多线程下载文件的过程是：
(1)首先获得下载文件的长度，然后设置本地文件的长度。
HttpURLConnection.getContentLength();//获取下载文件的长度
RandomAccessFile file = new RandomAccessFile("QQSetup.exe","rwd");
file.setLength(filesize);//设置本地文件的长度
(2)依据文件长度和线程数计算每条线程下载的数据长度和下载位置。
如：文件的长度为6M，线程数为3，那么。每条线程下载的数据长度为2M，每条线程開始下载的位置例如以下图所看到的。
比如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
下载開始位置：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開始下载，下载到什么位置为止，
如：指定从文件的2M位置開始下载。下载到位置(4M-1byte)为止
代码例如以下：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開始写入数据。
RandomAccessFile threadfile = new RandomAccessFile("QQWubiSetup.exe ","rwd");
threadfile.seek(2097152);//从文件的什么位置開始写入数据
/** *多线程下载，UI更新类 *@author young * */ public class MultiThreadDownload extends Thread{ private static final String TAG = "MultiThreadDownload"; /**每个线程须要下载的大小 */ private int blockSize; /*** 线程数量 默觉得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*/ 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; /**是否已经下载完毕*/ 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); //仅仅创建一个文件，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同样 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，直到全部下载线程都下载完毕。 */ boolean finished = false; //開始时间，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, "下载完毕"); 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; } /** * 改动默认线程数 * @param threadNum */ public void setThreadNum(int threadNum){ this.threadNum = threadNum; } /** * 分块下载完毕的标志 * @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; /**须要下载的URL*/ private URL url; /**缓存的FIle*/ private File file; /**開始位置*/ private int startPosition; /**结束位置*/ private int endPosition; /**当前位置*/ private int curPosition; /**完毕*/ private boolean finished = false; /**已经下载多少*/ private int downloadSize = 0; /*** * 分块文件下载，能够创建多线程模式 * @param url 下载的URL * @param file 下载的文件 * @param startPosition 開始位置 * @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 //设置从什么位置開始写入数据 rAccessFile.seek(startPosition); bis = new BufferedInputStream(conn.getInputStream(), BUFF_SIZE); while(curPosition endPosition) { //假设下载多了。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; //当前阶段下载完毕 Log.e(TAG, "当前"+this.getName()+"下载完毕"); } 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(); } /** * 是否完毕当前段下载完毕 * @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。 演示样例源代码：