0.使用多线程下载会提升文件下载的速度，那么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();RandomAccessFile file = new RandomAccessFile("QQWubiSetup.exe","rwd");file.setLength(filesize);//设置本地文件的长度(2)根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，每条线程开始下载的位置如下图所示。

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1.多线程下载的核心代码示例Java代码1.public class MulThreadDownload2.{3. /**4. * 多线程下载5. * @param args6. */7. public static void main(String[] args)8. {9. String path = "/QQWubiSetup.exe";10. try11. {12. new MulThreadDownload().download(path, 3);13. }14. catch (Exception e)15. {16. e.printStackTrace();17. }18. }19. /**20. * 从路径中获取文件名称21. * @param path 下载路径22. * @return23. */24. public static String getFilename(String path)25. {26. return path.substring(stIndexOf('/')+1);27. }28. /**29. * 下载文件30. * @param path 下载路径31. * @param threadsize 线程数32. */33. public void download(String path, int threadsize) throwsException34. {35. URL url = new URL(path);36. HttpURLConnection conn = (HttpURLConnection)url.openConnection();37. conn.setRequestMethod("GET");38. conn.setConnectTimeout(5 * 1000);39. //获取要下载的文件的长度40. int filelength = conn.getContentLength();41. //从路径中获取文件名称42. String filename = getFilename(path);43. File saveFile = new File(filename);44. RandomAccessFile accessFile = new RandomAccessFile(saveFile, "rwd");45. //设置本地文件的长度和下载文件相同46. accessFile.setLength(filelength);47. accessFile.close();48. //计算每条线程下载的数据长度49. int block = filelength%threadsize==0? filelength/threadsize : filelength/threadsize+1;50. for(int threadid=0 ; threadid < threadsize ; threadid++){51. new DownloadThread(url, saveFile, block, threadid).start();52. }53. }54.55. private final class DownloadThread extends Thread56. {57. private URL url;58. private File saveFile;59. private int block;//每条线程下载的数据长度60. private int threadid;//线程id61. public DownloadThread(URL url, File saveFile, int block, int threadid)62. {63. this.url = url;64. this.saveFile = saveFile;65. this.block = block;66. this.threadid = threadid;67. }68.@Override69. public void run()70. {71. //计算开始位置公式：线程id*每条线程下载的数据长度= ？72. //计算结束位置公式：（线程id +1）*每条线程下载的数据长度-1 =?73. int startposition = threadid * block;74. int endposition = (threadid + 1 ) * block - 1;75. try76. {77. RandomAccessFile accessFile = new RandomAccessFile(saveFile, "rwd");78. //设置从什么位置开始写入数据79. accessFile.seek(startposition);80. HttpURLConnection conn = (HttpURLConnection)url.openConnection();81. conn.setRequestMethod("GET");82. conn.setConnectTimeout(5 * 1000);83. conn.setRequestProperty("Range", "bytes="+ startposition+ "-"+ endposition);84. InputStream inStream = conn.getInputStream();85. byte[] buffer = new byte[1024];86. int len = 0;87. while( (len=inStream.read(buffer)) != -1 )88. {89. accessFile.write(buffer, 0, len);90. }91. inStream.close();92. accessFile.close();93. System.out.println("线程id:"+ threadid+ "下载完成");94. }95. catch (Exception e)96. {97. e.printStackTrace();98. }99. }100. }101.}2.多线程断点下载功能，这里把断点数据保存到数据库中：注意代码注释的理解(0)主Activity，关键点使用Handler更新进度条与开启线程下载避免ANR若不使用Handler却要立即更新进度条数据，可使用：//resultView.invalidate(); UI线程中立即更新进度条方法//resultView.postInvalidate(); 非UI线程中立即更新进度条方法Java代码1./**2. * 注意这里的设计思路，UI线程与参数保存问题，避免ANR问题，UI控件的显示3. * @author kay4. *5. */6.public class DownloadActivity extends Activity7.{8. private EditText downloadpathText;9. private TextView resultView;10. private ProgressBar progressBar;11.12.@Override13. public void onCreate(Bundle savedInstanceState)14. {15. super.onCreate(savedInstanceState);16. setContentView(yout.main);17.18. downloadpathText = (EditText) this.findViewById(R.id.downloadpath);19. progressBar = (ProgressBar) this.findViewById(R.id.downloadbar);20. resultView = (TextView) this.findViewById(R.id.result);21. Button button = (Button) this.findViewById(R.id.button);22. button.setOnClickListener(new View.OnClickListener()23. {24.@Override25. public void onClick(View v)26. {27. //取得下载路径28. String path = downloadpathText.getText().toString();29. //判断SDCard是否存在30. if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED))31. {32. //下载操作33. download(path, Environment.getExternalStorageDirectory());34. }35. else36. {37. Toast.makeText(DownloadActivity.this, R.string.sdcarderror, 1).show();38. }39. }40. });41. }42. //主线程(UI线程)43. //业务逻辑正确，但是该程序运行的时候有问题(可能出现ANR问题)44. //对于显示控件的界面更新只是由UI线程负责，如果是在非UI线程更新控件的属性值，更新后的显示界面不会反映到屏幕上45. /**46. * 参数类型：因为启动一个线程还需要使用到上面方法的参数，而主方法启动后很快就会销毁，47. * 那么使用Final可以解决参数丢失的问题48. * path 注意是Final类型49. * savedir 注意是Final类型50. */51. private void download(final String path, final File savedir)52. {53. //这里开启一个线程避免ANR错误54. new Thread(new Runnable()55. {56.@Override57. public void run()58. {59. FileDownloader loader = new FileDownloader(DownloadActivity.this, path, savedir, 3);60. //设置进度条的最大刻度为文件的长度61. progressBar.setMax(loader.getFileSize());62. try63. {64. loader.download(new DownloadProgressListener()65. {66. /**67. * 注意这里的设计，显示进度条数据需要使用Handler来处理68. * 因为非UI线程更新后的数据不能被刷新69. */70.@Override71. public void onDownloadSize(int size)72. {73. //实时获知文件已经下载的数据长度74. Message msg = new Message();75. //设置消息标签76. msg.what = 1;77. msg.getData().putInt("size", size);78. //使用Handler对象发送消息79. handler.sendMessage(msg);80. }81. });82. }83. catch (Exception e)84. {85. //发送一个空消息到消息队列86. handler.obtainMessage(-1).sendToTarget();87. /**88. * 或者使用下面的方法发送一个空消息89. * Message msg = new Message();90. * msg.what = 1;91. * handler.sendMessage(msg);92. */93. }94. }95. }).start();96. }97.98.99. /**Handler原理：当Handler被创建时会关联到创建它的当前线程的消息队列，该类用于往消息队列发送消息100. *101. * 消息队列中的消息由当前线程内部进行处理102. */103. private Handler handler = new Handler()104. {105. //重写Handler里面的handleMessage方法处理消息106.@Override107. public void handleMessage(Message msg)108. {109. switch (msg.what)110. {111. case 1:112. //进度条显示113. progressBar.setProgress(msg.getData().getInt("size"));114. float num = (float)progressBar.getProg ress()/(float)progressBar.getMax();115. int result = (int)(num*100);116. //resultView.invalidate(); UI线程中立即更新进度条方法117. //resultView.postInvalidate(); 非UI线程中立即更新进度条方法118. resultView.setText(result+ "%"); 119. //判断是否下载成功120. if(progressBar.getProgress()==progress Bar.getMax())121. {122. Toast.makeText(DownloadActivity.th is, R.string.success, 1).show();123. }124. break;125. case -1:126. Toast.makeText(DownloadActivity.this, R.string.error, 1).show();127. break;128. }129. }130. };131.132.}(1)下载类：注意计算每条线程的下载长度与下载起始位置的方法Java代码1.public class DownloadThread extends Thread2.{3. private static final String TAG = "DownloadThread";4. private File saveFile;5. private URL downUrl;6. private int block;7. //下载开始位置8. private int threadId = -1;9. //下载文件长度10. private int downLength;11. private boolean finish = false;12. private FileDownloader downloader;13. public DownloadThread(FileDownloader downloader, URL downUrl, File saveFile, int block, int downLength, int threadId)14. {15. this.downUrl = downUrl;16. this.saveFile = saveFile;17. this.block = block;18. this.downloader = downloader;19. this.threadId = threadId;20. this.downLength = downLength;21. }22.23.@Override24. public void run()25. {26. //未下载完成27. if(downLength < block)28. {29. try30. {31. HttpURLConnection http = (HttpURLConnection)downUrl.openConnection();32. http.setConnectTimeout(5 * 1000);33. http.setRequestMethod("GET");34. http.setRequestProperty("Accept", "image/gif,image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave -flash, application/xaml+xml, application/vnd.ms-xpsdocument, application/x-ms-xbap, application/x-ms-application, applicati on/vnd.ms-excel, application/vnd.ms-powerpoint, application/ms word, */*");35. http.setRequestProperty("Accept-Language", "zh-CN");36. http.setRequestProperty("Referer", downUrl.toString());37. http.setRequestProperty("Charset", "UTF-8");38. //下载开始位置：线程id*每条线程下载的数据长度 = ?39. int startPos = block * (threadId - 1) + downLength;40. //下载结束位置：（线程id+1）*每条线程下载的数据长度-1=?41. int endPos = block * threadId -1;42. //设置获取实体数据的范围43. http.setRequestProperty("Range", "bytes=" + startPos + "-"+ endPos);44. http.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");45. http.setRequestProperty("Connection", "Keep-Alive");46.47. InputStream inStream = http.getInputStream();48. byte[] buffer = new byte[1024];49. int offset = 0;50. print("Thread " + this.threadId + " start download from position "+ startPos);51. RandomAccessFile threadfile = new RandomAccessFile(this.saveFile, "rwd");52. threadfile.seek(startPos);53. while ((offset = inStream.read(buffer, 0, 1024)) != -1)54. {55. threadfile.write(buffer, 0, offset);56. downLength += offset;57. downloader.update(this.threadId, downLength);58. downloader.append(offset);59. }60. threadfile.close();61. inStream.close();62. print("Thread " + this.threadId + " downloadfinish");63. //标记是否完成64. this.finish = true;65. }66. catch (Exception e)67. {68. this.downLength = -1;69. print("Thread "+ this.threadId+ ":"+ e);70. }71. }72. }73.74. private static void print(String msg)75. {76. Log.i(TAG, msg);77. }78.79. /**80. * 下载是否完成81. * @return82. */83. public boolean isFinish()84. {85. return finish;86. }87.88. /**89. * 已经下载的内容大小90. * @return 如果返回值为-1,代表下载失败91. */92. public long getDownLength()93. {94. return downLength;95. }96.}文件下载器，使用Java代码1./**2. * 文件下载器,使用这个类的方法如下示例：3. * FileDownloader loader = new FileDownloader(context, "http:///ejb3/ActivePort.exe",4. * new File("D:\\androidsoft\\test"), 2);5. * loader.getFileSize();//得到文件总大小6. * try {7. * loader.download(new DownloadProgressListener(){8. * public void onDownloadSize(int size) {9. * print("已经下载："+ size);10. * }11. * });12. * }13. * catch (Exception e)14. * {15. * e.printStackTrace();16. * }17. */18.public class FileDownloader19.{20. private static final String TAG = "FileDownloader";21. private Context context;22. private FileService fileService;23. //已下载文件长度24. private int downloadSize = 0;25. //原始文件长度26. private int fileSize = 0;27. ///线程数28. private DownloadThread[] threads;29. //本地保存文件30. private File saveFile;31. //缓存各线程下载的长度32. private Map<Integer, Integer> data = new ConcurrentHashMap<Integer, Integer>();33. //每条线程下载的长度34. private int block;35. //下载路径36. private String downloadUrl;37. //获取线程数38. public int getThreadSize()39. {40. return threads.length;41. }42. /**43. * 获取文件大小44. * @return45. */46. public int getFileSize()47. {48. return fileSize;49. }50. /**51. * 累计已下载大小52. * @param size53. */54. protected synchronized void append(int size)55. {56. downloadSize += size;57. }58. /**59. * 更新指定线程最后下载的位置60. * @param threadId 线程id61. * @param pos 最后下载的位置62. */63. protected synchronized void update(int threadId, int pos)64. {65. this.data.put(threadId, pos);66. this.fileService.update(this.downloadUrl, this.data);67. }68. /**69. * 文件下载构造器70. * @param downloadUrl 下载路径71. * @param fileSaveDir 文件保存目录72. * @param threadNum 下载线程数73. */74. public FileDownloader(Context context, String downloadUrl, File fileSaveDir, int threadNum)75. {76. try77. {78. this.context = context;79. this.downloadUrl = downloadUrl;80. fileService = new FileService(this.context);81. URL url = new URL(this.downloadUrl);82. if(!fileSaveDir.exists()) fileSaveDir.mkdirs();83. this.threads = new DownloadThread[threadNum];84. HttpURLConnection conn = (HttpURLConnection) url.openConnection();85. conn.setConnectTimeout(5*1000);86. conn.setRequestMethod("GET");87. conn.setRequestProperty("Accept", "image/gif, image/jpeg, image/pjpeg, image/pjpeg, application/x-shockwave-fla sh, application/xaml+xml, application/vnd.ms-xpsdocument, appl ication/x-ms-xbap, application/x-ms-application, application/v nd.ms-excel, application/vnd.ms-powerpoint, application/msword , */*");88. conn.setRequestProperty("Accept-Language", "zh-CN");89. conn.setRequestProperty("Referer", downloadUrl);90. conn.setRequestProperty("Charset", "UTF-8");91. conn.setRequestProperty("User-Agent", "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR1.1.4322; .NET CLR2.0.50727; .NET CLR3.0.04506.30; .NET CLR3.0.4506.2152; .NET CLR 3.5.30729)");92. conn.setRequestProperty("Connection", "Keep-Alive");93. conn.connect();94. printResponseHeader(conn);95. if (conn.getResponseCode()==200)96. {97. //根据响应获取文件大小98. this.fileSize = conn.getContentLength();99. if (this.fileSize <= 0) throw new RuntimeException("Unkown file size ");100. //获取文件名称101. String filename = getFileName(conn); 102. //构建保存文件103. this.saveFile = new File(fileSaveDir, file name);104. //获取下载记录105. Map<Integer, Integer> logdata = fileService.getData(downloadUrl);106. //如果存在下载记录107. if(logdata.size()>0)108. {109. //把各条线程已经下载的数据长度放入data 中110. for(Map.Entry<Integer, Integer> entry : logdata.entrySet())111. data.put(entry.getKey(), entry.get Value());112. }113. //下面计算所有线程已经下载的数据长度114. if(this.data.size()==this.threads.length)115. {116. for (int i = 0; i < this.threads.lengt h; i++)117. {118. this.downloadSize += this.data.get (i+1);119. }120. print("已经下载的长度"+ this.downloadSize);121. }122. //计算每条线程下载的数据长度123. this.block = (this.fileSize % this.threads .length)==0? this.fileSize / this.threads.length : this.fileSi ze / this.threads.length + 1;124. }125. else126. {127. throw new RuntimeException("server no resp onse ");128. }129. }130. catch (Exception e)131. {132. print(e.toString());133. throw new RuntimeException("don't connection t his url");134. }135. }136. /**137. * 获取文件名138. */139. private String getFileName(HttpURLConnection conn) 140. {141. String filename = this.downloadUrl.substring(this.stIndexOf('/') + 1);142. if(filename==null || "".equals(filename.trim())){/ /如果获取不到文件名称143. for (int i = 0;; i++) {144. String mine = conn.getHeaderField(i); 145. if (mine == null) break;146. if("content-disposition".equals(conn.getHe aderFieldKey(i).toLowerCase())){147. Matcher m = pile(".*filenam e=(.*)").matcher(mine.toLowerCase());148. if(m.find()) return m.group(1); 149. }150. }151. filename = UUID.randomUUID()+ ".tmp";//默认取一个文件名152. }153. return filename;154. }155.156. /**157. * 开始下载文件158. * @param listener 监听下载数量的变化,如果不需要了解实时下载的数量,可以设置为null159. * @return 已下载文件大小160. * @throws Exception161. */162. public int download(DownloadProgressListener listener) throws Exception{163. try164. {165. //创建本地文件166. RandomAccessFile randOut = new RandomAccessFil e(this.saveFile, "rw");167. if(this.fileSize>0) randOut.setLength(this.fil eSize);168. randOut.close();169. URL url = new URL(this.downloadUrl);170. if(this.data.size() != this.threads.length) 171. {172. this.data.clear();173. for (int i = 0; i < this.threads.length; i ++)174. {175. //初始化每条线程已经下载的数据长度为176. this.data.put(i+1, 0);177. }178. }179. //开启线程进行下载180. for (int i = 0; i < this.threads.length; i++)181. {182. int downLength = this.data.get(i+1); 183. //判断线程是否已经完成下载,否则继续下载184. if(downLength < this.block && this.downloa dSize<this.fileSize)185. {186. this.threads[i] = new DownloadThread(t his, url, this.saveFile, this.block, this.data.get(i+1), i+1);187. this.threads[i].setPriority(7);//可删除这条188. this.threads[i].start();189. }190. else191. {192. this.threads[i] = null;193. }194. }195. this.fileService.save(this.downloadUrl, this.d ata);196. //下载未完成197. boolean notFinish = true;198. // 循环判断所有线程是否完成下载199. while (notFinish)200. {201. Thread.sleep(900);202. //假定全部线程下载完成203. notFinish = false;204. for (int i = 0; i < this.threads.length; i ++)205. {206. //如果发现线程未完成下载207. if (this.threads[i] != null && !this.t hreads[i].isFinish())208. {209. //设置标志为下载没有完成210. notFinish = true;211. //如果下载失败,再重新下载212. if(this.threads[i].getDownLength() == -1)213. {214. this.threads[i] = new Download Thread(this, url, this.saveFile, this.block, this.data.get(i+1 ), i+1);215. this.threads[i].setPriority(7) ;216. this.threads[i].start(); 217. }218. }219. }220. //通知目前已经下载完成的数据长度221. if(listener!=null) listener.onDownloadSize (this.downloadSize);222. }223. //删除数据库中下载信息224. fileService.delete(this.downloadUrl);225. }226. catch (Exception e)227. {228. print(e.toString());229. throw new Exception("file download fail"); 230. }231. return this.downloadSize;232. }233.234. /**235. * 获取Http响应头字段236. * @param http237. * @return238. */239. public static Map<String, String> getHttpResponseHeade r(HttpURLConnection http) {240. Map<String, String> header = new LinkedHashMap<Str ing, String>();241. for (int i = 0;; i++) {242. String mine = http.getHeaderField(i); 243. if (mine == null) break;244. header.put(http.getHeaderFieldKey(i), mine);245. }246. return header;247. }248. /**249. * 打印Http头字段250. * @param http251. */252. public static void printResponseHeader(HttpURLConnecti on http)253. {254. Map<String, String> header = getHttpResponseHeader (http);255. for(Map.Entry<String, String> entry : header.entry Set())256. {257. String key = entry.getKey()!=null ? entry.getK ey()+ ":" : "";258. print(key+ entry.getValue());259. }260. }261. private static void print(String msg)262. {263. Log.i(TAG, msg);264. }265.}Java代码1.public interface DownloadProgressListener2.{3. public void onDownloadSize(int size);4.}(2)文件操作，断点数据库存储Java代码1.public class DBOpenHelper extends SQLiteOpenHelper2.{3. private static final String DBNAME = "itcast.db";4. private static final int VERSION = 1;5.6. public DBOpenHelper(Context context)7. {8. super(context, DBNAME, null, VERSION);9. }10.11.@Override12. public void onCreate(SQLiteDatabase db)13. {14. db.execSQL("CREATE TABLE IF NOT EXISTS filedownlog (id integer primary key autoincrement, downpath varchar(100), threadid INTEGER, downlength INTEGER)");15. }16.@Override17. public void onUpgrade(SQLiteDatabase db, int oldVersion,int newVersion)18. {19. db.execSQL("DROP TABLE IF EXISTS filedownlog");20. onCreate(db);21. }22.}Java代码1./**2. * 文件下载业务bean3. */4.public class FileService5.{6. private DBOpenHelper openHelper;7. public FileService(Context context)8. {9. openHelper = new DBOpenHelper(context);10. }11. /**12. * 获取每条线程已经下载的文件长度13. * @param path14. * @return15. */16. public Map<Integer, Integer> getData(String path)17. {18. SQLiteDatabase db = openHelper.getReadableDatabase();19. Cursor cursor = db.rawQuery("select threadid, downlength from filedownlog where downpath=?", new String[]{path});20. Map<Integer, Integer> data = new HashMap<Integer, Integer>();21. while(cursor.moveToNext())22. {23. data.put(cursor.getInt(0), cursor.getInt(1));24. }25. cursor.close();26. db.close();27. return data;28. }29. /**30. * 保存每条线程已经下载的文件长度31. * @param path32. * @param map33. */34. public void save(String path, Map<Integer, Integer> map)35. {//int threadid, int position36. SQLiteDatabase db = openHelper.getWritableDatabase();37. db.beginTransaction();38. try39. {40. for(Map.Entry<Integer, Integer> entry : map.entrySet())41. {42. db.execSQL("insert into filedownlog(downpath,threadid, downlength) values(?,?,?)",43. new Object[]{path, entry.getKey(), entry.getValue()});44. }45. db.setTransactionSuccessful();46. }47. finally48. {49. db.endTransaction();50. }51. db.close();52. }53. /**54. * 实时更新每条线程已经下载的文件长度55. * @param path56. * @param map57. */58. public void update(String path, Map<Integer, Integer> map)59. {60. SQLiteDatabase db = openHelper.getWritableDatabase();61. db.beginTransaction();62. try{63. for(Map.Entry<Integer, Integer> entry : map.entrySet()){64. db.execSQL("update filedownlog set downlength=? where downpath=? and threadid=?",65. new Object[]{entry.getValue(), path,entry.getKey()});66. }67. db.setTransactionSuccessful();68. }finally{69. db.endTransaction();70. }71. db.close();72. }73. /**74. * 当文件下载完成后，删除对应的下载记录75. * @param path76. */77. public void delete(String path)78. {79. SQLiteDatabase db = openHelper.getWritableDatabase();80. db.execSQL("delete from filedownlog where downpath=?", new Object[]{path});81. db.close();82. }83.}。