堆栈内存溢出
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尝试将MySQL数据库连接到android时遇到NullPointerException错误

[英]Getting a NullPointerException error trying to connect a MySQL database to android

 原文  2015-03-21 17:17:50  1   1    java/ php/ android/ mysql/ database

问题描述

我知道有很多这样的问题，但我都看了看，没有一个答案对我有用。

这是我的java课 

public class AllBugsActivity extends ListActivity {

private ProgressDialog pDialog;

JSONParser jParser = new JSONParser();

ArrayList<HashMap<String, String>> bugsList;

private static String url_all_bugs = "http://10.0.2.2/FinalYearProject/FYPFinal/android_connect/get_all_bugs.php";

private static final String TAG_SUCCESS = "success";
private static final String TAG_BUGS = "bugs";
private static final String TAG_BID = "bid";
private static final String TAG_NAME = "name";

JSONArray bugs = null;

@Override
protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.all_bugs);

    bugsList = new ArrayList<HashMap<String, String>>();

    new LoadAllBugs().execute();

    ListView lv = getListView();

    lv.setOnItemClickListener(new OnItemClickListener() {

       @Override
        public void onItemClick(AdapterView<?> parent, View view,
                                int position, long id) {

            String bid = ((TextView) view.findViewById(R.id.bid)).getText()
                    .toString();

            Intent in = new Intent(getApplicationContext(),
                    EditBugActivity.class);
            in.putExtra(TAG_BID, bid);

            startActivityForResult(in, 100);
    }
});

}
@Override
protected void onActivityResult(int requestCode, int resultCode, Intent data) {
    super.onActivityResult(requestCode, resultCode, data);
    if (resultCode == 100) {
        Intent intent = getIntent();
        finish();
        startActivity(intent);
    }

}

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

    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        pDialog = new ProgressDialog(AllBugsActivity.this);
        pDialog.setMessage("Loading bugs. Please wait...");
        pDialog.setIndeterminate(false);
        pDialog.setCancelable(false);
        pDialog.show();
    }
    protected String doInBackground(String... args) {
        List<NameValuePair> params = new ArrayList<NameValuePair>();
        JSONObject json = jParser.makeHttpRequest(url_all_bugs, "GET", params);

        Log.d("All Bugs: ", json.toString());

        try {
            int success = json.getInt(TAG_SUCCESS);

            if (success == 1) {
                bugs = json.getJSONArray(TAG_BUGS);

                for (int i = 0; i < bugs.length(); i++) {
                    JSONObject c = bugs.getJSONObject(i);

                    String id = c.getString(TAG_BID);
                    String name = c.getString(TAG_NAME);

                    HashMap<String, String> map = new HashMap<String, String>();

                    map.put(TAG_BID, id);
                    map.put(TAG_NAME, name);

                    bugsList.add(map);
                }
            } else {
                Intent i = new Intent(getApplicationContext(),
                        NewBugActivity.class);
                i.addFlags(Intent.FLAG_ACTIVITY_CLEAR_TOP);
                startActivity(i);
            }
        } catch (JSONException e) {
            e.printStackTrace();
        }

        return null;
    }
    protected void onPostExecute(String file_url) {
        pDialog.dismiss();
        runOnUiThread(new Runnable() {
            public void run() {
                ListAdapter adapter = new SimpleAdapter(
                        AllBugsActivity.this, bugsList,
                        R.layout.list_bug, new String[] { TAG_BID,
                        TAG_NAME},
                        new int[] { R.id.bid, R.id.name });
                setListAdapter(adapter);
            }
        });

    }

}

}

这是我的JSONParser类 

public class JSONParser {

static InputStream is = null;
static JSONObject jObj = null;
static String json = "";

// constructor
public JSONParser() {

}

// function get json from url
// by making HTTP POST or GET mehtod
public JSONObject makeHttpRequest(String url, String method,
                                  List<NameValuePair> params) {

    // Making HTTP request
    try {

        // check for request method
        if(method == "POST"){
            // request method is POST
            // 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();

        }else if(method == "GET"){
            // request method is GET
            DefaultHttpClient httpClient = new DefaultHttpClient();
            String paramString = URLEncodedUtils.format(params, "utf-8");
            url += "?" + paramString;
            HttpGet httpGet = new HttpGet(url);

            HttpResponse httpResponse = httpClient.execute(httpGet);
            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();
    } 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;

}

}

继承人的错误即时通讯 

    03-21 17:06:15.158    1266-1280/com.example.neil.fypy4 E/AndroidRuntime﹕ FATAL EXCEPTION: AsyncTask #1
    java.lang.RuntimeException: An error occured while executing doInBackground()
            at android.os.AsyncTask$3.done(AsyncTask.java:299)
            at java.util.concurrent.FutureTask$Sync.innerSetException(FutureTask.java:273)
            at java.util.concurrent.FutureTask.setException(FutureTask.java:124)
            at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:307)
            at java.util.concurrent.FutureTask.run(FutureTask.java:137)
            at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
            at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
            at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
            at java.lang.Thread.run(Thread.java:856)
     Caused by: java.lang.NullPointerException
            at com.example.neil.fypy4.AllBugsActivity$LoadAllBugs.doInBackground(AllBugsActivity.java:98)
            at com.example.neil.fypy4.AllBugsActivity$LoadAllBugs.doInBackground(AllBugsActivity.java:83)
            at android.os.AsyncTask$2.call(AsyncTask.java:287)
            at java.util.concurrent.FutureTask$Sync.innerRun(FutureTask.java:305)
            at java.util.concurrent.FutureTask.run(FutureTask.java:137)
            at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:230)
            at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1076)
            at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:569)
            at java.lang.Thread.run(Thread.java:856)
03-21 17:06:15.511    1266-1266/com.example.neil.fypy4 W/EGL_emulation﹕ eglSurfaceAttrib not implemented
03-21 17:06:16.008    1266-1266/com.example.neil.fypy4 E/WindowManager﹕ Activity com.example.neil.fypy4.AllBugsActivity has leaked window com.android.internal.policy.impl.PhoneWindow$DecorView@41cd1748 that was originally added here
    android.view.WindowLeaked: Activity com.example.neil.fypy4.AllBugsActivity has leaked window com.android.internal.policy.impl.PhoneWindow$DecorView@41cd1748 that was originally added here
            at android.view.ViewRootImpl.<init>(ViewRootImpl.java:374)
            at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:292)
            at android.view.WindowManagerImpl.addView(WindowManagerImpl.java:224)
            at android.view.WindowManagerImpl$CompatModeWrapper.addView(WindowManagerImpl.java:149)
            at android.view.Window$LocalWindowManager.addView(Window.java:547)
            at android.app.Dialog.show(Dialog.java:277)
            at com.example.neil.fypy4.AllBugsActivity$LoadAllBugs.onPreExecute(AllBugsActivity.java:92)
            at android.os.AsyncTask.executeOnExecutor(AsyncTask.java:586)
            at android.os.AsyncTask.execute(AsyncTask.java:534)
            at com.example.neil.fypy4.AllBugsActivity.onCreate(AllBugsActivity.java:50)
            at android.app.Activity.performCreate(Activity.java:5008)
            at android.app.Instrumentation.callActivityOnCreate(Instrumentation.java:1079)
            at android.app.ActivityThread.performLaunchActivity(ActivityThread.java:2023)
            at android.app.ActivityThread.handleLaunchActivity(ActivityThread.java:2084)
            at android.app.ActivityThread.access$600(ActivityThread.java:130)
            at android.app.ActivityThread$H.handleMessage(ActivityThread.java:1195)
            at android.os.Handler.dispatchMessage(Handler.java:99)
            at android.os.Looper.loop(Looper.java:137)
            at android.app.ActivityThread.main(ActivityThread.java:4745)
            at java.lang.reflect.Method.invokeNative(Native Method)
            at java.lang.reflect.Method.invoke(Method.java:511)
            at com.android.internal.os.ZygoteInit$MethodAndArgsCaller.run(ZygoteInit.java:786)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:553)
            at dalvik.system.NativeStart.main(Native Method)

我的PHP课 

<?php
$response = array();
require_once __DIR__ . '/db_connect.php';
$db = new DB_CONNECT();
$result = mysql_query("SELECT *FROM bugs") or die(mysql_error());
if (mysql_num_rows($result) > 0) {
    $response["bugs"] = array();
    while ($row = mysql_fetch_array($result)) {
        $bug = array();
        $bug["bid"] = $row["bid"];
        $bug["name"] = $row["name"];
        $bug["severity"] = $row["severity"];
        $bug["description"] = $row["description"];
        $bug["created_at"] = $row["created_at"];
        $bug["updated_at"] = $row["updated_at"];
        array_push($response["bugs"], $bug);
    }
    $response["success"] = 1;
    echo json_encode($response);
} else {
    $response["success"] = 0;
    $response["message"] = "No bugs found";
    echo json_encode($response);
}
?>

任何帮助将不胜感激。

1 个解决方案

解决方案1
 1  2015-03-21 18:35:21

因此，在堆栈跟踪中，它将显示行号：从中可以对NPE来自的代码进行三角剖分。 由于您此处未提供行号，因此我将猜测int success = json.getInt(TAG_SUCCESS); 导致NPE。 原因是它是doInBackground(...)的第一个可能为null的对象doInBackground(...)如果查看JSONParser类，则返回jObj ，它是初始化为null的字段成员，并且仅在未发生错误时设置。 也就是说，您无需在doInBackground(...)检查JSONObject json是否从= jParser.makeHttpRequest(url_all_bugs, "GET", params);返回null = jParser.makeHttpRequest(url_all_bugs, "GET", params); 而是依靠TAG_SUCCESS 。 但这是一个鸡与蛋的问题，因为如果失败，它可以为null，并且没有标签来检查成功！

无论如何，我的建议是在doInBackground(...)的try / catch之前添加if (json != null) doInBackground(...) 。 您可能会发现JSONParser类无法正确解析。 您可以使用堆栈跟踪中的行号来查明问题的根源，也可以仅使用调试器逐步执行代码。

附带说明一下，您可以将public JSONObject makeHttpRequest(String url, String method, List<NameValuePair> params)静态方法，因为变量static InputStream is = null; static JSONObject jObj = null; static String json = ""; static InputStream is = null; static JSONObject jObj = null; static String json = ""; 不需要在类范围内（它们已经是静态的！）。 只需使它们成为方法范围，并有一个方便的静态方法调用即可解析您的json。

最后的评论：您不需要从onPostExecute(...)调用runOnUiThread(...) ，因为onPostExecute(...)在UI线程上针对每个异步任务运行。 这就是异步任务的工作原理。

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