Android 向 php 发送数据并接收问题 nullpointerexception

Question

我是 android 开发的新手，我不确定我缺少什么来让代码工作。尝试创建登录应用程序。我接收数据或使用 dataoutputstream 发送数据的方式是否存在问题？有没有更好的方法不被弃用和复杂。寻找最简单易懂的解决方案。

在后台代码中执行

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


    DataOutputStream printout;
    HttpURLConnection urlConn;
    BufferedReader reader;
    URL url;
    String json;
    JSONObject jObj;


    //send POST login details to server
    try {
        url = new URL(SERVER_ADDRESS + "login.php");
        urlConn = (HttpURLConnection) url.openConnection();
        urlConn.setRequestMethod("POST");
        urlConn.setDoOutput(true);
        urlConn.setRequestProperty("Accept-Charset", "UTF-8");
        urlConn.setReadTimeout(10000);
        urlConn.setConnectTimeout(15000);

        HashMap<String, String> map = new HashMap<String, String>();
        map.put("username", params[0]);
        map.put("password", params[1]);

        StringBuilder sb = new StringBuilder();
        for(HashMap.Entry<String, String> e : map.entrySet()){
            if(sb.length() > 0){
                sb.append("&");
            }
            sb.append(URLEncoder.encode(e.getKey(), "UTF-8")).append("=").append(URLEncoder.encode(e.getValue(), "UTF-8"));
        }


        String loginDetails = sb.toString();

        printout = new DataOutputStream(urlConn.getOutputStream());
        printout.writeBytes(loginDetails);
        printout.flush();
        printout.close();

        reader = new BufferedReader(new InputStreamReader(urlConn.getInputStream()));
        StringBuilder sb2 = new StringBuilder();
        String result = null;
        while((result=reader.readLine())!=null) {
            sb2.append(result + '\n');
            Log.d("Results", sb2.toString());
        };

        json = result.toString();

        jObj = new JSONObject(json);

        JSONLogin = jObj.getBoolean(TAG_LOGINSTATUS);
        Log.d("LoginStatus!!!!", JSONLogin.toString());

    } catch (MalformedURLException e) {
        e.printStackTrace();
    } catch (ProtocolException e) {
        e.printStackTrace();
    } catch (IOException e) {
        e.printStackTrace();
    } catch (JSONException e) {
        Log.d("JsonConvertion", "FAILED TO CONVERT JSON");
    }


    return JSONLogin;
}

堆栈错误

10-18 22:43:34.169 2459-2523/? E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #1
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime: Process: com.httpkhronolog.khronolog, PID: 2459
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime: java.lang.RuntimeException: An error occurred while executing doInBackground()
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at android.os.AsyncTask$3.done(AsyncTask.java:309)
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:354)
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at java.util.concurrent.FutureTask.setException(FutureTask.java:223)
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at java.util.concurrent.FutureTask.run(FutureTask.java:242)
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234)
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113)
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588)
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at java.lang.Thread.run(Thread.java:818)
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:  Caused by: java.lang.NullPointerException: Attempt to invoke virtual method 'java.lang.String java.lang.String.toString()' on a null object reference
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at com.httpkhronolog.khronolog.Login_Activity$checkUserDetails.doInBackground(Login_Activity.java:137)
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at com.httpkhronolog.khronolog.Login_Activity$checkUserDetails.doInBackground(Login_Activity.java:79)
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at android.os.AsyncTask$2.call(AsyncTask.java:295)
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at java.util.concurrent.FutureTask.run(FutureTask.java:237)
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:234) 
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1113) 
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:588) 
10-18 22:43:34.169 2459-2523/? E/AndroidRuntime:     at java.lang.Thread.run(Thread.java:818) 

login.php

<?php

    if($_SERVER["REQUEST_METHOD"] == "POST"){
       // Establishing Connection with Server by passing server_name, user_id and password as a parameter
       $connection = mysql_connect("localhost", "someusername", "Password");
       // Selecting Database
       $db = mysql_select_db("khronolo_systemdb", $connection);

       $username = $_POST["username"];
       $password =  $_POST["password"];

       $query = "SELECT * FROM user_table WHERE password='$password' AND email='$username';";
       $result = mysql_query($query, $connection) or die (mysql_error($connection));
       $rows = mysql_num_rows($result);
       if ($rows == 1) {
          $returnValue[] = array("loginStatus"=> "true");
          echo json_encode($returnValue);
       }
       else {
          $returnValue[] = array("loginStatus"=> "false");
          echo json_encode($returnValue);
       }

       mysql_close($connection);
    }
    ?>

Answer 1

使用Volley 来自 Google 的网络库。按照此tutorial 进行相同操作。要了解有关 Volley 的更多信息，请参阅此link

还有其他网络库，例如okHttp。

到目前为止，您的代码已被考虑：

在第 137 行将 json = result.toString(); 替换为 json = sb2.toString();

Answer 2

代替

json = result.toString();

你应该这样做

json = sb2.toString();

说明

你的while循环退出时，结果为null，所以result.toString()会导致nullpointerexception。

任何其他库

您可以使用 GSON 库来帮助您毫无障碍地获取/设置响应值。

GSON error