Java 在我的AsyncTask中，哪个变量读取为null？

每次我点击登录按钮调用AsyncTask时，应用程序都会崩溃。我不知道我是否传递了错误的字符串或者什么。任何想法都将不胜感激。如果我没有提供足够的信息，请告诉我。对不起，这是我的第一篇文章。多谢各位

package com.carrcodes.getgathering;

import android.app.Activity;
import android.content.Context;
import android.content.Intent;
import android.os.AsyncTask;
import android.os.Bundle;
import android.view.Menu;
import android.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.TextView;

import org.json.JSONException;
import org.json.JSONObject;

public class LoginActivity extends Activity {

    EditText usernameField,passwordField;
    Button login,register;
    TextView loginErrorMsg;

    // JSON Response node names
    private static String KEY_SUCCESS = "success";
    private static String KEY_ERROR = "error";
    private static String KEY_ERROR_MSG = "error_msg";
    private static String KEY_UID = "uid";
    private static String KEY_NAME = "name";
    private static String KEY_EMAIL = "email";
    private static String KEY_CREATED_AT = "created_at";

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

        usernameField = (EditText) findViewById(R.id.StudentEmailBox);
        passwordField = (EditText) findViewById(R.id.PasswordBox);
        login = (Button) findViewById(R.id.sign_in_button);
        register = (Button) findViewById(R.id.register_button);
        loginErrorMsg = (TextView) findViewById(R.id.login_error);

        login.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View view) {
             /*   Intent intent = new Intent(LoginActivity.this, GetGatheringActivity.class);
                Bundle login_info = new Bundle();
                String username = usernameField.getText().toString();
                String password = passwordField.getText().toString();
                login_info.putString("Username", username);
                login_info.putString("Password", password);
                intent.putExtras(login_info);
                UserFunctions userFunction = new UserFunctions();
                JSONObject json = userFunction.loginUser(username, password); */


                Async task = new Async();
                task.execute(KEY_SUCCESS);
            }});
/*

                StrictMode.ThreadPolicy policy = new StrictMode.ThreadPolicy.Builder().permitAll().build();
                StrictMode.setThreadPolicy(policy);
                // check for login response
                try {
                    if (json.getString(KEY_SUCCESS) != null) {
                        loginErrorMsg.setText("");
                        String res = json.getString(KEY_SUCCESS);
                        if (Integer.parseInt(res) == 1) {
                            // user successfully logged in
                            // Store user details in SQLite Database
                            DatabaseHandler db = new DatabaseHandler(getApplicationContext());
                            JSONObject json_user = json.getJSONObject("user");

                            // Clear all previous data in database
                            userFunction.logoutUser(getApplicationContext());
                            db.addUser(json_user.getString(KEY_EMAIL)
                                    , json.getString(KEY_UID), json_user.getString(KEY_CREATED_AT));

                            // Launch Dashboard Screen
                            Intent dashboard = new Intent(getApplicationContext(),
                                    GetGatheringActivity.class);

                            // Close all views before launching Dashboard
                            dashboard.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                            startActivity(dashboard);

                            // Close Login Screen
                            finish();
                        } else {
                            // Error in login
                            loginErrorMsg.setText("Incorrect username/password");
                        }
                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }
        });*/


                register.setOnClickListener(new View.OnClickListener() {
                    @Override
                    public void onClick(View view) {
                        Intent registerIntent = new Intent(getApplicationContext(),
                                RegisterActivity.class);

                        startActivity(registerIntent);
                        finish();
                    }
                });
            }


    private class Async extends AsyncTask<String, Void, Void>{

        private Context context;

        @Override
        protected Void doInBackground(String... sUrl) {

            Intent intent = new Intent (LoginActivity.this, GetGatheringActivity.class);
            Bundle login_info = new Bundle();
            String username = usernameField.getText().toString();
            String password = passwordField.getText().toString();
            login_info.putString("Username", username);
            login_info.putString("Password", password);
            intent.putExtras(login_info);
            UserFunctions userFunction = new UserFunctions();
            JSONObject json = userFunction.loginUser(username, password);


            // check for login response
            try {
                if (json.getString(KEY_SUCCESS) != null) {
                    loginErrorMsg.setText("");
                    String res = json.getString(KEY_SUCCESS);
                    if(Integer.parseInt(res) == 1){
                        // user successfully logged in
                        // Store user details in SQLite Database
                        DatabaseHandler db = new DatabaseHandler(getApplicationContext());
                        JSONObject json_user = json.getJSONObject("user");

                        // Clear all previous data in database
                        userFunction.logoutUser(getApplicationContext());
                        db.addUser(json_user.getString(KEY_EMAIL)
                                , json.getString(KEY_UID), json_user.getString(KEY_CREATED_AT));



                        // Launch Dashboard Screen
                        Intent dashboard = new Intent(getApplicationContext(),
                                GetGatheringActivity.class);

                        // Close all views before launching Dashboard
                        dashboard.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                        startActivity(dashboard);

                        // Close Login Screen
                        finish();

                    }else{
                        // Error in login
                        loginErrorMsg.setText("Incorrect username/password");
                    }

                }
            } catch (JSONException e) {
                e.printStackTrace();
            }
            return null;
        }
        }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.main, menu);
        return true;
    }

}

我怀疑json返回为null，因为在这一点之后，您对它调用了几个getString方法。您可以将其打印到日志中，还是使用调试器逐步执行

这是导致问题的线路，我认为没有线路号，很难确定：

if (json.getString(KEY_SUCCESS) != null) {

---

我认为问题在于空指针异常。 问题发生在 JSONObject json=userFunction.loginuserName，密码

---

像ifjson一样进行检查！=在调用任何方法之前为空。

那么，哪一行是LoginActivity.java行151？json_用户必须为空。从外观上看，这是您调用getString的唯一对象如果json.getStringKEY_成功！=无效的{@Carr then json为null.Hm。好的，谢谢。我会花一些时间回顾一下。调试器真正告诉我的就是我发布的logcat。有什么方法让我更深入地了解吗？是的，你发布的行是正确的@ToddI认为这一部分：输入结尾处的字符0表示你的json格式不好。如果我是你，我会深入研究userFunction是什么，看看你是否可以调试它或者修改它来记录更多关于它接收到的信息。祝你好运。

JSONObject json = userFunction.loginUser(username, password);

if (json.getString(KEY_SUCCESS) != null) {