Android下使用Http协议实现多线程断点续传下载

0.使用多线程下载会提升文件下载的速度，那么多线程下载文件的过程是：

(1)首先获得下载文件的长度，然后设置本地文件的长度

HttpURLConnection.getContentLength();

RandomAccessFile file = new RandomAccessFile("QQWubiSetup.exe","rwd");

file.setLength(filesize);//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=?

(3)使用Http的Range头字段指定每条线程从文件的什么位置开始下载，下载到什么位置为止，

如：指定从文件的2M位置开始下载，下载到位置(4M-1byte)为止

代码如下：HttpURLConnection.setRequestProperty("Range", "bytes=2097152-4194303");

(4)保存文件，使用RandomAccessFile类指定每条线程从本地文件的什么位置开始写入数据。

RandomAccessFile threadfile = new RandomAccessFile("QQWubiSetup.exe ","rwd");

threadfile.seek(2097152);//从文件的什么位置开始写入数据

1.多线程下载的核心代码示例

public class MulThreadDownload 
{
	/**
	 * 多线程下载
	 * @param args
	 */
	public static void main(String[] args) 
	{
		String path = "http://net.hoo.com/QQWubiSetup.exe";
		try 
		{
			new MulThreadDownload().download(path, 3);
		}
		catch (Exception e) 
		{
			e.printStackTrace();
		}
	}
	/**
	 * 从路径中获取文件名称
	 * @param path 下载路径
	 * @return
	 */
	public static String getFilename(String path)
	{
		return path.substring(path.lastIndexOf('/')+1);
	}
	/**
	 * 下载文件
	 * @param path 下载路径
	 * @param threadsize 线程数
	 */
	public void download(String path, int threadsize) throws Exception
	{
		URL url = new URL(path);
		HttpURLConnection conn = (HttpURLConnection)url.openConnection();
		conn.setRequestMethod("GET");
		conn.setConnectTimeout(5 * 1000);
		//获取要下载的文件的长度
		int filelength = conn.getContentLength();
		//从路径中获取文件名称
		String filename = getFilename(path);
		File saveFile = new File(filename);
		RandomAccessFile accessFile = new RandomAccessFile(saveFile, "rwd");
		//设置本地文件的长度和下载文件相同
		accessFile.setLength(filelength);
		accessFile.close();
		//计算每条线程下载的数据长度
		int block = filelength%threadsize==0? filelength/threadsize : filelength/threadsize+1;
		for(int threadid=0 ; threadid < threadsize ; threadid++){
			new DownloadThread(url, saveFile, block, threadid).start();
		}
	}
	
	private final class DownloadThread extends Thread
	{
		private URL url;
		private File saveFile;
		private int block;//每条线程下载的数据长度
		private int threadid;//线程id
		public DownloadThread(URL url, File saveFile, int block, int threadid) 
		{
			this.url = url;
			this.saveFile = saveFile;
			this.block = block;
			this.threadid = threadid;
		}
		@Override
		public void run() 
		{
			//计算开始位置公式：线程id*每条线程下载的数据长度= ？
		    //计算结束位置公式：（线程id +1）*每条线程下载的数据长度-1 =?
			int startposition = threadid * block;
			int endposition = (threadid + 1 ) * block - 1;
			try 
			{
				RandomAccessFile accessFile = new RandomAccessFile(saveFile, "rwd");
				//设置从什么位置开始写入数据
				accessFile.seek(startposition);
				HttpURLConnection conn = (HttpURLConnection)url.openConnection();
				conn.setRequestMethod("GET");
				conn.setConnectTimeout(5 * 1000);
				conn.setRequestProperty("Range", "bytes="+ startposition+ "-"+ endposition);
				InputStream inStream = conn.getInputStream();
				byte[] buffer = new byte[1024];
				int len = 0;
				while( (len=inStream.read(buffer)) != -1 )
				{
					accessFile.write(buffer, 0, len);
				}
				inStream.close();
				accessFile.close();
				System.out.println("线程id:"+ threadid+ "下载完成");
			}
			catch (Exception e) 
			{
				e.printStackTrace();
			}
		}		
	}
}

2.多线程断点下载功能，这里把断点数据保存到数据库中：注意代码注释的理解

(0)主Activity，关键点使用Handler更新进度条与开启线程下载避免ANR

若不使用Handler却要立即更新进度条数据，可使用：

//resultView.invalidate(); UI线程中立即更新进度条方法

//resultView.postInvalidate(); 非UI线程中立即更新进度条方法

/**
 * 注意这里的设计思路，UI线程与参数保存问题，避免ANR问题，UI控件的显示
 * @author kay
 *
 */
public class DownloadActivity extends Activity 
{
    private EditText downloadpathText;
    private TextView resultView;
    private ProgressBar progressBar;
    
    @Override
    public void onCreate(Bundle savedInstanceState) 
    {
        super.onCreate(savedInstanceState);
        setContentView(R.layout.main);
        
        downloadpathText = (EditText) this.findViewById(R.id.downloadpath);
        progressBar = (ProgressBar) this.findViewById(R.id.downloadbar);
        resultView = (TextView) this.findViewById(R.id.result);
        Button button = (Button) this.findViewById(R.id.button);
        button.setOnClickListener(new View.OnClickListener() 
        {			
			@Override
			public void onClick(View v) 
			{
				//取得下载路径
				String path = downloadpathText.getText().toString();
				//判断SDCard是否存在
				if(Environment.getExternalStorageState().equals(Environment.MEDIA_MOUNTED))
				{
					//下载操作
					download(path, Environment.getExternalStorageDirectory());
				}
				else
				{
					Toast.makeText(DownloadActivity.this, R.string.sdcarderror, 1).show();
				}				
			}
		});
    }
    //主线程(UI线程)
    //业务逻辑正确，但是该程序运行的时候有问题(可能出现ANR问题)
    //对于显示控件的界面更新只是由UI线程负责，如果是在非UI线程更新控件的属性值，更新后的显示界面不会反映到屏幕上
    /**
     * 参数类型：因为启动一个线程还需要使用到上面方法的参数，而主方法启动后很快就会销毁，
     * 那么使用Final可以解决参数丢失的问题
     * path 注意是Final类型
     * savedir 注意是Final类型
     */
    private void download(final String path, final File savedir)
    {
    	//这里开启一个线程避免ANR错误
    	new Thread(new Runnable() 
    	{			
			@Override
			public void run() 
			{
				FileDownloader loader = new FileDownloader(DownloadActivity.this, path, savedir, 3);
				//设置进度条的最大刻度为文件的长度
				progressBar.setMax(loader.getFileSize());
				try 
				{
					loader.download(new DownloadProgressListener() 
					{
						/**
						 * 注意这里的设计，显示进度条数据需要使用Handler来处理
						 * 因为非UI线程更新后的数据不能被刷新
						 */
						@Override
						public void onDownloadSize(int size) 
						{
							//实时获知文件已经下载的数据长度
							Message msg = new Message();
							//设置消息标签
							msg.what = 1;
							msg.getData().putInt("size", size);
							//使用Handler对象发送消息
							handler.sendMessage(msg);
						}
					});
				}
				catch (Exception e)
				{
					//发送一个空消息到消息队列
					handler.obtainMessage(-1).sendToTarget();
					/**
					 *  或者使用下面的方法发送一个空消息
					 *  Message msg = new Message();
					 *	msg.what = 1;
					 *	handler.sendMessage(msg);
					 */					
				}
			}
		}).start();
	}
    
    
    /**Handler原理：当Handler被创建时会关联到创建它的当前线程的消息队列，该类用于往消息队列发送消息
     * 
     * 消息队列中的消息由当前线程内部进行处理
     */
    private Handler handler = new Handler()
    {
    	//重写Handler里面的handleMessage方法处理消息
		@Override
		public void handleMessage(Message msg) 
		{			
			switch (msg.what) 
			{
				case 1:
					//进度条显示
					progressBar.setProgress(msg.getData().getInt("size"));
					float num = (float)progressBar.getProgress()/(float)progressBar.getMax();
					int result = (int)(num*100);
					//resultView.invalidate(); UI线程中立即更新进度条方法
					//resultView.postInvalidate(); 非UI线程中立即更新进度条方法
					resultView.setText(result+ "%");
					//判断是否下载成功
					if(progressBar.getProgress()==progressBar.getMax())
					{
						Toast.makeText(DownloadActivity.this, R.string.success, 1).show();
					}
					break;	
				case -1:
					Toast.makeText(DownloadActivity.this, R.string.error, 1).show();
					break;
			}
		}
    };
    
}

(1)下载类：

注意计算每条线程的下载长度与下载起始位置的方法

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;	
	//下载文件长度
	private int downLength;
	private boolean finish = false;
	private FileDownloader downloader;
	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;
	}
	
	@Override
	public void run() 
	{
		//未下载完成
		if(downLength < block)
		{
			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");
				//下载开始位置：线程id*每条线程下载的数据长度 = ?
				int startPos = block * (threadId - 1) + downLength;
				//下载结束位置：（线程id+1）*每条线程下载的数据长度-1=?
				int endPos = block * threadId -1;
				//设置获取实体数据的范围
				http.setRequestProperty("Range", "bytes=" + startPos + "-"+ endPos);
				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.append(offset);
				}
				threadfile.close();
				inStream.close();
				print("Thread " + this.threadId + " download finish");
				//标记是否完成
				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;
	}
}

文件下载器，使用

/**
 * 文件下载器,使用这个类的方法如下示例：
 * FileDownloader loader = new FileDownloader(context, "http://browse.babasport.com/ejb3/ActivePort.exe",
 *				new File("D:\\androidsoft\\test"), 2);
 *		loader.getFileSize();//得到文件总大小
 *		try {
 *			loader.download(new DownloadProgressListener(){
 *				public void onDownloadSize(int size) {
 *					print("已经下载："+ size);
 *				}			
 *			});
 *		}
 *		 catch (Exception e) 
 *		{
 *			e.printStackTrace();
 *		}
 */
public class FileDownloader 
{
	private static final String TAG = "FileDownloader";
	private Context context;
	private FileService fileService;	
	//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;
	//获取线程数
	public int getThreadSize() 
	{
		return threads.length;
	}
	/**
	 * 获取文件大小
	 * @return
	 */
	public int getFileSize() 
	{
		return fileSize;
	}
	/**
	 * 累计已下载大小
	 * @param size
	 */
	protected synchronized void append(int size) 
	{
		downloadSize += size;
	}
	/**
	 * 更新指定线程最后下载的位置
	 * @param threadId 线程id
	 * @param pos 最后下载的位置
	 */
	protected synchronized void update(int threadId, int pos) 
	{
		this.data.put(threadId, pos);
		this.fileService.update(this.downloadUrl, this.data);
	}
	/**
	 * 文件下载构造器
	 * @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;
			fileService = new FileService(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(5*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);
			if (conn.getResponseCode()==200) 
			{
				//根据响应获取文件大小
				this.fileSize = conn.getContentLength();
				if (this.fileSize <= 0) throw new RuntimeException("Unkown file size ");
				//获取文件名称		
				String filename = getFileName(conn);
				//构建保存文件
				this.saveFile = new File(fileSaveDir, filename);
				//获取下载记录
				Map<Integer, Integer> logdata = fileService.getData(downloadUrl);
				//如果存在下载记录
				if(logdata.size()>0)
				{
					//把各条线程已经下载的数据长度放入data中
					for(Map.Entry<Integer, Integer> entry : logdata.entrySet())
						data.put(entry.getKey(), entry.getValue());
				}
				//下面计算所有线程已经下载的数据长度
				if(this.data.size()==this.threads.length)
				{
					for (int i = 0; i < this.threads.length; i++) 
					{
						this.downloadSize += this.data.get(i+1);
					}
					print("已经下载的长度"+ this.downloadSize);
				}
				//计算每条线程下载的数据长度
				this.block = (this.fileSize % this.threads.length)==0? this.fileSize / this.threads.length : this.fileSize / this.threads.length + 1;
			}
			else
			{
				throw new RuntimeException("server no response ");
			}
		}
		catch (Exception e) 
		{
			print(e.toString());
			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 
		{
			//创建本地文件
			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++) 
				{
					//初始化每条线程已经下载的数据长度为0
					this.data.put(i+1, 0);
				}
			}
			//开启线程进行下载
			for (int i = 0; i < this.threads.length; i++) 
			{
				int downLength = this.data.get(i+1);
				//判断线程是否已经完成下载,否则继续下载
				if(downLength < this.block && this.downloadSize<this.fileSize)
				{	
					this.threads[i] = new DownloadThread(this, url, this.saveFile, this.block, this.data.get(i+1), i+1);
					this.threads[i].setPriority(7);//可删除这条
					this.threads[i].start();
				}
				else
				{
					this.threads[i] = null;
				}
			}
			this.fileService.save(this.downloadUrl, this.data);
			//下载未完成
			boolean notFinish = true;
			// 循环判断所有线程是否完成下载
			while (notFinish) 
			{
				Thread.sleep(900);
				//假定全部线程下载完成
				notFinish = false;
				for (int i = 0; i < this.threads.length; i++)
				{
					//如果发现线程未完成下载
					if (this.threads[i] != null && !this.threads[i].isFinish()) 
					{
						//设置标志为下载没有完成
						notFinish = true;
						//如果下载失败,再重新下载
						if(this.threads[i].getDownLength() == -1)
						{
							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();
						}
					}
				}
				//通知目前已经下载完成的数据长度
				if(listener!=null) listener.onDownloadSize(this.downloadSize);
			}
			//删除数据库中下载信息
			fileService.delete(this.downloadUrl);
		}
		catch (Exception e) 
		{
			print(e.toString());
			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()+ ":" : "";
			print(key+ entry.getValue());
		}
	}
	private static void print(String msg)
	{
		Log.i(TAG, msg);
	}
}

public interface DownloadProgressListener 
{
	public void onDownloadSize(int size);
}

(2)文件操作，断点数据库存储

public class DBOpenHelper extends SQLiteOpenHelper 
{
	private static final String DBNAME = "itcast.db";
	private static final int VERSION = 1;
	
	public DBOpenHelper(Context context) 
	{
		super(context, DBNAME, null, VERSION);
	}
	
	@Override
	public void onCreate(SQLiteDatabase db) 
	{
		db.execSQL("CREATE TABLE IF NOT EXISTS filedownlog (id integer primary key autoincrement, downpath varchar(100), threadid INTEGER, downlength INTEGER)");
	}
	@Override
	public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) 
	{
		db.execSQL("DROP TABLE IF EXISTS filedownlog");
		onCreate(db);
	}
}

/**
 * 文件下载业务bean
 */
public class FileService 
{
	private DBOpenHelper openHelper;
	public FileService(Context context) 
	{
		openHelper = new DBOpenHelper(context);
	}
	/**
	 * 获取每条线程已经下载的文件长度
	 * @param path
	 * @return
	 */
	public Map<Integer, Integer> getData(String path)
	{
		SQLiteDatabase db = openHelper.getReadableDatabase();
		Cursor cursor = db.rawQuery("select threadid, downlength from filedownlog where downpath=?", new String[]{path});
		Map<Integer, Integer> data = new HashMap<Integer, Integer>();
		while(cursor.moveToNext())
		{
			data.put(cursor.getInt(0), cursor.getInt(1));
		}
		cursor.close();
		db.close();
		return data;
	}
	/**
	 * 保存每条线程已经下载的文件长度
	 * @param path
	 * @param map
	 */
	public void save(String path,  Map<Integer, Integer> map)
	{//int threadid, int position
		SQLiteDatabase db = openHelper.getWritableDatabase();
		db.beginTransaction();
		try
		{
			for(Map.Entry<Integer, Integer> entry : map.entrySet())
			{
				db.execSQL("insert into filedownlog(downpath, threadid, downlength) values(?,?,?)",
						new Object[]{path, entry.getKey(), entry.getValue()});
			}
			db.setTransactionSuccessful();
		}
		finally
		{
			db.endTransaction();
		}
		db.close();
	}
	/**
	 * 实时更新每条线程已经下载的文件长度
	 * @param path
	 * @param map
	 */
	public void update(String path, Map<Integer, Integer> map)
	{
		SQLiteDatabase db = openHelper.getWritableDatabase();
		db.beginTransaction();
		try{
			for(Map.Entry<Integer, Integer> entry : map.entrySet()){
				db.execSQL("update filedownlog set downlength=? where downpath=? and threadid=?",
						new Object[]{entry.getValue(), path, entry.getKey()});
			}
			db.setTransactionSuccessful();
		}finally{
			db.endTransaction();
		}
		db.close();
	}
	/**
	 * 当文件下载完成后，删除对应的下载记录
	 * @param path
	 */
	public void delete(String path)
	{
		SQLiteDatabase db = openHelper.getWritableDatabase();
		db.execSQL("delete from filedownlog where downpath=?", new Object[]{path});
		db.close();
	}	
}