我已经扩展了一个Broadcastreceiver，以便我的应用程序知道在线状态何时被更改。问题是：每当在线状态发生变化时，我的广播接收器似乎都会被重建。因为我在稍后的时刻添加了一个监听器而不是构造函数，所以我的监听器是空的，我得到一个NullPointerException：
02-07 11:11:45.804: E/AndroidRuntime(21416): FATAL EXCEPTION: main
02-07 11:11:45.804: E/AndroidRuntime(21416): java.lang.RuntimeException: Unable to start receiver nl.raakict.android.dnd.util.NetworkStateReceiver: java.lang.NullPointerException
02-07 11:11:45.804: E/AndroidRuntime(21416): at android.app.ActivityThread.handleReceiver(ActivityThread.java:2699)
02-07 11:11:45.804: E/AndroidRuntime(21416): at android.app.ActivityThread.access$1500(ActivityThread.java:162)
02-07 11:11:45.804: E/AndroidRuntime(21416): at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1453)
02-07 11:11:45.804: E/AndroidRuntime(21416): at android.os.Handler.dispatchMessage(Handler.java:99)
02-07 11:11:45.804: E/AndroidRuntime(21416): at android.os.Looper.loop(Looper.java:158)
02-07 11:11:45.804: E/AndroidRuntime(21416): at android.app.ActivityThread.main(ActivityThread.java:5751)
02-07 11:11:45.804: E/AndroidRuntime(21416): at java.lang.reflect.Method.invokeNative(Native Method)
02-07 11:11:45.804: E/AndroidRuntime(21416): at java.lang.reflect.Method.invoke(Method.java:511)
02-07 11:11:45.804: E/AndroidRuntime(21416): at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:1083)
02-07 11:11:45.804: E/AndroidRuntime(21416): at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:850)
02-07 11:11:45.804: E/AndroidRuntime(21416): at dalvik.system.NativeStart.main(Native Method)
02-07 11:11:45.804: E/AndroidRuntime(21416): Caused by: java.lang.NullPointerException
02-07 11:11:45.804: E/AndroidRuntime(21416): at nl.raakict.android.dnd.util.NetworkStateReceiver.onReceive(NetworkStateReceiver.java:38)
02-07 11:11:45.804: E/AndroidRuntime(21416): at android.app.ActivityThread.handleReceiver(ActivityThread.java:2687)
02-07 11:11:45.804: E/AndroidRuntime(21416): ... 10 more
我的自定义广播接收器如下所示：
package nl.raakict.android.dnd.util;
import android.content.BroadcastReceiver;
import android.content.Context;
import android.content.Intent;
import android.net.ConnectivityManager;
import android.net.NetworkInfo;
import android.util.Log;
public class NetworkStateReceiver extends BroadcastReceiver{
private Listener mListener = null;
private static NetworkStateReceiver _instance;
public static NetworkStateReceiver getInstance()
{
if (_instance == null)
{
_instance = new NetworkStateReceiver();
}
return _instance;
}
public NetworkStateReceiver(){
}
@Override
public void onReceive(Context context, Intent intent) {
Log.d("app","Network connectivity change");
if(intent.getExtras()!=null) {
NetworkInfo ni=(NetworkInfo) intent.getExtras().get(ConnectivityManager.EXTRA_NETWORK_INFO);
if(ni!=null && ni.getState()==NetworkInfo.State.CONNECTED) {
Log.i("app","Network "+ni.getTypeName()+" connected");
mListener.onStateChange(true);
}
}
if(intent.getExtras().getBoolean(ConnectivityManager.EXTRA_NO_CONNECTIVITY,Boolean.FALSE)) {
Log.d("app","There's no network connectivity");
mListener.onStateChange(false);
}
}
public interface Listener {
public void onStateChange(boolean state);
}
public void registerListener (Listener listener) {
mListener = listener;
}
public boolean isOnline(Context context) {
ConnectivityManager cm =
(ConnectivityManager) context.getSystemService(Context.CONNECTIVITY_SERVICE);
NetworkInfo netInfo = cm.getActiveNetworkInfo();
if (netInfo != null && netInfo.isConnectedOrConnecting()) {
return true;
}
return false;
}
}
和我的清单：
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="nl.raakict.android.dnd"
android:versionCode="1"
android:versionName="1.0" >
<uses-sdk
android:minSdkVersion="14"
android:targetSdkVersion="19" />
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE" />
<uses-permission android:name="android.permission.INTERNET" />
<application
android:allowBackup="true"
android:icon="@drawable/ic_launcher"
android:label="@string/app_name"
android:theme="@style/AppTheme" >
<activity
android:name="nl.raakict.android.dnd.HomeActivity"
android:configChanges="orientation|keyboardHidden|screenSize"
android:label="@string/app_name"
android:theme="@style/FullscreenTheme" >
<intent-filter>
<action android:name="android.intent.action.MAIN" />
<category android:name="android.intent.category.LAUNCHER" />
</intent-filter>
</activity>
<receiver android:name="nl.raakict.android.dnd.util.NetworkStateReceiver">
<intent-filter>
<action android:name="android.net.conn.CONNECTIVITY_CHANGE" />
</intent-filter>
</receiver>
</application>
</manifest>
这是我的听众课程：
public class RequestHandler implements NetworkStateReceiver.Listener {
private static RequestHandler _instance;
private NetworkStateReceiver networkStateReveiver;
private ArrayList<CustomJsonRequest> delayedRequestQueue;
private boolean currentlyOnline;
private Context applicationContext;
public static RequestHandler getInstance(Context applicationContext)
{
if (_instance == null)
{
_instance = new RequestHandler(applicationContext);
}
return _instance;
}
private RequestHandler(Context applicationContext){
delayedRequestQueue= new ArrayList<CustomJsonRequest>();
this.applicationContext = applicationContext;
networkStateReveiver = NetworkStateReceiver.getInstance();
networkStateReveiver.registerListener(this);
currentlyOnline = networkStateReveiver.isOnline(applicationContext);
}
@Override
public void onStateChange(boolean online) {
// TODO Auto-generated method stub
currentlyOnline = online;
if(online){
sendRequestQueue();
}
}
public void newPostRequest(CustomJsonRequest request) {
delayedRequestQueue.add(request);
if(currentlyOnline)
sendRequestQueue();
}
public void sendRequestQueue(){
//code logic here
}
}
我做错了什么？
答案 0 :(得分：1)
问题在于，当网络状态完成时，Android将创建您的NetworkStateReceiver类实例，调用其onReceive然后销毁您的接收器实例。这就是为什么你的监听器总是为空的，如果你想通知网络变化的其他组件那么你应该在你的应用程序中发送本地广播。
您可能也会注册本地BroadcastReceiver：
“以编程方式注册您的BroadcastReceiver”： http://www.grokkingandroid.com/android-tutorial-broadcastreceiver/