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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