Android使用AsyncTask实现断点续传

 public abstract class AsyncTask 

 Header header_size = new BasicHeader("Range", "bytes=" + readedSize + "-");
 request.addHeader(header_size); 

package com.bbk.lling.myapplication;
import android.app.Activity;
import android.os.AsyncTask;
import android.os.Bundle;
import android.os.Environment;
import android.util.Log;
import android.view.View;
import android.widget.ProgressBar;
import android.widget.Toast;
import org.apache.http.Header;
import org.apache.http.HttpResponse;
import org.apache.http.client.HttpClient;
import org.apache.http.client.methods.HttpGet;
import org.apache.http.impl.client.DefaultHttpClient;
import org.apache.http.message.BasicHeader;
import java.io.File;
import java.io.FileOutputStream;
import java.io.IOException;
import java.io.InputStream;
import java.io.OutputStream;
import java.io.RandomAccessFile;
import java.net.MalformedURLException;
public class AsyncTaskDemoActivity extends Activity {
  private ProgressBar progressBar;
  //下载路径
  private String downloadPath = Environment.getExternalStorageDirectory() +
      File.separator + "download";
  private DownloadAsyncTask downloadTask;
  @Override
  protected void onCreate(Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_async_task_demo);
    progressBar = (ProgressBar) findViewById(R.id.progressbar);
    //开始下载
    findViewById(R.id.begin).setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        /**
         * 一个AsyncTask只能被执行一次，否则会抛异常
         * java.lang.IllegalStateException: Cannot execute task: the task is already running.
         * 如果要重新开始任务的话要重新创建AsyncTask对象
         */
        if(downloadTask != null && !downloadTask.isCancelled()) {
          return;
        }
        downloadTask = new DownloadAsyncTask("http://bbk-lewen.u.qiniudn.com/3d5b1a2c-4986-4e4a-a626-b504a36e600a.flv");
        downloadTask.execute();
      }
    });
    //暂停下载
    findViewById(R.id.end).setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        if(downloadTask != null && downloadTask.getStatus() == AsyncTask.Status.RUNNING) {
          downloadTask.cancel(true);
        }
      }
    });
  }
  /**
   * 下载的AsyncTask
   */
  private class DownloadAsyncTask extends AsyncTask {
    private static final String TAG = "DownloadAsyncTask";
    private String mUrl;
    public DownloadAsyncTask(String url) {
      this.mUrl = url;
    }
    @Override
    protected Long doInBackground(String... params) {
      Log.i(TAG, "downloading");
      if(mUrl == null) {
        return null;
      }
      HttpClient client = new DefaultHttpClient();
      HttpGet request = new HttpGet(mUrl);
      HttpResponse respOnse= null;
      InputStream is = null;
      RandomAccessFile fos = null;
      OutputStream output = null;
      try {
        //创建存储文件夹
        File dir = new File(downloadPath);
        if(!dir.exists()) {
          dir.mkdir();
        }
        //本地文件
        File file = new File(downloadPath + File.separator + mUrl.substring(mUrl.lastIndexOf("/") + 1));
        if(!file.exists()){
          //创建文件输出流
          output = new FileOutputStream(file);
          //获取下载输入流
          respOnse= client.execute(request);
          is = response.getEntity().getContent();
          //写入本地
          file.createNewFile();
          byte buffer [] = new byte[1024];
          int inputSize = -1;
          //获取文件总大小，用于计算进度
          long total = response.getEntity().getContentLength();
          int count = 0; //已下载大小
          while((inputSize = is.read(buffer)) != -1) {
            output.write(buffer, 0, inputSize);
            count += inputSize;
            //更新进度
            this.publishProgress((int) ((count / (float) total) * 100));
            //一旦任务被取消则退出循环，否则一直执行，直到结束
            if(isCancelled()) {
              output.flush();
              return null;
            }
          }
          output.flush();
        } else {
          long readedSize = file.length(); //文件大小，即已下载大小
          //设置下载的数据位置XX字节到XX字节
          Header header_size = new BasicHeader("Range", "bytes=" + readedSize + "-");
          request.addHeader(header_size);
          //执行请求获取下载输入流
          respOnse= client.execute(request);
          is = response.getEntity().getContent();
          //文件总大小=已下载大小+未下载大小
          long total = readedSize + response.getEntity().getContentLength();
          //创建文件输出流
          fos = new RandomAccessFile(file, "rw");
          //从文件的size以后的位置开始写入，其实也不用，直接往后写就可以。有时候多线程下载需要用
          fos.seek(readedSize);
          //这里用RandomAccessFile和FileOutputStream都可以，只是使用FileOutputStream的时候要传入第二哥参数true,表示从后面填充
//          output = new FileOutputStream(file, true);
          byte buffer [] = new byte[1024];
          int inputSize = -1;
          int count = (int)readedSize;
          while((inputSize = is.read(buffer)) != -1) {
            fos.write(buffer, 0, inputSize);
//            output.write(buffer, 0, inputSize);
            count += inputSize;
            this.publishProgress((int) ((count / (float) total) * 100));
            if(isCancelled()) {
//              output.flush();
              return null;
            }
          }
//          output.flush();
        }
      } catch (MalformedURLException e) {
        Log.e(TAG, e.getMessage());
      } catch (IOException e) {
        Log.e(TAG, e.getMessage());
      } finally{
        try{
          if(is != null) {
            is.close();
          }
          if(output != null) {
            output.close();
          }
          if(fos != null) {
            fos.close();
          }
        } catch(Exception e) {
          e.printStackTrace();
        }
      }
      return null;
    }
    @Override
    protected void onPreExecute() {
      Log.i(TAG, "download begin ");
      Toast.makeText(AsyncTaskDemoActivity.this, "开始下载", Toast.LENGTH_SHORT).show();
      super.onPreExecute();
    }
    @Override
    protected void onProgressUpdate(Integer... values) {
      super.onProgressUpdate(values);
      Log.i(TAG, "downloading " + values[0]);
      //更新界面进度条
      progressBar.setProgress(values[0]);
    }
    @Override
    protected void onPostExecute(Long aLong) {
      Log.i(TAG, "download success " + aLong);
      Toast.makeText(AsyncTaskDemoActivity.this, "下载结束", Toast.LENGTH_SHORT).show();
      super.onPostExecute(aLong);
    }
  }
}