How to create a download manager in Java

download manager in java

This example shows how to create a simple download manager in Java. It contains four classes in foru Java source files:

* Download.java: Contains Download class which downloads a file from a URL.
* DownloadManager.java: Contains the main class for download manager application.
* DownloadsTableModel.java: Contains the class which manages the download table's data.
* ProgressRenderer.java: Contains the class which is responsible to render a JProgressBar in a table cell.

The contents of the listed files are written below.

Download.java
import java.io.*;
import java.net.*;
import java.util.*;

// This class downloads a file from a URL.
class Download extends Observable implements Runnable {

// Max size of download buffer.
private static final int MAX_BUFFER_SIZE = 1024;

// These are the status names.
public static final String STATUSES[] = {"Downloading",
"Paused", "Complete", "Cancelled", "Error"};

// These are the status codes.
public static final int DOWNLOADING = 0;
public static final int PAUSED = 1;
public static final int COMPLETE = 2;
public static final int CANCELLED = 3;
public static final int ERROR = 4;

private URL url; // download URL
private int size; // size of download in bytes
private int downloaded; // number of bytes downloaded
private int status; // current status of download

// Constructor for Download.
public Download(URL url) {
this.url = url;
size = -1;
downloaded = 0;
status = DOWNLOADING;

// Begin the download.
download();
}

// Get this download's URL.
public String getUrl() {
return url.toString();
}

// Get this download's size.
public int getSize() {
return size;
}

// Get this download's progress.
public float getProgress() {
return ((float) downloaded / size) * 100;
}

// Get this download's status.
public int getStatus() {
return status;
}

// Pause this download.
public void pause() {
status = PAUSED;
stateChanged();
}

// Resume this download.
public void resume() {
status = DOWNLOADING;
stateChanged();
download();
}

// Cancel this download.
public void cancel() {
status = CANCELLED;
stateChanged();
}

// Mark this download as having an error.
private void error() {
status = ERROR;
stateChanged();
}

// Start or resume downloading.
private void download() {
Thread thread = new Thread(this);
thread.start();
}

// Get file name portion of URL.
private String getFileName(URL url) {
String fileName = url.getFile();
return fileName.substring(fileName.lastIndexOf('/') + 1);
}

// Download file.
public void run() {
RandomAccessFile file = null;
InputStream stream = null;

try {
// Open connection to URL.
HttpURLConnection connection =
(HttpURLConnection) url.openConnection();

// Specify what portion of file to download.
connection.setRequestProperty("Range",
"bytes=" + downloaded + "-");

// Connect to server.
connection.connect();

// Make sure response code is in the 200 range.
if (connection.getResponseCode() / 100 != 2) {
error();
}

// Check for valid content length.
int contentLength = connection.getContentLength();
if (contentLength < 1) {
error();
}

/* Set the size for this download if it
hasn't been already set. */
if (size == -1) {
size = contentLength;
stateChanged();
}

// Open file and seek to the end of it.
file = new RandomAccessFile(getFileName(url), "rw");
file.seek(downloaded);

stream = connection.getInputStream();
while (status == DOWNLOADING) {
/* Size buffer according to how much of the
file is left to download. */
byte buffer[];
if (size - downloaded > MAX_BUFFER_SIZE) {
buffer = new byte[MAX_BUFFER_SIZE];
} else {
buffer = new byte[size - downloaded];
}

// Read from server into buffer.
int read = stream.read(buffer);
if (read == -1)
break;

// Write buffer to file.
file.write(buffer, 0, read);
downloaded += read;
stateChanged();
}

/* Change status to complete if this point was
reached because downloading has finished. */
if (status == DOWNLOADING) {
status = COMPLETE;
stateChanged();
}
} catch (Exception e) {
error();
} finally {
// Close file.
if (file != null) {
try {
file.close();
} catch (Exception e) {}
}

// Close connection to server.
if (stream != null) {
try {
stream.close();
} catch (Exception e) {}
}
}
}

// Notify observers that this download's status has changed.
private void stateChanged() {
setChanged();
notifyObservers();
}
}

DownloadManager.java
import java.awt.*;
import java.awt.event.*;
import java.net.*;
import java.util.*;
import javax.swing.*;
import javax.swing.event.*;

// The Download Manager.
public class DownloadManager extends JFrame
implements Observer {

// Add download text field.
private JTextField addTextField;

// Download table's data model.
private DownloadsTableModel tableModel;

// Table listing downloads.
private JTable table;

// These are the buttons for managing the selected download.
private JButton pauseButton, resumeButton;
private JButton cancelButton, clearButton;

// Currently selected download.
private Download selectedDownload;

// Flag for whether or not table selection is being cleared.
private boolean clearing;

// Constructor for Download Manager.
public DownloadManager() {
// Set application title.
setTitle("Download Manager");

// Set window size.
setSize(640, 480);

// Handle window closing events.
addWindowListener(new WindowAdapter() {
public void windowClosing(WindowEvent e) {
actionExit();
}
});

// Set up file menu.
JMenuBar menuBar = new JMenuBar();
JMenu fileMenu = new JMenu("File");
fileMenu.setMnemonic(KeyEvent.VK_F);
JMenuItem fileExitMenuItem = new JMenuItem("Exit",
KeyEvent.VK_X);
fileExitMenuItem.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionExit();
}
});
fileMenu.add(fileExitMenuItem);
menuBar.add(fileMenu);
setJMenuBar(menuBar);

// Set up add panel.
JPanel addPanel = new JPanel();
addTextField = new JTextField(30);
addPanel.add(addTextField);
JButton addButton = new JButton("Add Download");
addButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionAdd();
}
});
addPanel.add(addButton);

// Set up Downloads table.
tableModel = new DownloadsTableModel();
table = new JTable(tableModel);
table.getSelectionModel().addListSelectionListener(new
ListSelectionListener() {
public void valueChanged(ListSelectionEvent e) {
tableSelectionChanged();
}
});
// Allow only one row at a time to be selected.
table.setSelectionMode(ListSelectionModel.SINGLE_SELECTION);

// Set up ProgressBar as renderer for progress column.
ProgressRenderer renderer = new ProgressRenderer(0, 100);
renderer.setStringPainted(true); // show progress text
table.setDefaultRenderer(JProgressBar.class, renderer);

// Set table's row height large enough to fit JProgressBar.
table.setRowHeight(
(int) renderer.getPreferredSize().getHeight());

// Set up downloads panel.
JPanel downloadsPanel = new JPanel();
downloadsPanel.setBorder(
BorderFactory.createTitledBorder("Downloads"));
downloadsPanel.setLayout(new BorderLayout());
downloadsPanel.add(new JScrollPane(table),
BorderLayout.CENTER);

// Set up buttons panel.
JPanel buttonsPanel = new JPanel();
pauseButton = new JButton("Pause");
pauseButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionPause();
}
});
pauseButton.setEnabled(false);
buttonsPanel.add(pauseButton);
resumeButton = new JButton("Resume");
resumeButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionResume();
}
});
resumeButton.setEnabled(false);
buttonsPanel.add(resumeButton);
cancelButton = new JButton("Cancel");
cancelButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionCancel();
}
});
cancelButton.setEnabled(false);
buttonsPanel.add(cancelButton);
clearButton = new JButton("Clear");
clearButton.addActionListener(new ActionListener() {
public void actionPerformed(ActionEvent e) {
actionClear();
}
});
clearButton.setEnabled(false);
buttonsPanel.add(clearButton);

// Add panels to display.
getContentPane().setLayout(new BorderLayout());
getContentPane().add(addPanel, BorderLayout.NORTH);
getContentPane().add(downloadsPanel, BorderLayout.CENTER);
getContentPane().add(buttonsPanel, BorderLayout.SOUTH);
}

// Exit this program.
private void actionExit() {
System.exit(0);
}

// Add a new download.
private void actionAdd() {
URL verifiedUrl = verifyUrl(addTextField.getText());
if (verifiedUrl != null) {
tableModel.addDownload(new Download(verifiedUrl));
addTextField.setText(""); // reset add text field
} else {
JOptionPane.showMessageDialog(this,
"Invalid Download URL", "Error",
JOptionPane.ERROR_MESSAGE);
}
}

// Verify download URL.
private URL verifyUrl(String url) {
// Only allow HTTP URLs.
if (!url.toLowerCase().startsWith("http://"))
return null;

// Verify format of URL.
URL verifiedUrl = null;
try {
verifiedUrl = new URL(url);
} catch (Exception e) {
return null;
}

// Make sure URL specifies a file.
if (verifiedUrl.getFile().length() < 2)
return null;

return verifiedUrl;
}

// Called when table row selection changes.
private void tableSelectionChanged() {
/* Unregister from receiving notifications
from the last selected download. */
if (selectedDownload != null)
selectedDownload.deleteObserver(DownloadManager.this);

/* If not in the middle of clearing a download,
set the selected download and register to
receive notifications from it. */
if (!clearing) {
selectedDownload =
tableModel.getDownload(table.getSelectedRow());
selectedDownload.addObserver(DownloadManager.this);
updateButtons();
}
}

// Pause the selected download.
private void actionPause() {
selectedDownload.pause();
updateButtons();
}

// Resume the selected download.
private void actionResume() {
selectedDownload.resume();
updateButtons();
}

// Cancel the selected download.
private void actionCancel() {
selectedDownload.cancel();
updateButtons();
}

// Clear the selected download.
private void actionClear() {
clearing = true;
tableModel.clearDownload(table.getSelectedRow());
clearing = false;
selectedDownload = null;
updateButtons();
}

/* Update each button's state based off of the
currently selected download's status. */
private void updateButtons() {
if (selectedDownload != null) {
int status = selectedDownload.getStatus();
switch (status) {
case Download.DOWNLOADING:
pauseButton.setEnabled(true);
resumeButton.setEnabled(false);
cancelButton.setEnabled(true);
clearButton.setEnabled(false);
break;
case Download.PAUSED:
pauseButton.setEnabled(false);
resumeButton.setEnabled(true);
cancelButton.setEnabled(true);
clearButton.setEnabled(false);
break;
case Download.ERROR:
pauseButton.setEnabled(false);
resumeButton.setEnabled(true);
cancelButton.setEnabled(false);
clearButton.setEnabled(true);
break;
default: // COMPLETE or CANCELLED
pauseButton.setEnabled(false);
resumeButton.setEnabled(false);
cancelButton.setEnabled(false);
clearButton.setEnabled(true);
}
} else {
// No download is selected in table.
pauseButton.setEnabled(false);
resumeButton.setEnabled(false);
cancelButton.setEnabled(false);
clearButton.setEnabled(false);
}
}

/* Update is called when a Download notifies its
observers of any changes. */
public void update(Observable o, Object arg) {
// Update buttons if the selected download has changed.
if (selectedDownload != null && selectedDownload.equals(o))
updateButtons();
}

// Run the Download Manager.
public static void main(String[] args) {
DownloadManager manager = new DownloadManager();
manager.show();
}
}

DownloadTableModel.java
import java.util.*;
import javax.swing.*;
import javax.swing.table.*;

// This class manages the download table's data.
class DownloadsTableModel extends AbstractTableModel
implements Observer {

// These are the names for the table's columns.
private static final String[] columnNames = {"URL", "Size",
"Progress", "Status"};

// These are the classes for each column's values.
private static final Class[] columnClasses = {String.class,
String.class, JProgressBar.class, String.class};

// The table's list of downloads.
private ArrayList downloadList = new ArrayList();

// Add a new download to the table.
public void addDownload(Download download) {

// Register to be notified when the download changes.
download.addObserver(this);

downloadList.add(download);

// Fire table row insertion notification to table.
fireTableRowsInserted(getRowCount() - 1, getRowCount() - 1);
}

// Get a download for the specified row.
public Download getDownload(int row) {
return (Download) downloadList.get(row);
}

// Remove a download from the list.
public void clearDownload(int row) {
downloadList.remove(row);

// Fire table row deletion notification to table.
fireTableRowsDeleted(row, row);
}

// Get table's column count.
public int getColumnCount() {
return columnNames.length;
}

// Get a column's name.
public String getColumnName(int col) {
return columnNames[col];
}

// Get a column's class.
public Class getColumnClass(int col) {
return columnClasses[col];
}

// Get table's row count.
public int getRowCount() {
return downloadList.size();
}

// Get value for a specific row and column combination.
public Object getValueAt(int row, int col) {

Download download = (Download) downloadList.get(row);
switch (col) {
case 0: // URL
return download.getUrl();
case 1: // Size
int size = download.getSize();
return (size == -1) ? "" : Integer.toString(size);
case 2: // Progress
return new Float(download.getProgress());
case 3: // Status
return Download.STATUSES[download.getStatus()];
}
return "";
}

/* Update is called when a Download notifies its
observers of any changes */
public void update(Observable o, Object arg) {
int index = downloadList.indexOf(o);

// Fire table row update notification to table.
fireTableRowsUpdated(index, index);
}
}

ProgressRenderer.java
import java.awt.*;
import javax.swing.*;
import javax.swing.table.*;

// This class renders a JProgressBar in a table cell.
class ProgressRenderer extends JProgressBar
implements TableCellRenderer {

// Constructor for ProgressRenderer.
public ProgressRenderer(int min, int max) {
super(min, max);
}

/* Returns this JProgressBar as the renderer
for the given table cell. */
public Component getTableCellRendererComponent(
JTable table, Object value, boolean isSelected,
boolean hasFocus, int row, int column) {
// Set JProgressBar's percent complete value.
setValue((int) ((Float) value).floatValue());
return this;
}
}

source: java-tips.org