多线程断点下载详解

首先先理清一下逻辑，后面会贴上源代码，里面有详细的解释，根据前面的逻辑去看后面的源码能够很快掌握断点下载。续传也是同样的道理。

看源码之前需要了解的知识：使用Http的Range头字段可以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");

 还有通知ProgressListener已经下载的文件大小。如果点击了暂停下载，下次再点击下载的时候会先去对应的路径读取相应的数据，然后继续下载。

下面是源码，里面有足够的注释。

import java.io.File;

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.view.View;
import android.widget.Button;
import android.widget.EditText;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;
import cn.itcast.net.download.DownloadProgressListener;
import cn.itcast.net.download.FileDownloader;

@SuppressLint("HandlerLea<pre name="code" class="java">import java.io.File;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.LinkedHashMap;
import java.util.Map;
import java.util.UUID;
import java.util.concurrent.ConcurrentHashMap;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import cn.itcast.service.FileService;

import android.content.Context;
import android.util.Log;

/**
 * 文件下载器
 * 
 * 使用方法 try { 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
 * loader.download(new DownloadProgressListener(){ public void
 * onDownloadSize(int size) { print("已经下载："+ size); } }); } catch (Exception e)
 * { e.printStackTrace(); }
 */
public class FileDownloader {
	//TAG ,方便调试
	private static final String TAG = "FileDownloader";
	//
	private Context context;
	//文件操作
	private FileService fileService;
	/* 停止下载 */
	private boolean exit;
	/* 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;
	}

	/**
	 * 退出下载
	 */
	public void exit() {
		this.exit = true;
	}

	public boolean getExit() {
		return this.exit;
	}

	/**
	 * 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, threadId, pos);
	}

	/**
	 * 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;
			
			fileService = new FileService(this.context);
			
			URL url = new URL(this.downloadUrl);
			if (!fileSaveDir.exists())
				fileSaveDir.mkdirs();
			
			
			//第一次只是执行一个初始化操作，并不执行DownloadThread里面的任何caozyuo
			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) {
				this.fileSize = conn.getContentLength();// 根据响应获取文件大小
				if (this.fileSize <= 0)
					throw new RuntimeException("Unkown file size ");

				String filename = getFileName(conn);// 获取文件名称
				
				this.saveFile = new File(fileSaveDir, filename);// 构建保存文件
			
				Map<Integer, Integer> logdata = fileService.getData(downloadUrl);// 获取下载记录
			
				
				
				
				
				//第一次判断就返回真
				if (logdata.size() > 0) {// 如果存在下载记录
					for (Map.Entry<Integer, Integer> entry : logdata.entrySet())
						data.put(entry.getKey(), entry.getValue());// 把各条线程已经下载的数据长度放入data中
				}
				
				//第一次判断就返回真
				if (this.data.size() == this.threads.length) {// 下面计算所有线程已经下载的数据总长度
					for (int i = 0; i < this.threads.length; i++) {
						//之所以加一是跳着取，因为第一个是线程id,第二个是下载大小，第三个是线程id，第四个是对应已下载大小
						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 {
			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) {// 如果原先未曾下载或者原先的下载线程数与现在的线程数不一致
				this.data.clear();
				for (int i = 0; i < this.threads.length; i++) {
					this.data.put(i + 1, 0);// 初始化每条线程已经下载的数据长度为0
				}
				this.downloadSize = 0;
			}
			for (int i = 0; i < this.threads.length; i++) {// 开启线程进行下载
				int downLength = this.data.get(i + 1);
				if (downLength < this.block
						&& this.downloadSize < this.fileSize) {// 判断线程是否已经完成下载,否则继续下载
					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;
				}
			}
			fileService.delete(this.downloadUrl);// 如果存在下载记录，删除它们，然后重新添加
			fileService.save(this.downloadUrl, this.data);
			boolean notFinish = true;// 下载未完成
			while (notFinish) {// 循环判断所有线程是否完成下载
				Thread.sleep(900);
				notFinish = false;// 假定全部线程下载完成
				for (int i = 0; i < this.threads.length; i++) {
					if (this.threads[i] != null && !this.threads[i].isFinish()) {// 如果发现线程未完成下载
						notFinish = true;// 设置标志为下载没有完成
						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();
						}
					}
				}
				if (listener != null)
					listener.onDownloadSize(this.downloadSize);// 通知目前已经下载完成的数据长度
			}
			if (downloadSize == this.fileSize)
				fileService.delete(this.downloadUrl);// 下载完成删除记录
		} catch (Exception e) {
			print(e.toString());
			throw new Exception("file download error");
		}
		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);
	}
}

<span style="font-family:KaiTi_GB2312;">import java.io.File;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

import android.util.Log;

public class DownloadThread extends Thread {
	//TAG，便于调试
	private static final String TAG = "DownloadThread";
	//文件在本地的保存名称
	private File saveFile;
	//资源的地址
	private URL downUrl;
	//每一条线程下载的文件大小
	private int block;
	//线程的id
	private int threadId = -1;	
	//每条线程已经下载的长度
	private int downLength;
	//是否已经完成
	private boolean finish = false;
	//文件下载类
	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() {
		
		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");
				//计算待会要写进文件的时候应该从哪个位置开始写
				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);
				//把该线程对应的下载数据存放到本地文件中
				//注意，三个子线程都是读入到同一个本地文件中，但读入的文件数据的位置不一样，所以最后刚好就是一个 文件
				RandomAccessFile threadfile = new RandomAccessFile(this.saveFile, "rwd");
				//跳到那个字节开始
				threadfile.seek(startPos);
				while (!downloader.getExit() && (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");
				this.finish = true;
			} catch (Exception e) {
				this.downLength = -1;
				print("Thread "+ this.threadId+ ":"+ e);
			}
		}
	}
	/***
	 * 
	 * @param msg
	 */
	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;
	}
}
</span>

<span style="font-family:KaiTi_GB2312;">import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

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);
	}

}
</span>

<span style="font-family:KaiTi_GB2312;">import java.util.HashMap;
import java.util.Map;

import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
/**
 * 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, int threadId, int pos){
		SQLiteDatabase db = openHelper.getWritableDatabase();
		db.execSQL("update filedownlog set downlength=? where downpath=? and threadid=?",
				new Object[]{pos, path, threadId});
		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();
	}
	
}
</span>

<span style="font-family:KaiTi_GB2312;">public interface DownloadProgressListener {
	public void onDownloadSize(int size);
}
</span>

多线程断点下载详解

原文：http://blog.csdn.net/chencangui/article/details/45440521