Android多线程下载

所用知识点：

1、设置http协议字段Range “bytes=“start+”-”+end

conn.addRequestProperty("Range", "bytes=" + start + "-" + end);

2、RandomAccessFile设置写入的位置

// 指定文件方式为可读写可操作
RandomAccessFile accessFile = new RandomAccessFile(new File(fileName),"rwd");
accessFile.seek(start);

3、开启多线程发送网络请求

// 启动下载

DownloadRunnable runnable = new DownloadRunnable(url,downLoadFile.getAbsolutePath(),start,end);
threadPool.execute(runnable);

DownLoad.java

import android.os.Environment;
import android.os.Handler;
import android.os.Message;

import java.io.File;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.URL;
import java.util.concurrent.Executor;
import java.util.concurrent.Executors;

/**
 * 
 */
public class DownLoad {

    // 创建线程池
    private Executor threadPool=  Executors.newFixedThreadPool(3);

   private Handler handler;

    public DownLoad(Handler handler)
    {
        this.handler = handler;
    }

    private class DownloadRunnable implements Runnable
    {
        private String url;

        private String fileName;

        private int start;

        private  int end;

        public DownloadRunnable(String url,String fileName,int start,int end)
        {
            this.url = url;
            this.fileName = fileName;
            this.start = start;
            this.end = end;
        }

        @Override
        public void run() {
                try {
                    URL httpUrl = new URL(url);
                    HttpURLConnection conn = (HttpURLConnection) httpUrl.openConnection();
                    conn.setRequestMethod("GET");
                    conn.setReadTimeout(5000);
                    conn.addRequestProperty("Range", "bytes=" + start + "-" + end);
                    // 指定文件方式为可读写可操作
                    RandomAccessFile accessFile = new RandomAccessFile(new File(fileName),"rwd");
                    accessFile.seek(start);

                    InputStream in = conn.getInputStream();
                    // 设置缓冲区
                    byte b[] = new byte[4 + 1024];
                    int len =0;
                    while ((len = in.read(b))!=-1)
                    {
                        accessFile.write(b,0,len);
                    }

                    if (accessFile!=null)
                    {
                        accessFile.close();
                    }
                    if (in!=null)
                    {
                        in.close();
                    }
                    Message message = Message.obtain();
                    message.what = 1;
                    handler.sendMessage(message);

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

    public void downLoadFile(String url)
    {
        try {
            URL httpUrl = new URL(url);
            try {
                HttpURLConnection conn =  (HttpURLConnection)httpUrl.openConnection();
                conn.setRequestMethod("GET");
                conn.setReadTimeout(5000);
                int length = conn.getContentLength();
                int block =  length/3;
                int start;
                int end;
                String fileName = getFileName(url);
                File parent = Environment.getExternalStorageDirectory();
                File downLoadFile = new File(parent,fileName);
                /**
                 * 假如总长度为11
                 * block = 11/3=3
                 * 第一个线程下载0-block=0-2
                 * 第二个线程下载3-2*block-1= 3-5
                 * 第三个线程下载6-length=6-11
                 * */
                for (int i=0;i<3;i++)
                {
                    start = i*block;
                    end = (i+1)*block -1;
                    if (i == 2)
                    {
                        end = length;
                    }
                    // 启动下载
                    DownloadRunnable runnable = new DownloadRunnable(url,downLoadFile.getAbsolutePath(),start,end);
                    threadPool.execute(runnable);
                }
            } catch (IOException e) {
                e.printStackTrace();
            }
        } catch (MalformedURLException e) {
            e.printStackTrace();
        }
    }

    private String getFileName(String url)
    {
        return url.substring(url.lastIndexOf("/"));
    }
}

MainActiviy.java 关键代码

private int count=0;

handler = new android.os.Handler()
{
    @Override
    public void handleMessage(Message msg) {
        super.handleMessage(msg);
        count +=msg.what;
        // 说明三个线程都下载完成
        if (count == 3)
        {
            textView.setText("Download Succ");
        }
    }
};

textView.setOnClickListener(new View.OnClickListener() {
            @Override
            public void onClick(View v) {
                new Thread(new Runnable() {
                    @Override
                    public void run() {
                        new DownLoad(handler).downLoadFile("http://192.168.0.103:8080/Pro_Test/aa.jpg");
                    }
                }).start();
            }
        });