SharedPreferences Java Android Studio

Question

我试图在他们登录/注册时创建一个用户会话，以便他们在关闭应用程序后自动进入应用程序。

我不断收到此错误：

Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'android.content.Context 
android.content.Context.getApplicationContext()' on a null object reference
    at com.example.myapplication.BackgroundWorker.<init>(BackgroundWorker.java:32)
    at com.example.myapplication.LoginActivity.OnLogin(LoginActivity.java:73)
    at java.lang.reflect.Method.invoke(Native Method) 
    at androidx.appcompat.app.AppCompatViewInflater$DeclaredOnClickListener

我正在关注这个答案https://stackoverflow.com/a/11027631/14270746

我已经在 php 和 Java 中创建了登录/注册脚本，所以我试图在我已经创建的代码中实现这些东西。 我在我的 BackgroundWorker.Java 类中执行此操作。 我尝试在我的 LoginActivity 中执行此操作，但遇到了同样的错误。 有人能帮我吗 ？

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

    Context context;
    AlertDialog alertDialog;
    BackgroundWorker(Context ctx) {

        context = ctx;
    }

    private SharedPreferences sharedpreferences;
    SharedPreferences preferences = PreferenceManager.getDefaultSharedPreferences(context.getApplicationContext());
    SharedPreferences.Editor editor = preferences.edit();

    @Override
    protected String doInBackground(String... params) {
        String type = params [0];
        String login_url = "http://192.168.1.37/byebye/login_handler.php";
        String register_url = "http://192.168.1.37/byebye/register_handler.php";
        if (type.equals("login")) {

            try {
                String username = params [1];
                String pw = params [2];

                editor.putString("username", String.valueOf(username));
                editor.apply();

                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("pw", "UTF-8")+"="+URLEncoder.encode(pw, "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")) {

            try {
                String name = params [1];
                String username = params [2];
                String email = params [3];
                String pw = 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(name, "UTF-8")+"&"
                        +URLEncoder.encode("username", "UTF-8")+"="+URLEncoder.encode(username, "UTF-8")+"&"
                        + URLEncoder.encode("email", "UTF-8")+"="+URLEncoder.encode(email, "UTF-8")+"&"
                        + URLEncoder.encode("pw", "UTF-8")+"="+URLEncoder.encode(pw, "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("Status");
    }

    @Override
    protected void onPostExecute(String result) {
        alertDialog.setMessage(result);
        alertDialog.show();

            Intent intent = new Intent(context.getApplicationContext(), ProfileActivity.class);
            context.startActivity(intent);
    }

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

}

这是第一个错误行（BackgroundWorker.java:32）：

SharedPreferences preferences = 
PreferenceManager.getDefaultSharedPreferences(context.getApplicationContext());

这是第二个：

BackgroundWorker backgroundWorker = new BackgroundWorker(this);

我的猜测是变量没有保存到 sharedpref。

Answer 1

内联变量初始值设定项在构造函数体运行之前被调用。 因此，当您尝试初始化preferences ， context的值仍然为 null，因为尚未调用构造函数。 将初始化程序放在文件中的构造函数之后并不重要，它仍然首先被调用。 您应该在构造函数内部初始化它。

此外，不要只保留对SharedPreferences.Editor的引用。 需要时在本地创建它。

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

    Context context;
    SharedPreferences preferences;
    AlertDialog alertDialog;

    BackgroundWorker(Context ctx) {
        context = ctx;
        preferences = PreferenceManager.getDefaultSharedPreferences(ctx.getApplicationContext());
    }
    
    @Override
    protected String doInBackground(String... params) {
        SharedPreferences.Editor editor = preferences.edit();

        String type = params [0];
        String login_url = "http://192.168.1.37/byebye/login_handler.php";
        String register_url = "http://192.168.1.37/byebye/register_handler.php";
        // ...
    }
    
    // ...
}

Answer 2

您应该在您的活动类中初始化您的SharedPreference 。 因为 AsyncTask 在后台工作。 因此，当您的应用程序关闭时，AsyncTask 仍然存在，但您的应用程序不在前台。 这就是getApplicationContext()提供 null 的原因。 因此，为了防止此错误，请在活动onCreate()方法中初始化SharedPreference然后使用它。