Android App Development: Calling Web Services

One of the most common functionalities required in mobile applications is to call a web service to retrieve data. This process involves requesting the web service with parameters, receiving the response and parsing it to obtain data.
Today the most common web services types are SOAP and REST. Android does not provide a built in SOAP client, there are many third party libraries that can be used, but we’ll see how to call a SOAP web service with native android APIs.

Requesting SOAP web service:

Before proceeding to the code, let’s take a look at the SOAP structure:

A soap request can be something like this:

POST /InStock HTTP/1.1
Host: www.example.org
Content-Type: application/soap+xml; charset=utf-8
Content-Length: length
SOAPAction: "http://www.w3schools.com/GetItems"

<?xml version="1.0"?>
<soap:Envelope
xmlns:soap="http://www.w3.org/2001/12/soap-envelope"
soap:encodingStyle="http://www.w3.org/2001/12/soap-encoding">
<soap:Header>
  <m:Trans xmlns:m="http://www.w3schools.com/transaction/"
  soap:mustUnderstand="1">234
  </m:Trans>
</soap:Header>
<soap:Body>
  <m:GetPrice xmlns:m="http://www.w3schools.com/prices">
    <m:Item>Apples</m:Item>
  </m:GetPrice>
</soap:Body></soap:Envelope>

The SOAP request/response is sent as a SOAP Envelope which consists of a SOAP Header and a SOAP Body.

1. SOAP Header: optional component of the envelop, contains application specific information, such as authentication.
2. SOAP Body: the actual message sent to/received from the service.
3. The header can contain a SOAP Action which identifies the desired function to be called by the service.

Calling the service:

To call the SOAP web service you have to do the following:
First: construct the SOAP envelope manually like this:

String envelope="<?xml version=\"1.0\" encoding=\"utf-8\"?>"+
"<soap:Envelope xmlns:xsi=\"http://www.w3.org/2001/XMLSchema-instance\"

xmlns:xsd=\"http://www.w3.org/2001/XMLSchema\"

xmlns:soap=\"http://schemas.xmlsoap.org/soap/envelope/\">"+
  "<soap:Body>"+
    "<GetItems xmlns=\"http://tempuri.org/\">"+
      "<startDate>%s</ startDate>"+
      "<getAll>%s</getAll>"+
    "</Items>"+
  "</soap:Body>"+
"</soap:Envelope>";

where %s are place holders where you substitute request parameters in like this

String requestEnvelope=String.format(envelope, "10-5-2011","true");

Second: call the web service like this:
Second: call the web service like this:

String CallWebService(String url,
    String soapAction,
   String envelope)  {
  final DefaultHttpClient httpClient=new DefaultHttpClient();
  // request parameters
  HttpParams params = httpClient.getParams();
     HttpConnectionParams.setConnectionTimeout(params, 10000);
     HttpConnectionParams.setSoTimeout(params, 15000);
     // set parameter
  HttpProtocolParams.setUseExpectContinue(httpClient.getParams(), true);

  // POST the envelope
  HttpPost httppost = new HttpPost(url);
  // add headers
     httppost.setHeader("soapaction", soapAction);
     httppost.setHeader("Content-Type", "text/xml; charset=utf-8");

     String responseString="";
     try {

      // the entity holds the request
   HttpEntity entity = new StringEntity(envelope);
   httppost.setEntity(entity);

   // Response handler
   ResponseHandler<string> rh=new ResponseHandler<string>() {
    // invoked when client receives response
    public String handleResponse(HttpResponse response)
      throws ClientProtocolException, IOException {

     // get response entity
     HttpEntity entity = response.getEntity();

     // read the response as byte array
           StringBuffer out = new StringBuffer();
           byte[] b = EntityUtils.toByteArray(entity);

           // write the response byte array to a string buffer
           out.append(new String(b, 0, b.length));
           return out.toString();
    }
   };

   responseString=httpClient.execute(httppost, rh); 

  }
     catch (Exception e) {
      Log.v("exception", e.toString());
  }

     // close the connection
  httpClient.getConnectionManager().shutdown();
  return responseString;
 }

After calling this function, you will have the response as a String, something like this:

<?xml version="1.0" encoding="utf-8"?>
<soap:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"

xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:soap="http://schemas.xmlsoap.org/soap/envelope/">
  <soap:Body>
    <GetItemsResponse xmlns="http://tempuri.org/">
      <GetItemsResult>

        <Items>
          <Item>
            <name>string</name>
            <description>string</ description >
          </iPhoneCategory>
          <iPhoneCategory>
            <name>string</name>
            <description>string</ description >
          </ Item >
        </Items>
      </GetItemsResult>
    </ GetItemsResponse >
  </soap:Body>
</soap:Envelope>

This response needs to be parsed to extract the data.

Requesting REST web service:

REST web services are much simpler, you request the service by calling a URL with the parameters. like this

http://example.com/resources/getitems

an example of calling a REST web service:

String callWebErvice(String serviceURL){
		// http get client
        	HttpClient client=new DefaultHttpClient();
        	HttpGet getRequest=new HttpGet();

        	try {
        		// construct a URI object
				getRequest.setURI(new URI(serviceURL));
			} catch (URISyntaxException e) {
				Log.e("URISyntaxException", e.toString());
			}

			// buffer reader to read the response
        	BufferedReader in=null;
        	// the service response
        	HttpResponse response=null;
			try {
				// execute the request
				response = client.execute(getRequest);
			} catch (ClientProtocolException e) {
				Log.e("ClientProtocolException", e.toString());
			} catch (IOException e) {
				Log.e("IO exception", e.toString());
			}
        	try {
				in=new BufferedReader(new InputStreamReader(response.getEntity().getContent()));
			} catch (IllegalStateException e) {
				Log.e("IllegalStateException", e.toString());
			} catch (IOException e) {
				Log.e("IO exception", e.toString());
			}
        	StringBuffer buff=new StringBuffer("");
        	String line="";
        	try {
				while((line=in.readLine())!=null)
				{
					buff.append(line);
				}
			} catch (IOException e) {
				Log.e("IO exception", e.toString());
				return e.getMessage();
			}

        	try {
				in.close();
			} catch (IOException e) {
				Log.e("IO exception", e.toString());
			}
        	// response, need to be parsed
        	return buff.toString();
	}

 

Connecting to a web service over a Secure Sockets Layer (SSL): protocol:

Android default HttpClinet does not support SSL connections, so if you have a secured web service, you need to connect to it via javax.net.ssl.HttpsURLConnection.
if you want to call a SSL SOAP web service:

String CallWebService(String url,
			 String soapAction,
			String envelope) throws IOException  {
		URL address=new URL(url);
		URLConnection connection=address.openConnection();
		HttpsURLConnection post=(HttpsURLConnection)connection;
		post.setDoInput(true);
		post.setDoOutput(true);
		post.setRequestMethod("POST");
		post.setRequestProperty("SOAPAction", soapAction);
		post.setRequestProperty( "Content-type", "text/xml; charset=utf-8" );
		post.setRequestProperty( "Content-Length", String.valueOf(envelope.length()));
		post.setReadTimeout(4000);

		OutputStream outStream=post.getOutputStream();
		Writer out=new OutputStreamWriter(outStream);
		out.write(envelope);
		out.flush();
		out.close();

		InputStream inStream = post.getInputStream();
		BufferedInputStream in = new BufferedInputStream(inStream,4);
		StringBuffer buffer=new StringBuffer();
		// read 4 bytes a time
		byte[] buffArray=new byte[4];
		int c=0;
			while((c=in.read(buffArray))!=-1){
				for(int i=0;i<c;i++)
					buffer.append((char)buffArray[i]);
			}

			return buffer.toString();
	}

In this post we saw how to request a web service, in the next post we’re going to see how to parser the different (SOAP, XML, JSon) types of responses.