I already created an android app which uses a web service to send and retrieve JSON data.
When I make a request while the device is online it works fine but when the device goes offline the app is stuck and prints Null Pointer Exception error.
Is there a way to listen to the internet connection?
From your question and comments, looks like you have a problem if the connection is lost during the request/response process. So in order to listen for this change you need to crate a BroadcastReceiver (NetworkStateReceiver) to listen to network state change, something like this: NetworkStateReceiver.java
package your.package.name;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import java.util.ArrayList;
import java.util.List;
public class NetworkStateReceiver extends BroadcastReceiver {
// Listeners list
protected List<NetworkStateReceiverListener> listeners;
// Connection flag
protected Boolean connected;
/**
* Public constructor
*/
public NetworkStateReceiver() {
listeners = new ArrayList<NetworkStateReceiverListener>();
connected = null;
}
/**
*
* @param context Context - Application context
* @param intent Intent - Manages application actions on network state changes
*/
public void onReceive(Context context, Intent intent) {
if(intent == null || intent.getExtras() == null) return;
ConnectivityManager manager = (ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo ni = manager.getActiveNetworkInfo();
if((ni != null) && ni.isConnected()) {
connected = true;
} else {
connected = false;
}
mNotifyStateToAll();
}
/**
* Notify the state to all needed methods
*/
private void mNotifyStateToAll() {
for(NetworkStateReceiverListener listener : listeners)
mNotifyState(listener);
}
/**
* Notify the network state
* @param listener NetworkStateReceiverListener - receives network state change
*/
private void mNotifyState(NetworkStateReceiverListener listener) {
if(connected == null || listener == null) return;
if(connected == true) {
listener.networkAvailable();
} else {
listener.networkUnavailable();
}
}
/**
* Add listener once it is needed
* @param l NetworkStateReceiverListener - receives network state change
*/
public void addListener(NetworkStateReceiverListener l) {
listeners.add(l);
mNotifyState(l);
}
/**
* Remove the listener once it is not needed anymore
* @param l NetworkStateReceiverListener - receives network state change
*/
public void removeListener(NetworkStateReceiverListener l) {
listeners.remove(l);
}
/**
* Set interface to communicate with Main methods
*/
public interface NetworkStateReceiverListener {
public void networkAvailable();
public void networkUnavailable();
}
}
Your activity needs to implement this:
public class MyActivity extends Activity implements NetworkStateReceiver.NetworkStateReceiverListener {
// Receiver that detects network state changes
private NetworkStateReceiver networkStateReceiver;
private boolean mNetworkAvailable;
...
// What ever the code you want or need
...
/**
* Call back for NetworkStateReceiver to set the network state to available
*/
@Override
public void networkAvailable() {
Log.d(TAG, "I'm in, baby! Dance, dance revolution!");
sNetworkAvailable = true;
// Network available again do things here
}
/**
* Call back for NetworkStateReceiver to set the network state to unavailable
*/
@Override
public void networkUnavailable() {
Log.d(TAG, "I'm dancing with myself, noone can see me.");
sNetworkAvailable = false;
// Network broke, warn the user, or do alternative action
}
/**
* Need to register the receiver
*/
@Override
public void onResume() {
super.onResume();
// Register the network state receiver to listen to network state change
if (networkStateReceiver == null) {
networkStateReceiver = new NetworkStateReceiver();
networkStateReceiver.addListener(this);
this.registerReceiver(networkStateReceiver, new IntentFilter(android.net.ConnectivityManager.CONNECTIVITY_ACTION));
}
}
/**
* Unregister the receiver as you do not need it anymore
*/
@Override
public void onDestroy() {
super.onDestroy();
// Remove network state receiver and listener as we don't need them at this point
if (networkStateReceiver != null) {
networkStateReceiver.removeListener(this);
this.unregisterReceiver(networkStateReceiver);
networkStateReceiver = null;
}
}
...
// What ever the code you want or need
...
}
You can check if it is connected to internet through this.
private boolean isNetworkAvailable() {
ConnectivityManager manager = (ConnectivityManager)
getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo networkInfo = manager.getActiveNetworkInfo();
boolean isAvailable = false;
if(networkInfo != null && networkInfo.isConnected()) {
isAvailable = true;
}
return isAvailable;
}
In your manifest add permission
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
Java
public boolean networkstatus() {
ConnectivityManager connectivityManager = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo activeNetworkInfo = connectivityManager.getActiveNetworkInfo();
return activeNetworkInfo != null && activeNetworkInfo.isConnected();
}
if(networkstatus())
{
// do your process
}
else
{
//alert message for no internet connection
}
before request the web service call this method
private boolean isOnline() {
ConnectivityManager cm = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo NInfo = cm.getActiveNetworkInfo();
if (NInfo != null && NInfo.isConnectedOrConnecting())
return true;
else
return false;
}
if(isOnline()){
// request the service
// but make sure that U have surrounded the calling web-service by try and catch
try{
// here make your request when the connection go offline the app will catch the error and ignore the process
}catch (Exception e) {
}
add this permission
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />