多线程下载器

package com.java;
import java.awt.FlowLayout;
import java.awt.Font;
import java.awt.event.ActionEvent;
import java.awt.event.ActionListener;
import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
import java.net.URLConnection;
 
import javax.swing.JButton;
import javax.swing.JFrame;
import javax.swing.JLabel;
import javax.swing.JOptionPane;
import javax.swing.JPanel;
import javax.swing.JTextField;
 
public class DownMultiThreadFrame extends JFrame implements ActionListener{
	private static final long serialVersionUID = 9055094226537574068L;

	public DownMultiThreadFrame(){
        panel.setLayout(new FlowLayout());
        label1.setFont(new Font("雅黑", Font.BOLD, 15));
        panel.add(label1);
        panel.add(label2);
        panel.add(urlField);
        panel.add(StartButton);
        panel.add(resetButton);
        panel.add(exitButton);
        setContentPane(panel);
        StartButton.addActionListener(this);
        resetButton.addActionListener(this);
        exitButton.addActionListener(this);
        setSize(400, 400);
        setVisible(true);
        setDefaultCloseOperation(JFrame.EXIT_ON_CLOSE);
    }
 
    public static void main(String[] args){
        new DownMultiThreadFrame();
 
    }
 
    private final JPanel panel = new JPanel();
    private final JLabel label1 = new JLabel("网络资源的多线程下载：");
    private final JLabel label2 = new JLabel("网络资源的网址：");
    JButton StartButton = new JButton("点击开始下载");
    JButton resetButton = new JButton("清空");
    JButton exitButton = new JButton("退出");
    JTextField urlField = new JTextField(20);
 
    @Override
    public void actionPerformed(ActionEvent e){
        if(e.getSource() == StartButton){
            if("".equals(urlField.getText())){
                JOptionPane.showMessageDialog(this, "请输入资源地址");
            }
            String url = urlField.getText();
            int pos = url.lastIndexOf("/");
            String fileName = url.substring(pos + 1);
            try{
                download(url, "d:\\" + fileName, 2);
            }catch(Exception e1){
                e1.printStackTrace();
            }
        }else if(e.getSource() == resetButton){
            urlField.setText("");
        }else{
            System.exit(0);
        }
    }
 
    /**
     * 下载网络资源
     * */
    public void download(String url, String dest, int threadNum)
            throws Exception{
        URL downURL = new URL(url);
        // 打开网络连接
        HttpURLConnection conn = (HttpURLConnection) downURL.openConnection();
        // 创建文件长度的变量
        long fileLength = -1;
        // 获得连接状态标记代码
        int stateFlagCode = conn.getResponseCode();
        if(stateFlagCode == 200){
            // 网络连接正常
            fileLength = conn.getContentLength();
            // 取笑网络连接
            conn.disconnect();
        }
        if(fileLength > 0){
            // 计算每个线程的字节数
            long byteCounts = fileLength / threadNum + 1;
            File file = new File(dest);
            int i = 0;
            while(i < threadNum){
                long startPosition = byteCounts * i;
                long endPosition = byteCounts * (i + 1);
                if(i == threadNum - 1){
                    DownMulltiThread fileThread = new DownMulltiThread(url,
                            file, startPosition, 0);
                    new Thread(fileThread).start();
                }else{
                    DownMulltiThread fileThread = new DownMulltiThread(url,
                            file, startPosition, endPosition);
                    new Thread(fileThread).start();
                }
                i++;
            }
        }
    }
 
}// end class
 
/**
 * 用于实现网络资源的下载
 * */
class DownMulltiThread implements Runnable{
 
    public DownMulltiThread(){
 
    }
 
    /**
     * @param sUrl
     *            网络资源位置
     * @param desFile
     *            需要写入的文件对象
     * @param startPos
     *            写入的开始位置
     * @param endPos
     *            写入文件的结束位置
     * */
    public DownMulltiThread(String sUrl, File desFile, long startPos,
            long endPos){
        this.desFile = desFile;
        this.sUrl = sUrl;
        this.startPos = startPos;
        this.endPos = endPos;
    }
 
    private String sUrl = "";
    private File desFile;
    private long startPos;
    private long endPos;
 
    @Override
    public void run(){
        RandomAccessFile out;
        try{
            out = new RandomAccessFile(desFile, "rw");
            out.seek(startPos);
            URL url = new URL(sUrl);
            URLConnection urlcon = url.openConnection();
            urlcon.connect();
            InputStream in = urlcon.getInputStream();
            BufferedInputStream buf = new BufferedInputStream(in);
            byte[] bytes = new byte[1024];
            int len = buf.read(bytes);
            long diffLength=endPos-startPos;
            while(len != -1){
                out.write(bytes, 0, len);
                diffLength-=1024;
                if(diffLength<=0){
                	break;
                }
                len = buf.read(bytes);
            }
 
        }catch(FileNotFoundException e){
            e.printStackTrace();
        }catch(IOException e){
            e.printStackTrace();
        }
    }
}