Android线程池控制并发数多线程下载

多线程下载并不是并发下载线程越多越好，因为当用户开启太多的并发线程之后，应用程序需要维护每条线程的开销，线程同步的开销。

这些开销反而会导致下载速度降低。因此需要避免在代码中直接开启大量线程执行下载。

主要实现步奏：

1、定义一个downutil类，下载工作基本在此类完成，在构造器中初始化ui线程的handler。用于子线程和ui线程传递下载进度值。

2、所有的下载任务都保存在linkedlist。在init（）方法中开启一个后台线程，不断地从linkedlist中取任务交给线程池中的空闲线程执行。

3、每当addtask方法添加一个任务，就向 mpoolthreadhandler发送条消息，就从任务队列中取出一个任务交给线程池执行。这里使用了使用了semaphore信号量，也就是说只有当一个任务执行完成之后，release（）一个信号量，才能从linkedlist中取出一个任务再去执行，否则acquire（）方法会一直阻塞线程，直到上一个任务完成。

public class downutil
{
 //定义下载资源的路径
 private string path;
 //指定下载文件的保存位置
 private string targetfile;
 //定义下载文件的总大小
 private int filesize;

 //线程池
 private executorservice mthreadpool;
 //线程数量
 private static final int default_thread_count = 5;
 //任务队列
 private linkedlist<runnable> mtasks;

 //后台轮询线程
 private thread mpoolthread;
 //后台线程的handler
 private handler mpoolthreadhandler;
 //ui线程的handler
 private handler muithreadhandler;
 //信号量
 private semaphore semaphore;
 private semaphore mhandlersemaphore = new semaphore(0);
 //下载线程数量
 private int threadnum;

 public downutil(string path , string targetfile , int threadnum , final progressbar bar)
 {
  this.path = path;
  this.targetfile = targetfile;
  this.threadnum = threadnum;
  init();

  muithreadhandler = new handler()
  {
   int sumsize = 0;
   @override
   public void handlemessage(message msg)
   {
    if (msg.what == 0x123)
    {
     int size = msg.getdata().getint("upper");
     sumsize += size;
     log.d("sumsize" , sumsize + "");
     bar.setprogress((int) (sumsize * 1.0 / filesize * 100));
    }
   }
  };
 }

 private void init()
 {
  mpoolthread = new thread()
  {
   public void run()
   {
    looper.prepare();
    mpoolthreadhandler = new handler()
    {
     public void handlemessage(message msg)
     {
      if (msg.what == 0x111)
      {
       mthreadpool.execute(gettask());
       try
       {
        semaphore.acquire();
       }
       catch (interruptedexception e)
       {
        e.printstacktrace();
       }
      }
     }
    };
    mhandlersemaphore.release();
    looper.loop();
   }
  };
  mpoolthread.start();

  mthreadpool = executors.newfixedthreadpool(default_thread_count);
  mtasks = new linkedlist<>();
  semaphore = new semaphore(default_thread_count);
 }

 public void download()
 {


  try {
   url url = new url(path);
   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("charset", "utf-8");
   conn.setrequestproperty("connection", "keep-alive");

   //得到文件的大小
   filesize = conn.getcontentlength();
   conn.disconnect();

   int currentpartsize = filesize / threadnum + 1;
   randomaccessfile file = new randomaccessfile(targetfile , "rw");
   file.setlength(filesize);
   file.close();


   for (int i = 0 ; i < threadnum ; i++)
   {
    //计算每条线程下载的开始位置
    int startpos = i * currentpartsize;
    //每条线程使用一个randomaccessfile进行下载
    randomaccessfile currentpart = new randomaccessfile(targetfile , "rw");
    //定位该线程的下载位置
    currentpart.seek(startpos);

    //将任务添加到任务队列中
    addtask(new downthread(startpos , currentpartsize , currentpart));
   }
  }
  catch (ioexception e)
  {
   e.printstacktrace();
  }
 }

 private runnable gettask()
 {
  if (!mtasks.isempty())
  {
   return mtasks.removefirst();
  }
  return null;
 }

 private synchronized void addtask(runnable task)
 {
  mtasks.add(task);
  try
  {
   if (mpoolthreadhandler == null)
   {
    mhandlersemaphore.acquire();
   }
  }
  catch (interruptedexception e)
  {
   e.printstacktrace();
  }
  mpoolthreadhandler.sendemptymessage(0x111);
 }

 private class downthread implements runnable
 {
  //当前线程的下载位置
  private int startpos;
  //定义当前线程负责下载的文件大小
  private int currentpartsize;
  //当前线程需要下载的文件块
  private randomaccessfile currentpart;
  //定义该线程已经下载的字节数
  private int length;

  public downthread(int startpos , int currentpartsize , randomaccessfile currentpart)
  {
   this.startpos = startpos;
   this.currentpartsize = currentpartsize;
   this.currentpart = currentpart;
  }

  @override
  public void run()
  {
   try
   {
    url url = new url(path);
    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("charset", "utf-8");
    conn.setrequestproperty("connection", "keep-alive");

    inputstream instream = conn.getinputstream();
    //跳过startpos个字节
    skipfully(instream , this.startpos);

    byte[] buffer = new byte[1024];
    int hasread = 0;
    while (length < currentpartsize && (hasread = instream.read(buffer)) > 0)
    {
     currentpart.write(buffer , 0 , hasread);
     //累计该线程下载的总大小
     length += hasread;
    }

    log.d("length" , length + "");

    //创建消息
    message msg = new message();
    msg.what = 0x123;
    bundle bundle = new bundle();
    bundle.putint("upper" , length);
    msg.setdata(bundle);
    //向ui线程发送消息
    muithreadhandler.sendmessage(msg);

    semaphore.release();
    currentpart.close();
    instream.close();
   }
   catch (exception e)
   {
    e.printstacktrace();
   }
  }
 }

 public static void skipfully(inputstream in , long bytes) throws ioexception
 {
  long remaining = bytes;
  long len = 0;
  while (remaining > 0)
  {
   len = in.skip(remaining);
   remaining -= len;
  }
 }
}

以下是mainactivity的代码：

public class mainactivity extends activity
{
 edittext url;
 edittext target;
 button downbn;
 progressbar bar;
 downutil downutil;
 private string savepath;

 @override
 protected void oncreate(bundle savedinstancestate)
 {
  super.oncreate(savedinstancestate);
  setcontentview(r.layout.main);
  //获取界面中的四个界面控件
  url = (edittext) findviewbyid(r.id.address);
  target = (edittext) findviewbyid(r.id.target);
  try
  {
   file sdcarddir = environment.getexternalstoragedirectory();
   savepath = sdcarddir.getcanonicalpath() + "/d.chm";
  }
  catch (exception e)
  {
   e.printstacktrace();
  }
  target.settext(savepath);
  downbn = (button) findviewbyid(r.id.down);
  bar = (progressbar) findviewbyid(r.id.bar);
  downbn.setonclicklistener(new view.onclicklistener()
  {
   @override
   public void onclick(view view)
   {
    downutil = new downutil(url.gettext().tostring() , target.gettext().tostring() , 7 , bar);
    new thread()
    {
     @override
     public void run()
     {
      try
      {
       downutil.download();
      }
      catch (exception e)
      {
       e.printstacktrace();
      }
     }
    }.start();
   }
  });
 }
}

页面布局比较简单这里一并贴出：

<linearlayout xmlns:android="http://schemas.android.com/apk/res/android"
 android:layout_width="match_parent"
 android:layout_height="match_parent"
 android:orientation="vertical">

 <textview
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:text="@string/title1"/>

 <edittext
  android:id="@+id/address"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:text="@string/address"/>

 <textview
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:text="@string/targetaddress"/>

 <edittext
  android:id="@+id/target"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"/>

 <button
  android:id="@+id/down"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:text="@string/down"/>

 <!-- 定义一个水平进度条，用于显示下载进度 -->
 <progressbar
  android:id="@+id/bar"
  android:layout_width="match_parent"
  android:layout_height="wrap_content"
  android:max="100"
  style="?android:attr/progressbarstylehorizontal"/>

</linearlayout>

此例主要是在李刚老师的《疯狂java的讲义》的多线程的例子上修改，感谢李刚老师，如有不足之处，欢迎批评指正。

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