Android原生实现多线程断点下载实例代码

各位父老乡亲，我单汉三又回来了，今天为大家带来一个用原生的安卓写的多线程断点下载demo。

通过本文你可以学习到：

1. sqlite的基本使用，数据库的增删改查。
2. handler的消息处理与更新ui。
3. service（主要用于下载）的进阶与使用。
4. 原生的json文件解析（多层嵌套）。
5. randomaccessfile的基本使用，可以将文件分段。
6. 基于httpurlconnection的大文件下载。
7. 上面内容结合，实现多线程，断点下载。
   

demo是在tv上运行的，图片显示的问题不要纠结了。

文件下载的demo已完成，没时间上传与讲解，今天为您展示并讲解一下，纯原生的东西来下载文件，希望可以帮你理解更多安卓比较基础的问题。

我们的思路：建立一个数据库，两个表，一个用来保存网络数据，一个保存本地下载的进度等等。在点击下载按钮的时候启动downloadservice，进行比对之后下载

先看一下demo的目录结构：

所有的步骤在代码里有非常详细的讲解，一定要看代码（下面是抽取的几个重要的类讲解）！

数据库的建立与dao

/**
 * created by administrator on 2017/3/6 0006.
 */
public class downloaddbhelper extends sqliteopenhelper {
  /**
   * downloaddbhelper用于创建数据库，如果不会使用原生的建库的话
   *
   * 跟随小司机我的脚步来一起练一练
   * 建两个表：
   * download_info表存储下载信息
   * localdownload_info表存储本地下载信息
   * 之后对比两个表进行继续下载等等
   */
  public static string database_name = "downloadfiles.db";
  public static string table_download_info = "download_info";
  public static string table_localdownload_info = "localdownload_info";
  private static int version = 1;
  public downloaddbhelper(context context) {
    super(context, database_name, null, version);
  }
  @override
  public void oncreate(sqlitedatabase db) {
    /*在此进行创建数据库和表格,来一起动手写一遍,就是两个sqlite语句*/
    db.execsql("create table " + table_download_info + "(" + "id integer primary key autoincrement," +
        "thread_id integer," + "start_position integer," + "end_position integer," + " completed_size integer," + "url varchar(100))");
    db.execsql("create table " + table_localdownload_info + "(" + "id integer primary key autoincrement," + "name varchar(50)," +
        "url varchar(100)," + "completedsize integer," + "filesize integer," + "status integer)");
  }
  @override
  public void onupgrade(sqlitedatabase db, int oldversion, int newversion) {
    /*数据库更新升级，检测到版本的变化，发现版本号不一样，就会自动调用onupgrade函数
    * 新版本号和老版本号都会作为onupgrade函数的参数传进来，便于开发者知道数据库应该从哪个版本升级到哪个版本。
    * */
    string sql = "drop table if exists " + table_download_info + "";
    string sqlone = "drop table if exists " + table_localdownload_info + "";
    db.execsql(sql);
    db.execsql(sqlone);
    oncreate(db);//删除数据库，重新创建。这里只是简单的，并没有添加或者减少数据库中的其他字段
  }
}

dao对数据库进行增删改查

/**
 * created by shancancan on 2017/3/6 0006.
 */
public class dao {
  /*
  * dao层主要是做数据持久层的工作，负责与数据库进行联络的一些任务都封装在此。
  * dao层所定义的接口里的方法都大同小异，这是由我们在dao层对数据库访问的操作来决定的，
  * 对数据库的操作，我们基本要用到的就是新增，更新，删除，查询等方法。
  * 因而dao层里面基本上都应该要涵盖这些方法对应的操作。
  * */
  private static dao dao;
  private static downloaddbhelper dbhelper;
  public static final byte[] lock = new byte[0]; //新建两个字节作为对象锁
  public static final byte[] file_lock = new byte[0];
  public dao() {//空构造方法，
  }
  public static synchronized dao getinstance(context context) {//本demo用单例模式中的懒汉模式+线程不安全 线程安全的代价是效率变低
    if (dao == null) {
      dao = new dao();
      dbhelper = new downloaddbhelper(context);
    }
    return dao;
  }
  /* public static synchronized dao getinstance(context context) {//本demo用单例模式中的懒汉模式+线程安全 线程安全的代价是效率变低，99%情况下不需要同步
    if (dao == null) { //你可以在这两个方法中随便选择一个
      dao = new dao();
      dbhelper = new downloaddbhelper(context);
    }
    return dao;
  }*/
  /***************************************  下方dao层中对数据库的增、删、改、查  *********************************************************/
  /**
   * 检查本地下载记录，是否下载过
   *
   * @param url
   * @return
   */
  public boolean isexist(string url) {
    sqlitedatabase database = dbhelper.getreadabledatabase(); //获取本app所创建的数据库
    string sql = "select count(*) from " + table_localdownload_info + " where url=?"; //查询语句,查询总共有多少条的语句
    cursor cursor = database.rawquery(sql, new string[]{url});
    /**
     *
     * @cursor
     * cursor 是每行的集合。
     * 使用 movetofirst() 定位第一行。
     * 你必须知道每一列的名称。
     * 你必须知道每一列的数据类型。
     * cursor 是一个随机的数据源。
     * 所有的数据都是通过下标取得。
     * cursor按照我的理解就是一个箭头，指到哪一行就是那一行的集合
     * 比较重要的方法有：close(),movetofirst(),movetonext(),movetolast(),movetoprevious(),getcolumncount()等。
     *
     * @rawquery
     * rawquery是直接使用sql语句进行查询的，也就是第一个参数字符串，
     * 在字符串内的“？”会被后面的string[]数组逐一对换掉
     * cursor用完之后要关闭，cursor用完之后要关闭，cursor用完之后要关闭。重要的事情说三遍！！！
     *
     * */
    cursor.movetofirst();
    int count = cursor.getint(0);
    cursor.close();
    return count > 0;
  }
  /**
   * 是否为首次下载
   *
   * @param url
   * @return
   */
  public boolean isfirstdownload(string url) {
    sqlitedatabase database = dbhelper.getreadabledatabase();
    string sql = "select count(*) from " + table_download_info + " where url=?";
    cursor cursor = database.rawquery(sql, new string[]{url});
    cursor.movetofirst();
    int count = cursor.getint(0);
    cursor.close();
    return count == 0;
  }
  /**
   * 保存下载的具体信息 保存所下载的list集合中的数据
   *
   * @param infos
   * @param context
   */
  public void saveinfos(list<downloadinfo> infos, context context) {
    /**
     * 事务（transaction）是并发控制的单位，是用户定义的一个操作序列。
     * 这些操作要么都做，要么都不做，是一个不可分割的工作单位。
     * 通过事务，sql server能将逻辑相关的一组操作绑定在一起，
     * 以便保持数据的完整性。
     *
     * 事务具有四个特征：原子性（ atomicity ）、一致性（ consistency ）、
     * 隔离性（ isolation ）和持续性（ durability ）。这四个特性简称为 acid 特性。
     *
     * */
    synchronized (lock) {
      sqlitedatabase database = dbhelper.getwritabledatabase();
      database.begintransaction();//开启事务
      try {//如果有异常，在这里捕获
        for (downloadinfo info : infos) {//for循环将数据存入数据库
          string sql = "insert into " + table_download_info + "(thread_id,start_position, end_position, completed_size, url) values (?,?,?,?,?)";
          object[] bindargs = {info.getthreadid(), info.getstartposition(), info.getendposition(), info.getcompletedsize(), info.geturl()};
          database.execsql(sql, bindargs);
        }
        database.settransactionsuccessful();//结束事务
      } catch (sqlexception e) {
        e.printstacktrace();
      } finally {
        database.endtransaction();//关闭事务
      }
    }
  }
  /**
   * 得到下载具体信息
   *
   * @param urlstr
   * @return list<downloadinfo> 一个下载器信息集合器,里面存放了每条线程的下载信息
   */
  public list<downloadinfo> getinfos(string urlstr) {
    list<downloadinfo> list = new arraylist<downloadinfo>();
    sqlitedatabase database = dbhelper.getreadabledatabase();
    string sql = "select thread_id, start_position, end_position, completed_size, url from " + table_download_info + " where url=?";
    cursor cursor = database.rawquery(sql, new string[]{urlstr});
    while (cursor.movetonext()) {//通过cursor取到下载器信息，循环遍历，得到下载器集合
      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;
  }
  /**
   * 本地下载列表添加记录,添加本地数据库信息，完成度等等
   *
   * @param filestatus
   **/
  public void insertfilestatus(filestatus filestatus) {
    synchronized (file_lock) {//异步加开启事务，保证数据的完整性
      sqlitedatabase database = dbhelper.getwritabledatabase();
      database.begintransaction();
      try {
        string sql = "insert into " + table_localdownload_info + " (name,url,completedsize,filesize,status) values(?,?,?,?,?)";
        object[] bindargs = {filestatus.getname(), filestatus.geturl(), filestatus.getcompletedsize(), filestatus.getfilesize(), filestatus.getstatus()};
        database.execsql(sql, bindargs);
        database.settransactionsuccessful();
      } catch (sqlexception e) {
        e.printstacktrace();
      } finally {
        database.endtransaction();
      }
    }
  }
  /**
   * @param context
   * @param compeletedsize
   * @param threadid
   * @param urlstr     这里是更新数据库，建议在保存一个表格的时候就对另一个表格数据库进行更新
   */
  public void updatainfos(int threadid, int compeletedsize, string urlstr, context context) {
    synchronized (lock) {
      string sql = "update " + table_download_info + "set completed_size = ? where thread_id =? and url=?";
      string localsql = "update " + table_localdownload_info + "set completedsize = (select sum(completed_size) from " +
          table_download_info + "where url=? group by url ) where url=?";
      object[] bindargs = {compeletedsize, threadid, urlstr};
      object[] localargs = {urlstr, urlstr};
      sqlitedatabase database = dbhelper.getwritabledatabase();
      database.begintransaction();
      try {
        database.execsql(sql, bindargs);
        database.execsql(localsql, localargs);
        database.settransactionsuccessful();
      } catch (sqlexception e) {
        e.printstacktrace();
      } finally {
        database.endtransaction();
      }
    }
  }
  /**
   * @param url 更新文件的状态，0为正在下载，1为已经下载完成,2为下载出错
   **/
  public void updatefilestatus(string url) {
    synchronized (file_lock) {
      string sql = "update " + table_localdownload_info + " set status = ? where url = ?";
      object[] bindargs = {1, url};
      sqlitedatabase database = dbhelper.getwritabledatabase();
      database.begintransaction();
      try {
        database.execsql(sql, bindargs);
        database.settransactionsuccessful();
      } catch (sqlexception e) {
        e.printstacktrace();
      } finally {
        database.endtransaction();
      }
    }
  }
  /**
   * @return list<filestatus>
   * 取出本地下载列表数据，如在重新进入应用时，要重新把进度之类的设置好
   **/
  public list<filestatus> getfilestatus() {
    list<filestatus> list = new arraylist<filestatus>();
    sqlitedatabase database = dbhelper.getreadabledatabase();
    //string sql = "slect * from " + table_localdownload_info + ""; //不能用，需要哪些条件就在语句中写出哪些条件
    string sql = "select name, url, status, completedsize, filesize from " + table_localdownload_info + "";
    cursor cursor = database.rawquery(sql, null);
    while (cursor.movetonext()) {
      filestatus filestate = new filestatus(cursor.getstring(0), cursor.getstring(1), cursor.getint(2), cursor.getint(3), cursor.getint(4));
      list.add(filestate);
    }
    cursor.close();
    return list;
  }
  /**
   * @param url
   * @param completesize
   * @param status    更新文件的下载状态
   **/
  public void updatefiledownstatus(int completesize, int status, string url) {
    synchronized (file_lock) {
      string sql = "update " + table_localdownload_info + " set completedsize = ?,status = ? where url = ?";
      sqlitedatabase database = dbhelper.getwritabledatabase();
      database.begintransaction();
      try {
        object[] bindargs = {completesize, status, url};
        database.execsql(sql, bindargs);
        database.delete(table_download_info, "url = ?", new string[]{url});
        database.settransactionsuccessful();
      } catch (sqlexception e) {
        e.printstacktrace();
      } finally {
        database.endtransaction();
      }
    }
  }
  /**
   * @param url 获取文件名称
   **/
  public string getfilename(string url) {
    string result = "";
    string sql = "select name from " + table_localdownload_info + " where url = ?";
    sqlitedatabase database = dbhelper.getreadabledatabase();
    cursor cursor = database.rawquery(sql, new string[]{url});
    if (cursor.movetonext()) {
      result = cursor.getstring(0);
    }
    cursor.close();
    return result;
  }
  /**
   * 删除文件之后，要删除下载的数据，一个是用户可以重新下载
   * 另一个是表再次添加一条数据的时候不出现错误
   *
   * @param url
   */
  public void deletefile(string url) {
    sqlitedatabase database = dbhelper.getwritabledatabase();
    database.begintransaction();
    try {
      database.delete(table_download_info, " url = ?", new string[]{url});
      database.delete(table_localdownload_info, " url = ?", new string[]{url});
      database.settransactionsuccessful();
    } catch (exception e) {
      e.printstacktrace();
    } finally {
      database.endtransaction();
    }
  }
  /**
   * 关闭数据库
   *
   * @close
   */
  public void closedb() {
    dbhelper.close();
  }
}

downloadservice 主要代码

@suppresslint("handlerleak")
public class downloadservice extends service
{
 public ibinder binder = new mybinder();
 public class mybinder extends binder
 {
 public downloadservice getservice()
 {
  return downloadservice.this;
 }
 }
 @override
 public ibinder onbind(intent intent)
 {
 return binder;
 }
 public static int number = 0;
 // 文件保存地址
 public final string savepath = "/mnt/sdcard/multifiledownload/";
 // 存放下载列表的引用
 public static list<filestatus> list = new arraylist<filestatus>();
 public static map<string, string> localdownlist = new hashmap<string, string>();
 // 保存每个文件下载的下载器
 public static map<string, downloader> downloaders = new hashmap<string, downloader>();
 // 每个下载文件完成的长度
 private map<string, integer> completesizes = new hashmap<string, integer>();
 // 每个下载文件的总长度
 private map<string, integer> filesizes = new hashmap<string, integer>();
 private downloader downloader;
 private int threadcount = 5;
 private dao dao;
 private downloadcallback loadcallback;
 
 private filestatus mfilestatus = null;
 private handler handler = new handler()
 {
 @override
 public void handlemessage(message msg)
 {
  super.handlemessage(msg);
  if (msg.what == 1)
  {
  string url = (string) msg.obj;
  int length = msg.arg1;
  int completesize = completesizes.get(url);
  int filesize = filesizes.get(url);
  completesize += length;
  completesizes.put(url, completesize);
  synchronized (list)
  {
   for (int i = 0; i < list.size(); i++)
   {
   filestatus filestatus = list.get(i);
   if (filestatus.geturl().equals(url))
   {
    if (completesize == filestatus.getfilesize())
    {
    system.out.println("-----------下载完成:"+filestatus.getname()+":"+completesize+"-----"+filestatus.getfilesize());
    list.set(i, new filestatus(filestatus.getname(), filestatus.geturl(), 1, completesize, filestatus.getfilesize()));
    dao.updatefiledownstatus(completesize, 1, url);
    }
    else
    {
    list.set(i, new filestatus(filestatus.getname(), filestatus.geturl(), 0, completesize, filestatus.getfilesize()));
    }
    mfilestatus = list.get(i);
   }
   }
   
   this.postdelayed(new runnable()
   {
   @override
   public void run()
   {
    if (loadcallback != null && mfilestatus != null)
    {
    loadcallback.refreshui(mfilestatus);
    }
   }
   }, 1000);
  }
  
  }
 }
 };
 @override
 public void oncreate()
 {
 super.oncreate();
 dao = dao.getinstance(this);
 list = dao.getfilestatus();
 for (filestatus filestatus : list)
 {
  localdownlist.put(filestatus.geturl(), filestatus.geturl());
 }
 
 timer timer = new timer();
 timer.schedule(new timertask()
 {
  @override
  public void run()
  {
  number++;
  }
 }, 0, 1000);
 }
 public void download(final button button, final string url, final string name, final handler mhandler)
 {
 if (dao.isexist(url))
 {
  toast.maketext(this, "别点了，已经在下载了", toast.length_short).show();
  return;
 }
 final string filename = name + url.substring(url.lastindexof("."));
 new thread(new runnable()
 {
  @override
  public void run()
  {
  downloader = downloaders.get(url);
  if (downloader == null)
  {
   downloader = new downloader(url, savepath, filename, threadcount, downloadservice.this, handler);
   downloaders.put(url, downloader);
  }
  if (downloader.isdownloading())
   return;
  
  loadinfo loadinfo = downloader.getdownloaderinfors();
  
  if(loadinfo != null)
  {
   filestatus filestatus = new filestatus(filename, url, 0, loadinfo.getcomplete(), loadinfo.getfilesize());
   dao.insertfilestatus(filestatus);
   completesizes.put(url, loadinfo.getcomplete());
   filesizes.put(url, filestatus.getfilesize());
   list.add(filestatus);
   localdownlist.put(url, url);
   downloader.download();
   message msg = new message();
   msg.what = 1;
   msg.obj = button;
   mhandler.sendmessage(msg);
  }
  else
  {
   message msg = new message();
   msg.what = 2;
   msg.obj = button;
   mhandler.sendmessage(msg);
  }
  }
 }).start();
 }
 
 //暂停下载
 public void pause(downloader downloader)
 {
 downloader.pause();
 }
 
 //继续下载
 public void redownload(final button button, final string url, final string name, final handler mhandler)
 {
 new thread(new runnable()
 {
  @override
  public void run()
  {
  string filename = dao.getfilename(url);
  
  downloader = downloaders.get(url);
  if (downloader == null)
  {
   downloader = new downloader(url, savepath, filename, threadcount, downloadservice.this, handler);
   downloaders.put(url, downloader);
  }
  if (downloader.isdownloading())
   return;
  
  loadinfo loadinfo = downloader.getdownloaderinfors();
  
  if(loadinfo != null && !filename.equals(""))
  {
   if(!completesizes.containskey(url))
   {
   completesizes.put(url, loadinfo.getcomplete());
   }
   if(!filesizes.containskey(url))
   {
   filesizes.put(url, loadinfo.getfilesize());
   }
   downloader.download();
   message msg = new message();
   msg.what = 1;
   msg.obj = button;
   mhandler.sendmessage(msg);
  }
  else
  {
   message msg = new message();
   msg.what = 2;
   msg.obj = button;
   mhandler.sendmessage(msg);
  }
  }
 }).start();
 }
 
 public void delete(final string url)
 {
 downloader down = downloaders.get(url);
 if(down != null)
 {
  down.pause();
 }
 
 handler.postdelayed(new runnable()
 {
  @override
  public void run()
  {
  dao.deletefile(url);
  
  for (int i = 0; i < list.size(); i++)
  {
   filestatus filestatus = list.get(i);
   if (filestatus.geturl().equals(url))
   {
   list.remove(i);
   }
  }
  
  localdownlist.remove(url);
  downloaders.remove(url);
  completesizes.remove(url);
  filesizes.remove(url);
  
  if(loadcallback != null)
  {
   loadcallback.deletefile(url);
  }
  }
 }, 1000);
 }
 public interface downloadcallback
 {
 public void refreshui(filestatus filestatus);
 
 public void deletefile(string url);
 }
 public void setloadcallback(downloadcallback loadcallback)
 {
 this.loadcallback = loadcallback;
 }
}

下载工具类downloadutil

/**
 * created by shancancan on 2017/3/7 0007.
 */
public class downloadutil {
  /**
   * 此类的主要功能
   * 1、检查是否下载
   * 2、下载文件,文件的下载采用httpurlconnection
   */
  private string downpath;// 下载路径
  private string savepath;// 保存路径
  private string filename;// 文件名称
  private int threadcount;// 线程数
  private handler mhandler;
  private dao dao;
  private context context;
  private int filesize;// 文件大小
  private int range;
  private list<downloadinfo> infos;// 存放下载信息类的集合
  private int state = init;
  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;
  /**
   * 构造方法，获取dao的对象
   *
   * @param downpath
   * @param savepath
   * @param filename
   * @param threadcount
   * @param context
   * @param mhandler
   */
  public downloadutil(string downpath, string savepath, string filename, int threadcount, handler mhandler, context context) {
    this.downpath = downpath;
    this.savepath = savepath;
    this.filename = filename;
    this.threadcount = threadcount;
    this.mhandler = mhandler;
    this.context = context;
    dao = dao.getinstance(context);
  }
  /**
   * 判断是否pause
   **/
  public boolean ispause() {
    return state == pause;
  }
  /**
   * 判断是否downloading
   */
  public boolean isdownloading() {
    return state == downloading;
  }
  /**
   * @param url 判断是否是第一次下载，利用dao查询数据库中是否有下载这个地址的记录
   */
  private boolean isfirst(string url) {
    return dao.isfirstdownload(url);
  }
  /**
   * 获取要下载的东西
   */
  public loaditeminfo getdownloadinfos() {
    if (isfirst(downpath)) {
      if (initfirst()) {//如果是第一次下载的话，要进行初始化,1.获得下载文件的长度 2.创建文件,设置文件的大小
        range = this.filesize / this.threadcount;
        infos = new arraylist<downloadinfo>();
        //这里就是启动多线程下载，看出来了吗？配合randomaccessfile。每一个downloadinfo就是randomaccessfile文件的一部分
        for (int i = 0; i < this.threadcount - 1; i++) {
          downloadinfo info = new downloadinfo(i, i * range, (i + 1) * range - 1, 0, downpath);
          infos.add(info);
        }
        downloadinfo info = new downloadinfo(this.threadcount - 1, (this.threadcount - 1) * range, this.filesize, 0, downpath);
        infos.add(info);
        dao.saveinfos(infos, this.context);
        //(string urldownload, int completepercent, int filesize)
        loaditeminfo loadinfo = new loaditeminfo(this.downpath, 0, this.filesize);
        return loadinfo;
      } else {
        return null;
      }
    } else {
      //不是第一次下载，我们应该怎么做呢？从数据库里面取回来
      infos = dao.getinfos(this.downpath);
      if (infos != null && infos.size() > 0) {
        int size = 0;
        int completesize = 0;
        for (downloadinfo info : infos) {
          completesize += info.getcompletedsize();
          size += info.getendposition() - info.getstartposition() + this.threadcount - 1;
        }
        loaditeminfo loadinfo = new loaditeminfo(this.downpath, completesize, size);
        return loadinfo;
      } else {
        return null;
      }
    }
  }
  // 设置暂停
  public void pause() {
    state = pause;
  }
  // 重置下载状态,将下载状态设置为init初始化状态
  public void reset() {
    state = init;
  }
  /**
   * 基本上，randomaccessfile的工作方式是，把datainputstream和dataoutputstream结合起来，再加上它自己的一些方法，
   * 比如定位用的getfilepointer( )，在文件里移动用的seek( )，以及判断文件大小的length( )、skipbytes()跳过多少字节数。
   * 此外，它的构造函数还要一个表示以只读方式("r")，还是以读写方式("rw")打开文件的参数 (和c的fopen( )一模一样)。它不支持只写文件。
   */
  private boolean initfirst() {
    boolean result = true;
    httpurlconnection conn = null;
    randomaccessfile randomfile = null;
    url url = null;
    try {
      url = new url(downpath);
      conn = (httpurlconnection) url.openconnection();
      conn.setconnecttimeout(5 * 1000);
      conn.setrequestmethod("get");
      // 如果http返回的代码是200或者206则为连接成功
      if (conn.getresponsecode() == 200 || conn.getresponsecode() == 206) //状态码(206)，表示服务器已经执行完部分对资源的get请求
      {
        filesize = conn.getcontentlength();// 得到文件的大小
        if (filesize <= 0) {
          //("网络故障,无法获取文件大小");
          return false;
        }
        file dir = new file(savepath);
        // 如果文件目录不存在,则创建
        if (!dir.exists()) {
          if (dir.mkdirs()) {
            //("mkdirs success.");
          }
        }
        file file = new file(this.savepath, this.filename);
        randomfile = new randomaccessfile(file, "rwd");
        randomfile.setlength(filesize);// 设置保存文件的大小
        randomfile.close();
        conn.disconnect();
      }
    } catch (exception e) {
      e.printstacktrace();
      result = false;
    } finally {
      if (randomfile != null) {
        try {
          randomfile.close();
        } catch (ioexception e) {
          e.printstacktrace();
        }
      }
      if (conn != null) {
        conn.disconnect();
      }
    }
    return result;
  }
  /**
   * 下面的这个方法就是开启多线程进行下载了数据了
   */
  public void download() {
    if (infos != null) {
      if (state == downloading) {
        return;
      }
      state = downloading;// 把状态设置为正在下载
      for (downloadinfo info : infos) {//为什么说我们是多线程呢？因为我们分别用新线程去下载刚才分割好的一个randomaccessfile文件
        new downloadthread(info.getthreadid(), info.getstartposition(), info.getendposition(), info.getcompletedsize(), info.geturl(), this.context).start();
      }
    }
  }
  /**
   * 现在要创建线程用来下载了，这里采用内部类
   */
  public class downloadthread extends thread {
    private int threadid;
    private int startpostion;
    private int endpostion;
    private int compeletedsize;
    private string url;
    private context context;
    public static final int progress = 1;
    public downloadthread(int threadid, int startpostion, int endpostion, int compeletedsize, string url, context context) {//构造方法，传入特定的参数
      this.threadid = threadid;
      this.startpostion = startpostion;
      this.endpostion = endpostion;
      this.compeletedsize = compeletedsize;
      this.url = url;
      this.context = context;
    }
    //开始下载
    @override
    public void run() {
      httpurlconnection conn = null;
      randomaccessfile randomaccessfile = null;
      inputstream instream = null;
      file file = new file(savepath, filename);
      url url = null;
      try {
        url = new url(this.url);
        conn = (httpurlconnection) url.openconnection();
        constructconnection(conn);
        if (conn.getresponsecode() == 200 || conn.getresponsecode() == 206) {
          randomaccessfile = new randomaccessfile(file, "rwd");
          randomaccessfile.seek(this.startpostion + this.compeletedsize);//randomaccessfile移动指针，到需要下载的块
          instream = conn.getinputstream();
          byte buffer[] = new byte[4096];//这个4096为么子呢？我也不知道，就是看阿里的人下载apk的时候都用4096，我也用
          int length = 0;
          while ((length = instream.read(buffer, 0, buffer.length)) != -1) {
            randomaccessfile.write(buffer, 0, length);
            compeletedsize += length;
            // 更新数据库中的下载信息
            dao.updatainfos(threadid, compeletedsize, this.url, this.context);
            // 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 = progress;
            message.obj = this.url;
            message.arg1 = length;
            mhandler.sendmessage(message);// 给downloadservice发送消息
            if (state == pause) {
              //("-----pause-----");
              return;
            }
          }
          // ("------------线程:" + this.threadid + "下载完成");
        }
      } catch (ioexception e) {
        e.printstacktrace();
        //("-----下载异常-----"); 这里下载异常我就不处理了，你可以发一条重新下载的消息
      } finally {//用完只后流要关闭，不然容易造成资源抢占，内存泄漏
        try {
          if (instream != null) {
            instream.close();
          }
          if (randomaccessfile != null) {
            randomaccessfile.close();
          }
          if (conn != null) {
            conn.disconnect();
          }
        } catch (ioexception e) {
          e.printstacktrace();
        }
      }
    }
    /**
     * 构建请求连接时的参数 返回开始下载的位置
     *
     * @param conn
     */
    private void constructconnection(httpurlconnection conn) throws ioexception {
      conn.setconnecttimeout(5 * 1000);// 设置连接超时5秒
      conn.setrequestmethod("get");// get方式提交，如果你是用post请求必须添加 conn.setdooutput(true); conn.setdoinput(true);
      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", this.url);
      conn.setrequestproperty("charset", "utf-8");
      int startpositionnew = this.startpostion + this.compeletedsize;
      // 设置获取实体数据的范围
      conn.setrequestproperty("range", "bytes=" + startpositionnew + "-" + this.endpostion);
      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();
    }
  }
}

github地址：https://github.com/shanlovana/downloadfiles/

以上就是本文的全部内容，希望对大家的学习有所帮助，也希望大家多多支持。