使用ConnectivityManager监听网络状态变化

Java代码  

mIntenFilter = new IntentFilter(ConnectivityManager.CONNECTIVITY_ACTION);
   mConnectivityManager = (ConnectivityManager) getSystemService(CONNECTIVITY_SERVICE);
   mReceiver = new BroadcastReceiver() {

    @Override
    public void onReceive(Context context, Intent intent) {

        if (intent.getBooleanExtra(ConnectivityManager.EXTRA_NO_CONNECTIVITY, false)) {
            Log.i(TAG, "netWork has lost");
        }

        NetworkInfo tmpInfo = (NetworkInfo) intent.getExtras().get(ConnectivityManager.EXTRA_NETWORK_INFO);
        Log.i(TAG, tmpInfo.toString() + " {isConnected = " + tmpInfo.isConnected() + "}");
    }
};

注册Receiver，监听网络变化： 

Java代码  

registerReceiver(mReceiver, mIntenFilter);

取消注册，不再监听网络变化： 

Java代码  

unregisterReceiver(mReceiver);

也可在AndroidManifest.xml中注册BroadcastReceiver 

Xml代码  

<receiver android:name=".ConnectionReceiver" >
    <intent-filter>
        <action android:name="android.net.conn.CONNECTIVITY_CHANGE" />
    </intent-filter>
</receiver>

程序第一次启动时会收到一个Broadcast

从 GPRS 到 WIFI，程序至少会收到3个Broadcast 
第一个是连接到WIFI 
第二个是断开GPRS 
第三个是连接到WIFI 

从WIFI到GPRS，程序至少会收到2个Broadcast 
第一个是断开Wifi 
第二个是连接到GPRS