所以我正在尝试创建一个应用程序，用于与服务器通信，通过网络发送QR扫描结果。此刻我可以登录。但是当我尝试请求运行时权限来使用相机时，我的错误日志表明它由于nullpointer异常而无法调用onPause（）方法。虽然我已经指定在未授予权限时不启动QR扫描程序...所以我猜这是UI线程崩溃。单击扫描按钮时，将提示权限窗口（方法getScanPermit（）），如果是，则调用ScanQR（）方法，初始化扫描程序。这是我的代码：
public class Client extends AppCompatActivity implements ZXingScannerView.ResultHandler{
public static final int CONNECTIONPORT = 5000;
public static final int CAMERA_REQUEST_CODE = 10;
private Socket socket = null;
private ObjectOutputStream outputStream;
private ObjectInputStream inputStream;
private ZXingScannerView scannerView;
public boolean connected = false;
private static String ip = "192.168.0.191";
String message = "";
EditText username;
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
StrictMode.setThreadPolicy(policy);
setContentView(R.layout.loginscene);
username = (EditText) findViewById(R.id.usernameText);
final Button loginbutton = (Button) findViewById(R.id.loginbutton);
loginbutton.setOnClickListener(new View.OnClickListener() {
@Override
public void onClick(View v) {
InitConnection();
}
});
}
public void InitConnection() {
try {
socket = new Socket(ip, CONNECTIONPORT);
outputStream = new ObjectOutputStream(socket.getOutputStream());
outputStream.flush();
inputStream = new ObjectInputStream(socket.getInputStream());
connected = true;
socket.setSoTimeout(1000);
} catch (IOException connecterror) {
Toast.makeText(getApplicationContext(), "could not connect to server", Toast.LENGTH_SHORT).show();
}
send("login");
send(username.getText().toString());
try {
String stringy = (String) inputStream.readObject();
if (stringy.equals("granted")) {
setContentView(R.layout.connectedscene);
} else {
Toast.makeText(getApplicationContext(), "Invalid Username", Toast.LENGTH_SHORT).show();
outputStream.close();
inputStream.close();
socket.close();
}
} catch (ClassNotFoundException | IOException e1) {
Toast.makeText(getApplicationContext(), "could not read answer from server", Toast.LENGTH_SHORT).show();
}
}
public void send(Object o) {
if (connected) {
try {
outputStream.reset();
outputStream.writeObject(o);
outputStream.flush();
} catch (IOException e) {
connected = false;
Toast.makeText(getApplicationContext(), "could not send data to server", Toast.LENGTH_SHORT).show();
try {
outputStream.close();
inputStream.close();
} catch (IOException e3) {
Toast.makeText(getApplicationContext(), "could not close streams", Toast.LENGTH_SHORT).show();
}
}
} else {
Toast.makeText(getApplicationContext(), "you are not connected to the server", Toast.LENGTH_SHORT).show();
}
}
public void getScanPermit(View v){
if(checkSelfPermission(Manifest.permission.CAMERA)== PackageManager.PERMISSION_GRANTED){
ScanQR();
}else{
String[] permissionRequested = {Manifest.permission.CAMERA};
requestPermissions(permissionRequested, CAMERA_REQUEST_CODE);
}
}
@Override
public void onRequestPermissionsResult(int requestCode, @NonNull String[] permissions, @NonNull int[] grantResults) {
super.onRequestPermissionsResult(requestCode, permissions, grantResults);
if(requestCode==CAMERA_REQUEST_CODE){
if(grantResults[0]==PackageManager.PERMISSION_GRANTED){
ScanQR();
}else{
Toast.makeText(getApplicationContext(),"Unable to scan QR without camera permission",Toast.LENGTH_SHORT).show();
}
}
}
public void ScanQR(){
scannerView = new ZXingScannerView(this);
setContentView(scannerView);
scannerView.setResultHandler(this);
scannerView.startCamera();
}
@Override
protected void onPause() {
super.onPause();
scannerView.stopCamera();
}
@Override
public void handleResult(Result result) {
Toast.makeText(getApplicationContext(), result.getText().toString(), Toast.LENGTH_SHORT).show();
}}
这是错误日志（对不起白空间）：
E/AndroidRuntime: FATAL EXCEPTION: main Process:
java.lang.RuntimeException: Unable to pause activity {hofman.jasper.app/hofman.jasper.app.Client}: java.lang.NullPointerException: Attempt to invoke virtual method 'void me.dm7.barcodescanner.zxing.ZXingScannerView.stopCamera()' on a null object reference
at android.app.ActivityThread.performPauseActivityIfNeeded(ActivityThread.java:3751)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3717)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3691)
at android.app.ActivityThread.handlePauseActivity(ActivityThread.java:3665)
at android.app.ActivityThread.-wrap16(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1489)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'void me.dm7.barcodescanner.zxing.ZXingScannerView.stopCamera()' on a null object reference
at hofman.jasper.app.Client.onPause(Client.java:158)
at android.app.Activity.performPause(Activity.java:6819)
at android.app.Instrumentation.callActivityOnPause(Instrumentation.java:1322)
at android.app.ActivityThread.performPauseActivityIfNeeded(ActivityThread.java:3740)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3717)
at android.app.ActivityThread.performPauseActivity(ActivityThread.java:3691)
at android.app.ActivityThread.handlePauseActivity(ActivityThread.java:3665)
at android.app.ActivityThread.-wrap16(ActivityThread.java)
at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1489)
at android.os.Handler.dispatchMessage(Handler.java:102)
at android.os.Looper.loop(Looper.java:154)
at android.app.ActivityThread.main(ActivityThread.java:6119)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:886)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:776)
我希望你能提前给我一些提示！谢谢！