不能从JSONObject中检索值

我通过PHP给android中的java类(Login screen)传递值，但是给出错误JSONException Error，不能从下面的代码中定义。请问如何修改呢？谢谢！

LoginActivity.java

@Override
public void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.login);

    // Importing all assets like buttons, text fields
    inputUid = (EditText) findViewById(R.id.loginUid);
    inputPassword = (EditText) findViewById(R.id.loginPassword);
    btnLogin = (Button) findViewById(R.id.btnLogin);

    btnLinkToRegister = (Button) findViewById(R.id.btnLinkToRegisterScreen);

    loginErrorMsg = (TextView) findViewById(R.id.login_error);

    // Login button Click Event
    btnLogin.setOnClickListener(new View.OnClickListener() {

        public void onClick(View view) {
            String id = inputUid.getText().toString();
            String pswd = inputPassword.getText().toString();
            UserFunctions userFunction = new UserFunctions();
            Log.d("Button", "Login");
            JSONObject json = userFunction.loginUser(id, pswd);

            // 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.getString(KEY_ID), json_user.getString(KEY_NAME), json_user.getString(KEY_EMAIL), json_user.getString(KEY_CREATED_AT));                     

                        // Launch Dashboard Screen
                        Intent dashboard = new Intent(getApplicationContext(), DashboardActivity.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();
            }
        }
    });

这个错误很可能是由于服务器返回的JSON字符串格式不正确导致的。在这里，它是在解析JSON对象时引发的。您需要检查服务器返回的JSON字符串是否符合JSON规范。您也可以使用在线工具来验证JSON字符串的有效性。

此外，您还需要检查json.getString(KEY_SUCCESS)是否为null，如果是null，则可能会抛出异常。

修改代码如下：

  if (json.has(KEY_SUCCESS)) {
                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.getString(KEY_ID), json_user.getString(KEY_NAME), json_user.getString(KEY_EMAIL), json_user.getString(KEY_CREATED_AT));                     
                    // Launch Dashboard Screen
                    Intent dashboard = new Intent(getApplicationContext(), DashboardActivity.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");
                }
            }