Android快速实现断点续传的方法

compile 'com.loopj.android:android-async-http:1.4.9'

<uses-permission android:name="android.permission.INTERNET"></uses-permission>
<uses-permission android:name="android.permission.WRITE_EXTERNAL_STORAGE" />

<?xml version="1.0" encoding="utf-8"?>
<LinearLayout xmlns:android="http://schemas.android.com/apk/res/android"
  xmlns:app="http://schemas.android.com/apk/res-auto"
  xmlns:tools="http://schemas.android.com/tools"
  android:layout_width="match_parent"
  android:layout_height="match_parent"
  android:orientation="vertical"
  tools:context="com.five.fashion.duandianxuchuan.MainActivity">
<ProgressBar
  style="@style/Widget.AppCompat.ProgressBar.Horizontal"
  android:layout_width="match_parent"
  android:layout_height="100dp"
  android:id="@+id/pb"
  />
<TextView
  android:layout_width="wrap_content"
  android:layout_height="wrap_content"
  android:id="@+id/tv_info"
  />
  <Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="下载"
    android:id="@+id/bt_download"
    />
  <Button
    android:layout_width="wrap_content"
    android:layout_height="wrap_content"
    android:text="暂停"
    android:id="@+id/bt_pause"
    />
</LinearLayout>

package com.five.fashion.duandianxuchuan;

import android.os.Bundle;
import android.os.Environment;
import android.os.Handler;
import android.os.Message;
import android.os.SystemClock;
import android.support.annotation.Nullable;
import android.support.v7.app.AppCompatActivity;
import android.text.TextUtils;
import android.util.Log;
import android.view.View;
import android.widget.Button;
import android.widget.ProgressBar;
import android.widget.TextView;

import com.loopj.android.http.HttpGet;

import java.io.BufferedReader;
import java.io.File;
import java.io.FileReader;
import java.io.FileWriter;
import java.io.InputStream;
import java.io.RandomAccessFile;

import cz.msebera.android.httpclient.Header;
import cz.msebera.android.httpclient.HttpResponse;
import cz.msebera.android.httpclient.client.HttpClient;
import cz.msebera.android.httpclient.client.methods.HttpHead;
import cz.msebera.android.httpclient.impl.client.DefaultHttpClient;


public class MainActivity extends AppCompatActivity {
  protected static final String TAG = "OtherActivity";
  //下载线程的数量
  private final static int threadsize = 3;
  protected static final int SET_MAX = 0;
  public static final int UPDATE_VIEW = 1;
  private ProgressBar pb;
  private Button bt_download;
  private Button bt_pause;
  private TextView tv_info;
  //显示进度和更新进度
  private Handler mHandler = new Handler(){
    public void handleMessage(android.os.Message msg) {
      switch (msg.what) {
        case SET_MAX://设置进度条的最大值
          int filelength = msg.arg1;
          pb.setMax(filelength);
          break;
        case UPDATE_VIEW://更新进度条 和 下载的比率
          int len = msg.arg1;//新下载的长度
          pb.setProgress(pb.getProgress()+len);//设置进度条的刻度

          int max = pb.getMax();//获取进度的最大值
          int progress = pb.getProgress();//获取已经下载的数据量
          // 下载：30  总：100
          int result = (progress*100)/max;

          tv_info.setText("下载:"+result+"%");

          break;

        default:
          break;
      }
    };
  };
  String uri = "http://c.hiphotos.baidu.com/image/pic/item/b90e7bec54e736d1e51217c292504fc2d46269f3.jpg";
  @Override
  protected void onCreate(@Nullable Bundle savedInstanceState) {
    super.onCreate(savedInstanceState);
    setContentView(R.layout.activity_main);
    //找到控件
    pb = (ProgressBar) findViewById(R.id.pb);
    tv_info = (TextView) findViewById(R.id.tv_info);
    bt_download = (Button) findViewById(R.id.bt_download);
    bt_pause = (Button) findViewById(R.id.bt_pause);
    //数据的回显
    //确定下载的文件
    String name = getFileName(uri);
    File file = new File(Environment.getExternalStorageDirectory(), name);
    if (file.exists()){//文件存在回显
      //获取文件的大小
      int filelength = (int) file.length();
      pb.setMax(filelength);
      try {
        //统计原来所有的下载量
        int count = 0;
        //读取下载记录文件
        for (int threadid = 0; threadid < threadsize; threadid++) {
          //获取原来指定线程的下载记录
          int existDownloadLength = readDownloadInfo(threadid);
          count = count + existDownloadLength;
        }
        //设置进度条的刻度
        pb.setProgress(count);

        //计算比率
        int result = (count * 100) / filelength;
        tv_info.setText("下载:" + result + "%");
      } catch (Exception e) {
        e.printStackTrace();
      }
    }
    bt_download.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        download(v);
      }
    });
    bt_pause.setOnClickListener(new View.OnClickListener() {
      @Override
      public void onClick(View v) {
        pause(v);
      }
    });
  }
  //暂停
  private boolean flag = false;//是否在下载

  public void pause(View v){
    flag = false;
    bt_download.setEnabled(true);
    bt_pause.setEnabled(false);
  }

  //下载
  public void download(View v){
    flag = true; //是在下载
    bt_download.setEnabled(false);//一点击变成不可点击
    bt_pause.setEnabled(true);//一点击变成可点击
    new Thread(){//子线程
      public void run() {
        try {
          //获取服务器上文件的大小
          HttpClient client = new DefaultHttpClient();
          HttpHead request = new HttpHead(uri);
          HttpResponse response = client.execute(request);
          //response 只有响应头 没有响应体
          if(response.getStatusLine().getStatusCode() == 200){
            Header[] headers = response.getHeaders("Content-Length");
            String value = headers[0].getValue();
            //文件大小
            int filelength = Integer.parseInt(value);
            Log.i(TAG, "filelength:"+filelength);

            //设置进度条的最大值
            Message msg_setmax = Message.obtain(mHandler, SET_MAX, filelength, 0);
            msg_setmax.sendToTarget();


            //处理下载记录文件
            for(int threadid=0;threadid<threadsize;threadid++){
              //对应的下载记录文件
              File file = new File(Environment.getExternalStorageDirectory(),threadid+".txt");
              //判断文件是否存在
              if(!file.exists()){
                //创建文件
                file.createNewFile();
              }
            }

            //在sdcard创建和服务器大小一样的文件
            String name = getFileName(uri);
            File file = new File(Environment.getExternalStorageDirectory(),name);
            //随机访问文件
            RandomAccessFile raf = new RandomAccessFile(file, "rwd");
            //设置文件的大小
            raf.setLength(filelength);
            //关闭
            raf.close();

            //计算每条线程的下载量
            int block = (filelength%threadsize == 0)?(filelength/threadsize):(filelength/threadsize+1);
            //开启三条线程执行下载
            for(int threadid=0;threadid<threadsize;threadid++){
              new DownloadThread(threadid, uri, file, block).start();
            }
          }
        } catch (Exception e) {
          e.printStackTrace();
        }
      };
    }.start();
  }


  //线程下载类
  private class DownloadThread extends Thread{
    private int threadid;//线程的id
    private String uri;//下载的地址
    private File file;//下载文件
    private int block;//下载的块
    private int start;
    private int end;

    public DownloadThread(int threadid, String uri, File file, int block) {
      super();
      this.threadid = threadid;
      this.uri = uri;
      this.file = file;
      this.block = block;
      //计算下载的开始位置和结束位置
      start = threadid * block;
      end = (threadid + 1)*block -1;

      try {
        //读取该条线程原来的下载记录
        int existDownloadLength = readDownloadInfo(threadid);

        //修改下载的开始位置  从新下载
        start = start + existDownloadLength;
      } catch (Exception e) {
        e.printStackTrace();
      }
    }

    //下载  状态码：200是普通的下载   206是分段下载    Range:范围
    @Override
    public void run() {
      super.run();
      try {
        RandomAccessFile raf = new RandomAccessFile(file, "rwd");
        //跳转到起始位置
        raf.seek(start);

        //分段下载
        HttpClient client = new DefaultHttpClient();
        HttpGet request = new HttpGet(uri);
        request.addHeader("Range", "bytes:"+start+"-"+end);//添加请求头
        HttpResponse response = client.execute(request);
        if(response.getStatusLine().getStatusCode() == 200){
          InputStream inputStream = response.getEntity().getContent();
          //把流写入到文件
          byte[] buffer = new byte[1024];
          int len = 0;
          while((len = inputStream.read(buffer)) != -1){
            //如果暂停下载 点击暂停 false 就直接return 点击下载true接着下载
            if(!flag){
              return;//标准线程结束
            }
            //写数据
            raf.write(buffer, 0, len);

            //读取原来下载的数据量 这里读取是为了更新下载记录
            int existDownloadLength = readDownloadInfo(threadid);//原来下载的数据量

            //计算最新的下载
            int newDownloadLength = existDownloadLength + len;

            //更新下载记录 从新记录最新下载位置
            updateDownloadInfo(threadid, newDownloadLength);
            //更新进度条的显示  下载的百分比
            Message update_msg = Message.obtain(mHandler, UPDATE_VIEW, len, 0);
            update_msg.sendToTarget();
            //模拟 看到进度条动的效果
            SystemClock.sleep(50);
          }
          inputStream.close();
          raf.close();
          Log.i(TAG, "第"+threadid+"条线程下载完成");
        }

      } catch (Exception e) {
        e.printStackTrace();
      }
    }
  }


  /**
   * 读取指定线程的下载数据量
   * @param threadid 线程的id
   * @return
   * @throws Exception
   */
  public int readDownloadInfo(int threadid) throws Exception{
    //下载记录文件
    File file = new File(Environment.getExternalStorageDirectory(),threadid+".txt");
    BufferedReader br = new BufferedReader(new FileReader(file));
    //读取一行数据
    String content = br.readLine();

    int downlength = 0;
    //如果该文件第一次创建去执行读取操作 文件里面的内容是 null
    if(!TextUtils.isEmpty(content)){
      downlength = Integer.parseInt(content);
    }
    //关闭流
    br.close();
    return downlength;
  }


  /**
   * 更新下载记录
   * @param threadid
   * @param newDownloadLength
   */
  public void updateDownloadInfo(int threadid,int newDownloadLength) throws Exception{
    //下载记录文件
    File file = new File(Environment.getExternalStorageDirectory(),threadid+".txt");
    FileWriter fw = new FileWriter(file);
    fw.write(newDownloadLength+"");
    fw.close();
  }

  /**
   * 获取文件的名称
   * @param uri
   * @return
   */
  private String getFileName(String uri){
    return uri.substring(uri.lastIndexOf("/")+1);
  }

}