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??? 在我们做开发的时候经常遇到的就是下载了，现在下载的方法有很多很多，那么怎么做到断点续传下载呢！很多人都头疼这个问题，如果我们没有很好的逻辑真不是很容易解决啊。我参考了一下前辈们的资料了整理了一个项目，能实现多个文件的同时下载。
????断点续传下载，顾名思义，那就是我们在一次下载未结束时，退出下载，第二次下载时会接着第一次下载的进度继续下载。那么怎么记录第一次下载的数据呢，这里肯定就要用到数据库了。下面就是我创建数据库的一个SQLiteOpenHelper类。用来首次运行时创建数据库。
DBHelper.java
package com.icss.DBHelper;import android.content.Context;import android.database.sqlite.SQLiteDatabase;import android.database.sqlite.SQLiteOpenHelper;/** * 建立一个数据库帮助类 */public class DBHelper extends SQLiteOpenHelper { // download.db-->数据库名 public DBHelper(Context context) { super(context, "download.db", null, 1); } /** * 在download.db数据库下创建一个download_info表存储下载信息 */ @Override public void onCreate(SQLiteDatabase db) { db.execSQL("create table download_info(_id integer PRIMARY KEY AUTOINCREMENT, thread_id integer, " + "start_pos integer, end_pos integer, compelete_size integer,url char)"); } @Override public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) { }}?下面看主界面的布局,在这里，我只设计了一个ListView来显示下载的音乐的名称，和一个开始下载按钮和一个暂停按钮。
布局文件如下：
main.xml：
<?xml version="1.0" encoding="utf-8"?><LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:orientation="vertical" android:layout_width="fill_parent" android:layout_height="fill_parent" android:weightSum="1"> <ListView android:id="@android:id/list" android:layout_height="wrap_content" android:layout_width="match_parent" android:layout_weight="0.70"></ListView></LinearLayout>
?list_item.xml：
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android" android:layout_height="fill_parent" android:id="@+id/linearLayout1" android:layout_width="fill_parent" android:orientation="vertical"> <LinearLayout android:layout_width="fill_parent" android:id="@+id/linearLayout2" android:layout_height="wrap_content" android:orientation="horizontal" android:layout_marginBottom="5dip"> <TextView android:layout_width="fill_parent" android:layout_height="wrap_content" android:layout_weight="1" android:id="@+id/tv_resouce_name" /> <Button android:layout_width="fill_parent" android:layout_height="wrap_content" android:layout_weight="1" android:text="下载" android:id="@+id/btn_start" android:onClick="startDownload" /> <Button android:layout_width="fill_parent" android:layout_height="wrap_content" android:layout_weight="1" android:text="暂停" android:id="@+id/btn_pause" android:onClick="pauseDownload" /> </LinearLayout></LinearLayout>
???? 我们要定义一个记录在下载时各个时期的数据的类，这里我创建了一个DownloadInfo类来记录。代码如下：
DownloadInfo：
package com.icss.entity;public class DownloadInfo { private int threadId;// 下载器id private int startPos;// 开始点 private int endPos;// 结束点 private int compeleteSize;// 完成度 private String url;// 下载器网络标识 public DownloadInfo(int threadId, int startPos, int endPos, int compeleteSize, String url) { super(); this.threadId = threadId; this.startPos = startPos; this.endPos = endPos; this.compeleteSize = compeleteSize; this.url = url; } public int getThreadId() { return threadId; } public void setThreadId(int threadId) { this.threadId = threadId; } public int getStartPos() { return startPos; } public void setStartPos(int startPos) { this.startPos = startPos; } public int getEndPos() { return endPos; } public void setEndPos(int endPos) { this.endPos = endPos; } public int getCompeleteSize() { return compeleteSize; } public void setCompeleteSize(int compeleteSize) { this.compeleteSize = compeleteSize; } public String getUrl() { return url; } public void setUrl(String url) { this.url = url; } @Override public String toString() { return "DownloadInfo [threadId=" + threadId + ", startPos=" + startPos + ", endPos=" + endPos + ", compeleteSize=" + compeleteSize +"]"; }}???? 在下载时，我们有进度条来显示进度，怎么确定进度条的进度，大小和起始位置呢？这里我定义了一个LoadInfo类来记录下载器详细信息。代码如下：
LoadInfo：
package com.icss.entity;public class LoadInfo { public int fileSize;// 文件大小 private int complete;// 完成度 private String urlstring;// 下载器标识 public LoadInfo() { super(); // TODO Auto-generated constructor stub } public LoadInfo(int fileSize, int complete, String urlstring) { super(); this.fileSize = fileSize; this.complete = complete; this.urlstring = urlstring; } public int getFileSize() { return fileSize; } public void setFileSize(int fileSize) { this.fileSize = fileSize; } public int getComplete() { return complete; } public void setComplete(int complete) { this.complete = complete; } public String getUrlstring() { return urlstring; } public void setUrlstring(String urlstring) { this.urlstring = urlstring; } @Override public String toString() { return "LoadInfo [fileSize=" + fileSize + ", complete=" + complete + ", urlstring=" + urlstring + "]"; }}??? 下面是最最重要的一步，那就是定义一个下载器来进行下载了，这里我就不多说，具体解释在代码中都有注解，供大家研究参考。
Downloader:
package com.icss.service;import java.io.File;import java.io.InputStream;import java.io.RandomAccessFile;import java.net.HttpURLConnection;import java.net.URL;import java.util.ArrayList;import java.util.List;import com.icss.dao.Dao;import com.icss.entity.DownloadInfo;import com.icss.entity.LoadInfo;import android.content.Context;import android.os.Handler;import android.os.Message;import android.util.Log;public class Downloader { private String urlstr;// 下载的地址 private String localfile;// 保存路径 private int threadcount;// 线程数 private Handler mHandler;// 消息处理器 private Dao dao;// 工具类 private int fileSize;// 所要下载的文件的大小 private List<DownloadInfo> infos;// 存放下载信息类的集合 private static final int INIT = 1;// 定义三种下载的状态：初始化状态，正在下载状态，暂停状态 private static final int DOWNLOADING = 2; private static final int PAUSE = 3; private int state = INIT; public Downloader(String urlstr, String localfile, int threadcount, Context context, Handler mHandler) { this.urlstr = urlstr; this.localfile = localfile; this.threadcount = threadcount; this.mHandler = mHandler; dao = new Dao(context); } /** * 判断是否正在下载 */ public boolean isdownloading() { return state == DOWNLOADING; } /** * 得到downloader里的信息 首先进行判断是否是第一次下载，如果是第一次就要进行初始化，并将下载器的信息保存到数据库中 * 如果不是第一次下载，那就要从数据库中读出之前下载的信息（起始位置，结束为止，文件大小等），并将下载信息返回给下载器 */ public LoadInfo getDownloaderInfors() { if (isFirst(urlstr)) { init(); int range = fileSize / threadcount; infos = new ArrayList<DownloadInfo>(); for (int i = 0; i < threadcount - 1; i++) { DownloadInfo info = new DownloadInfo(i, i * range, (i + 1) * range - 1, 0, urlstr); infos.add(info); } DownloadInfo info = new DownloadInfo(threadcount - 1, (threadcount - 1) * range, fileSize - 1, 0, urlstr); infos.add(info); // 保存infos中的数据到数据库 dao.saveInfos(infos); // 创建一个LoadInfo对象记载下载器的具体信息 LoadInfo loadInfo = new LoadInfo(fileSize, 0, urlstr); return loadInfo; } else { // 得到数据库中已有的urlstr的下载器的具体信息 infos = dao.getInfos(urlstr); Log.v("TAG", "not isFirst size=" + infos.size()); int size = 0; int compeleteSize = 0; for (DownloadInfo info : infos) { compeleteSize += info.getCompeleteSize(); size += info.getEndPos() - info.getStartPos() + 1; } return new LoadInfo(size, compeleteSize, urlstr); } } /** */ private void init() { try { URL url = new URL(urlstr); HttpURLConnection connection = (HttpURLConnection) url .openConnection(); connection.setConnectTimeout(5000); connection.setRequestMethod("GET"); fileSize = connection.getContentLength(); File file = new File(localfile); if (!file.exists()) { file.createNewFile(); } // 本地访问文件 RandomAccessFile accessFile = new RandomAccessFile(file, "rwd"); accessFile.setLength(fileSize); accessFile.close(); connection.disconnect(); } catch (Exception e) { e.printStackTrace(); } } /** * 判断是否是第一次 下载 */ private boolean isFirst(String urlstr) { return dao.isHasInfors(urlstr); } /** * 114 * 利用线程开始下载数据 115 */ public void download() { if (infos != null) { if (state == DOWNLOADING) return; state = DOWNLOADING; for (DownloadInfo info : infos) { new MyThread(info.getThreadId(), info.getStartPos(), info.getEndPos(), info.getCompeleteSize(), info.getUrl()).start(); } } } public class MyThread extends Thread { private int threadId; private int startPos; private int endPos; private int compeleteSize; private String urlstr; public MyThread(int threadId, int startPos, int endPos, int compeleteSize, String urlstr) { this.threadId = threadId; this.startPos = startPos; this.endPos = endPos; this.compeleteSize = compeleteSize; this.urlstr = urlstr; } @Override public void run() { HttpURLConnection connection = null; RandomAccessFile randomAccessFile = null; InputStream is = null; try { URL url = new URL(urlstr); connection = (HttpURLConnection) url.openConnection(); connection.setConnectTimeout(5000); connection.setRequestMethod("GET"); // 设置范围，格式为Range：bytes x-y; connection.setRequestProperty("Range", "bytes=" + (startPos + compeleteSize) + "-" + endPos); randomAccessFile = new RandomAccessFile(localfile, "rwd"); randomAccessFile.seek(startPos + compeleteSize); // 将要下载的文件写到保存在保存路径下的文件中 is = connection.getInputStream(); byte[] buffer = new byte[4096]; int length = -1; while ((length = is.read(buffer)) != -1) { randomAccessFile.write(buffer, 0, length); compeleteSize += length; // 更新数据库中的下载信息 dao.updataInfos(threadId, compeleteSize, urlstr); // 用消息将下载信息传给进度条，对进度条进行更新 Message message = Message.obtain(); message.what = 1; message.obj = urlstr; message.arg1 = length; mHandler.sendMessage(message); if (state == PAUSE) { return; } } } catch (Exception e) { e.printStackTrace(); } finally { try { is.close(); randomAccessFile.close(); connection.disconnect(); dao.closeDb(); } catch (Exception e) { e.printStackTrace(); } } } } // 删除数据库中urlstr对应的下载器信息 public void delete(String urlstr) { dao.delete(urlstr); } // 设置暂停 public void pause() { state = PAUSE; } // 重置下载状态 public void reset() { state = INIT; }}???? 在这边下载器类的定义中，我们用到了许多关于进行数据库操作的方法，这里我定义了一个数据库工具类，来提供这些方法，代码如下：
Dao：
package com.icss.dao;import java.util.ArrayList;import java.util.List;import javax.crypto.spec.DESKeySpec;import com.icss.DBHelper.DBHelper;import com.icss.entity.DownloadInfo; import android.content.Context; import android.database.Cursor;import android.database.sqlite.SQLiteDatabase; /** * * 一个业务类 */ public class Dao { private DBHelper dbHelper; public Dao(Context context) { dbHelper = new DBHelper(context); } /** * 查看数据库中是否有数据 */ public boolean isHasInfors(String urlstr) { SQLiteDatabase database = dbHelper.getReadableDatabase(); String sql = "select count(*) from download_info where url=?"; Cursor cursor = database.rawQuery(sql, new String[] { urlstr }); cursor.moveToFirst(); int count = cursor.getInt(0); cursor.close(); return count == 0; } /** * 保存 下载的具体信息 */ public void saveInfos(List<DownloadInfo> infos) { SQLiteDatabase database = dbHelper.getWritableDatabase(); for (DownloadInfo info : infos) { String sql = "insert into download_info(thread_id,start_pos, end_pos,compelete_size,url) values (?,?,?,?,?)"; Object[] bindArgs = { info.getThreadId(), info.getStartPos(), info.getEndPos(), info.getCompeleteSize(), info.getUrl() }; database.execSQL(sql, bindArgs); } } /** * 得到下载具体信息 */ public List<DownloadInfo> getInfos(String urlstr) { List<DownloadInfo> list = new ArrayList<DownloadInfo>(); SQLiteDatabase database = dbHelper.getReadableDatabase(); String sql = "select thread_id, start_pos, end_pos,compelete_size,url from download_info where url=?"; Cursor cursor = database.rawQuery(sql, new String[] { urlstr }); while (cursor.moveToNext()) { DownloadInfo info = new DownloadInfo(cursor.getInt(0), cursor.getInt(1), cursor.getInt(2), cursor.getInt(3), cursor.getString(4)); list.add(info); } cursor.close(); return list; } /** * 更新数据库中的下载信息 */ public void updataInfos(int threadId, int compeleteSize, String urlstr) { SQLiteDatabase database = dbHelper.getReadableDatabase(); String sql = "update download_info set compelete_size=? where thread_id=? and url=?"; Object[] bindArgs = { compeleteSize, threadId, urlstr }; database.execSQL(sql, bindArgs); } /** * 关闭数据库 */ public void closeDb() { dbHelper.close(); } /** * 下载完成后删除数据库中的数据 */ public void delete(String url) { SQLiteDatabase database = dbHelper.getReadableDatabase(); database.delete("download_info", "url=?", new String[] { url }); database.close(); } }?下面的是程序的主程序，其他的不说了，直接上代码：
package com.icss;import java.util.ArrayList;import java.util.HashMap;import java.util.List;import java.util.Map;import android.app.ListActivity;import android.os.Bundle;import android.os.Handler;import android.os.Message;import android.view.View;import android.widget.LinearLayout;import android.widget.LinearLayout.LayoutParams;import android.widget.ProgressBar;import android.widget.SimpleAdapter;import android.widget.TextView;import android.widget.Toast;import com.icss.entity.LoadInfo;import com.icss.service.Downloader;public class MainActivity extends ListActivity { // 固定下载的资源路径，这里可以设置网络上的地址 private static final String URL = "http://10.0.0.80:8080/xiangce/"; // 固定存放下载的音乐的路径：SD卡目录下 private static final String SD_PATH = "/mnt/sdcard/"; // 存放各个下载器 private Map<String, Downloader> downloaders = new HashMap<String, Downloader>(); // 存放与下载器对应的进度条 private Map<String, ProgressBar> ProgressBars = new HashMap<String, ProgressBar>(); /** * 31 * 利用消息处理机制适时更新进度条 32 */ private Handler mHandler = new Handler() { public void handleMessage(Message msg) { if (msg.what == 1) { String url = (String) msg.obj; int length = msg.arg1; ProgressBar bar = ProgressBars.get(url); if (bar != null) { // 设置进度条按读取的length长度更新 bar.incrementProgressBy(length); if (bar.getProgress() == bar.getMax()) { Toast.makeText(MainActivity.this, "下载完成！", 0).show(); // 下载完成后清除进度条并将map中的数据清空 LinearLayout layout = (LinearLayout) bar.getParent(); layout.removeView(bar); ProgressBars.remove(url); downloaders.get(url).delete(url); downloaders.get(url).reset(); downloaders.remove(url); } } } } }; @Override public void onCreate(Bundle savedInstanceState) { super.onCreate(savedInstanceState); setContentView(R.layout.main); showListView(); } // 显示listView，这里可以随便添加音乐 private void showListView() { List<Map<String, String>> data = new ArrayList<Map<String, String>>(); Map<String, String> map = new HashMap<String, String>(); map.put("name", "mm.mp3"); data.add(map); map = new HashMap<String, String>(); map.put("name", "pp.mp3"); data.add(map); map = new HashMap<String, String>(); map.put("name", "tt.mp3"); data.add(map); map = new HashMap<String, String>(); map.put("name", "You.mp3"); data.add(map); SimpleAdapter adapter = new SimpleAdapter(this, data, R.layout.list_item, new String[] { "name" }, new int[] { R.id.tv_resouce_name }); setListAdapter(adapter); } /** * 83 * 响应开始下载按钮的点击事件 84 */ public void startDownload(View v) { // 得到textView的内容 LinearLayout layout = (LinearLayout) v.getParent(); String musicName = ((TextView) layout .findViewById(R.id.tv_resouce_name)).getText().toString(); String urlstr = URL + musicName; String localfile = SD_PATH + musicName; // 设置下载线程数为4，这里是我为了方便随便固定的 int threadcount = 4; // 初始化一个downloader下载器 Downloader downloader = downloaders.get(urlstr); if (downloader == null) { downloader = new Downloader(urlstr, localfile, threadcount, this, mHandler); downloaders.put(urlstr, downloader); } if (downloader.isdownloading()) return; // 得到下载信息类的个数组成集合 LoadInfo loadInfo = downloader.getDownloaderInfors(); // 显示进度条 showProgress(loadInfo, urlstr, v); // 调用方法开始下载 downloader.download(); } /** * 显示进度条 */ private void showProgress(LoadInfo loadInfo, String url, View v) { ProgressBar bar = ProgressBars.get(url); if (bar == null) { bar = new ProgressBar(this, null, android.R.attr.progressBarStyleHorizontal); bar.setMax(loadInfo.getFileSize()); bar.setProgress(loadInfo.getComplete()); System.out.println(loadInfo.getFileSize()+"--"+loadInfo.getComplete()); ProgressBars.put(url, bar); LinearLayout.LayoutParams params = new LayoutParams( LayoutParams.FILL_PARENT, 5); ((LinearLayout) ((LinearLayout) v.getParent()).getParent()) .addView(bar, params); } } /** * 响应暂停下载按钮的点击事件 */ public void pauseDownload(View v) { LinearLayout layout = (LinearLayout) v.getParent(); String musicName = ((TextView) layout .findViewById(R.id.tv_resouce_name)).getText().toString(); String urlstr = URL + musicName; downloaders.get(urlstr).pause(); }}???? 最后我们需要在android Manifest.xml中添加这两个权限一个是访问internet的权限，另一个是写外存的权限。
<uses-permission android:name="android.permission.INTERNET"></uses-permission> <uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE"></uses-permission>
???? 万事俱备只欠东风啦！哈哈，我需要在我的web工程里放几个音乐文件。
???下载后得到的文件可在sdcard下面找到！