你的是一个经典的Swing并发问题。解决方案是一样的 - 在后台线程中执行长时间运行的代码，例如可以使用SwingWorker找到。

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.

例如，

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;
   }
}