HttpURLConnection.getContentLength();RandomAccessFile file = new RandomAccessFile("youdao.exe","rw");file.setLength(filesize);//設置本地文件的長度

RandomAccessFile threadfile = new RandomAccessFile("<span style="font-family: Arial, Helvetica, sans-serif;">youdao.exe</span><span style="font-family: Arial, Helvetica, sans-serif;"> ","rw");</span>threadfile.seek(2097152);//從文件的什么位置開始寫入數據

package www.csdn.net.down;import java.io.File;import java.io.FileInputStream;import java.io.FileOutputStream;import java.io.IOException;import java.io.InputStream;import java.io.RandomAccessFile;import java.net.HttpURLConnection;import java.net.MalformedURLException;import java.net.URL;public class DownLoadTest { public File file; public RandomAccessFile accessFile; // 線程的數量 public static int threadNum = 3; // 每個線程負責下載的大小 int blockSize; // 創建訪問的路徑 public String path = "http://localhost:8080/doudou/youdao.exe"; public static int threadCount;// 數量 public void testDown() {  try {   // 創建出URL對象   URL url = new URL(path);   // 創建出 HttpURLConnection對象   HttpURLConnection httpURLConnection = (HttpURLConnection) url     .openConnection();   // 設置 發請求發送的方式   httpURLConnection.setRequestMethod("GET");   // 設置請求是否超時時間   httpURLConnection.setConnectTimeout(5000);   // 設置   httpURLConnection     .setRequestProperty("User-Agent",       " Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; Trident/6.0)");   // 是否響應成功   if (httpURLConnection.getResponseCode() == 200) {    // 獲取文件的大小    int size = httpURLConnection.getContentLength();    System.out.println("文件的大小" + size);    // 創建文件    file = new File("youdao.exe");    accessFile = new RandomAccessFile(file, "rwd");    // 設置文件的大小    accessFile.setLength(size);    // 每個線程下載的大小    blockSize = size / threadNum;    // 開三個線程 操作此文件    for (int i = 1; i <= threadNum; i++) {     // 1 2 3     // 計算出每個線程開始的位置     int startSize = (i - 1) * blockSize;     // 結束位置     int endSize = (i) * blockSize;     // 當線程是最后一個線程的時候     if (i == threadNum) {      // 判斷文件的大小是否大于計算出來的結束位置      if (size > endSize) {       // 結束位置 等于 文件的大小       endSize = size;      }     }     // 為每個線程創建一個隨機的讀取     RandomAccessFile threadAccessFile = new RandomAccessFile(       file, "rwd");     new Thread(new DownLoadThread(i, threadAccessFile,       startSize, endSize, path)).start();    }   }  } catch (MalformedURLException e) {   // TODO Auto-generated catch block   e.printStackTrace();  } catch (IOException e) {   // TODO Auto-generated catch block   e.printStackTrace();  } } public static void main(String[] args) {  DownLoadTest downLoadTest = new DownLoadTest();  // 調用下載方法  downLoadTest.testDown(); }}class DownLoadThread implements Runnable { // 下載文件的封裝 public RandomAccessFile accessFile; // 每個線程 都擁有一個accessFile的文件對象 線程1 線程2 線程3 // 線程下載文件的起始位置 public int startSize; public int endSize; // 文件下載的path路徑 public String path; public int threadId; // 線程的標識 public DownLoadThread(int threadId, RandomAccessFile accessFile,   int startSize, int endSize, String path) {  this.threadId = threadId;  this.accessFile = accessFile;  this.startSize = startSize;  this.endSize = endSize;  this.path = path; } @Override public void run() {  // 執行run方法  try {   // 創建文件   File threadFile = new File(threadId + ".txt");   if (threadFile.exists()) {    // 讀取該文件的內容    // 創建文件的輸入流對象    FileInputStream fis = new FileInputStream(threadFile);    // 采用工具類讀取    byte data[] = StreamTools.isToData(fis);    // 轉化成字符串    String threadLen = new String(data);    if ((threadLen != null) && (!"".equals(threadLen))) {     startSize = Integer.valueOf(threadLen);     // 解決 416bug的錯誤     if (startSize > endSize) {      startSize = endSize - 1;     }    }   }   // 創建URL對象   URL url = new URL(path);   // 創建HttpURLConnection對象   HttpURLConnection httpURLConnection = (HttpURLConnection) url     .openConnection();   // 設置請求的頭   httpURLConnection.setRequestMethod("GET");   // 設置請求是否超時時間   httpURLConnection.setConnectTimeout(5000);   // 設置   httpURLConnection     .setRequestProperty("User-Agent",       " Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; Trident/6.0)");   // 關鍵的設置   httpURLConnection.setRequestProperty("Range", "bytes=" + startSize     + "-" + endSize);   // 輸出當前線程   System.out.println("當前線程" + threadId + " 下載開始位置:" + startSize     + " 下載結束位置:" + endSize);   // 響應成功   // 設置隨機讀取文件的 開始位置   accessFile.seek(startSize);   // 獲取相應流對象   InputStream is = httpURLConnection.getInputStream();   // 創建輸出流對象   byte buffer[] = new byte[1024];   int len = 0;   int threadTotal = 0;// 每個線程下載后保存記錄 /   while ((len = is.read(buffer)) != -1) {    accessFile.write(buffer, 0, len);    threadTotal += len;// 記錄你寫入的長度 //xml文件    // 通過文件記錄文件下載的長度    FileOutputStream fos = new FileOutputStream(threadFile);    fos.write((threadTotal + "").getBytes());    fos.flush();    fos.close();   }   accessFile.close();   is.close();   System.out.println(threadId + "線程執行完畢");   //線程操作   synchronized (DownLoadTest.class) {    DownLoadTest.threadCount++;    if (DownLoadTest.threadCount >= DownLoadTest.threadNum) {     for(int i=1;i<=DownLoadTest.threadNum;i++){      File file = new File(i+".txt");      if(file.exists()){       file.delete();      }     }    }   }  } catch (MalformedURLException e) {   // TODO Auto-generated catch block   e.printStackTrace();  } catch (IOException e) {   // TODO Auto-generated catch block   e.printStackTrace();  } }}

package www.csdn.net.down;import java.io.ByteArrayOutputStream;import java.io.IOException;import java.io.InputStream;public class StreamTools { public static byte[] isToData(InputStream is) throws IOException{  // 字節輸出流  ByteArrayOutputStream bops = new ByteArrayOutputStream();  // 讀取數據的緩存區  byte buffer[] = new byte[1024];  // 讀取長度的記錄  int len = 0;  // 循環讀取  while ((len = is.read(buffer)) != -1) {   bops.write(buffer, 0, len);  }  // 把讀取的內容轉換成byte數組  byte data[] = bops.toByteArray();  bops.flush();  bops.close();  is.close();  return data; }}