Android AsyncTask ListView – JSON

Many times we need to populate Android Listview using AsyncTask. This is the case when we have to invoke a remote server and exchange information using JSON.
In this post, I want to go a bit deeper into the ListView analysis. In the previous posts, I described how to use ListView in several ways using standard and custom adapters. In all the example I’ve supposed to have a fixed items set. In this post, I will describe how it is possible to retrieve items directly from JEE Server.
To make things simple let’s suppose we have a simple JEE server that manages some contacts. Our contact is made by

  • name
  • surname,
  • email
  • phone number

How to make this server is out of the scope of this post and I will simply make available the source code. Just to give you some details, I can say that the server is a JEE server developed using RESTFul web services (in our case jersey API).
In this case, the app behaves like a client that invokes the remote service to get the contacts list and show it using the ListView. The data sent from the server to the client is formatted using JSON.

Here you can download the source code:

Implement Android AsyncTask ListView

As always we start creating our custom adapter, that uses a custom layout. If you need more information about creating a custom adapter you can give a look here and here. Here’s the code:

[java]public class SimpleAdapter extends ArrayAdapter<Contact> {
private List<Contact> itemList;
private Context context;
public SimpleAdapter(List<Contact> itemList, Context ctx) {
super(ctx, android.R.layout.simple_list_item_1, itemList);
this.itemList = itemList;
this.context = ctx;
}

public int getCount() {
if (itemList != null)
return itemList.size();
return 0;
}

public Contact getItem(int position) {
if (itemList != null)
return itemList.get(position);

return null;
}

public long getItemId(int position) {
if (itemList != null)
return itemList.get(position).hashCode();
return 0;
}

@Override
public View getView(int position, View convertView, ViewGroup parent) {
View v = convertView;
if (v == null) {
LayoutInflater inflater = (LayoutInflater)
context.getSystemService(Context.LAYOUT_INFLATER_SERVICE);
v = inflater.inflate(R.layout.list_item, null);
}

Contact c = itemList.get(position);
TextView text = (TextView) v.findViewById(R.id.name);
text.setText(c.getName());
TextView text1 = (TextView) v.findViewById(R.id.surname);
text1.setText(c.getSurname());
TextView text2 = (TextView) v.findViewById(R.id.email);
text2.setText(c.getEmail());
TextView text3 = (TextView) v.findViewById(R.id.phone);
text3.setText(c.getPhoneNum());
return v;
}

public List<Contact> getItemList() {
return itemList;
}

public void setItemList(List<Contact> itemList) {
this.itemList = itemList;
}
}[/java]

By now everything is smooth and easy.

How to develop an HTTP Client to retrieve data form ListView Adapter

One thing we need to do is creating our HTTP client in order to make a request to the JEE server. As we all know an HTTP request can require a long time before the answer is available, so we need to take this into the account so that Android OS won’t stop our application. The simplest thing is using Android AsyncTask listview, or in more details, creating an AsyncTask that makes this request and waits for the response.

[java] private class AsyncListViewLoader extends
AsyncTask<String, Void, List<Contact>> {
private final ProgressDialog dialog = new ProgressDialog(MainActivity.this);

@Override
protected void onPostExecute(List<Contact> result) {
super.onPostExecute(result);
dialog.dismiss();
adpt.setItemList(result);
adpt.notifyDataSetChanged();
}

@Override
protected void onPreExecute() {
super.onPreExecute();
dialog.setMessage("Downloading contacts…");
dialog.show();
}

@Override
protected List<Contact> doInBackground(String… params) {
List<Contact> result = new ArrayList<Contact>();
try {
URL u = new URL(params[0]);
HttpURLConnection conn = (HttpURLConnection) u.openConnection();
conn.setRequestMethod("GET");
conn.connect();
InputStream is = conn.getInputStream();

// Read the stream
byte[] b = new byte[1024];
ByteArrayOutputStream baos = new ByteArrayOutputStream();
while ( is.read(b) != -1)
baos.write(b);

String JSONResp = new String(baos.toByteArray());

JSONArray arr = new JSONArray(JSONResp);
for (int i=0; i < arr.length(); i++) {
result.add(convertContact(arr.getJSONObject(i)));
}

return result;
}
catch(Throwable t) {
t.printStackTrace();
}

return null;
}

private Contact convertContact(JSONObject obj) throws JSONException {
String name = obj.getString("name");
String surname = obj.getString("surname");
String email = obj.getString("email");
String phoneNum = obj.getString("phoneNum");

return new Contact(name, surname, email, phoneNum);
}
}[/java]

Let’s analyze the code.

In the initial step (onPreExecute()) before the task starts we simply show a dialog to inform the user that the app is downloading the contact list. The most interesting part is in the doInBackground method where the app makes the HTTP connection. We first create an HTTPConnection and set the GET method like that:

[java]HttpURLConnection conn = (HttpURLConnection) u.openConnection();
conn.setRequestMethod("GET");
conn.connect();
[/java]

Afterward, we have to create an input stream and then read the byte stream:

[java]InputStream is = conn.getInputStream();
// Read the stream
byte[] b = new byte[1024];
ByteArrayOutputStream baos = new ByteArrayOutputStream();
while ( is.read(b) != -1)
baos.write(b);
[/java]

Now we have all the stream in our byte array and we need simply parse it using JSON.

[java]JSONArray arr = new JSONArray(JSONResp);
for (int i=0; i<arr.length(); i++) {
result.add(convertContact(arr.getJSONObject(i)));
}
return result;
[/java]

Done! What’s next?…Well we need to inform the adapter that we’ve a new contact list and it has to show it. We can do it in the onPostExecute(List<Contact> result) where:

[java]super.onPostExecute(result);
dialog.dismiss();
adpt.setItemList(result);
adpt.notifyDataSetChanged();[/java]

And finally in the onCreate method we have:

[java] public void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_main);

adpt = new SimpleAdapter(new ArrayList(), this);
ListView lView = (ListView) findViewById(R.id.listview);

lView.setAdapter(adpt);

// Exec async load task
(new AsyncListViewLoader()).execute("http://10.0.2.2:8080/JSONServer/rest/ContactWS/get");
}
[/java]

Edit: Some readers asked me how the Contact.java looks like. I add it here:

[java]public class Contact implements Serializable {

private String name;
private String surname;
private String email;
private String phoneNum;

public Contact(String name, String surname, String email, String phoneNum) {
super();
this.name = name;
this.surname = surname;
this.email = email;
this.phoneNum = phoneNum;
}

// get and set methods
}
[/java]

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