在 Android Studio 中，我在“doInBackground 方法”中收到此錯誤

Question

這里首先執行 getWeather 方法，該方法執行 DownloadTask 方法，當用戶傳遞一些 null 值並且 getWeather 中的鏈接不正確時，在 doInBackground 方法中獲取輸入 stream 時顯示致命錯誤。

package com.ssd.weatherapp;

import androidx.appcompat.app.AppCompatActivity;

import android.content.Context;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.view.View;
import android.view.inputmethod.InputMethodManager;
import android.widget.EditText;
import android.widget.TextView;
import android.widget.Toast;

import org.json.JSONArray;
import org.json.JSONObject;

import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLEncoder;

public class MainActivity extends AppCompatActivity {

    EditText editText;
    TextView resultTextView;

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

        editText = findViewById(R.id.editText);
        resultTextView = findViewById(R.id.resultTextView);

     }

    public void getWeather (View view){
        try {
            DownloadTask task = new DownloadTask();

            String encodedCityName = URLEncoder.encode(editText.getText().toString(), "UTF-8");

            task.execute("https://openweathermap.org/data/2.5/weather?q=" + encodedCityName + "&appid=439d4b804bc8187953eb36d2a8c26a02\n");

            InputMethodManager mgr = (InputMethodManager) getSystemService(Context.INPUT_METHOD_SERVICE);

            mgr.hideSoftInputFromWindow(editText.getWindowToken(), 0);

        }catch(Exception e){
            e.printStackTrace();
            Log.i("info","1st one");
            Toast.makeText(getApplicationContext(),"Could not find weather :(",Toast.LENGTH_SHORT).show();

        }
    }


    public class DownloadTask extends AsyncTask<String,Void,String> {

        @Override
        protected String doInBackground(String... urls) {
            String result = "";
            URL url;
            HttpURLConnection urlConnection = null;



            try{
                url = new URL(urls[0]);

                urlConnection= (HttpURLConnection) url.openConnection();
                System.out.println(urlConnection.getInputStream());

                InputStream in =  urlConnection.getInputStream();

                InputStreamReader reader= new InputStreamReader(in);

                int data = reader.read();

                while(data != -1){
                    char current =(char) data;
                    result += current;
                    data = reader.read();
                }

                return result;

            }catch(Exception e){
                e.printStackTrace();


                Toast.makeText(getApplicationContext(),"Could not find weather :(",Toast.LENGTH_SHORT).show();
                return null;

            }

        }

        @Override
        protected void onPostExecute(String s) {
            super.onPostExecute(s);

            try {

                JSONObject jsonObject = new JSONObject(s);

                String weatherInfo = jsonObject.getString("weather");
                Log.i("info",weatherInfo);

                JSONArray arr = new JSONArray(weatherInfo);

                String msg ="";

                for(int i=0; i < arr.length(); i++){
                    JSONObject jsonPart= arr. getJSONObject(i);

                   String main = jsonPart.getString("main");
                   String description =jsonPart.getString("description");

                   if(!main.equals("") && !description.equals("")){
                       msg += main + ": " + description + "\r\n";
                   }
                }

                if(!msg.equals("")){
                    resultTextView.setText(msg);
                }else{
                    Log.i("info","3rd one");
                    Toast.makeText(getApplicationContext(),"Could not find weather :(",Toast.LENGTH_SHORT).show();

                }

            }catch(Exception e){
                e.printStackTrace();
                Log.i("info","4th one");
                Toast.makeText(getApplicationContext(),"Could not find weather :(",Toast.LENGTH_SHORT).show();
            }

        }
    }

}

當我沒有在 textView 中鍵入任何內容並將其發送到 downloadTask Class 時發生錯誤，而不是顯示異常和敬酒，應用程序崩潰並顯示此錯誤：

2020-06-16 00:01:46.117 7948-7986/com.ssd.weatherapp D/NetworkSecurityConfig: No Network Security Config specified, using platform default
2020-06-16 00:01:47.529 7948-7986/com.ssd.weatherapp W/System.err: java.io.FileNotFoundException: https://openweathermap.org/data/2.5/weather?q=&appid=439d4b804bc8187953eb36d2a8c26a02
2020-06-16 00:01:47.529 7948-7986/com.ssd.weatherapp W/System.err:     at com.android.okhttp.internal.huc.HttpURLConnectionImpl.getInputStream(HttpURLConnectionImpl.java:251)
2020-06-16 00:01:47.529 7948-7986/com.ssd.weatherapp W/System.err:     at com.android.okhttp.internal.huc.DelegatingHttpsURLConnection.getInputStream(DelegatingHttpsURLConnection.java:210)
2020-06-16 00:01:47.529 7948-7986/com.ssd.weatherapp W/System.err:     at com.android.okhttp.internal.huc.HttpsURLConnectionImpl.getInputStream(HttpsURLConnectionImpl.java:26)
2020-06-16 00:01:47.529 7948-7986/com.ssd.weatherapp W/System.err:     at com.ssd.weatherapp.MainActivity$DownloadTask.doInBackground(MainActivity.java:74)
2020-06-16 00:01:47.529 7948-7986/com.ssd.weatherapp W/System.err:     at com.ssd.weatherapp.MainActivity$DownloadTask.doInBackground(MainActivity.java:60)
2020-06-16 00:01:47.529 7948-7986/com.ssd.weatherapp W/System.err:     at android.os.AsyncTask$2.call(AsyncTask.java:333)
2020-06-16 00:01:47.529 7948-7986/com.ssd.weatherapp W/System.err:     at java.util.concurrent.FutureTask.run(FutureTask.java:266)
2020-06-16 00:01:47.529 7948-7986/com.ssd.weatherapp W/System.err:     at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:245)
2020-06-16 00:01:47.529 7948-7986/com.ssd.weatherapp W/System.err:     at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
2020-06-16 00:01:47.529 7948-7986/com.ssd.weatherapp W/System.err:     at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
2020-06-16 00:01:47.530 7948-7986/com.ssd.weatherapp W/System.err:     at java.lang.Thread.run(Thread.java:764)

    --------- beginning of crash
2020-06-16 00:01:47.532 7948-7986/com.ssd.weatherapp E/AndroidRuntime: FATAL EXCEPTION: AsyncTask #1
    Process: com.ssd.weatherapp, PID: 7948
    java.lang.RuntimeException: An error occurred while executing doInBackground()
        at android.os.AsyncTask$3.done(AsyncTask.java:354)
        at java.util.concurrent.FutureTask.finishCompletion(FutureTask.java:383)
        at java.util.concurrent.FutureTask.setException(FutureTask.java:252)
        at java.util.concurrent.FutureTask.run(FutureTask.java:271)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:245)
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167)
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641)
        at java.lang.Thread.run(Thread.java:764)
     Caused by: java.lang.RuntimeException: Can't toast on a thread that has not called Looper.prepare()
        at android.widget.Toast$TN.<init>(Toast.java:393)
        at android.widget.Toast.<init>(Toast.java:117)
        at android.widget.Toast.makeText(Toast.java:280)
        at android.widget.Toast.makeText(Toast.java:270)
        at com.ssd.weatherapp.MainActivity$DownloadTask.doInBackground(MainActivity.java:94)
        at com.ssd.weatherapp.MainActivity$DownloadTask.doInBackground(MainActivity.java:60)
        at android.os.AsyncTask$2.call(AsyncTask.java:333)
        at java.util.concurrent.FutureTask.run(FutureTask.java:266)
        at android.os.AsyncTask$SerialExecutor$1.run(AsyncTask.java:245) 
        at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1167) 
        at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:641) 
        at java.lang.Thread.run(Thread.java:764) 
2020-06-16 00:01:47.919 7948-7976/com.ssd.weatherapp D/EGL_emulation: eglMakeCurrent: 0xe7740080: ver 2 0 (tinfo 0xdfb95320)

（注意：單擊按鈕時，它運行 getWeather 方法）但是如果我在 textView 中給出正確的值，它就可以正常工作。

Answer 1

正如其他人已經指出的那樣，您必須注意在主 (UI) 線程上運行的內容以及在后台線程上運行的內容。

AsyncTask的doInBackground在后台線程中運行，因此您無法在此處執行任何與 UI 相關的操作，例如顯示Toast或更新視圖。

---

其他一些需要注意的事情是：

• AsyncTask現在已被棄用，最終將不可用。
• java 中的內部類應為 static（除非有意）以避免泄漏其父級 class。 在上面的代碼中，如果Activity完成后 AsyncTask 仍在運行，則AsyncTask將阻止 GC 清除Activity引用。
• 與前一點類似，如果 Activity 在AsyncTask完成之前被銷毀，應用程序將崩潰。 因為當AsyncTask完成並嘗試在onPostExecute中更新 UI 時，將沒有要更新的視圖（null）。