在SharedPreferences中存储访问令牌会返回null对象引用错误

Question

我试图存储登录后从oAuth2服务器收到的访问令牌，但当我尝试存储它时，我收到了一个错误java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.dentdevils.helper.retrofit.oauth2.PreferenceClass.setToken(java.lang.String)' on a null object reference，我不确定原因。

以下是我的课程：PreferenceClass.java

package com.example.dentdevils.helper.retrofit.oauth2;
import android.content.Context;
import android.content.SharedPreferences;
public class PreferenceClass {
public static final String PREFERENCE = "accessTokenPref";
private SharedPreferences sharedPreferences;
public PreferenceClass(Context context) {
sharedPreferences = context.getSharedPreferences(PREFERENCE, Context.MODE_PRIVATE);
}
public String getToken() {
String token = sharedPreferences.getString("token", "");
return token;
}
public void setToken(String token) {
SharedPreferences.Editor editor = sharedPreferences.edit();
editor.putString("token", token);
editor.commit();
}
public void clearToken() {
SharedPreferences.Editor editor = sharedPreferences.edit();
editor.remove("token");
editor.commit();
}
}

MainActivity.Java

package com.example.dentdevils;
import android.Manifest;
import androidx.appcompat.app.AppCompatActivity;
import androidx.core.app.ActivityCompat;
import androidx.core.content.ContextCompat;
import android.app.AlertDialog;
import android.app.ProgressDialog;
import android.content.Context;
import android.content.DialogInterface;
import android.content.Intent;
import android.content.SharedPreferences;
import android.content.pm.PackageManager;
import android.os.Bundle;
import android.util.Log;
import android.widget.Button;
import android.widget.EditText;
import android.widget.Toast;
import com.example.dentdevils.helper.retrofit.oauth2.PreferenceClass;
import com.example.dentdevils.helper.retrofit.oauth2.client.cOauthService;
import com.example.dentdevils.helper.retrofit.oauth2.request.AccessToken;
import com.example.dentdevils.helper.retrofit.oauth2.response.AccessTokenResponse;
import com.example.dentdevils.helper.retrofit.oauth2.service.OauthService;
import com.example.dentdevils.ui.HomeActivity;
import retrofit2.Call;
import retrofit2.Callback;
public class MainActivity extends AppCompatActivity {
SharedPreferences sharedPreferences;
public static final String mypreference = "mypref";
public static final String email = "emailKey";
public static final String access = "accessKey";
public static final int MY_PERMISSIONS_REQUEST_CAMERA = 99;
private static final String TAG = MainActivity.class.getSimpleName();
private Button btnLogin;
private EditText inputEmail;
private EditText inputPassword;
private ProgressDialog progressDialog;
private PreferenceClass preferenceClass;


@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_login);
checkCameraPermission();
sharedPreferences = getSharedPreferences(mypreference,
Context.MODE_PRIVATE);
inputEmail = findViewById(R.id.email);
inputPassword = findViewById(R.id.txtEditPassword);
btnLogin = findViewById(R.id.btnLogin);
progressDialog = new ProgressDialog(this);
progressDialog.setCancelable(false);
btnLogin.setOnClickListener(v -> getAccessToken());
}
public void getAccessToken() {
progressDialog.show();
AccessToken accessToken = new AccessToken();
accessToken.setClient_id("client");
accessToken.setClient_secret("client");
accessToken.setGrant_type("client_credentials");
OauthService oauthService = cOauthService.createService(OauthService.class, inputEmail.getText().toString(), inputPassword.getText().toString());
Call<AccessTokenResponse> call = oauthService.fetchAccessToken(accessToken);
call.enqueue(new Callback<AccessTokenResponse>() {
@Override
public void onResponse(Call<AccessTokenResponse> call, retrofit2.Response<AccessTokenResponse> response) {
if (response.isSuccessful()) {
String a = response.body().getAccess_token();
preferenceClass.setToken(a);
progressDialog.dismiss();
Intent i = new Intent(getApplicationContext(), HomeActivity.class);
startActivity(i);
} else {
progressDialog.dismiss();
Toast.makeText(MainActivity.this, "There was an error, please try again!", Toast.LENGTH_LONG).show();
}
}
@Override
public void onFailure(Call<AccessTokenResponse> call, Throwable t) {
Log.d("Error", t.getMessage());
}
});
}
public boolean checkCameraPermission() {
if (ContextCompat.checkSelfPermission(this, Manifest.permission.CAMERA) != PackageManager.PERMISSION_GRANTED) {
if (ActivityCompat.shouldShowRequestPermissionRationale(this, Manifest.permission.CAMERA)) {
new AlertDialog.Builder(this)
.setTitle("Permission Required!")
.setMessage("This app requires the camera permission to be able to capture images of car damage. Please grant the app access to the camera by going into your Settings > Apps > Apps > Dent Devils > Permissions > Camera, then press Allow.")
.setPositiveButton("", new DialogInterface.OnClickListener() {
@Override
public void onClick(DialogInterface dialog, int which) {
ActivityCompat.requestPermissions(MainActivity.this, new String[]{Manifest.permission.CAMERA}, MY_PERMISSIONS_REQUEST_CAMERA);
}
})
.create()
.show();
} else {
ActivityCompat.requestPermissions(this, new String[]{Manifest.permission.CAMERA}, MY_PERMISSIONS_REQUEST_CAMERA);
}
return false;
} else {
return true;
}
}
}

为什么这不起作用，并且它不将我的访问令牌存储在SharedPreferences中，有什么明显的原因吗？我知道访问令牌会被返回，因为我可以记录它并显示它。

完整的logcat输出如下所示：

2020-08-25 16:22:20.724 7197-7197/com.example.dentdevils E/AndroidRuntime: FATAL EXCEPTION: main
Process: com.example.dentdevils, PID: 7197
java.lang.NullPointerException: Attempt to invoke virtual method 'void com.example.dentdevils.helper.retrofit.oauth2.PreferenceClass.setToken(java.lang.String)' on a null object reference
at com.example.dentdevils.MainActivity$1.onResponse(MainActivity.java:76)
at retrofit2.DefaultCallAdapterFactory$ExecutorCallbackCall$1.lambda$onResponse$0$DefaultCallAdapterFactory$ExecutorCallbackCall$1(DefaultCallAdapterFactory.java:89)
at retrofit2.-$$Lambda$DefaultCallAdapterFactory$ExecutorCallbackCall$1$3wC8FyV4pyjrzrYL5U0mlYiviZw.run(Unknown Source:6)
at android.os.Handler.handleCallback(Handler.java:888)
at android.os.Handler.dispatchMessage(Handler.java:100)
at android.os.Looper.loop(Looper.java:213)
at android.app.ActivityThread.main(ActivityThread.java:8178)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:513)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1101)

Answer 1

您永远不会在MainActivity中为preferenceClass赋值。

在onCreate:中添加类似内容

preferenceClass = new PreferenceClass(this);