Using swing GUI to make a progress bar show up while downloading a file

Question

I currently have a class that should show me a simple form while downloading a file. It is working, but the progress bar is not updating, and I can only see it once the download is finished. Can someone help me?

import java.awt.FlowLayout;
import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import javax.swing.JFrame;
import static javax.swing.JFrame.EXIT_ON_CLOSE;
import javax.swing.JProgressBar;

public class Downloader {

    public Downloader(String site, File file) {
        JFrame frm = new JFrame();
        JProgressBar current = new JProgressBar(0, 100);
        current.setSize(50, 100);
        current.setValue(0);
        current.setStringPainted(true);
        frm.add(current);
        frm.setVisible(true);
        frm.setLayout(new FlowLayout());
        frm.setSize(50, 100);
        frm.setDefaultCloseOperation(EXIT_ON_CLOSE);
        try {
            URL url = new URL(site);
            HttpURLConnection connection
                    = (HttpURLConnection) url.openConnection();
            int filesize = connection.getContentLength();
            float totalDataRead = 0;
            try (java.io.BufferedInputStream in = new java.io.BufferedInputStream(connection.getInputStream())) {
                java.io.FileOutputStream fos = new java.io.FileOutputStream(file);
                try (java.io.BufferedOutputStream bout = new BufferedOutputStream(fos, 1024)) {
                    byte[] data = new byte[1024];
                    int i;
                    while ((i = in.read(data, 0, 1024)) >= 0) {
                        totalDataRead = totalDataRead + i;
                        bout.write(data, 0, i);
                        float Percent = (totalDataRead * 100) / filesize;
                        current.setValue((int) Percent);
                    }
                }
            }
        } catch (IOException e) {
            javax.swing.JOptionPane.showConfirmDialog((java.awt.Component) null, e.getMessage(), "Error",
                    javax.swing.JOptionPane.DEFAULT_OPTION);
        }
    }
}

Answer 1

Yours is a classic Swing concurrency issue. The solution is as always -- do the long running code in a background thread such as can be found with a SwingWorker.

e.g.,

import java.awt.FlowLayout;
import java.beans.PropertyChangeEvent;
import java.beans.PropertyChangeListener;
import java.io.*;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.concurrent.ExecutionException;

import javax.swing.JFrame;
import javax.swing.SwingWorker;

import static javax.swing.JFrame.EXIT_ON_CLOSE;

import javax.swing.JProgressBar;

public class Downloader {

   public Downloader(String site, File file) {
      JFrame frm = new JFrame();
      final JProgressBar current = new JProgressBar(0, 100);
      current.setSize(50, 100);
      current.setValue(0);
      current.setStringPainted(true);
      frm.add(current);
      frm.setVisible(true);
      frm.setLayout(new FlowLayout());
      frm.setSize(50, 100);
      frm.setDefaultCloseOperation(EXIT_ON_CLOSE);
      final Worker worker = new Worker(site, file);
      worker.addPropertyChangeListener(new PropertyChangeListener() {

         @Override
         public void propertyChange(PropertyChangeEvent pcEvt) {
            if ("progress".equals(pcEvt.getPropertyName())) {
               current.setValue((Integer) pcEvt.getNewValue());
            } else if (pcEvt.getNewValue() == SwingWorker.StateValue.DONE) {
               try {
                  worker.get();
               } catch (InterruptedException | ExecutionException e) {
                  // handle any errors here
                  e.printStackTrace(); 
               }
            }

         }
      });
      worker.execute();
   }
}

class Worker extends SwingWorker<Void, Void> {
   private String site;
   private File file;

   public Worker(String site, File file) {
      this.site = site;
      this.file = file;
   }

   @Override
   protected Void doInBackground() throws Exception {
      URL url = new URL(site);
      HttpURLConnection connection = (HttpURLConnection) url
            .openConnection();
      int filesize = connection.getContentLength();
      int totalDataRead = 0;
      try (java.io.BufferedInputStream in = new java.io.BufferedInputStream(
            connection.getInputStream())) {
         java.io.FileOutputStream fos = new java.io.FileOutputStream(file);
         try (java.io.BufferedOutputStream bout = new BufferedOutputStream(
               fos, 1024)) {
            byte[] data = new byte[1024];
            int i;
            while ((i = in.read(data, 0, 1024)) >= 0) {
               totalDataRead = totalDataRead + i;
               bout.write(data, 0, i);
               int percent = (totalDataRead * 100) / filesize;
               setProgress(percent);
            }
         }
      }
      return null;
   }
}