目前在尝试验证用户凭据时获取nullpointer异常，我相信它是在OnPostExecute方法中引起的，但我无法弄清楚导致它的原因。这是我的BackgroundWorker类，以及我的LoginActivity。用于登录的PHP文件返回＆＃34; LoginSuccess＆＃34;或＆＃34; LoginFailed＆＃34;。当OnPostExecute尝试从Login Activity调用方法时，似乎发生了错误，是否知道如何解决这个问题？
public class BackgroundWorker extends AsyncTask<String, Void, String>{
Context context;
AlertDialog alertDialog;
LoginActivity loginActivity;
BackgroundWorker (Context ctx) {
context = ctx;
}
@Override
protected String doInBackground(String... params) {
String type = params[0];
if(type.equals("login")) {
//If a registered user is trying to login
try {
String username = params[1];
String password = params[2];
URL url = new URL(login_url);
HttpURLConnection httpURLConnection = (HttpURLConnection)url.openConnection();
httpURLConnection.setRequestMethod("POST");
httpURLConnection.setDoOutput(true);
httpURLConnection.setDoInput(true);
OutputStream outputStream = httpURLConnection.getOutputStream();
BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
String post_data = URLEncoder.encode("username", "UTF-8")+"="+URLEncoder.encode(username, "UTF-8")+"&"
+URLEncoder.encode("password", "UTF-8")+"="+URLEncoder.encode(password, "UTF-8");
bufferedWriter.write(post_data);
bufferedWriter.flush();
bufferedWriter.close();
outputStream.close();
InputStream inputStream = httpURLConnection.getInputStream();
BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream, "iso-8859-1"));
String result="";
String line="";
while((line = bufferedReader.readLine()) != null) {
result+=line;
}
bufferedReader.close();
inputStream.close();
httpURLConnection.disconnect();
return result;
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
} else if(type.equals("register")) {
//If a new user to trying to register
try {
String str_name = params[1];
String str_username = params[2];
String str_password = params[3];
String str_email = params[4];
URL url = new URL(register_url);
HttpURLConnection httpURLConnection = (HttpURLConnection)url.openConnection();
httpURLConnection.setRequestMethod("POST");
httpURLConnection.setDoOutput(true);
httpURLConnection.setDoInput(true);
OutputStream outputStream = httpURLConnection.getOutputStream();
BufferedWriter bufferedWriter = new BufferedWriter(new OutputStreamWriter(outputStream, "UTF-8"));
String post_data = URLEncoder.encode("name", "UTF-8")+"="+URLEncoder.encode(str_name, "UTF-8")+"&"+
URLEncoder.encode("user_name", "UTF-8")+"="+URLEncoder.encode(str_username, "UTF-8")+"&"
+URLEncoder.encode("user_pass", "UTF-8")+"="+URLEncoder.encode(str_password, "UTF-8")
+"&"+URLEncoder.encode("user_email", "UTF-8")+"="+URLEncoder.encode(str_email, "UTF-8");
bufferedWriter.write(post_data);
bufferedWriter.flush();
bufferedWriter.close();
outputStream.close();
InputStream inputStream = httpURLConnection.getInputStream();
BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(inputStream, "iso-8859-1"));
String result="";
String line="";
while((line = bufferedReader.readLine()) != null) {
result+=line;
}
bufferedReader.close();
inputStream.close();
httpURLConnection.disconnect();
return result;
} catch (MalformedURLException e) {
e.printStackTrace();
} catch (IOException e) {
e.printStackTrace();
}
}
return null;
}
@Override
protected void onPreExecute() {
alertDialog = new AlertDialog.Builder(context).create();
alertDialog.setTitle("Login Status");
}
@Override
protected void onPostExecute(String result) {
switch(result){
case "LoginSuccess":
loginActivity.OnLoginSuccess();
break;
case "LoginFailed":
loginActivity.OnLoginFailed();
break;
}
}
@Override
protected void onProgressUpdate(Void... values) {
super.onProgressUpdate(values);
}
}
LoginActivity
public class LoginActivity extends AppCompatActivity{
EditText etUsername, etPassword;
Button bLogin;
String tmpUsername;
@RequiresApi(api = Build.VERSION_CODES.M)
@Override
protected void onCreate(Bundle savedInstanceState) {
super.onCreate(savedInstanceState);
setContentView(R.layout.activity_login);
etUsername = (EditText) findViewById(R.id.etUsername);
etPassword = (EditText) findViewById(R.id.etPassword);
bLogin = (Button) findViewById(R.id.bLogin);
final TextView registerLink = (TextView) findViewById(R.id.tvRegisterHere);
tmpUsername = "";
requestPermissions();
}
@RequiresApi(api = Build.VERSION_CODES.M)
private boolean requestPermissions() {
if (ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_FINE_LOCATION) != PackageManager.PERMISSION_GRANTED && ActivityCompat.checkSelfPermission(this, Manifest.permission.ACCESS_COARSE_LOCATION) != PackageManager.PERMISSION_GRANTED) {
requestPermissions(new String[]{
Manifest.permission.ACCESS_FINE_LOCATION, Manifest.permission.ACCESS_COARSE_LOCATION, Manifest.permission.INTERNET
}, 1 );
// TODO: Consider calling
// ActivityCompat#requestPermissions
// here to request the missing permissions, and then overriding
// public void onRequestPermissionsResult(int requestCode, String[] permissions,
// int[] grantResults)
// to handle the case where the user grants the permission. See the documentation
// for ActivityCompat#requestPermissions for more details.
return true;
} else {
return false;
}
}
public void OnLogin(View view){
String username = etUsername.getText().toString();
String password = etPassword.getText().toString();
String type = "login";
tmpUsername = username;
BackgroundWorker loginRequest = new BackgroundWorker(this);
loginRequest.execute(type, username, password);
}
public void OpenReg(View view) {
startActivity(new Intent(this, RegisterActivity.class));
}
public void OnLoginSuccess(){
Intent intent = new Intent(LoginActivity.this, ViewGroupActivity.class);
intent.putExtra("username", tmpUsername);
startActivity(intent);
}
public void OnLoginFailed(){
Toast.makeText(this, "Username or password doesn't match. Please try again",Toast.LENGTH_SHORT).show();
}
这是我的logcat
04-04 15:02:11.180 6775-6775/com.example.myproject.findme E/AndroidRuntime:
FATAL EXCEPTION: main
Process: com.example.myproject.findme, PID: 6775
java.lang.NullPointerException: Attempt to invoke virtual method
'java.lang.String android.content.Context.getPackageName()' on a null object
reference
at android.content.ContextWrapper.getPackageName(ContextWrapper.java:135)
at android.widget.Toast.<init>(Toast.java:114)
at android.widget.Toast.makeText(Toast.java:277)
at android.widget.Toast.makeText(Toast.java:267)
at com.example.myproject.findme.LoginActivity.OnLoginFailed(LoginActivity.java:97)
at com.example.myproject.findme.BackgroundWorker.onPostExecute(BackgroundWorker.java:288)
at com.example.myproject.findme.BackgroundWorker.onPostExecute(BackgroundWorker.java:25)
at android.os.AsyncTask.finish(AsyncTask.java:695)
at android.os.AsyncTask.-wrap1(Unknown Source:0)
at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:712)
at android.os.Handler.dispatchMessage(Handler.java:106)
at android.os.Looper.loop(Looper.java:164)
at android.app.ActivityThread.main(ActivityThread.java:6494)
at java.lang.reflect.Method.invoke(Native Method)
at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:438)
at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:807)
答案 0 :(得分：0)
我注意到你没有初始化你的loginActivity实例变量。您可以使用witb setter方法或在BackgroundWorker类的构造函数中执行此操作。目前，您只传递Context对象的引用。 您应该像这样编写BackgroundWorker构造函数
BackgroundWorker(LoginActivity loginActivity){
this.loginActivity = loginActivity;
}
并在登录活动中传递参数 - 这 - 像那样
new BackgroundWorker(this)
并且您还可以使用loginActivity引用作为BackgroundWorker中的上下文。
它将100％工作。
但是如果你将接口方法用作回调会更好。
例如：
interface BackgroundWorkerCallback{
Contrxt getContext();
void success();
void failed();
}
在LoginActivity中实现此接口，根据需要覆盖最后两个方法，第一个方法应返回上下文引用。
在您的BackgroundWorker类
中更改构造函数，如下所示：
private BackgroundWorkerCallback callback;
public BackgroundWorker(BackgroundWorkerCallback callback){
this.callback = callback;
}
并在你的帖子中执行方法调用这样的方法
if success callback.success();
else if failed callback.failed();
和上下文使用
callback.getContext();