Android DownloadFilesTask 概念

Question

在教程的帮助下尝试使用客户端服务器（获取 mysql 数据）时，我遇到了很多错误。我刚刚在这里阅读了一些主题，我了解到我遇到的最大错误

Error in http connectionandroid.os.NetworkOnMainThreadException E/log_tag: Error converting result java.lang.NullPointerException

可能是因为我需要线程和处理程序之间的辅助任务。

我有以下代码，我理解这个概念，通常它应该可以工作。

    package de.irgendwas;

import android.app.Activity;
import android.net.ParseException;
import android.net.Uri;
import android.os.AsyncTask;
import android.os.Bundle;
import android.util.Log;
import android.widget.TextView;
import android.widget.Toast;

import com.google.android.gms.appindexing.Action;
import com.google.android.gms.appindexing.AppIndex;
import com.google.android.gms.appindexing.Thing;
import com.google.android.gms.common.api.GoogleApiClient;

import org.apache.http.HttpEntity;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpPost;
import org.apache.http.impl.client.DefaultHttpClient;
import org.json.JSONArray;
import org.json.JSONException;
import org.json.JSONObject;

import java.io.BufferedReader;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.net.URL;

public class MainActivity extends Activity {
    private GoogleApiClient client;
    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        /** Called when the activity is first created. */

        String result = null;
        InputStream is = null;
        StringBuilder sb = null;

        //http post
        try {
            HttpClient httpclient = new DefaultHttpClient();
            HttpPost httppost = new HttpPost("http://localhost/connect_db.php");
            HttpResponse response = httpclient.execute(httppost);
            HttpEntity entity = response.getEntity();
            is = entity.getContent();
            Log.e("log_tag", "connection established");
        } catch (Exception e) {
            Log.e("log_tag", "Error in http connection" + e.toString());
        }

        //convert response to string
        try {
            BufferedReader reader = new BufferedReader(new InputStreamReader(is, "utf-8"), 8);
            sb = new StringBuilder();
            sb.append(reader.readLine() + "\n");
            String line = "0";
            while ((line = reader.readLine()) != null) {
                sb.append(line + "\n");
            }
            is.close();
            result = sb.toString();
        } catch (Exception e) {
            Log.e("log_tag", "Error converting result " + e.toString());
        }

        //paring data
        int fd_id = 0;
        String fd_name = "";
        try {
            JSONArray jArray = new JSONArray(result);
            JSONObject json_data = null;
            Toast.makeText(getBaseContext(), "1. try Block", Toast.LENGTH_LONG).show();
            for (int i = 0; i < jArray.length(); i++) {
                json_data = jArray.getJSONObject(i);
                fd_id = json_data.getInt("artist");
                fd_name = json_data.getString("title");
            }
        } catch (JSONException e1) {
            Toast.makeText(getBaseContext(), "No Data Found", Toast.LENGTH_LONG).show();
        } catch (ParseException e1) {
            e1.printStackTrace();
        }

        //print received data
        TextView t = (TextView) findViewById(R.id.text);
        t.setText("ID: " + fd_id + " Name: " + fd_name);
        // ATTENTION: This was auto-generated to implement the App Indexing API.
        // See https://g.co/AppIndexing/AndroidStudio for more information.
        client = new GoogleApiClient.Builder(this).addApi(AppIndex.API).build();
    }

    /**
     * ATTENTION: This was auto-generated to implement the App Indexing API.
     * See https://g.co/AppIndexing/AndroidStudio for more information.
     */
    public Action getIndexApiAction() {
        Thing object = new Thing.Builder()
                .setName("Main Page") // TODO: Define a title for the content shown.
                // TODO: Make sure this auto-generated URL is correct.
                .setUrl(Uri.parse("http://[ENTER-YOUR-URL-HERE]"))
                .build();
        return new Action.Builder(Action.TYPE_VIEW)
                .setObject(object)
                .setActionStatus(Action.STATUS_TYPE_COMPLETED)
                .build();
    }

    @Override
    public void onStart() {
        super.onStart();

        // ATTENTION: This was auto-generated to implement the App Indexing API.
        // See https://g.co/AppIndexing/AndroidStudio for more information.
        client.connect();
        AppIndex.AppIndexApi.start(client, getIndexApiAction());
    }

    @Override
    public void onStop() {
        super.onStop();

        // ATTENTION: This was auto-generated to implement the App Indexing API.
        // See https://g.co/AppIndexing/AndroidStudio for more information.
        AppIndex.AppIndexApi.end(client, getIndexApiAction());
        client.disconnect();
    }

}

我不会 post 服务器上的 php 脚本，因为如果我通过浏览器调用 URL 它会正确传送我的测试数据，所以应该没问题。

现在，在执行应用程序时出现上述错误。在网上搜索时，我发现了 DownloadFilesTask 概念，听起来它应该是解决方案。但我不会得到它，我无法在我的代码中实现它。有人可以提供帮助吗，或者除了 developer.android.

上的教程之外还有什么好的教程吗？

Answer 1

发生错误是因为您想在 UI 线程中执行与网络相关的过程。您需要将它从您的 UI 主题中移开。对于您的情况，您可以使用 AsyncTask.

AsyncTask 是一种在后台线程中执行操作而无需手动处理线程创建或执行的机制。 AsyncTasks 被设计用于短期操作（最多几秒钟），您可能希望使用服务 and/or 执行器来执行非常长的运行任务。

AsyncTask 通常用于无法在 UI 线程上完成的长运行任务，例如从 API 下载网络数据或从设备上的其他位置索引数据。

请阅读Creating and Executing Async Tasks。

这里有一个example for using AsyncTask在网络相关的过程中：

public class MainActivity extends Activity {
    private ImageView ivBasicImage;

    @Override
    protected void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);
        ivBasicImage = (ImageView) findViewById(R.id.ivBasicImage);
        String url = "https://i.imgur.com/tGbaZCY.jpg";
        // Download image from URL and display within ImageView
        new ImageDownloadTask(ivBasicImage).execute(url);
    }

    // Defines the background task to download and then load the image within the ImageView
    private class ImageDownloadTask extends AsyncTask<String, Void, Bitmap> {
        ImageView imageView;

        public ImageDownloadTask(ImageView imageView) {
            this.imageView = imageView;
        }

        protected Bitmap doInBackground(String... addresses) {
            Bitmap bitmap = null;
            InputStream in;
            try {
                // 1. Declare a URL Connection
                URL url = new URL(addresses[0]);
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                // 2. Open InputStream to connection
                conn.connect();
                in = conn.getInputStream();
                // 3. Download and decode the bitmap using BitmapFactory
                bitmap = BitmapFactory.decodeStream(in);
            } catch (IOException e) {
                e.printStackTrace();
            } finally {
              if(in != null)
                 in.close();
            }
            return bitmap;
        }

        // Fires after the task is completed, displaying the bitmap into the ImageView
        @Override
        protected void onPostExecute(Bitmap result) {
            // Set bitmap image for the result
            imageView.setImageBitmap(result);
        }
    }
}

Answer 2

您还应该记录此异常 Log.e("log_tag", "Error in http connection" + e.toString());
这是因为这一行

HttpPost httppost = new HttpPost("http://localhost/connect_db.php");

这将始终指向设备的本地主机而不是服务器。所以你应该使用：

HttpPost httppost = new HttpPost("http://10.0.2.2/connect_db.php");

这就是 result

上空指针异常的原因

Android DownloadFilesTask 概念

Android DownloadFilesTask concept

java

android

androidhttpclient

android-handlerthread