Android实现断点下载的方法

最近做的项目中需要实现断点下载，即用户一次下载可以分多次进行，下载过程可以中断，在目前大多数的带离线缓存的软件都是需要实现这一功能。本文阐述了通过sqlite3简单实现了一个具有断点下载功能的demo。言归正传，开始正文。

设计

数据库表存储元数据
DBHelper.java
用于业务存储的Dao
Dao.java
抽象下载信息的Bean
LoadInfo.java
呈现下载信息View
MainActivity.java
存储下载信息Bean
DownloadInfo.java
封装好的下载类
Downloader.java

代码结构

具体实现

下载信息类：DownloadInfo.java

这里的代码很简单，就是一个用来保存下载信息的类，很简单，没啥好说的，具体看注释

package entity;
public class DownloadInfo {
  private int threadId;//线程ID
  private int startPos;//下载起始位置
  private int endPos;//下载结束位置
  private int completeSize;//下载完成量
  private String url;//资源URL
  public DownloadInfo(int tId,int sp, int ep,int cSize,String address){
    threadId=tId;
    startPos=sp;
    endPos=ep;
    completeSize = cSize;
    url=address;
  }
  
  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 getCompleteSize() {
    return completeSize;
  }
  
  public void setCompleteSize(int completeSize) {
    this.completeSize = completeSize;
  }
  
  public String getUrl() {
    return url;
  }
  
  public void setUrl(String url) {
    this.url = url;
  }
  @Override
  public String toString() {
    // TODO Auto-generated method stub
    return "threadId:"+threadId+",startPos:"+startPos+",endPos："+endPos+",completeSize:"+completeSize+",url:"+url;
  }
}

数据库 DBHelper.java

package db;
import android.content.Context;
import android.database.sqlite.SQLiteDatabase;
import android.database.sqlite.SQLiteOpenHelper;
public class DBHelper extends SQLiteOpenHelper {
  String sql ="create table download_info (id integer PRIMARY KEY AUTOINCREMENT,"
      + "thread_id integer,"
      + "start_pos integer,"
      + "end_pos integer,"
      + "complete_size integer,"
      + "url char)";
  public DBHelper(Context context) {
    // TODO Auto-generated constructor stub
    super(context, "download.db", null, 1);
  }
  @Override
  public void onCreate(SQLiteDatabase db) {
    // TODO Auto-generated method stub
    db.execSQL(sql);
  }
  @Override
  public void onUpgrade(SQLiteDatabase db, int oldVersion, int newVersion) {
    // TODO Auto-generated method stub
  }
}

数据库业务管理类 Dao.java

package db;
import java.util.ArrayList;
import java.util.List;
import android.content.Context;
import android.database.Cursor;
import android.database.sqlite.SQLiteDatabase;
import entity.DownloadInfo;
public class Dao {
  private DBHelper dbHelper;
  public Dao(Context context){
    dbHelper = new DBHelper(context);
  }
  public boolean isNewTask(String url){
    SQLiteDatabase db = dbHelper.getReadableDatabase();
    String sql = "select count(*) from download_info where url=?";
    Cursor cursor = db.rawQuery(sql, new String[]{url});
    cursor.moveToFirst();
    int count = cursor.getInt(0);
    cursor.close();
    return count == 0;
  }
  public void saveInfo(List<DownloadInfo> infos){
    SQLiteDatabase db = dbHelper.getWritableDatabase();
    String sql = "insert into download_info(thread_id,start_pos,end_pos,complete_size,url) values(?,?,?,?,?)";
    Object[] bindArgs= null;
    for(DownloadInfo info:infos){
      bindArgs=new Object[]{info.getThreadId(),info.getStartPos(),info.getEndPos(),info.getCompleteSize(),info.getUrl()};
      db.execSQL(sql, bindArgs);
    }
  }
  public List<DownloadInfo> getInfo(String url){
    SQLiteDatabase db = dbHelper.getReadableDatabase();
    List<DownloadInfo> infos = new ArrayList<DownloadInfo>();
    String sql ="select thread_id,start_pos,end_pos,complete_size,url from download_info where url=?";
    Cursor cursor=db.rawQuery(sql, new String[]{url});
    while(cursor.moveToNext()){
      DownloadInfo info = new DownloadInfo(cursor.getInt(0), cursor.getInt(1), cursor.getInt(2), cursor.getInt(3),cursor.getString(4));
      infos.add(info);
    }
    cursor.close();
    return infos;
  }
  public void deleteInfo(String url){
    SQLiteDatabase db = dbHelper.getWritableDatabase();
    db.delete("download_info", "url=?", new String[]{url});
    db.close();
  }
  public void updateInfo(int completeSize,int threadId,String url){
    SQLiteDatabase db = dbHelper.getWritableDatabase();
    String sql ="update download_info set complete_size=? where thread_id=? and url=?";
    db.execSQL(sql, new Object[]{completeSize,threadId,url});
  }
  public void closeDB(){
    dbHelper.close();
  }
}

当前状态保存类 LoadInfo.java

package entity;
public class LoadInfo {
  private int fileSize;
  private int completeSize;  
  private String url;
  public LoadInfo(int fs,int cSize,String address){    
    fileSize=fs;    
    completeSize = cSize;      
    url=address;
  }  
  
  public int getFileSize() {
    return fileSize;
  }
  
  public void setFileSize(int fileSize) {
    this.fileSize = fileSize;
  }
  
  public int getCompleteSize() {
    return completeSize;
  }
  
  public void setCompleteSize(int completeSize) {
    this.completeSize = completeSize;
  }
  
  public String getUrl() {
    return url;
  }
  
  public void setUrl(String url) {
    this.url = url;
  }
}

下载助手类：Downloader.java

package com.winton.downloadmanager;
import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.ArrayList;
import java.util.List;
import android.content.Context;
import android.os.Handler;
import android.os.Message;
import db.Dao;
import entity.DownloadInfo;
import entity.LoadInfo;
public class Downloader {
  private String url;
  private String localPath;
  private int threadCount;
  private Handler mHanler;
  private Dao dao;
  private int fileSize;
  private List<DownloadInfo> infos;
  private final static int INIT = 1;
  private final static int DOWNLOADING =2;
  private final static int PAUSE =3;
  private int state = INIT;
  public Downloader(String address,String lPath,int thCount,Context context,Handler handler){
    url =address;
    localPath = lPath;
    threadCount = thCount;
    mHanler = handler;
    dao = new Dao(context);
  }
  public boolean isDownloading(){
    return state == DOWNLOADING;
  }
  public LoadInfo getDownLoadInfos(){
    if(isFirstDownload(url)){
      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, url);
        infos.add(info);
      }
      dao.saveInfo(infos);
      return new LoadInfo(fileSize, 0, url);
    }else{
      infos = dao.getInfo(url);
      int size = 0;
      int completeSize =0;
      for(DownloadInfo info:infos){
        completeSize += info.getCompleteSize();
        size += info.getEndPos()-info.getStartPos()+1;
      }
      return new LoadInfo(size, completeSize, url);
    }
  }
  public boolean isFirstDownload(String url){
    return dao.isNewTask(url);
  }
  public void init(){
    try {
      URL mUrl = new URL(this.url);
      HttpURLConnection connection = (HttpURLConnection)mUrl.openConnection();
      connection.setConnectTimeout(5000);
      connection.setRequestMethod("GET");
      fileSize = connection.getContentLength();
      File file = new File(localPath);
      if(!file.exists()){
        file.createNewFile();
      }
      RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");
      accessFile.setLength(fileSize);
      accessFile.close();
      connection.disconnect();
    } catch (MalformedURLException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } catch (IOException e) {
      // TODO Auto-generated catch block
      e.printStackTrace();
    } 
  }
  public void download(){
    if(infos != null){
      if(state ==DOWNLOADING){
        return;
      }
      state = DOWNLOADING;
      for(DownloadInfo info:infos){
        new DownloadThread(info.getThreadId(), info.getStartPos(), info.getEndPos(), info.getCompleteSize(), info.getUrl()).start();
      }
    }
  }
  class DownloadThread extends Thread{
    private int threadId;
    private int startPos;
    private int endPos;
    private int completeSize;
    private String url;
    public DownloadThread(int tId,int sp,int ep,int cSize,String address) {
      // TODO Auto-generated constructor stub
      threadId=tId;
      startPos=sp;
      endPos = ep;
      completeSize = cSize;
      url = address;
    }
    @Override
    public void run() {
      // TODO Auto-generated method stub
      HttpURLConnection connection = null;
      RandomAccessFile randomAccessFile = null;
      InputStream is = null;
      try {
        URL mUrl = new URL(url);
        connection = (HttpURLConnection)mUrl.openConnection();
        connection.setConnectTimeout(5000);
        connection.setRequestMethod("GET");
        connection.setRequestProperty("Range", "bytes="+(startPos+completeSize)+"-"+endPos);
        randomAccessFile = new RandomAccessFile(localPath, "rwd");
        randomAccessFile.seek(startPos+completeSize);
        is=connection.getInputStream();
        byte[] buffer = new byte[4096];
        int length =-1;
        while((length=is.read(buffer)) != -1){
          randomAccessFile.write(buffer, 0, length);
          completeSize +=length;
          dao.updateInfo(threadId, completeSize, url);
          Message msg = Message.obtain();
          msg.what=1;
          msg.obj=url;
          msg.arg1=length;
          mHanler.sendMessage(msg);
          if(state==PAUSE){
            return;
          }
        }
      } catch (MalformedURLException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      } catch (IOException e) {
        // TODO Auto-generated catch block
        e.printStackTrace();
      }finally{
        try {
          is.close();
          randomAccessFile.close();
          connection.disconnect();
          dao.closeDB();
        } catch (IOException e) {
          // TODO Auto-generated catch block
          e.printStackTrace();
        }
      }
    }
  }
  public void delete(String url){
    dao.deleteInfo(url);
  }
  public void reset(){
    state=INIT;
  }
  public void pause(){
    state=PAUSE;
  }
}

View呈现类：MainActivity.java

package com.winton.downloadmanager;
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.view.View.OnClickListener;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;
import android.widget.Toast;
import entity.LoadInfo;
public class MainActivity extends Activity implements OnClickListener{
  private TextView name;
  private ProgressBar process;
  private Button start,stop;
  private Downloader downloader;
  //处理下载进度UI的
  Handler handler = new Handler(){
    public void handleMessage(android.os.Message msg) {
      if(msg.what==1){
        name.setText((String)msg.obj);
        int lenght = msg.arg1;
        process.incrementProgressBy(lenght);
      }
      if(msg.what==2){
        int max =msg.arg1;
        process.setMax(max);
        Toast.makeText(getApplicationContext(), max+"", 1).show();
      }
    };
  };
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    name=(TextView)findViewById(R.id.tv_name);
    process=(ProgressBar)findViewById(R.id.pb_download);
    start = (Button)findViewById(R.id.bt_start);
    stop = (Button)findViewById(R.id.bt_stop);
    start.setOnClickListener(this);
    stop.setOnClickListener(this);
    downloader=new Downloader("http://img4.duitang.com/uploads/item/201206/11/20120611174542_5KRMj.jpeg", Environment.getExternalStorageDirectory().getPath()+"/test1.jpg", 1, getApplicationContext(), handler);
  }
  @Override
  public void onClick(View v) {
    // TODO Auto-generated method stub
    if(v==start){
      new Thread(new Runnable() {
        @Override
        public void run() {
          // TODO Auto-generated method stub
          LoadInfo loadInfo = downloader.getDownLoadInfos();
          Message msg =handler.obtainMessage();
          msg.what=2;
          msg.arg1=loadInfo.getFileSize();
          handler.sendMessage(msg);
          downloader.download();
        }
      }).start();
      return;
    }
    if(v==stop){
      downloader.pause();
      return;
    }
  }  
}

运行效果

总体比较简单，基本实现了 断点下载的功能，而且开启了多个线程去实现分块下载，对初学的同学有一定的帮助。

这些代码我也是从网上各种搜集而来，自己亲自动手敲了一遍，并做了一些小的改动，对整个断点下载的过程有了一个深刻的认识。因此平时要多敲代码，善于总结，必能有所收获。

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