java - TextView NullPointerException Context.getApplicationInfo 错误

Question

我正在执行一个LoginActivity，以允许用户通过自己在远程数据库中注册的帐户登录。

当数据库(当然是 PHP，但这不是主题)返回LOGIN_FALSE时，我检查用户是否连接到互联网。如果他连接到互联网，我想显示 TextView loginWrongIdentity。如果用户没有连接到互联网，我想显示 TextView loginNoInternet。

问题:当涉及到 TextView 时，我收到错误。我在网上搜索了一个半小时才找到解决方案，但不幸的是我找不到解决方案。

预先感谢您的帮助:)

--

堆栈跟踪:

E/AndroidRuntime: FATAL EXCEPTION: main
Process: fr.sotot.farmertycoon, PID: 21914
java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.pm.ApplicationInfo android.content.Context.getApplicationInfo()' on a null object reference
    at android.content.ContextWrapper.getApplicationInfo(ContextWrapper.java:169)
    at android.view.ContextThemeWrapper.getTheme(ContextThemeWrapper.java:174)
    at android.content.Context.obtainStyledAttributes(Context.java:740)
    at androidx.appcompat.app.AppCompatDelegateImpl.createSubDecor(AppCompatDelegateImpl.java:692)
    at androidx.appcompat.app.AppCompatDelegateImpl.ensureSubDecor(AppCompatDelegateImpl.java:659)
    at androidx.appcompat.app.AppCompatDelegateImpl.findViewById(AppCompatDelegateImpl.java:479)
    at androidx.appcompat.app.AppCompatActivity.findViewById(AppCompatActivity.java:214)
    at fr.sotot.farmertycoon.connect.LoginActivity.resultLogin(LoginActivity.java:51)
    at fr.sotot.farmertycoon.utils.DatabaseManager.onPostExecute(DatabaseManager.java:92)
    at fr.sotot.farmertycoon.utils.DatabaseManager.onPostExecute(DatabaseManager.java:21)
    at android.os.AsyncTask.finish(AsyncTask.java:755)
    at android.os.AsyncTask.access$900(AsyncTask.java:192)
    at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:772)
    at android.os.Handler.dispatchMessage(Handler.java:107)
    at android.os.Looper.loop(Looper.java:237)
    at android.app.ActivityThread.main(ActivityThread.java:7804)
    at java.lang.reflect.Method.invoke(Native Method)
    at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:493)
    at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:1068)

登录 Activity xml

<TextView
    android:id="@+id/loginWrongIdentity"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/wrongIdentity"
    app:layout_constraintBottom_toTopOf="@id/loginUsername"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@id/connectText"
    app:layout_constraintVertical_bias="1"
    android:textColor="#FF0000"
    android:visibility="invisible" />

<TextView
    android:id="@+id/loginNoInternet"
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="@string/noInternet"
    app:layout_constraintBottom_toTopOf="@id/loginUsername"
    app:layout_constraintEnd_toEndOf="parent"
    app:layout_constraintStart_toStartOf="parent"
    app:layout_constraintTop_toBottomOf="@id/connectText"
    app:layout_constraintVertical_bias="1"
    android:textColor="#FF0000"
    android:visibility="invisible" />

LoginActivity 类:

package fr.sotot.farmertycoon.connect;

import androidx.appcompat.app.AppCompatActivity;

import android.os.Bundle;
import android.view.View;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import fr.sotot.farmertycoon.R;
import fr.sotot.farmertycoon.utils.DatabaseManager;
import fr.sotot.farmertycoon.utils.NetworkCheck;

public class LoginActivity extends AppCompatActivity {

    NetworkCheck networkCheck = new NetworkCheck();
    EditText usernameText, passwordText;

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

        usernameText = findViewById(R.id.loginUsername);
        passwordText = findViewById(R.id.loginPassword);

    }

    public void login(View view) {

        String username = usernameText.getText().toString();
        String password = passwordText.getText().toString();
        String operation = "login";

        DatabaseManager databaseManager = new DatabaseManager(this);
        databaseManager.execute(operation, username, password);

    }

    public void resultLogin(String result) {

        if(result != null) {
            if(result.equals("LOGIN_TRUE")) {
                Toast.makeText(this,  "Bonjour "+usernameText.getText().toString()+", vous êtes désormais connecté", Toast.LENGTH_SHORT).show();
            }else if(result.equals("LOGIN_FALSE")) {
                if(networkCheck.haveNetwork()) {
                    TextView wrongIdentity = findViewById(R.id.loginWrongIdentity);
                    wrongIdentity.setVisibility(View.VISIBLE);
                }else{
                    TextView noInternet = findViewById(R.id.loginNoInternet);
                    noInternet.setVisibility(View.VISIBLE);
                }
            }
        }else{
            System.out.println("NULL LOL");
        }
    }

}

DatabaseManager 类:

package fr.sotot.farmertycoon.utils;

import android.content.Context;
import android.os.AsyncTask;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.lang.ref.WeakReference;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.nio.charset.StandardCharsets;

import fr.sotot.farmertycoon.connect.LoginActivity;

public class DatabaseManager extends AsyncTask<String, Void, String> {

    private String operation;

    Context context;
    public DatabaseManager(Context context) {
        this.context = context;
    }

    @Override
    protected String doInBackground(String... params) {

        [...]

    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }

    @Override
    protected void onPostExecute(String result) {

        if(operation.equals("login")) {
            LoginActivity loginActivity = new LoginActivity();
            loginActivity.resultLogin(result);
        }

    }

    @Override
    protected void onProgressUpdate(Void... values) {
        super.onProgressUpdate(values);
    }

}

Answer 1

感谢 Steve Benett，问题已解决:)!

您所要做的就是将 LoginActivity 而不是 Context 放入 DatabaseManager 构造函数中。
如果它可以帮助某些遇到相同问题的人，这里是有效的代码:

package fr.sotot.farmertycoon.utils;

import android.os.AsyncTask;

import java.io.BufferedReader;
import java.io.BufferedWriter;
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.OutputStream;
import java.io.OutputStreamWriter;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;
import java.nio.charset.StandardCharsets;

import fr.sotot.farmertycoon.connect.LoginActivity;

public class DatabaseManager extends AsyncTask<String, Void, String> {

    private String operation;

    public LoginActivity loginActivity;
    public DatabaseManager(LoginActivity activity) {
        this.loginActivity = activity;
    }

    @Override
    protected String doInBackground(String... params) {

        [...]
    }

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
    }

    @Override
    protected void onPostExecute(String result) {

        if(operation.equals("login")) {
            loginActivity.resultLogin(result);
        }

    }

    @Override
    protected void onProgressUpdate(Void... values) {
        super.onProgressUpdate(values);
    }

}