android多线程断点下载

多线程下载是为了抢占服务器的更多资源，达到最快的下载速度，但是手机相比较PC的执行效率还是存在一定的差异，如果开启过多线程，CPU会不堪重负，从而影响整个文件的下载速度也有可能出现未响应。

多线程下载原理：

1.首先读取网络文件的长度，然后在本地生成一个与网络文件长度相等的本地文件

2.开启N多个线程下载文件，计算每条线程下载的数据量，公式：int block=文件长度%N==0 ？ 文件长度/N ：文件长度/N+1

3.开启多线程分别从网络不同的位置下载数据，并从本地文件相同的位置写入数据，要计算出每条线程从网络文件的什么位置开始下载数据，到什么位置结束

实例参考：

1.为实现断点下载，需要实时保存每个线程下载的数据长度，这里使用数据库来进行保存

/**
 * 创建数据库，目的是实时保存每个线程下载的字节数，有助于实现断点下载
 *
 */
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);
	}
	/**
	 * 存放断点记录,记录每一条线程的下载进度
	 */
	@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);
	}
}

2.操作数据库，也就是增删改查

/**
 * 操作数据库业务类
 * 保存、更新、删除每个线程的当前下载字节数
 * 在第一次下载时进行保存，然后对每一次下载进度进行更新，下载完成后删除数据库保存的下载进度
 */
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
	 * 调用频率非常高,几百毫秒就会调用一次
	 * @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();
	}
}

3.实现文件下载线程类

import java.io.File;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

import android.util.Log;
/**
 * 下载线程类，用于在文件下载器中开启多个线程
 *
 */
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 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");
				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 (!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);
			}
		}
	}
	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;
	}
}

4.在文件下载线程的基础上封装文件下载器，对外提供文件下载操作

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.kafei.service.FileService;

import android.content.Context;
import android.util.Log;
/**
 * 文件下载器
 * 
	try {
		FileDownloader loader = new FileDownloader(context, "http://192.168.1.110:8080/jyt2013.rar",
				new File("D:\\文件\\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 {
	private static final String TAG = "FileDownloader";
	private Context context;
	private FileService fileService;//文件下载进度保存至数据库
	/* 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();
			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
				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
				}
				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++) {
						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);
	}
}

5.实现文件下载进度接口监听器，用于在文件下载过程中实时获取文件的下载进度

/**
 * 文件下载进度接口监听器
 */
public interface DownloadProgressListener {
	public void onDownloadSize(int size);
}

6.在activity中实现文件下载

/**
 * 执行次序：点击开始按钮==》dowload方法==》DownloadTask（开启子线程）==》UIHander（重绘屏幕显示进度）
 *
 */
public class MainActivity extends Activity {
	private EditText pathEdit;// 下载路径
	private TextView resultView;// 下载进度显示
	private Button downloadButton;// 下载按钮
	private Button stopButton;//停止按钮
	private ProgressBar progressBar;// 进度条
	private Handler handler=new UIHander();//handler的作用是用于往创建Hander对象所在的线程所绑定的消息队列发送消息

	@Override
	public void onCreate(Bundle savedInstanceState) {
		super.onCreate(savedInstanceState);
		setContentView(R.layout.main);
		pathEdit = (EditText) this.findViewById(R.id.path);
		resultView = (TextView) this.findViewById(R.id.resultView);
		downloadButton = (Button) this.findViewById(R.id.downloadbutton);
		stopButton = (Button) this.findViewById(R.id.stopbutton);
		progressBar = (ProgressBar) this.findViewById(R.id.progressBar);
		ButtonClickListener listener=new ButtonClickListener();
		stopButton.setOnClickListener(listener);
		downloadButton.setOnClickListener(listener);
	}

	/**
	 * 开始和停止按钮监听器
	 *
	 */
	private final class ButtonClickListener implements View.OnClickListener {

		public void onClick(View v) {
			switch (v.getId()) {
			case R.id.downloadbutton://如果是点击开始按钮就执行下载操作
				String path = pathEdit.getText().toString();
				if (Environment.getExternalStorageState().equals(
						Environment.MEDIA_MOUNTED)) {
					File saveDir = Environment.getExternalStorageDirectory();
					dowload(path, saveDir);//开始下载
				} else {
					Toast.makeText(getApplicationContext(), R.string.sdcarderror, 1)
							.show();
				}
				downloadButton.setEnabled(false);
				stopButton.setEnabled(true);
				break;
			case R.id.stopbutton://如果是点击停止按钮，就执行停止下载操作
				exit();//停止下载
				downloadButton.setEnabled(true);
				stopButton.setEnabled(false);
				break;
			}
		}
		/*
		由于用户的输入事件(点击button, 触摸屏幕....)是由主线程负责处理的，如果主线程处于工作状态，
		此时用户产生的输入事件如果没能在5秒内得到处理，系统就会报“应用无响应”错误。
		所以在主线程里不能执行一件比较耗时的工作，否则会因主线程阻塞而无法处理用户的输入事件，
		导致“应用无响应”错误的出现。耗时的工作应该在子线程里执行。
		 */
		private DownloadTask task;
		/**
		 * 退出下载
		 */
		public void exit(){
			if(task!=null)task.exit();
		}
		private void dowload(String path, File saveDir) {
			 task=new DownloadTask(path,saveDir);//开始下载任务线程类
			new Thread(task).start();//开启子线程进行下载
			
		}
		/**
		 * 下载任务线程，主要负责任务的下载
		 * 把耗时的工作交给子线程
		 * UI控件画面的重绘（更新）是由主线程负责处理的，
		 * 如果在子线程中更新UI控件的值，更新后的值不会重绘到屏幕上
		 * 一定要在主线程里更新UI控件的值，这样才能在屏幕上显示出来
		 * 不能在子线程中更新UI控件的值
		 */
		private final class DownloadTask implements Runnable{
			private String path;//下载路径
			private File saveDir;//存放位置
			private FileDownloader loader;//文件下载器
			public DownloadTask(String path, File saveDir) {
				this.path=path;
				this.saveDir=saveDir;
			}
			/**
			 * 退出下载
			 */
			public void exit(){
				if(loader!=null)loader.exit();
			
			}
			public void run() {
				try {
					 loader = new FileDownloader(
							getApplicationContext(), path, saveDir, 2);//2表示开启的下载线程数，因为手机的执行效率有限，不建议开启过多线程
					progressBar.setMax(loader.getFileSize());// 得到文件总大小,设置为进度条的最大刻度
					//监听下载文件大小变化
					loader.download(new DownloadProgressListener() {
						public void onDownloadSize(int size) {
							Message msg=new Message();//消息对象
							msg.what=1;//定义消息标识ID为1
							msg.getData().putInt("size", size);
							handler.sendMessage(msg);//将消息发送出去
						}
					});
				} catch (Exception e) {
					e.printStackTrace();
					handler.sendMessage(handler.obtainMessage(-1));//-1表示定义的消息标识ID
				}
			}
		}
	}
	/**
	 * 在下载过程中，实现不断的重绘UI进度的值
	 * 如果把显示进度放在子线程中，重绘不起作用
	 * 因为重绘UI只能在主线程中进行
	 */
	private final class UIHander extends Handler{
		public void handleMessage(Message msg){
			switch (msg.what) {
			case 1://如果是1，则表示重绘UI界面的下载进度
				int size=msg.getData().getInt("size");//从消息队列中取出当前已下载的文件大小，作为进度条的当前刻度
				progressBar.setProgress(size);//进度条的当前刻度
				float num=(float)progressBar.getProgress()/(float)progressBar.getMax();//计算当前下载进度
				int result=(int)(num*100);
				resultView.setText(result+"%");
				//如果下载的进度等于进度条的最大刻度。说明下载完成，给出提示
				if(progressBar.getProgress()==progressBar.getMax()){
					Toast.makeText(getApplicationContext(), R.string.success, 1).show();
				}
				break;
			case -1://如果下载出错，给出提示
				Toast.makeText(getApplicationContext(), R.string.error, 1).show();
				break;
			}
		}
	}
}

界面如下图所示：

源码下载：android多线程断点下载