android应用多线程和http断点续传下载数据

断点续传的原理很简单，就是在Http的请求上和一般的下载有所不同而已。注：需要web容器的支持，现在绝大多数都支持此项
以例子说明断点续传。
例如使用本地的服务器127.0.0.1，文件名为data.zip。下载该文件所发出的头信息如下：

GET /data.zip HTTP/1.1 
Accept: image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-excel, application/msword, application/vnd.ms-powerpoint, */* 
Accept-Language: zh-cn 
Accept-Encoding: gzip, deflate 
User-Agent: Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0) 
Connection: Keep-Alive 

在服务器端接收到请求后，会按要求寻找请求的文件，并提取文件的信息，然后返回给浏览器，返回信息如下：

200 
Content-Length=105555555 
Accept-Ranges=bytes 
Date=Sun, 28 Aug 2012 10:56:16 GMT 
ETag=W/"031a54e173c11:df5" 
Content-Type=application/octet-stream 
Server=Apache/1.3.14(Unix)
Last-Modified= Sun, 28 Aug 2012 10:56:16 GMT 

断点续传，就是要将文件从已经下载的地方开始，继续下载。所在客户端在发给服务器的信息中，要指明下载开始的地方。
下面是用自己模拟一个"浏览器"来传递请求信息给Web服务器，要求从20120828字节开始。

GET /data.zip HTTP/1.0 
User-Agent:  Mozilla/4.0 (compatible; MSIE 5.01; Windows NT 5.0)RANGE: bytes=20120828- 

请仔细查看一下，就会发现上面多了一行RANGE: bytes=20120828- 的信息
这一行的意思是告诉服务器下载data.zip这个文件从20120828字节开始传，前面的字节就不需要用传了。
服务器收到这个请求以后，返回的信息如下：

206 
Content-Length=105555555
Content-Range=bytes 20120828-105555554/105555555
Date= Sun, 28 Aug 2012 11:04:18 GMT 
ETag=W/"031a54e173c11:df5" 
Content-Type=application/octet-stream 
Server=Apache/1.3.14(Unix) 
Last-Modified= Sun, 28 Aug 2012 10:56:16 GMT 

和前面服务器返回的信息比较一下，就会发现增加了一行：

Content-Range=bytes 20120828-10555554/10555555

返回的代码也改为206了，而不再是200了。
知道了以上原理，就可以进行断点续传的编程了。

至于多线程，不多做介绍了，以下是实现的多线程断点续的部分代码：
文件下载类的主要代码

	private static final String TAG = "FileDownloader";
	private Context context;
	private DownloadDao downloadDao;	
	/* 已下载文件长度 */
	private int downloadSize = 0;
	/* 原始文件长度 */
	private int fileSize = 0;
	/* 线程数 */
	private DownloadThread[] threads;
	/* 本地保存文件 */
	private File saveFile;
	/* 缓存各线程下载的长度*/
	private Map<Integer, Integer> data = new ConcurrentHashMap<Integer, Integer>();
	/* 每条线程下载的长度 */
	private int block;
	/* 下载路径  */
	private String downloadUrl;
	/* 压缩包名称 */
	private String filename;
/**
 * 构建文件下载器
 * @param downloadUrl 下载路径
 * @param fileSaveDir 文件保存目录
 * @param threadNum 下载线程数
 */
public FileDownloader(Context context, String downloadUrl, File fileSaveDir, int threadNum) {
	try {
		this.context = context;
		this.downloadUrl = downloadUrl;
		downloadDao = new DownloadDao(this.context);
		URL url = new URL(this.downloadUrl);
		if(!fileSaveDir.exists()) fileSaveDir.mkdirs();
		this.threads = new DownloadThread[threadNum];					
		HttpURLConnection conn = (HttpURLConnection) url.openConnection();
		conn.setConnectTimeout(10*1000);
		//conn.setReadTimeout(50*1000);
		conn.setRequestMethod("GET");
		conn.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, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*");
		conn.setRequestProperty("Accept-Language", "zh-CN");
		conn.setRequestProperty("Referer", downloadUrl); 
		conn.setRequestProperty("Charset", "UTF-8");
		conn.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)");
		conn.setRequestProperty("Connection", "Keep-Alive");
		conn.connect();
		printResponseHeader(conn);
		Log.w("conn.getResponseCode()",String.valueOf(conn.getResponseCode()));
		if (conn.getResponseCode()==200) {
			this.fileSize = conn.getContentLength();//根据响应获取文件大小
			if (this.fileSize <= 0) throw new RuntimeException("Unkown file size ");
					
			this.filename = getFileName(conn);
			this.saveFile = new File(fileSaveDir, filename);/* 保存文件 */
			ArrayList<Map<String,String>> logdata = downloadDao.getData(downloadUrl);
			if(logdata.size()>0){
				for(int i=0; i<logdata.size(); i++){
					int threadId = 0;
					int pos = 0;
					for(Map.Entry<String, String> entry : logdata.get(i).entrySet())
					{
						if(entry.getKey().equals("THREADID")){
							threadId = Integer.parseInt(entry.getValue());
						}else{
							pos = Integer.parseInt(entry.getValue());
						}
					}
					data.put(threadId,pos);
				}
//					for(Map.Entry<String, String> entry : logdata.get(0).entrySet())
//						data.put(Integer.parseInt(entry.getKey()), Integer.parseInt(entry.getValue()));
			}
			this.block = (this.fileSize % this.threads.length)==0? this.fileSize / this.threads.length : this.fileSize / this.threads.length + 1;
			if(this.data.size()==this.threads.length){
				for (int i = 0; i < this.threads.length; i++) {
					this.downloadSize += this.data.get(i+1);
				}
				Log.w(TAG, "已经下载的长度"+ this.downloadSize);
			}				
		}else{
			throw new RuntimeException("server no response ");
		}
	} catch (Exception e) {
		Log.w(TAG, e.getMessage());
		throw new RuntimeException("don't connection this url");
	}
}
/**
 * 获取文件名
 */
private String getFileName(HttpURLConnection conn) {
	String filename = this.downloadUrl.substring(this.downloadUrl.lastIndexOf('/') + 1);
	if(filename==null || "".equals(filename.trim())){//如果获取不到文件名称
		for (int i = 0;; i++) {
			String mine = conn.getHeaderField(i);
			if (mine == null) break;
			if("content-disposition".equals(conn.getHeaderFieldKey(i).toLowerCase())){
				Matcher m = Pattern.compile(".*filename=(.*)").matcher(mine.toLowerCase());
				if(m.find()) return m.group(1);
			}
		}
		filename = UUID.randomUUID()+ ".tmp";//默认取一个文件名
	}
	return filename;
}

/**
 *  开始下载文件
 * @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null
 * @return 已下载文件大小
 * @throws Exception
 */
public int download(DownloadProgressListener listener) throws Exception{
	try {
		RandomAccessFile randOut = new RandomAccessFile(this.saveFile, "rw");
		if(this.fileSize>0) randOut.setLength(this.fileSize);
		randOut.close();
		URL url = new URL(this.downloadUrl);
		if(this.data.size() != this.threads.length){
			this.data.clear();
			for (int i = 0; i < this.threads.length; i++) {
				this.data.put(i+1, 0);
			}
		}
		for (int i = 0; i < this.threads.length; i++) {
			int downLength = this.data.get(i+1);
			if(downLength < this.block && this.downloadSize<this.fileSize){ //该线程未完成下载时,继续下载						
				this.threads[i] = new DownloadThread(this, url, this.saveFile, this.block, this.data.get(i+1), i+1);
				this.threads[i].setPriority(7);
				this.threads[i].start();
			}else{
				this.threads[i] = null;
			}
		}
		this.downloadDao.saveData(this.downloadUrl, this.data);
		boolean notFinish = true;//下载未完成
		while (notFinish) {// 循环判断是否下载完毕
			Thread.sleep(900);
			notFinish = false;//假定下载完成
			for (int i = 0; i < this.threads.length; i++){
				if (this.threads[i] != null && !this.threads[i].isFinish()) {
					notFinish = true;//下载没有完成
					if(this.threads[i].getDownLength() == -1){//如果下载失败,再重新下载
						this.threads[i] = new DownloadThread(this, url, this.saveFile, this.block, this.data.get(i+1), i+1);
						this.threads[i].setPriority(7);
						this.threads[i].start();
					}
				}
			}
			
			Log.w("this.downloadSize", String.valueOf(this.downloadSize));
			Log.w("this.fileSize", String.valueOf(this.fileSize));
			if(this.downloadSize==this.fileSize){
				//解压zip文件
				/*listener.onDownloadSize(this.downloadSize,true);
				String unZipfileName=this.saveFile.getPath();//压缩文件路径
				this.unZip(unZipfileName, this.tempPath);
				listener.onDownloadSize(this.downloadSize,false);*/
				
				listener.onDownloadSize(this.downloadSize,false);
				String unZipfileName=this.saveFile.getPath();//压缩文件路径
				this.unZip(unZipfileName, this.tempPath);
				listener.onDownloadSize(this.downloadSize,true);
			}else{
				listener.onDownloadSize(this.downloadSize,false);
			}
			
		}
		
		downloadDao.deleteData(this.downloadUrl);
		
	} catch (Exception e) {
		Log.w(TAG, e.getMessage());
		throw new Exception("file download fail");
	}
	return this.downloadSize;
}
/**
 * 获取Http响应头字段
 * @param http
 * @return
 */
public static Map<String, String> getHttpResponseHeader(HttpURLConnection http) {
	Map<String, String> header = new LinkedHashMap<String, String>();
	for (int i = 0;; i++) {
		String mine = http.getHeaderField(i);
		if (mine == null) break;
		header.put(http.getHeaderFieldKey(i), mine);
	}
	return header;
}
/**
 * 打印Http头字段
 * @param http
 */
public static void printResponseHeader(HttpURLConnection http){
	Map<String, String> header = getHttpResponseHeader(http);
	for(Map.Entry<String, String> entry : header.entrySet()){
		String key = entry.getKey()!=null ? entry.getKey()+ ":" : "";
		Log.w(TAG, key+ entry.getValue());
	}
}

下载的线程类的主要代码

/**
 * 下载线程类
 * @author zhaidi
 *
 */
public class DownloadThread extends Thread {
	private static final String TAG = "DownloadThread";
	private File saveFile;
	private URL downUrl;
	private int block;
	/* 下载开始位置  */
	private int threadId = -1;	
	private int downLength;
	private boolean finish = false;
	private FileUpOrDown downloader;
	//private FileUpdater updater;

	public DownloadThread(FileDownloader downloader, URL downUrl, File saveFile, int block, int downLength, int threadId) {
		this.downUrl = downUrl;
		this.saveFile = saveFile;
		this.block = block;
		this.downloader = downloader;
		this.threadId = threadId;
		this.downLength = downLength;
	}
	public DownloadThread(FileUpdater updater, URL downUrl, File saveFile, int block, int downLength, int threadId) {
		this.downUrl = downUrl;
		this.saveFile = saveFile;
		this.block = block;
		this.downloader = updater;
		this.threadId = threadId;
		this.downLength = downLength;
	}
	@Override
	public void run() {
		if(downLength < block){//未下载完成
			try {
				HttpURLConnection http = (HttpURLConnection) downUrl.openConnection();
				http.setConnectTimeout(5 * 1000);
				http.setRequestMethod("GET");
				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, application/vnd.ms-excel, application/vnd.ms-powerpoint, application/msword, */*");
				http.setRequestProperty("Accept-Language", "zh-CN");
				http.setRequestProperty("Referer", downUrl.toString()); 
				http.setRequestProperty("Charset", "UTF-8");
				int startPos = block * (threadId - 1) + downLength;//开始位置
				int endPos = block * threadId -1;//结束位置
				http.setRequestProperty("Range", "bytes=" + startPos + "-"+ endPos);//设置获取实体数据的范围
				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)");
				http.setRequestProperty("Connection", "Keep-Alive");
				
				InputStream inStream = http.getInputStream();
				byte[] buffer = new byte[1024];
				int offset = 0;
				print("Thread " + this.threadId + " start download from position "+ startPos);
				RandomAccessFile threadfile = new RandomAccessFile(this.saveFile, "rwd");
				threadfile.seek(startPos);
				while ((offset = inStream.read(buffer, 0, 1024)) != -1) {
					threadfile.write(buffer, 0, offset);
					downLength += offset;
					downloader.update(this.threadId, downLength);
					downloader.saveLogFile();
					downloader.append(offset);
				}
				threadfile.close();
				inStream.close();			
				print("Thread " + this.threadId + " download finish");
				this.finish = true;
			} catch (Exception e) {
				this.downLength = -1;
				print("Thread "+ this.threadId+ ":"+ e);
			}
		}
	}
	private static void print(String msg){
		Log.i(TAG, msg);
	}
	/**
	 * 下载是否完成
	 * @return
	 */
	public boolean isFinish() {
		return finish;
	}
	/**
	 * 已经下载的内容大小
	 * @return 如果返回值为-1,代表下载失败
	 */
	public long getDownLength() {
		return downLength;
	}
}

下载的调用如下：
其中downloadbar是一个进度条控件，SpecialActivity则是android中的一个控件

FileDownloader loader = new FileDownloader(SpecialActivity.this, path, dir, 5);
					int length = loader.getFileSize();//获取文件的长度
					downloadbar.setMax(length);
					loader.download(new DownloadProgressListener(){
						@Override
						/*public void onDownloadSize(int size) {//可以实时得到文件下载的长度
							Message msg = new Message();
							msg.what = 1;
							msg.getData().putInt("size", size);							
							handler.sendMessage(msg);
						}});*/
						public void onDownloadSize(int size,boolean upzip) {//可以实时得到文件下载的长度
							Message msg = new Message();
							msg.what = 1;
							msg.getData().putInt("size", size);							
							handler.sendMessage(msg);
						}});