java – 使用AsyncTask下载文件

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" />

这是我得到的日志(注意我添加了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

为了防止这有用,这里是完整的MainActivity.java代码,对我有用. (url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> {

        /**
         * 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,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();
        }

    }

}