoModel(
	photo.attributes.getNamedItem("id").nodeValue,
	photo.attributes.getNamedItem("server").nodeValue,
	photo.attributes.getNamedItem("secret").nodeValue
	))
	}
	return result
	}
	}

view raw Flickr parsing XML response hosted with ❤ by GitHub

While this approach may not be ideal if we need to work extensively with XML API, it is a proper fallback to use for just a bunch of requests.