HttpURLConnection.getContentLength();
RandomAccessFile file = new RandomAccessFile("youdao.exe","rw");
file.setLength(filesize);//設(shè)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(shù)據(jù)

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(shù)量
 public static int threadNum = 3;
 // 每個(gè)線程負(fù)責(zé)下載的大小
 int blockSize;
 // 創(chuàng)建訪問的路徑
 public String path = "http://localhost:8080/doudou/youdao.exe";
 public static int threadCount;// 數(shù)量
 public void testDown() {
  try {
   // 創(chuàng)建出URL對象
   URL url = new URL(path);
   // 創(chuàng)建出 HttpURLConnection對象
   HttpURLConnection httpURLConnection = (HttpURLConnection) url
     .openConnection();
   // 設(shè)置 發(fā)請求發(fā)送的方式
   httpURLConnection.setRequestMethod("GET");
   // 設(shè)置請求是否超時(shí)時(shí)間
   httpURLConnection.setConnectTimeout(5000);
   // 設(shè)置
   httpURLConnection
     .setRequestProperty("User-Agent",
       " Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; Trident/6.0)");
   // 是否響應(yīng)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(chuàng)建文件
    file = new File("youdao.exe");
    accessFile = new RandomAccessFile(file, "rwd");
    // 設(shè)置文件的大小
    accessFile.setLength(size);
    // 每個(gè)線程下載的大小
    blockSize = size / threadNum;
    // 開三個(gè)線程 操作此文件
    for (int i = 1; i <= threadNum; i++) {
     // 1 2 3
     // 計(jì)算出每個(gè)線程開始的位置
     int startSize = (i - 1) * blockSize;
     // 結(jié)束位置
     int endSize = (i) * blockSize;
     // 當(dāng)線程是最后一個(gè)線程的時(shí)候
     if (i == threadNum) {
      // 判斷文件的大小是否大于計(jì)算出來的結(jié)束位置
      if (size > endSize) {
       // 結(jié)束位置 等于 文件的大小
       endSize = size;
      }
     }
     // 為每個(gè)線程創(chuàng)建一個(gè)隨機(jī)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(diào)用下載方法
  downLoadTest.testDown();
 }
}
class DownLoadThread implements Runnable {
 // 下載文件的封裝
 public RandomAccessFile accessFile; // 每個(gè)線程 都擁有一個(gè)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(biāo)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(zhí)行run方法
  try {
   // 創(chuàng)建文件
   File threadFile = new File(threadId + ".txt");
   if (threadFile.exists()) {
    // 讀取該文件的內(nèi)容
    // 創(chuàng)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(zhuǎn)化成字符串
    String threadLen = new String(data);
    if ((threadLen != null) && (!"".equals(threadLen))) {
     startSize = Integer.valueOf(threadLen);
     // 解決 416bug的錯(cuò)誤
     if (startSize > endSize) {
      startSize = endSize - 1;
     }
    }
   }
   // 創(chuàng)建URL對象
   URL url = new URL(path);
   // 創(chuàng)建HttpURLConnection對象
   HttpURLConnection httpURLConnection = (HttpURLConnection) url
     .openConnection();
   // 設(shè)置請求的頭
   httpURLConnection.setRequestMethod("GET");
   // 設(shè)置請求是否超時(shí)時(shí)間
   httpURLConnection.setConnectTimeout(5000);
   // 設(shè)置
   httpURLConnection
     .setRequestProperty("User-Agent",
       " Mozilla/5.0 (compatible; MSIE 10.0; Windows NT 6.2; Trident/6.0)");
   // 關(guān)鍵的設(shè)置
   httpURLConnection.setRequestProperty("Range", "bytes=" + startSize
     + "-" + endSize);
   // 輸出當(dāng)前線程
   System.out.println("當(dāng)前線程" + threadId + " 下載開始位置:" + startSize
     + " 下載結(jié)束位置:" + endSize);
   // 響應(yīng)成功
   // 設(shè)置隨機(jī)讀取文件的 開始位置
   accessFile.seek(startSize);
   // 獲取相應(yīng)流對象
   InputStream is = httpURLConnection.getInputStream();
   // 創(chuàng)建輸出流對象
   byte buffer[] = new byte[1024];
   int len = 0;
   int threadTotal = 0;// 每個(gè)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(zhí)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(jié)輸出流
  ByteArrayOutputStream bops = new ByteArrayOutputStream();
  // 讀取數(shù)據(jù)的緩存區(qū)
  byte buffer[] = new byte[1024];
  // 讀取長度的記錄
  int len = 0;
  // 循環(huán)讀取
  while ((len = is.read(buffer)) != -1) {
   bops.write(buffer, 0, len);
  }
  // 把讀取的內(nèi)容轉(zhuǎn)換成byte數(shù)組
  byte data[] = bops.toByteArray();
  bops.flush();
  bops.close();
  is.close();
  return data;
 }
}