如何修复 FATAL EXCEPTION: AsyncTask #2 错误

Question

我只想使用 PHP 访问我的数据库，并在 Toast 中查看结果， 但我不知道应该在哪里查找错误 (FATAL EXCEPTION: AsyncTask #2)。

安卓代码：

btnLogin.setOnClickListener(new View.OnClickListener() {

        @Override
        public void onClick(View arg0) {
            // TODO Auto-generated method stub

            if (  ( !username.getText().toString().equals("")) && ( !password.getText().toString().equals("")) )
              {
                 new Connect().execute();
              }
              else if ( ( !username.getText().toString().equals("")) )
              {
                  Toast.makeText(getApplicationContext(),
                          "Password field empty", Toast.LENGTH_SHORT).show();
              }
              else if ( ( !password.getText().toString().equals("")) )
              {
                  Toast.makeText(getApplicationContext(),
                          "User Name field empty", Toast.LENGTH_SHORT).show();
              }
              else
              {
                  Toast.makeText(getApplicationContext(),
                          "User Name and Password field are empty", Toast.LENGTH_SHORT).show();
              }


        }
    });
}


class Connect extends AsyncTask<String, String, String>
{
    @Override
    protected String doInBackground(String... arg0) {
        // TODO Auto-generated method stub
        List<NameValuePair> params= new ArrayList<NameValuePair>();
        params.add(new BasicNameValuePair("username", username.getText().toString()));
        params.add(new BasicNameValuePair("password", password.getText().toString()));

        JSONObject json=JSONParser.getJSONFromUrl(loginURL, params);

        try {
            String login = json.getString(sTag_Login);
            if (login=="Admin") {
                Toast.makeText(getApplicationContext(), "Welcome Admin", Toast.LENGTH_LONG).show();
            }
            else if (login=="Customer") {
                Toast.makeText(getApplicationContext(), "Welcome", Toast.LENGTH_LONG).show();
            }
            {
                Toast.makeText(getApplicationContext(), "error ", Toast.LENGTH_LONG).show();
            }
        } catch(JSONException e) {
            e.printStackTrace();
        }

        return null;
    }
}

这是我的 PHP 代码：

<?php

 // array for JSON response
$response = array();
$db = mysqli_connect('mysql.hostinger.ae','u641845309_ur','q1p0w2o9','u641845309_song');

// username and password sent from Form and protect MySQL injection for Security purpose
$username = $_POST['username']; 
$password = $_POST['password']; 

$sql = "SELECT * FROM customer WHERE UName='$username' and Password='$password'";

// Establishing Connection with Server by passing server_name, user_id and password as a parameter
$result=mysqli_query($db,$sql);  

// If result matched $myusername and $mypassword, table row must be 1 row
if ($result)
{ 
    while($row = mysqli_fetch_array($result)) {
        if(strtolower($username) == 'admin') 
            $response["login"] =   "Admin"; 
        else 
            $response["login"] = "Customer"; 
    }
}
else 
{
    $response["login"] ="error"; 
}

// echoing JSON response
echo json_encode($response);

JSON 代码：

public class JSONParser {
    static InputStream is = null;
    static JSONObject jObj = null;
    static String json = "";

    // constructor
    public JSONParser() {

    }

    public static JSONObject getJSONFromUrl(String url, List<NameValuePair> params) {
        // Making HTTP request
        try {
            // defaultHttpClient
            DefaultHttpClient httpClient = new DefaultHttpClient();
            HttpPost httpPost = new HttpPost(url);
            httpPost.setEntity(new UrlEncodedFormEntity(params));

            HttpResponse httpResponse = httpClient.execute(httpPost);
            HttpEntity httpEntity = httpResponse.getEntity();
            is = httpEntity.getContent();

        } catch (UnsupportedEncodingException e) {
            e.printStackTrace();
        } catch (ClientProtocolException e) {
            e.printStackTrace();
        } catch (IOException e) {
            e.printStackTrace();
        }

        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(
                    is, "iso-8859-1"), 8);
            StringBuilder sb = new StringBuilder();
            String line = null;
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
            is.close();
            json = sb.toString();
            Log.e("JSON", json);
        } catch (Exception e) {
            Log.e("Buffer Error", "Error converting result " + e.toString());
        }

        // try parse the string to a JSON object
        try {
            jObj = new JSONObject(json);
        } catch (JSONException e) {
            Log.e("JSON Parser", "Error parsing data " + e.toString());
        }

        // return JSON String
        return jObj;

    }
}

这是logcat：

01-15 13:49:35.770: D/gralloc_goldfish(2098): Emulator without GPU emulation detected.
01-15 13:49:39.800: D/dalvikvm(2098): GC_FOR_ALLOC freed 166K, 7% free 3117K/3348K, paused 54ms, total 68ms
01-15 13:49:39.800: D/InputEventConsistencyVerifier(2098): KeyEvent: ACTION_UP but key was not down.
01-15 13:49:39.800: D/InputEventConsistencyVerifier(2098):   in android.widget.LinearLayout{b3d26008 V.E..... ......I. 0,59-455,118}
01-15 13:49:39.800: D/InputEventConsistencyVerifier(2098):   0: sent at 8592505000000, KeyEvent { action=ACTION_UP, keyCode=KEYCODE_TAB, scanCode=15, metaState=0, flags=0x8, repeatCount=0, eventTime=8592505, downTime=8592401, deviceId=0, source=0x101 }
01-15 13:50:01.560: W/System.err(2098): java.net.UnknownHostException: Unable to resolve host "mwssong.esy.es": No address associated with hostname
01-15 13:50:01.570: W/System.err(2098):     at java.net.InetAddress.lookupHostByName(InetAddress.java:424)
01-15 13:50:01.580: W/System.err(2098):     at java.net.InetAddress.getAllByNameImpl(InetAddress.java:236)
01-15 13:50:01.580: W/System.err(2098):     at java.net.InetAddress.getAllByName(InetAddress.java:214)
01-15 13:50:01.580: W/System.err(2098):     at org.apache.http.impl.conn.DefaultClientConnectionOperator.openConnection(DefaultClientConnectionOperator.java:137)
01-15 13:50:01.590: W/System.err(2098):     at org.apache.http.impl.conn.AbstractPoolEntry.open(AbstractPoolEntry.java:164)
01-15 13:50:01.590: W/System.err(2098):     at org.apache.http.impl.conn.AbstractPooledConnAdapter.open(AbstractPooledConnAdapter.java:119)
01-15 13:50:01.590: W/System.err(2098):     at org.apache.http.impl.client.DefaultRequestDirector.execute(DefaultRequestDirector.java:360)
01-15 13:50:01.600: W/System.err(2098):     at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:555)
01-15 13:50:01.600: W/System.err(2098):     at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:487)
01-15 13:50:01.600: W/System.err(2098):     at org.apache.http.impl.client.AbstractHttpClient.execute(AbstractHttpClient.java:465)
01-15 13:50:01.600: W/System.err(2098):     at com.example.e_music.JSONParser.getJSONFromUrl(JSONParser.java:44)
01-15 13:50:01.610: W/System.err(2098):     at com.example.e_music.MainActivity$Connect.doInBackground(MainActivity.java:126)
01-15 13:50:01.620: W/System.err(2098):     at com.example.e_music.MainActivity$Connect.doInBackground(MainActivity.java:1)
01-15 13:50:01.620: W/System.err(2098):     at android.os.AsyncTask$2.call(AsyncTask.java:288)
01-15 13:50:01.620: W/System.err(2098):     at java.util.concurrent.FutureTask.run(FutureTask.java:237)
01-15 13:50:01.640: W/System.err(2098):     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
01-15 13:50:01.640: W/System.err(2098):     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
01-15 13:50:01.650: W/System.err(2098):     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
01-15 13:50:01.650: W/System.err(2098):     at java.lang.Thread.run(Thread.java:841)
01-15 13:50:01.670: W/System.err(2098): Caused by: libcore.io.GaiException: getaddrinfo failed: EAI_NODATA (No address associated with hostname)
01-15 13:50:01.670: W/System.err(2098):     at libcore.io.Posix.getaddrinfo(Native Method)
01-15 13:50:01.670: W/System.err(2098):     at libcore.io.ForwardingOs.getaddrinfo(ForwardingOs.java:61)
01-15 13:50:01.670: W/System.err(2098):     at java.net.InetAddress.lookupHostByName(InetAddress.java:405)
01-15 13:50:01.670: W/System.err(2098):     ... 18 more
01-15 13:50:01.680: E/Buffer Error(2098): Error converting result java.lang.NullPointerException: lock == null
01-15 13:50:01.680: E/JSON Parser(2098): Error parsing data org.json.JSONException: End of input at character 0 of 
01-15 13:50:01.680: W/dalvikvm(2098): threadid=12: thread exiting with uncaught exception (group=0xb3a22ba8)
01-15 13:50:01.750: E/AndroidRuntime(2098): FATAL EXCEPTION: AsyncTask #2
01-15 13:50:01.750: E/AndroidRuntime(2098): Process: com.example.e_music, PID: 2098
01-15 13:50:01.750: E/AndroidRuntime(2098): java.lang.RuntimeException: An error occured while executing doInBackground()
01-15 13:50:01.750: E/AndroidRuntime(2098):     at android.os.AsyncTask$3.done(AsyncTask.java:300)
01-15 13:50:01.750: E/AndroidRuntime(2098):     at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:355)
01-15 13:50:01.750: E/AndroidRuntime(2098):     at java.util.concurrent.FutureTask.setException(FutureTask.java:222)
01-15 13:50:01.750: E/AndroidRuntime(2098):     at java.util.concurrent.FutureTask.run(FutureTask.java:242)
01-15 13:50:01.750: E/AndroidRuntime(2098):     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:231)
01-15 13:50:01.750: E/AndroidRuntime(2098):     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1112)
01-15 13:50:01.750: E/AndroidRuntime(2098):     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:587)
01-15 13:50:01.750: E/AndroidRuntime(2098):     at java.lang.Thread.run(Thread.java:841)
01-15 13:50:01.750: E/AndroidRuntime(2098): Caused by: java.lang.NullPointerException
01-15 13:50:01.750: E/AndroidRuntime(2098):     at com.example.e_music.MainActivity$Connect.doInBackground(MainActivity.java:129)
01-15 13:50:01.750: E/AndroidRuntime(2098):     at com.example.e_music.MainActivity$Connect.doInBackground(MainActivity.java:1)
01-15 13:50:01.750: E/AndroidRuntime(2098):     at android.os.AsyncTask$2.call(AsyncTask.java:288)
01-15 13:50:01.750: E/AndroidRuntime(2098):     at java.util.concurrent.FutureTask.run(FutureTask.java:237)
01-15 13:50:01.750: E/AndroidRuntime(2098):     ... 4 more

Answer 1

从 doInBackground() 中移除你的 Toast，AsyncTask 独立于你的 UI 活动工作，所以如果你想在其中调用 Toast，你至少应该在这个块中运行它：

                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(getApplicationContext(),
                                       "Your code is looking very bad!",
                                       Toast.LENGTH_LONG);
                    }
                });

另一个问题是，如果您的 getJSONFromUrl() 函数中发生任何异常，它可能会返回 null，因为您在异常处理程序中没有做任何事情来防止它返回 null。
在 doInBackground() 方法中，您试图捕获 JSONException，但它可能会是 NullPointerException，因为 getJSONFromUrl() 可能会返回 null。您需要简要检查您的代码。考虑使用 isEmpty() 而不是与空字符串进行比较，正确处理异常。
您清楚了 mwssong.esy.es 的 UnknownHostException，您是否将 http:// 添加到其中？

Answer 2

where I should search when appear FATAL EXCEPTION: AsyncTask #2 error

通常直接在后面的行中查看原因，这会给你一个文件和行号，说明哪里出错了

Caused by: java.lang.NullPointerException
01-15 13:50:01.750: E/AndroidRuntime(2098): at
   com.example.e_music.MainActivity$Connect.doInBackground(MainActivity.java:129)

不要从后台线程更新用户界面。修理 通过在doInBackground 中返回一个字符串以供onPostExecute 使用，分别将您的长时间运行的操作（登录检查）和 ui 更新（toast）分隔为适当的方法，doInBackground 和onPostExecute

class Connect extends AsyncTask<String, String, String> {

        @Override
        protected String doInBackground(String... arg0) {
            // TODO Auto-generated method stub
            List<NameValuePair> params = new ArrayList<NameValuePair>();
            params.add(new BasicNameValuePair("username", username.getText().toString()));
            params.add(new BasicNameValuePair("password", password.getText().toString()));


            try {
                JSONObject json = JSONParser.getJSONFromUrl(loginURL, params);
                String login = json.getString(sTag_Login);
                if ("Admin".equals(login)) {
                    return "Welcome Admin";
                } else if ("Customer".equals(login)) {
                    return "Welcome";
                }
            } catch (JSONException e) {
                e.printStackTrace();
            }

            return "error";
        }

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);
            Toast.makeText(MainActivity.this, s, Toast.LENGTH_LONG).show();
        }
}