java异常：尝试调用虚拟方法“安卓”。所容纳之物资源安卓。所容纳之物上下文空对象引用上的getResources（）

Question

当我试图通过MainActivity中的setter方法在文本视图中添加两个字符串变量msg和phoneNo时。java（由MyReceiver中的dow（）方法调用）。java）运行时异常（如下所示） 当MyReceiver中的onReceive方法时发生。java文件由广播接收器调用。我无法理解这个例外

主要活动。爪哇

package com.example.smsreceiver;

import 安卓x.annotation.NonNull;
import 安卓x.appcompat.app.AppCompatActivity;
import 安卓x.core.app.ActivityCompat;
import 安卓x.core.content.ContextCompat;

import 安卓.Manifest;
import 安卓.content.pm.PackageManager;
import 安卓.os.Bundle;
import 安卓.widget.TextView;
import 安卓.widget.Toast;


public class MainActivity extends AppCompatActivity {

    private static final int MY_PERMISSION_REQUEST_RECEIVE_SMS = 0;
    TextView textView;

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

        textView = findViewById(R.id.textView2);

        //check if the permission is not granted
        if(ContextCompat.checkSelfPermission(this, Manifest.permission.RECEIVE_SMS) != PackageManager.PERMISSION_GRANTED) {
            if(!ActivityCompat.shouldShowRequestPermissionRationale(this, Manifest.permission.RECEIVE_SMS)) {
                //a pop up will appear asking for required permission i.e. Allow or Deny
                ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.RECEIVE_SMS}, MY_PERMISSION_REQUEST_RECEIVE_SMS);
            }
        }

        //this.setter();
    }//onCreate
    //after getting the result of permission request the result will be passed through this method

    @Override
    public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults)
    {

        //will check the requestCode
        if (requestCode == MY_PERMISSION_REQUEST_RECEIVE_SMS) {//check whether the length of grantResults is greater than 0 and is equal to PERMISSION_GRANTED
            if (grantResults.length > 0 && grantResults[0] == PackageManager.PERMISSION_GRANTED) {
                //Now broadcast receiver work in background
                Toast.makeText(this, "Thank you for permitting!", Toast.LENGTH_LONG).show();

            } else {
                Toast.makeText(this, "Well I can't do anything until you permit me", Toast.LENGTH_LONG).show();
            }
        }
    }

    public void setter(String msg, String phoneNo){
        setContentView(R.layout.activity_main);
        textView = findViewById(R.id.textView2);
        String hi = msg + phoneNo;
        textView.setText(hi);
        Toast.makeText(this, "message: " + msg + "\n Number: " + phoneNo, Toast.LENGTH_LONG).show();

    }

}

我的接收器。爪哇

package com.example.smsreceiver;

import 安卓.content.BroadcastReceiver;
import 安卓.content.Context;
import 安卓.content.Intent;
import 安卓.os.Bundle;
import 安卓.telephony.SmsMessage;
import 安卓.util.Log;
import 安卓.widget.Toast;

import java.util.Objects;

public class MyReceiver extends BroadcastReceiver {

    private static final  String SMS_RECEIVED = "安卓.provider.Telephony.SMS_RECEIVED";
    private static final String TAG = "SmsBroadcastReceiver";
    public static String msg, phoneNo = "";

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

        Log.i(TAG, "Intent Received: " + intent.getAction());
        if(Objects.equals(intent.getAction(), SMS_RECEIVED)) {

            //retrieves a map of extended data from the intent
            Bundle dataBundle = intent.getExtras();

            if (dataBundle != null) {

                //create an PDU(Protocol Data Unit) object which is a protocol for transferring message
                Object[] mypdu = (Object[])dataBundle.get("pdus");
                final SmsMessage[] message = new SmsMessage[mypdu.length];

                for (int i = 0; i < mypdu.length; i++) {

                    //for build version >= API Level 23 (Build.VERSION_CODES.M)

                    String format = dataBundle.getString("format");
                    //from PDU we get all abject and SmsMessage Object using following line of code
                    message[i] = SmsMessage.createFromPdu((byte[]) mypdu[i], format);
                    msg = message[i].getMessageBody();
                    phoneNo = message[i].getOriginatingAddress();
                }
                Toast.makeText( context, "message: " + msg + "\n NUmber: " + phoneNo, Toast.LENGTH_LONG).show();
                dow(msg, phoneNo);
            }
        }
    }
    public void  dow(String msg, String phoneNo){
        MainActivity mainActivity = new MainActivity();
        mainActivity.setter(msg, phoneNo);
    }

}

主要活动。xml

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:安卓="http://schemas.安卓.com/apk/res/安卓"
    xmlns:app="http://schemas.安卓.com/apk/res-auto"
    xmlns:tools="http://schemas.安卓.com/tools"
    安卓:layout_width="match_parent"
    安卓:layout_height="match_parent"
    安卓:orientation="vertical"
    tools:context=".MainActivity">


    <TextView
        安卓:id="@+id/textView2"
        安卓:layout_width="match_parent"
        安卓:layout_height="wrap_content"
        安卓:gravity="center"
        安卓:textSize="24sp"
        />

</LinearLayout>

Logcat显示的异常

Process: com.example.smsreceiver, PID: 18414
    java.lang.RuntimeException: Unable to start receiver com.example.smsreceiver.MyReceiver: java.lang.NullPointerException: Attempt to invoke virtual method '安卓.content.res.Resources 安卓.content.Context.getResources()' on a null object reference
        at 安卓.app.ActivityThread.handleReceiver(ActivityThread.java:3665)
        at 安卓.app.ActivityThread.access$1500(ActivityThread.java:219)
        at 安卓.app.ActivityThread$H.handleMessage(ActivityThread.java:1850)
        at 安卓.os.Handler.dispatchMessage(Handler.java:106)
        at 安卓.os.Looper.loop(Looper.java:227)
        at 安卓.app.ActivityThread.main(ActivityThread.java:7200)
        at java.lang.reflect.Method.invoke(Native Method)
        at com.安卓.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:575)
        at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:903)
     Caused by: java.lang.NullPointerException: Attempt to invoke virtual method '安卓.content.res.Resources 安卓.content.Context.getResources()' on a null object reference
        at 安卓.content.ContextWrapper.getResources(ContextWrapper.java:91)
        at 安卓.view.ContextThemeWrapper.getResourcesInternal(ContextThemeWrapper.java:127)
        at 安卓.view.ContextThemeWrapper.getResources(ContextThemeWrapper.java:121)
        at 安卓x.appcompat.app.AppCompatActivity.getResources(AppCompatActivity.java:543)
        at 安卓.widget.Toast.<init>(Toast.java:139)
        at 安卓.widget.Toast.makeText(Toast.java:306)
        at 安卓.widget.Toast.makeText(Toast.java:296)
        at com.example.smsreceiver.MainActivity.setter(MainActivity.java:60)
        at com.example.smsreceiver.MyReceiver.dow(MyReceiver.java:51)
        at com.example.smsreceiver.MyReceiver.onReceive(MyReceiver.java:45)
        at 安卓.app.ActivityThread.handleReceiver(ActivityThread.java:3649)
        at 安卓.app.ActivityThread.access$1500(ActivityThread.java:219) 
        at 安卓.app.ActivityThread$H.handleMessage(ActivityThread.java:1850) 
        at 安卓.os.Handler.dispatchMessage(Handler.java:106) 
        at 安卓.os.Looper.loop(Looper.java:227) 
        at 安卓.app.ActivityThread.main(ActivityThread.java:7200) 
        at java.lang.reflect.Method.invoke(Native Method) 
        at com.安卓.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:575) 
        at com.安卓.internal.os.ZygoteInit.main(ZygoteInit.java:903) 

Answer 1

您正在尝试自己实例化MainActivity。相反，请执行以下操作： 1） 在MainActivity中创建MyReceiver作为内部类，并像往常一样注册它。 2） 接收广播并从onReceive（）中更新UI