Android BroadcastReceiver nullpointerexception在注册

Question

I'm Trying to receive a download completed action but it says the broadcast receiver is null on registering.
But I checked it and it clearly isn't.

public class UpdateHandler extends Activity{

    private DownloadManager manager;

    /**
     * Class Constructor will only be used for non-static Referencing in a static condition.
     */
    public UpdateHandler(DownloadManager dm){
        this.manager = dm;  
    }


    public void updateApp(final String appname) {

        String apkurl = apk_url + appname;
        DownloadManager.Request request = new DownloadManager.Request(
                Uri.parse(apkurl));
        request.setDescription("Even geduld aub...");
        request.setTitle("Bimii-app wordt gedownload");
        request.setDestinationInExternalPublicDir(
                Environment.DIRECTORY_DOWNLOADS, appname);

        File folder1 = android.os.Environment
                .getExternalStoragePublicDirectory(android.os.Environment.DIRECTORY_DOWNLOADS
                        + "/" + appname);
        if (folder1.exists()) {
            Log.d("!--->", Environment.DIRECTORY_DOWNLOADS + "/" + appname
                    + " pre-existant, delete!");
            folder1.delete();
        } else {
            Log.d("!--->", Environment.DIRECTORY_DOWNLOADS + "/" + appname
                    + " is new.");
        }

        registerBroadcastReceiver(request, appname);
    }


    /**
     * Installing a specific Application by name.
     * @param appname
     */
    private void installUpdates(String appname) {
        Log.d("installing ", appname);
        File filepath = android.os.Environment
                .getExternalStoragePublicDirectory(android.os.Environment.DIRECTORY_DOWNLOADS
                        + "/" + appname);

        Intent install_intent = new Intent(Intent.ACTION_VIEW);
        install_intent.setDataAndType(Uri.fromFile(filepath),
                "application/vnd.android.package-archive");
        install_intent.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK);     
        startActivity(install_intent);
    }


    private void registerBroadcastReceiver(DownloadManager.Request request, final String appname){
        // get download service and enqueue file
        manager.enqueue(request);

        BroadcastReceiver downloadDone = new BroadcastReceiver() {
            public void onReceive(Context ctxt, Intent intent) {
                Log.d("!--->", "File Download Completed + appname");
                installUpdates(appname);
            }
        };

        registerReceiver(downloadDone, new IntentFilter(
                DownloadManager.ACTION_DOWNLOAD_COMPLETE));
    }
}

And this is how I initialize my UpdateHandler as requested. This happens in my home activity.

    @Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);

    //Initialization
    init();

    loadApps();
    update_handler = new UpdateHandler((DownloadManager) getSystemService(Context.DOWNLOAD_SERVICE));
    setupButtons();
    loadListView();
}

The android version I'm running is 4.2.2 in case that matters.

Answer 1

You need to have an activity/Application context for registering the broadcast receiver.

As I can see from screenshot, you are registering receiver from utils.UpdateHandler file so you need to pass application context to this method.

So code will be

/**
     * Class Constructor will only be used for non-static Referencing in a static condition.
     */
    public UpdateHandler(Context context, DownloadManager dm){
        this.mContext = context;
        this.manager = dm;  
    }

and then call context.registerReceiver()

Answer 2

In your UpdateHandler class the context is not initialized yet. Because you are just extending the Activity class. Without calling the Activity's lifecycle method (eg: onCreate ) the context will not be available to you. It will be null.

So I suggest you pass the context from your HomeActivity to the UpdateHandler class through the constructor and use that context to register the receiver.

change your UpdateHandler like this

public class UpdateHandler{
    private DownloadManager manager;
    private Context mContext
    public UpdateHandler(Context mContext,DownloadManager dm){
        this.manager = dm;  
        this.mContext = mContext;
    }

and to register your receiver use this

context.registerReceiver(downloadDone, new IntentFilter(
                DownloadManager.ACTION_DOWNLOAD_COMPLETE));

Also you dont want to extend UpdateHandler class with Activity