为什么我无法从android中的服务器下载文件

Question

为什么我不能从android中的服务器下载文件，而我认为我的代码是正确的。我的代码在我的设备中运行良好，甚至当我点击按钮时，我看到对话框窗口。但是当我在外部存储器中搜索时，我找不到此应用程序的结果。即使我在本地目录中检查了我的文件，但它的名称是正确的，我检查了服务器是否运行，或者没有运行。

public class MainActivity extends Activity {

    ProgressBar pb;
    Dialog dialog;
    int downloadedSize = 0;
    int totalSize = 0;
    TextView cur_val;
    String dwnload_file_path = "http://127.0.0.1:8080/apps/a.jpg";
    @Override
    public void onCreate(Bundle savedInstanceState) {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.activity_main);

        Button b = (Button) findViewById(R.id.button);
        b.setOnClickListener(new OnClickListener() {
            @Override
            public void onClick(View v) {
                showProgress(dwnload_file_path);

                new Thread(new Runnable() {
                    public void run() {
                        downloadFile();
                    }
                }).start();
            }
        });
    }

    void downloadFile(){

        try {
            URL url = new URL(dwnload_file_path);
            HttpURLConnection urlConnection = (HttpURLConnection) url.openConnection();

            urlConnection.setRequestMethod("GET");
            urlConnection.setDoOutput(true);

            //connect
            urlConnection.connect();

            //set the path where we want to save the file
            File SDCardRoot = Environment.getExternalStorageDirectory();
            //create a new file, to save the downloaded file
            File file = new File(SDCardRoot,"Image");

            FileOutputStream fileOutput = new FileOutputStream(file);

            //Stream used for reading the data from the internet
            InputStream inputStream = urlConnection.getInputStream();

            //this is the total size of the file which we are downloading
            totalSize = urlConnection.getContentLength();

            runOnUiThread(new Runnable() {
                public void run() {
                    pb.setMax(totalSize);
                }
            });

            //create a buffer...
            byte[] buffer = new byte[1024];
            int bufferLength = 0;

            while ( (bufferLength = inputStream.read(buffer)) > 0 ) {
                fileOutput.write(buffer, 0, bufferLength);
                downloadedSize += bufferLength;
                // update the progressbar //
                runOnUiThread(new Runnable() {
                    public void run() {
                        pb.setProgress(downloadedSize);
                        float per = ((float)downloadedSize/totalSize) * 100;
                        cur_val.setText("Downloaded " + downloadedSize + "KB / " + totalSize + "KB (" + (int)per + "%)" );
                    }
                });
            }
            //close the output stream when complete //
            fileOutput.close();
            runOnUiThread(new Runnable() {
                public void run() {
                    // pb.dismiss(); // if you want close it..
                }
            });

        } catch (final MalformedURLException e) {
            showError("Error : MalformedURLException " + e);
            e.printStackTrace();
        } catch (final IOException e) {
            showError("Error : IOException " + e);
            e.printStackTrace();
        }
        catch (final Exception e) {
            showError("Error : Please check your internet connection " + e);
        }
    }

    void showError(final String err){
        runOnUiThread(new Runnable() {
            public void run() {
                Toast.makeText(MainActivity.this, err, Toast.LENGTH_LONG).show();
            }
        });
    }

    void showProgress(String file_path){
        dialog = new Dialog(MainActivity.this);
        dialog.requestWindowFeature(Window.FEATURE_NO_TITLE);
        dialog.setContentView(R.layout.myprogressdialog);
        dialog.setTitle("Download Progress");

        TextView text = (TextView) dialog.findViewById(R.id.tv1);
        text.setText("Downloading file from ... " + file_path);
        cur_val = (TextView) dialog.findViewById(R.id.cur_pg_tv);
        cur_val.setText("Starting download...");
        dialog.show();

        pb = (ProgressBar)dialog.findViewById(R.id.progress_bar);
        pb.setProgress(0);

    }
}