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

问题:

This question already has an answer here:

• JProgressBar wont update 3 answers

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);         }     } } 

回答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;    } } 

文章来源: Using swing GUI to make a progress bar show up while downloading a file [duplicate]