Range 

　　 用于请求头中，指定第一个字节的位置和最后一个字节的位置，一般格式：

　　 Range:(unit=first byte pos)-[last byte pos] 

比如你要请求下载一个MP3文件，url为：http://abv.cn/music/光辉岁月.mp3

你需要通过http发送get请求，请求头字段可能如下

关于其他字段的含义，我在这里就不解释了，自己动手查去，哪个不懂就查哪个。上面很直观的展示了请求很响应的内容。

package com.wwj.download.db;

import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;

public class DBOpenHelper extends SQLiteOpenHelper {
	private static final String DBNAME = "eric.db";
	private static final int VERSION = 1;

	public DBOpenHelper(Context context) {
		super(context, DBNAME, null, VERSION);
	}

	@Override
	public void onCreate(SQLiteDatabase db) {
		// 创建filedownlog表
		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);
	}
}

package com.wwj.net.download;

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

/**
 * 文件下载器
 * 
 */
public class FileDownloader {
	private static final String TAG = "FileDownloader";
	private Context context;
	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()) // 判断目录是否存在，如果不存在，创建目录
				fileSaveDir.mkdirs();
			this.threads = new DownloadThread[threadNum];// 实例化线程数组
			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(); // 连接
			printResponseHeader(conn);
			if (conn.getResponseCode() == 200) { // 响应成功
				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); // 预分配fileSize大小
			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); // 设置线程优先级
					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);
	}
}

package com.wwj.net.download;

import java.util.HashMap;
import java.util.Map;

import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;

import com.wwj.download.db.DBOpenHelper;

/**
 * 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();
	}

}

package com.wwj.net.download;

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 {
	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); // 设置连接超时
				http.setRequestMethod("GET"); // 设置请求方法，这里是“GET”
				// 浏览器可接受的MIME类型
				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"); // 浏览器所希望的语言种类，当服务器能够提供一种以上的语言版本时要用到
				http.setRequestProperty("Referer", downUrl.toString());// 包含一个URL，用户从该URL代表的页面出发访问当前请求的页面。
				http.setRequestProperty("Charset", "UTF-8"); // 字符集
				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
				// 浏览器类型，如果Servlet返回的内容与浏览器类型有关则该值非常有用。
				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"); // 设置为持久连接

				// 得到输入流
				InputStream inStream = http.getInputStream();
				byte[] buffer = new byte[1024];
				int offset = 0;
				print("Thread " + this.threadId
						+ " start download from position " + startPos);
				// 随机访问文件
				RandomAccessFile threadfile = new RandomAccessFile(
						this.saveFile, "rwd");
				// 定位到pos位置
				threadfile.seek(startPos);
				while (!downloader.getExit()
						&& (offset = inStream.read(buffer, 0, 1024)) != -1) {
					// 写入文件
					threadfile.write(buffer, 0, offset);
					downLength += offset; // 累加下载的大小
					downloader.update(this.threadId, downLength); // 更新指定线程下载最后的位置
					downloader.append(offset); // 累加已下载大小
				}
				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;
	}
}

package com.wwj.net.download;


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

package com.wwj.download;

import java.io.File;
import java.io.UnsupportedEncodingException;
import java.net.URLEncoder;

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 com.wwj.net.download.DownloadProgressListener;
import com.wwj.net.download.FileDownloader;

public class MainActivity extends Activity {
	private static final int PROCESSING = 1;
	private static final int FAILURE = -1;

	private EditText pathText; // url地址
	private TextView resultView;
	private Button downloadButton;
	private Button stopButton;
	private ProgressBar progressBar;

	private Handler handler = new UIHandler();

	private final class UIHandler extends Handler {
		public void handleMessage(Message msg) {
			switch (msg.what) {
			case PROCESSING: // 更新进度
				progressBar.setProgress(msg.getData().getInt("size"));
				float num = (float) progressBar.getProgress()
						/ (float) progressBar.getMax();
				int result = (int) (num * 100); // 计算进度
				resultView.setText(result + "%");
				if (progressBar.getProgress() == progressBar.getMax()) { // 下载完成
					Toast.makeText(getApplicationContext(), R.string.success,
							Toast.LENGTH_LONG).show();
				}
				break;
			case FAILURE: // 下载失败
				Toast.makeText(getApplicationContext(), R.string.error,
						Toast.LENGTH_LONG).show();
				break;
			}
		}
	}

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

	private final class ButtonClickListener implements View.OnClickListener {
		@Override
		public void onClick(View v) {
			switch (v.getId()) {
			case R.id.downloadbutton: // 开始下载
				// http://abv.cn/music/光辉岁月.mp3，可以换成其他文件下载的链接
				String path = pathText.getText().toString();
				String filename = path.substring(path.lastIndexOf(‘/‘) + 1);

				try {
					// URL编码（这里是为了将中文进行URL编码）
					filename = URLEncoder.encode(filename, "UTF-8");
				} catch (UnsupportedEncodingException e) {
					e.printStackTrace();
				}

				path = path.substring(0, path.lastIndexOf("/") + 1) + filename;
				if (Environment.getExternalStorageState().equals(
						Environment.MEDIA_MOUNTED)) {
					// File savDir =
					// Environment.getExternalStoragePublicDirectory(Environment.DIRECTORY_MOVIES);
					// 保存路径
					File savDir = Environment.getExternalStorageDirectory();
					download(path, savDir);
				} else {
					Toast.makeText(getApplicationContext(),
							R.string.sdcarderror, Toast.LENGTH_LONG).show();
				}
				downloadButton.setEnabled(false);
				stopButton.setEnabled(true);
				break;
			case R.id.stopbutton: // 暂停下载
				exit();
				Toast.makeText(getApplicationContext(),
						"Now thread is Stopping!!", Toast.LENGTH_LONG).show();
				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;

		private void exit() {
			if (task != null)
				task.exit();
		}

		private void download(String path, File savDir) {
			task = new DownloadTask(path, savDir);
			new Thread(task).start();
		}

		/**
		 * 
		 * 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;
			private File saveDir;
			private FileDownloader loader;

			public DownloadTask(String path, File saveDir) {
				this.path = path;
				this.saveDir = saveDir;
			}

			/**
			 * 退出下载
			 */
			public void exit() {
				if (loader != null)
					loader.exit();
			}

			DownloadProgressListener downloadProgressListener = new DownloadProgressListener() {
				@Override
				public void onDownloadSize(int size) {
					Message msg = new Message();
					msg.what = PROCESSING;
					msg.getData().putInt("size", size);
					handler.sendMessage(msg);
				}
			};

			public void run() {
				try {
					// 实例化一个文件下载器
					loader = new FileDownloader(getApplicationContext(), path,
							saveDir, 3);
					// 设置进度条最大值
					progressBar.setMax(loader.getFileSize());
					loader.download(downloadProgressListener);
				} catch (Exception e) {
					e.printStackTrace();
					handler.sendMessage(handler.obtainMessage(FAILURE)); // 发送一条空消息对象
				}
			}
		}
	}

}