Java 使用 AsyncTask 下载文件

Download file with AsyncTask

提问by Matheus Weber

I am trying to download a file with asyncTask but is not working, there is no error messages or nothing, just dont download the file... i try everything but it appears that is not entering on the while... anybody know what can be the problem? I tested on my mobile, the url is ok too.

我正在尝试使用 asyncTask 下载文件，但无法正常工作，没有错误消息或什么也没有，只是不要下载文件...我尝试了所有方法，但似乎暂时没有输入...任何人都知道可以是问题吗？我在我的手机上测试过，网址也可以。

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

    /**
     * Before starting background thread
     * */
    @Override
    protected void onPreExecute() {
        super.onPreExecute();
        System.out.println("Starting download");
    }

    /**
     * Downloading file in background thread
     * */
    @Override
    protected String doInBackground(String... f_url) {
        int count;
        try {
            String root = Environment.getExternalStorageDirectory().toString();

            System.out.println("Downloading");
            URL url = new URL(f_url[0]);

            URLConnection conection = url.openConnection();
            conection.connect();
            // getting file length
            int lenghtOfFile = conection.getContentLength();

            // input stream to read file - with 8k buffer
            InputStream input = new BufferedInputStream(url.openStream(), 8192);

            // Output stream to write file

            OutputStream output = new         FileOutputStream(root+"/downloadedfile.jpg");
            byte data[] = new byte[1024];

            long total = 0;
            while ((count = input.read(data)) != -1) {
                total += count;

                // writing data to file
                output.write(data, 0, count);

            }

            // flushing output
            output.flush();

            // closing streams
            output.close();
            input.close();

        } catch (Exception e) {
            Log.e("Error: ", e.getMessage());
        }

        return null;
    }



    /**
     * After completing background task
     * **/
    @Override
    protected void onPostExecute(String file_url) {
       System.out.println("Downloaded");
    }

}

采纳答案by Daniel Nugent

I just ran your code, and it works fine for me. The image was downloaded to the sdcard.

我刚刚运行了你的代码，它对我来说很好用。图像已下载到 SD 卡。

Just to note, make sure you have these permissions set in your AndroidManifest.xml:

请注意，请确保您在 AndroidManifest.xml 中设置了以下权限：

<uses-permission android:name="android.permission.INTERNET" />

<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

Here are the logs I got (note that I added a ProgressDialog):

这是我得到的日志（注意我添加了一个ProgressDialog）：

03-21 16:53:46.422  21017-21017/com.imagedownload.danielnugent.imagedownload D/Activity﹕ #1 setTransGradationModeColor false
03-21 16:53:56.211  21017-21017/com.imagedownload.danielnugent.imagedownload D/Activity﹕ #1 setTransGradationModeColor false
03-21 16:54:06.441  21017-21017/com.imagedownload.danielnugent.imagedownload I/System.out﹕ Starting download
03-21 16:54:06.441  21017-21017/com.imagedownload.danielnugent.imagedownload D/Dialog﹕ checkMirrorLinkEnabled returns : false
03-21 16:54:06.441  21017-21017/com.imagedownload.danielnugent.imagedownload D/Dialog﹕ showing allowed
03-21 16:54:06.461  21017-25126/com.imagedownload.danielnugent.imagedownload I/System.out﹕ Downloading
03-21 16:54:06.461  21017-21017/com.imagedownload.danielnugent.imagedownload D/Activity﹕ #1 setTransGradationModeColor false
03-21 16:54:06.481  21017-21017/com.imagedownload.danielnugent.imagedownload D/ProgressBar﹕ updateDrawableBounds: left = 0
03-21 16:54:06.481  21017-21017/com.imagedownload.danielnugent.imagedownload D/ProgressBar﹕ updateDrawableBounds: top = 0
03-21 16:54:06.491  21017-21017/com.imagedownload.danielnugent.imagedownload D/ProgressBar﹕ updateDrawableBounds: right = 144
03-21 16:54:06.491  21017-21017/com.imagedownload.danielnugent.imagedownload D/ProgressBar﹕ updateDrawableBounds: bottom = 144
03-21 16:54:11.596  21017-21017/com.imagedownload.danielnugent.imagedownload I/System.out﹕ Downloaded

Just in case this will be useful, here is the full MainActivity.java code that worked for me. (url is a placeholder):

以防万一这会有用，这里是对我有用的完整 MainActivity.java 代码。（网址是占位符）：

import android.app.ProgressDialog;
import android.os.AsyncTask;
import android.os.Environment;
import android.support.v7.app.ActionBarActivity;
import android.os.Bundle;
import android.util.Log;
import android.view.Menu;
import android.view.MenuItem;

import java.io.BufferedInputStream;
import java.io.FileOutputStream;
import java.io.InputStream;
import java.io.OutputStream;
import java.net.URL;
import java.net.URLConnection;


public class MainActivity extends ActionBarActivity {

    private ProgressDialog pDialog;

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

        new DownloadFileFromURL().execute("http://www.example.com/IMG.jpg");
    }


    @Override
    public boolean onCreateOptionsMenu(Menu menu) {
        // Inflate the menu; this adds items to the action bar if it is present.
        getMenuInflater().inflate(R.menu.menu_main, menu);
        return true;
    }

    @Override
    public boolean onOptionsItemSelected(MenuItem item) {
        // Handle action bar item clicks here. The action bar will
        // automatically handle clicks on the Home/Up button, so long
        // as you specify a parent activity in AndroidManifest.xml.
        int id = item.getItemId();

        //noinspection SimplifiableIfStatement
        if (id == R.id.action_settings) {
            return true;
        }

        return super.onOptionsItemSelected(item);
    }

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

        /**
         * Before starting background thread
         * */
        @Override
        protected void onPreExecute() {
            super.onPreExecute();
            System.out.println("Starting download");

            pDialog = new ProgressDialog(MainActivity.this);
            pDialog.setMessage("Loading... Please wait...");
            pDialog.setIndeterminate(false);
            pDialog.setCancelable(false);
            pDialog.show();
        }

        /**
         * Downloading file in background thread
         * */
        @Override
        protected String doInBackground(String... f_url) {
            int count;
            try {
                String root = Environment.getExternalStorageDirectory().toString();

                System.out.println("Downloading");
                URL url = new URL(f_url[0]);

                URLConnection conection = url.openConnection();
                conection.connect();
                // getting file length
                int lenghtOfFile = conection.getContentLength();

                // input stream to read file - with 8k buffer
                InputStream input = new BufferedInputStream(url.openStream(), 8192);

                // Output stream to write file

                OutputStream output = new FileOutputStream(root+"/downloadedfile.jpg");
                byte data[] = new byte[1024];

                long total = 0;
                while ((count = input.read(data)) != -1) {
                    total += count;

                    // writing data to file
                    output.write(data, 0, count);

                }

                // flushing output
                output.flush();

                // closing streams
                output.close();
                input.close();

            } catch (Exception e) {
                Log.e("Error: ", e.getMessage());
            }

            return null;
        }



        /**
         * After completing background task
         * **/
        @Override
        protected void onPostExecute(String file_url) {
            System.out.println("Downloaded");

            pDialog.dismiss();
        }

    }

}