例如10M大小，使用3个线程来下载，

线程下载的数据长度   (10%3 == 0 ？ 10/3:10/3+1) ,第1，2个线程下载长度是4M，第三个线程下载长度为2M

下载开始位置：线程id*每条线程下载的数据长度 = ?

下载结束位置：（线程id+1）*每条线程下载的数据长度-1=?

(3)使用Http的Range头字段指定每条线程从文件的什么位置开始下载，下载到什么位置为止，

如：指定从文件的2M位置开始下载，下载到位置(4M-1byte)为止

代码如下：HttpURLConnection.setRequestProperty("Range", "bytes=2097152-4194303");

(4)保存文件，使用RandomAccessFile类指定每条线程从本地文件的什么位置开始写入数据。

RandomAccessFile threadfile = new RandomAccessFile("QQWubiSetup.exe ","rwd");

threadfile.seek(2097152);//从文件的什么位置开始写入数据

/** *多线程下载，UI更新类  *@author young * */
public class MultiThreadDownload extends Thread{  private static final String TAG = "MultiThreadDownload";  /**每一个线程需要下载的大小 */  private int blockSize;  /*** 线程数量<br> 默认为5个线程下载*/  private int threadNum = 5;  /*** 文件大小 */  private int fileSize;  /** * 已经下载多少 */  private int downloadSize;  /**文件的url,线程编号，文件名称*/  private String UrlStr,ThreadNo,fileName;  /***保存的路径*/  private String savePath;  /**下载的百分比*/  private int downloadPercent = 0;  /**下载的 平均速度*/  private int downloadSpeed = 0;  /**下载用的时间*/  private int usedTime = 0;  /**当前时间*/  private long curTime;  /**是否已经下载完成*/  private boolean completed = false;  private Handler handler ;  /** * 下载的构造函数   * @param url  请求下载的URL * @param handler   UI更新使用 * @param savePath  保存文件的路径 */  public MultiThreadDownload(Handler handler,String url,String savePath)  {  this.handler = handler;  this.UrlStr = url;  this.savePath = savePath;  Log.e(TAG, toString());  }  @Override  public void run() {  FileDownloadThread[] fds = new FileDownloadThread[threadNum];//设置线程数量  try {  URL url = new URL(UrlStr);  URLConnection conn = url.openConnection();  fileSize = conn.getContentLength();  this.fileName = FileUtil.getFileName(UrlStr);  //只创建一个文件，saveFile下载内容  File saveFile = new File(savePath+"/"+fileName);  Log.e(TAG, "文件一共："+fileSize+" savePath "+savePath+"  fileName  "+fileName);  RandomAccessFile accessFile = new RandomAccessFile(saveFile,"rwd");  //设置本地文件的长度和下载文件相同     accessFile.setLength(fileSize);    accessFile.close();  //Handler更新UI，发送消息  sendMsg(FileUtil.startDownloadMeg);  //每块线程下载数据  blockSize = ((fileSize%threadNum)==0)?(fileSize/threadNum):(fileSize/threadNum+1);  Log.e(TAG, "每个线程分别下载 ："+blockSize);  for (int i = 0; i < threadNum; i++) {  int curThreadEndPosition = (i+1)!=threadNum ? ((i+1)*blockSize-1) : fileSize;  FileDownloadThread fdt = new FileDownloadThread(url, saveFile, i*blockSize, curThreadEndPosition);  fdt.setName("thread"+i);  fdt.start();  fds[i]=fdt;  }  /** * 获取数据，更新UI，直到所有下载线程都下载完成。 */  boolean finished = false;  //开始时间，放在循环外，求解的usedTime就是总时间  long startTime = System.currentTimeMillis();  while(!finished)  {  downloadSize = 0;  finished = true;  for (int i = 0; i < fds.length; i++) {  downloadSize+= fds[i].getDownloadSize();  if(!fds[i].isFinished())  {  finished = false;  }  }  downloadPercent = (downloadSize*100)/fileSize;  curTime = System.currentTimeMillis();  System.out.println("curTime = "+curTime+" downloadSize = "+downloadSize+" usedTime "+(int) ((curTime-startTime)/1000));  usedTime = (int) ((curTime-startTime)/1000);  if(usedTime==0)usedTime = 1;    downloadSpeed = (downloadSize/usedTime)/1024;  sleep(1000);/*1秒钟刷新一次界面*/  sendMsg(FileUtil.updateDownloadMeg);  }  Log.e(TAG, "下载完成");  completed = true;  sendMsg(FileUtil.endDownloadMeg);  } catch (Exception e) {  Log.e(TAG, "multi file error  Exception  "+e.getMessage());  e.printStackTrace();  }  super.run();  }  /** * 得到文件的大小 * @return */  public int getFileSize()  {  return this.fileSize;  }  /** * 得到已经下载的数量 * @return */  public int getDownloadSize()  {  return this.downloadSize;  }  /** * 获取下载百分比 * @return */  public int getDownloadPercent(){  return this.downloadPercent;  }  /** * 获取下载速度 * @return */  public int getDownloadSpeed(){  return this.downloadSpeed;  }  /** * 修改默认线程数 * @param threadNum */  public void setThreadNum(int threadNum){  this.threadNum = threadNum;  }  /** * 分块下载完成的标志 * @return */  public boolean isCompleted(){  return this.completed;  }  @Override  public String toString() {  return "MultiThreadDownload [threadNum=" + threadNum + ", fileSize="  + fileSize + ", UrlStr=" + UrlStr + ", ThreadNo=" + ThreadNo  + ", savePath=" + savePath + "]";  }  /** * 发送消息，用户提示 * */  private void sendMsg(int what)  {  Message msg = new Message();  msg.what = what;  handler.sendMessage(msg);  } 

public class FileDownloadThread extends Thread{  private static final String TAG = "FileDownloadThread";  /**缓冲区 */  private static final int BUFF_SIZE = 1024;  /**需要下载的URL*/  private URL url;  /**缓存的FIle*/  private File file;  /**开始位置*/  private int startPosition;  /**结束位置*/  private int endPosition;  /**当前位置*/  private int curPosition;  /**完成*/  private boolean finished = false;  /**已经下载多少*/  private int downloadSize = 0;  /*** * 分块文件下载，可以创建多线程模式 * @param url   下载的URL * @param file  下载的文件 * @param startPosition 开始位置 * @param endPosition   结束位置 */  public FileDownloadThread(URL url, File file, int startPosition,  int endPosition) {  this.url = url;  this.file = file;  this.startPosition = startPosition;  this.curPosition = startPosition;  this.endPosition = endPosition;  Log.e(TAG, toString());  }  @Override  public void run() {  BufferedInputStream bis = null;  RandomAccessFile rAccessFile = null;  byte[] buf = new byte[BUFF_SIZE];  URLConnection conn = null;  try {  conn = url.openConnection();  conn.setConnectTimeout(10000);//设置超时  conn.setReadTimeout(10000);  conn.setAllowUserInteraction(true);  System.out.println(this.getName()+" startPosition "+startPosition+" endPosition "+endPosition);  conn.setRequestProperty("Range", "bytes="+(startPosition)+"-"+endPosition);  //取剩余未下载的  rAccessFile = new RandomAccessFile(file,"rwd");//读写  //设置从什么位置开始写入数据   rAccessFile.seek(startPosition);  bis = new BufferedInputStream(conn.getInputStream(), BUFF_SIZE);  while(curPosition<endPosition)  //当前位置小于结束位置  继续下载  {  int len = bis.read(buf,0,BUFF_SIZE);  if(len==-1)   //下载完成    {   break;  }  rAccessFile.write(buf,0,len);  curPosition = curPosition +len;  if(curPosition > endPosition)  {   //如果下载多了，则减去多余部分  System.out.println("  curPosition > endPosition  !!!!");  int extraLen = curPosition-endPosition;  downloadSize += (len-extraLen+1);  }else{  downloadSize+=len;  }  }  this.finished = true;  //当前阶段下载完成  Log.e(TAG, "当前"+this.getName()+"下载完成");  } catch (Exception e) {  Log.e(TAG, "download error Exception "+e.getMessage());  e.printStackTrace();  }finally{  //关闭流  FileUtil.closeInputStream(bis);  try {  rAccessFile.close();  } catch (IOException e) {  // TODO Auto-generated catch block  Log.e("AccessFile", "AccessFile IOException "+e.getMessage());  }  }  super.run();  }  /** * 是否完成当前段下载完成 * @return */  public boolean isFinished() {  return finished;  }  /** * 已经下载多少 * @return */  public int getDownloadSize() {  return downloadSize;  }  @Override  public String toString() {  return "FileDownloadThread [url=" + url + ", file=" + file  + ", startPosition=" + startPosition + ", endPosition="  + endPosition + ", curPosition=" + curPosition + ", finished="  + finished + ", downloadSize=" + downloadSize + "]";  }

多线程下载是分段下载，创建保存一个文件，子线程分别通过RandomAccessFile类进行写入操作。

示例源码：demo下载