实现原理
(1)首先获得下载文件的长度，然后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=?之前练习时的一个demo，不多说了，直接上代码吧，有关断点续传，需要使用，不再加了，网上有很多成熟的项目可以直接用。
实例
MainApp：
package com.amos.app;import java.io.File;import java.io.IOException;import java.net.MalformedURLException;import java.net.URL;import java.net.URLConnection;import com.amos.download.R;import android.annotation.SuppressLint;import android.app.Activity;import android.os.Bundle;import android.os.Environment;import android.os.Handler;import android.os.Message;import android.util.Log;import android.view.View;import android.view.View.OnClickListener;import android.widget.ProgressBar;import android.widget.TextView;import android.widget.Toast;/** * @author yangxiaolong * @2014-5-6 */public class MainApp extends Activity implements OnClickListener { private static final String TAG = MainApp.class.getSimpleName(); /** 显示下载进度TextView */ private TextView mMessageView; /** 显示下载进度ProgressBar */ private ProgressBar mProgressbar; @Override protected void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.progress_activity); findViewById(R.id.download_btn).setOnClickListener(this); mMessageView = (TextView) findViewById(R.id.download_message); mProgressbar = (ProgressBar) findViewById(R.id.download_progress); } @Override public void onClick(View v) { if (v.getId() == R.id.download_btn) { doDownload(); } } /** * 使用Handler更新UI界面信息 */ @SuppressLint("HandlerLeak") Handler mHandler = new Handler() { @Override public void handleMessage(Message msg) { mProgressbar.setProgress(msg.getData().getInt("size")); float temp = (float) mProgressbar.getProgress() / (float) mProgressbar.getMax(); int progress = (int) (temp * 100); if (progress == 100) { Toast.makeText(MainApp.this, "下载完成！", Toast.LENGTH_LONG).show(); } mMessageView.setText("下载进度:" + progress + " %"); } }; /** * 下载准备工作，获取SD卡路径、开启线程 */ private void doDownload() { // 获取SD卡路径 String path = Environment.getExternalStorageDirectory() + "/amosdownload/"; File file = new File(path); // 如果SD卡目录不存在创建 if (!file.exists()) { file.mkdir(); } // 设置progressBar初始化 mProgressbar.setProgress(0); // 简单起见，我先把URL和文件名称写死，其实这些都可以通过HttpHeader获取到 String downloadUrl = "http://gdown.baidu.com/data/wisegame/91319a5a1dfae322/baidu_16785426.apk"; String fileName = "baidu_16785426.apk"; int threadNum = 5; String filepath = path + fileName; Log.d(TAG, "download file path:" + filepath); downloadTask task = new downloadTask(downloadUrl, threadNum, filepath); task.start(); } /** * 多线程文件下载 * * @author yangxiaolong * @2014-8-7 */ class downloadTask extends Thread { private String downloadUrl;// 下载链接地址 private int threadNum;// 开启的线程数 private String filePath;// 保存文件路径地址 private int blockSize;// 每一个线程的下载量 public downloadTask(String downloadUrl, int threadNum, String fileptah) { this.downloadUrl = downloadUrl; this.threadNum = threadNum; this.filePath = fileptah; } @Override public void run() { FileDownloadThread[] threads = new FileDownloadThread[threadNum]; try { URL url = new URL(downloadUrl); Log.d(TAG, "download file http path:" + downloadUrl); URLConnection conn = url.openConnection(); // 读取下载文件总大小 int fileSize = conn.getContentLength(); if (fileSize <= 0) { System.out.println("读取文件失败"); return; } // 设置ProgressBar最大的长度为文件Size mProgressbar.setMax(fileSize); // 计算每条线程下载的数据长度 blockSize = (fileSize % threadNum) == 0 ? fileSize / threadNum : fileSize / threadNum + 1; Log.d(TAG, "fileSize:" + fileSize + " blockSize:"); File file = new File(filePath); for (int i = 0; i < threads.length; i++) { // 启动线程，分别下载每个线程需要下载的部分 threads[i] = new FileDownloadThread(url, file, blockSize, (i + 1)); threads[i].setName("Thread:" + i); threads[i].start(); } boolean isfinished = false; int downloadedAllSize = 0; while (!isfinished) { isfinished = true; // 当前所有线程下载总量 downloadedAllSize = 0; for (int i = 0; i < threads.length; i++) { downloadedAllSize += threads[i].getDownloadLength(); if (!threads[i].isCompleted()) { isfinished = false; } } // 通知handler去更新视图组件 Message msg = new Message(); msg.getData().putInt("size", downloadedAllSize); mHandler.sendMessage(msg); // Log.d(TAG, "current downloadSize:" + downloadedAllSize); Thread.sleep(1000);// 休息1秒后再读取下载进度 } Log.d(TAG, " all of downloadSize:" + downloadedAllSize); } catch (MalformedURLException e) { e.printStackTrace(); } catch (IOException e) { e.printStackTrace(); } catch (InterruptedException e) { e.printStackTrace(); } } }} FileDownloadThread:
package com.amos.app;import java.io.BufferedInputStream;import java.io.File;import java.io.IOException;import java.io.RandomAccessFile;import java.net.URL;import java.net.URLConnection;import android.util.Log;/** * 文件下载类 * * @author yangxiaolong * @2014-5-6 */public class FileDownloadThread extends Thread { private static final String TAG = FileDownloadThread.class.getSimpleName(); /** 当前下载是否完成 */ private boolean isCompleted = false; /** 当前下载文件长度 */ private int downloadLength = 0; /** 文件保存路径 */ private File file; /** 文件下载路径 */ private URL downloadUrl; /** 当前下载线程ID */ private int threadId; /** 线程下载数据长度 */ private int blockSize; /** * * @param url:文件下载地址 * @param file:文件保存路径 * @param blocksize:下载数据长度 * @param threadId:线程ID */ public FileDownloadThread(URL downloadUrl, File file, int blocksize, int threadId) { this.downloadUrl = downloadUrl; this.file = file; this.threadId = threadId; this.blockSize = blocksize; } @Override public void run() { BufferedInputStream bis = null; RandomAccessFile raf = null; try { URLConnection conn = downloadUrl.openConnection(); conn.setAllowUserInteraction(true); int startPos = blockSize * (threadId - 1);//开始位置 int endPos = blockSize * threadId - 1;//结束位置 //设置当前线程下载的起点、终点 conn.setRequestProperty("Range", "bytes=" + startPos + "-" + endPos); System.out.println(Thread.currentThread().getName() + " bytes=" + startPos + "-" + endPos); byte[] buffer = new byte[1024]; bis = new BufferedInputStream(conn.getInputStream()); raf = new RandomAccessFile(file, "rwd"); raf.seek(startPos); int len; while ((len = bis.read(buffer, 0, 1024)) != -1) { raf.write(buffer, 0, len); downloadLength += len; } isCompleted = true; Log.d(TAG, "current thread task has finished,all size:" + downloadLength); } catch (IOException e) { e.printStackTrace(); } finally { if (bis != null) { try { bis.close(); } catch (IOException e) { e.printStackTrace(); } } if (raf != null) { try { raf.close(); } catch (IOException e) { e.printStackTrace(); } } } } /** * 线程文件是否下载完毕 */ public boolean isCompleted() { return isCompleted; } /** * 线程下载文件长度 */ public int getDownloadLength() { return downloadLength; }} 效果图：
