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Working with HTTP
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Here we re going to use some of the most common methods and concepts to talk to network resources from Android applications. To begin, we ll retrieve data using HTTP GET requests to a simple HTML page, using the standard java.net API. From there, we ll look at using the Android-included Apache HttpClient API. After we use HttpClient directly to get a feel for it, we ll also make a helper class, HttpRequestHelper, that you can use to simplify the process and encapsulate the details. This class and the Apache networking API in general has a few advantages over rolling your own networking with java.net, as you ll see. When the helper class is in place, we ll use it to make additional HTTP and HTTPS requests, both GET and POST, and we ll look at basic authentication. Our first HTTP request will be an HTTP GET call using an HttpUrlConnection.
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Simple HTTP and java.net
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The most basic HTTP request method is GET. In this type of request, any data that s sent is embedded in the URL, using the query string. The next class in our NetworkExplorer application, which is shown in the following listing, has an Activity that demonstrates the GET request method.
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Listing 6.4 The SimpleGet Activity showing java.net.UrlConnection
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public class SimpleGet extends Activity { . . . other portions of onCreate omitted for brevity this.getButton.setOnClickListener(new OnClickListener() { public void onClick(View v) { Invoke getHttpResponse getOutput.setText(""); method String output = getHttpResponse(getInput.getText().toString()); if (output != null) { getOutput.setText(output); } } }); }; . . . private String getHttpResponse(String location) { String result = null; URL url = null; Construct try { URL object url = new URL(location);
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} catch (MalformedURLException e) { // log and or handle } if (url != null) { Open connection using try { HttpURLConnection HttpURLConnection urlConn = (HttpURLConnection) url.openConnection(); BufferedReader in = new BufferedReader( new InputStreamReader(
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urlConn.getInputStream())); String inputLine; int lineCount = 0; // limit lines for example while ((lineCount < 10) && ((inputLine = in.readLine()) != null)) { lineCount++; result += "\n" + inputLine; } in.close(); urlConn.disconnect(); } catch (IOException e) { // log and or handle } } else { // log and or handle } return result; } }
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Append to result
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Read data
To get an HTTP response and show the first few lines of it in our SimpleGet class, we call a getHttpResponse method that we ve built B. Within this method, we construct a java.net.URL object C, which takes care of many of the details for us, and then we open a connection to a server using an HttpURLConnection D. We then use a BufferedReader to read data from the connection one line at a time E. Keep in mind that as we re doing this, we re using the same thread as the UI and therefore blocking the UI. This isn t a good idea. We re using the same thread here only to demonstrate the network operation; we ll explain more about how to use a separate thread shortly. After we have the data, we append it to the result String that our method returns F, and we close the reader and the connection. Using the plain and simple java.net support that has been ported to Android this way provides quick and dirty access to HTTP network resources. Communicating with HTTP this way is fairly easy, but it can quickly get cumbersome when you need to do more than just retrieve simple data, and, as noted, the blocking nature of the call is bad form. You could get around some of the problems with this approach on your own by spawning separate threads and keeping track of them and by writing your own small framework/API structure around that concept for each HTTP request, but you don t have to. Fortunately, Android provides another set of APIs in the form of the Apache HttpClient3 library that abstract the java.net classes further and are designed to offer more robust HTTP support and help handle the separate-thread issue.
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