可以参考下：

package com.download.services;

import java.io.File;

import java.io.InputStream;

import java.io.RandomAccessFile;

import java.net.HttpURLConnection;

import java.net.URL;

import java.util.List;

import org.apache.http.HttpStatus;

import android.content.Context;

import android.content.Intent;

import com.download.db.ThreadDAO;

import com.download.db.ThreadDAOImpl;

import com.download.entities.FileInfo;

import com.download.entities.ThreadInfo;

/**

 * 下载任务类

 *

 */

public class DownloadTask {

private Context mContext = null; //程序上下文

private FileInfo mFileInfo = null; //下载文件的信息

private ThreadDAO mDao = null; //数据库操作对象

public boolean isPause = false; //是否暂停任务的标识

private int mFinished = 0; //文件下载进度

public DownloadTask(Context mContext, FileInfo mFileInfo) {

this.mContext = mContext;

this.mFileInfo = mFileInfo;

mDao = new ThreadDAOImpl(mContext);

}

/**

* 开始下载

*/

public void download(){

this.isPause = false;

//从数据库中通过文件URL获得下载线程记录

List<ThreadInfo> threads = mDao.getThreadInfos(mFileInfo.getUrl());

ThreadInfo threadInfo = null;

if(threads.size() > 0){

//读取数据库中下载线程记录，因为是单线程下载所以get（0）

threadInfo = threads.get(0);

}else{

//如果数据库中没有记录，创建新的线程信息记录

threadInfo = new ThreadInfo(0,mFileInfo.getUrl(),0,mFileInfo.getLength(),0);

//插入下载线程信息

mDao.insertThreadInfo(threadInfo);

}

//启动线程进行下载

DownloadThread thread = new DownloadThread(threadInfo);

thread.start();

}

/**

* 数据下载线程

*/

class DownloadThread extends Thread{

private ThreadInfo mThreadInfo = null; //线程下载记录

public DownloadThread(ThreadInfo threadInfo){

mThreadInfo = threadInfo;

}

public void run(){

HttpURLConnection conn = null;

RandomAccessFile raf = null;

InputStream input = null;

try {

// //插入下载线程信息

// if(!mDao.isExists(mThreadInfo.getUrl(), mThreadInfo.getId())){

// mDao.insertThreadInfo(mThreadInfo);

// }

//打开连接

URL url = new URL(mThreadInfo.getUrl());

conn = (HttpURLConnection) url.openConnection();

conn.setConnectTimeout(3000);

conn.setRequestMethod("GET");

//设置下载位置

int start = mThreadInfo.getStart()+mThreadInfo.getFinished();

conn.setRequestProperty("Range", "bytes="+start+"-"+mThreadInfo.getEnd());

//设置本地文件写入位置

File file = new File(DownloadService.DOWNLOAD_DIR, mFileInfo.getName());

raf = new RandomAccessFile(file,"rwd");

raf.seek(start);

//累加完成进度

mFinished += mThreadInfo.getFinished();

if(conn.getResponseCode() == HttpStatus.SC_PARTIAL_CONTENT){

//下载文件

int len = 0;

byte[] buffer = new byte[1024];

Intent intent = new Intent(DownloadService.ACTION_UPDATE);

long time = System.currentTimeMillis();

input = conn.getInputStream();

//读取文件中的数据

while((len = input.read(buffer))!=-1){

//写入本地文件

raf.write(buffer,0,len);

//累加整个文件完成进度

mFinished += len;

//间隔500毫秒更新一次进度，减少UI更新频率

if(System.currentTimeMillis() - time > 500){

time = System.currentTimeMillis();

//发送进度到Activity

intent.putExtra("finished", mFinished * 100 / mFileInfo.getLength());

mContext.sendBroadcast(intent);

}

//下载任务暂停，结束下载

if(isPause){

//保存进度到数据库

mDao.updateThreadInfo(mThreadInfo.getId(), 

mThreadInfo.getUrl(), 

mFinished);

return;

}

}

}

//删除下载记录

mDao.deleteThreadInfo(mThreadInfo.getUrl());

//提示下载完成

Intent intent = new Intent(DownloadService.ACTION_FINISH);

mContext.sendBroadcast(intent);

} catch (Exception e) {

e.printStackTrace();

} finally{

try {

conn.disconnect();

raf.close();

input.close();

} catch (Exception e) {

e.printStackTrace();

}

}

}

}

}