我已经创建了一个使用网络服务发送和检索 JSON 数据的 Android 应用。
当我在设备在线时发出请求时，它工作正常，但当设备离线时，应用程序卡住并打印 Null Pointer Exception 错误。
有没有办法收听互联网连接？
【问题讨论】：
标签： php android json web-services
你可以通过这个检查它是否连接到互联网。
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;
}
【讨论】：
从您的问题和 cmets 来看，如果在请求/响应过程中连接丢失，您可能会遇到问题。所以为了监听这个变化，你需要创建一个 BroadcastReceiver (NetworkStateReceiver) 来监听网络状态变化，如下所示： 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();
}
}
您的活动需要实现这一点：
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
...
}
【讨论】：
在请求网络服务之前调用这个方法
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) {
}
添加此权限
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
【讨论】：
在你的清单中添加权限
<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
}
【讨论】：
由 (Seshu Vinay) 回答
此方法检查手机是否已连接到互联网，如果已连接则返回 true：
private boolean isNetworkConnected() {
ConnectivityManager cm = (ConnectivityManager) getSystemService(Context.CONNECTIVITY_SERVICE);
return cm.getActiveNetworkInfo() != null;
}
在清单中，
<uses-permission android:name="android.permission.ACCESS_WIFI_STATE" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
编辑：此方法实际上检查设备是否连接到互联网（有可能它连接到网络但未连接到互联网）。
public boolean isInternetAvailable() {
try {
InetAddress ipAddr = InetAddress.getByName("google.com"); //You can replace it with your name
if (ipAddr.equals("")) {
return false;
} else {
return true;
}
} catch (Exception e) {
return false;
}
}
【讨论】：