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;
  }

  /**
   * @return the threadid
   */
  public int getthreadid() {
    return threadid;
  }

  /**
   * @param threadid the threadid to set
   */
  public void setthreadid(int threadid) {
    this.threadid = threadid;
  }

  /**
   * @return the startpos
   */
  public int getstartpos() {
    return startpos;
  }

  /**
   * @param startpos the startpos to set
   */
  public void setstartpos(int startpos) {
    this.startpos = startpos;
  }

  /**
   * @return the endpos
   */
  public int getendpos() {
    return endpos;
  }

  /**
   * @param endpos the endpos to set
   */
  public void setendpos(int endpos) {
    this.endpos = endpos;
  }

  /**
   * @return the completesize
   */
  public int getcompletesize() {
    return completesize;
  }

  /**
   * @param completesize the completesize to set
   */
  public void setcompletesize(int completesize) {
    this.completesize = completesize;
  }

  /**
   * @return the url
   */
  public string geturl() {
    return url;
  }

  /**
   * @param url the url to set
   */
  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;
  }  
  /**
   * @return the filesize
   */
  public int getfilesize() {
    return filesize;
  }
  /**
   * @param filesize the filesize to set
   */
  public void setfilesize(int filesize) {
    this.filesize = filesize;
  }
  /**
   * @return the completesize
   */
  public int getcompletesize() {
    return completesize;
  }

  /**
   * @param completesize the completesize to set
   */
  public void setcompletesize(int completesize) {
    this.completesize = completesize;
  }

  /**
   * @return the url
   */
  public string geturl() {
    return url;
  }

  /**
   * @param url the url to set
   */
  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。