Question

我对编程有点陌生，试图使项目成为一个Android应用程序，但是当我尝试在设备上运行它时，它就会崩溃。

我在logcat中收到这些错误：

java.lang.NullPointerException: Attempt to get length of null array
    2020-07-12 19:52:04.128 12417-12417/? E/AndroidRuntime: FATAL EXCEPTION: main
        Process: com.example.temperatura, PID: 12417
        java.lang.NullPointerException: Attempt to invoke virtual method 'int java.lang.String.length()' on a null object reference
            at org.json.JSONTokener.nextCleanInternal(JSONTokener.java:121)
            at org.json.JSONTokener.nextValue(JSONTokener.java:98)
            at org.json.JSONArray.<init>(JSONArray.java:94)
            at org.json.JSONArray.<init>(JSONArray.java:110)
            at com.example.temperatura.MainActivity.loadIntoListView(MainActivity.java:116)
            at com.example.temperatura.MainActivity.access$200(MainActivity.java:21)
            at com.example.temperatura.MainActivity$1DownloadJSON.onPostExecute(MainActivity.java:68)
            at com.example.temperatura.MainActivity$1DownloadJSON.onPostExecute(MainActivity.java:52)
            at android.os.AsyncTask.finish(AsyncTask.java:755)
            at android.os.AsyncTask.access$900(AsyncTask.java:192)
            at android.os.AsyncTask$InternalHandler.handleMessage(AsyncTask.java:772)
            at android.os.Handler.dispatchMessage(Handler.java:107)
            at android.os.Looper.loop(Looper.java:224)
            at android.app.ActivityThread.main(ActivityThread.java:7520)
            at java.lang.reflect.Method.invoke(Native Method)
            at com.android.internal.os.RuntimeInit$MethodAndArgsCaller.run(RuntimeInit.java:539)
            at com.android.internal.os.ZygoteInit.main(ZygoteInit.java:950)

这是我的主要活动。java：

package com.example.temperatura;
import android.os.AsyncTask;
import android.os.Bundle;
import android.widget.ArrayAdapter;
import android.widget.ListView;
import android.widget.Toast;
import androidx.appcompat.app.AppCompatActivity;
import androidx.swiperefreshlayout.widget.SwipeRefreshLayout;

import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;
import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;


public class MainActivity extends AppCompatActivity {

    ListView listView;
    List<String> sensor = new ArrayList<>();
    ArrayAdapter arrayAdapter;
    boolean needRefresh;        // whether refresh is needed


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

        final SwipeRefreshLayout pullToRefresh = findViewById(R.id.pullToRefresh);

        listView = (ListView) findViewById(R.id.listView);
        downloadJSON("http://pillsmanager.com/temperatura/conn_app.php");

        pullToRefresh.setOnRefreshListener(new SwipeRefreshLayout.OnRefreshListener() {
            @Override
            public void onRefresh() {
                downloadJSON("http://pillsmanager.com/temperatura/conn_app.php");
                pullToRefresh.setRefreshing(false);
                needRefresh = true;
            }
        });
    }


    private void downloadJSON(final String urlWebService) {

        class DownloadJSON extends AsyncTask<Void, Void, String> {

            @Override
            protected void onPreExecute() {
                super.onPreExecute();
            }


            @Override
            protected void onPostExecute(String s) {
                super.onPostExecute(s);
                Toast.makeText(getApplicationContext(), s, Toast.LENGTH_SHORT).show();
                try {
                    if (needRefresh) {
                        updateAndLoadIntoListView(s);
                    } else {
                        loadIntoListView(s);
                    }
                } catch (JSONException e) {
                    e.printStackTrace();
                }
            }


            @Override
            protected String doInBackground(Void... voids) {
                try {
                    URL url = new URL(urlWebService);
                    HttpURLConnection con = (HttpURLConnection) url.openConnection();
                    StringBuilder sb = new StringBuilder();
                    BufferedReader bufferedReader = new BufferedReader(new InputStreamReader(con.getInputStream()));
                    String json;
                    while ((json = bufferedReader.readLine()) != null) {
                        sb.append(json + "\n");
                    }
                    return sb.toString().trim();
                } catch (Exception e) {
                    return null;
                }
            }
        }
        DownloadJSON getJSON = new DownloadJSON();
        getJSON.execute();

    }

    private void updateAndLoadIntoListView(String json) throws JSONException {
        JSONArray jsonArray = new JSONArray(json);
        sensor.clear();
        for (int i = 0; i < jsonArray.length(); i++) {
            final JSONObject obj = jsonArray.getJSONObject(i);
            sensor.add(obj.getString("temperature") + " " + obj.getString("humidity"));

        }


        arrayAdapter.notifyDataSetChanged();
        needRefresh = false;
    }


    private void loadIntoListView(String json) throws JSONException {


        JSONArray jsonArray = new JSONArray(json);
        for (int i = 0; i < jsonArray.length(); i++) {
            final JSONObject obj = jsonArray.getJSONObject(i);
            sensor.add(obj.getString("temperature") + " " + obj.getString("humidity"));


        }

        arrayAdapter = new ArrayAdapter<String>(this, android.R.layout.simple_list_item_1, sensor);
        listView.setAdapter(arrayAdapter);


    }
}

非常感谢所有帮助，我已经尝试了所有方法，但仍然无法解决问题

Answer 1

从堆栈跟踪中，您似乎正在将null传递给loadIntoListView方法。

只有在您正在处理下载的doInBackground方法遇到异常时，它才会在代码中发生。

在设备上无法运行并在仿真器上运行的原因可能有很多。我建议切换到一些网络库来处理下载并处理那些极端情况。真的很烦。尝试使用OkHttp

Android应用在真实设备上崩溃但在模拟器上运行

1 个答案: