BroadcastReceiver is not working when app is closed

Question

I have one MainActivity.java and one SmsReceiver.java I want to start the MainActivity when any message is recieved in phone.

MainActivity.java -->

public class MainActivity extends AppCompatActivity{

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        String sms = getIntent().getStringExtra("message");
    }
}

SmsReceiver.java -->

public class SmsReceiver extends BroadcastReceiver{

    String str = "";

    @Override
    public void onReceive(Context context, Intent intent) {
        Bundle bundle = intent.getExtras();
        SmsMessage msgs = null;
        if (bundle != null) {
            Object[] pdus = (Object[]) bundle.get("pdus");
            for (int i = 0; i < pdus.length; i++) {
                msgs = SmsMessage.createFromPdu((byte[]) pdus[i]);
                str += msgs.getMessageBody().toString();
            }
        sendToMain(context,intent);
        }
    }

    public void sendToMain(Context context,Intent intent){
        Toast.makeText(context,"Message received", Toast.LENGTH_SHORT).show();
        Intent i = new Intent(context,MainActivity.class);
        i.putExtra("message",str);
        i.setFlags(Intent.FLAG_ACTIVITY_NEW_TASK|   Intent.FLAG_ACTIVITY_CLEAR_TOP| Intent.FLAG_ACTIVITY_SINGLE_TOP);
        context.startActivity(i);
    }
}

and AndroidManifest.xml contains -->

    <receiver android:name=".SmsReceiver"
                android:exported="true">
        <intent-filter>
            <action android:name=
                "android.provider.Telephony.SMS_RECEIVED" />
        </intent-filter>
    </receiver>

and permissions are also given -->

<uses-permission android:name="android.permission.RECEIVE_SMS"/>
<uses-permission android:name="android.permission.READ_SMS"/>

But when any new Message is received, nothing happens...

Please suggest if any thing is wrong here.