HTTP多线程断点下载

原理：获取目标文件的大小,在本地创建一个相同大小的文件,并计算每个线程需要下载的起始位置及大小,然后分配至每个线程独立下载,全部下载完毕则自动合并.

实现步骤

1. 查看并计算目标文件的大小

URL url = new URL(mPath);
HttpURLConnection connection = (HttpURLConnection) 
url.openConnection();
connection.setRequestMethod("GET");
int code = connection.getResponseCode();
if (code == 200) {
    int length = connection.getContentLength();
    System.out.println("文件总长度为：" + length);

2. 设置目标文件在本地的映射

RandomAccessFile raf = new RandomAccessFile(
        Environment.getExternalStorageDirectory().
        getAbsolutePath()+"/"+getDownlaodName(mPath), "rw");
raf.setLength(length);

3. 开启子线程并分配下载任务

int blokeSize = length / mTotalCount;
System.out.println("每一块大小为：" + blokeSize);
runningThreadCount = mTotalCount;
for (int threadid = 0; threadid < mTotalCount; threadid++) {
    int startPosition = threadid * blokeSize;
    int endPosition = (threadid + 1) * blokeSize - 1;
    //最后一个线程应该下载至末尾
    if (threadid == mTotalCount - 1) {
        endPosition = length - 1;
    }
    System.out.println("线程编号：" + threadid + ""
            + ",下载范围：" + startPosition + "~~" + endPosition);
    //开启下载子线程
    new DownloadThread(threadid, startPosition, 
        endPosition).start();

4. 子线程中的具体逻辑

public void run() {
    System.out.println("线程"+threadid+"开始运行了");
    try{
        //读存有历史下载进度的文件,判断是否已经下载过
        File finfo=new File(Environment
            .getExternalStorageDirectory()
            .getAbsolutePath() +"/"+mTotalCount 
            + getDownlaodName(mPath)+threadid+".txt");
        if (finfo.exists()&&finfo.length()>0) {
            //获取文件输入流
            FileInputStream fis=new FileInputStream(finfo);
            //读取缓冲
            BufferedReader br=new BufferedReader(
                new InputStreamReader(fis));
            //得到文件内容
            String lasrposition=br.readLine();
            //转化为int值
            int intlastposition=Integer.parseInt(lasrposition);
            lastLoadSize=intlastposition-startPosition;
            startPosition=intlastposition;
            fis.close();
        }
        URL url=new URL(mPath);
        HttpURLConnection conn=(HttpURLConnection) 
        url.openConnection();
        conn.setRequestMethod("GET");
        System.out.println("线程实际下载："+threadid+"，范围："+startPosition
                +"~~"+endPosition);
        //设置http请求头部参数
        conn.setRequestProperty("Range", "bytes="
        +startPosition+"-"+endPosition);
        int code=conn.getResponseCode();
        //206代表请求部分数据成功
        if (code==206) {
            //拿到链接返回的输入流
            InputStream is=conn.getInputStream();
            //指向要写的文件（abc.exe）
            RandomAccessFile raf=new RandomAccessFile(
                    Environment.getExternalStorageDirectory().getAbsolutePath()
                            +"/"+getDownlaodName(mPath), "rw");
            //指定文件开始写的位置
            raf.seek(startPosition);
            //下载缓冲区，越大下载越快，对硬盘损耗越小，但是越容易丢失数据
            byte[] buffer=new byte[1024*1024];
            int len=-1;
            int total=0;//当前线程本次下载数据总量
            while ((len=is.read(buffer))!=-1) {
                raf.write(buffer,0,len);
                total+=len;
                //将每次更新的数据同步到底层硬盘
                RandomAccessFile inforaf=new RandomAccessFile(
                        Environment.getExternalStorageDirectory().getAbsolutePath()
                        +"/"+mTotalCount +getDownlaodName(mPath)+threadid+".txt","rwd");
                //保存当前线程下载到什么位置
                inforaf.write(String.valueOf(startPosition+total).getBytes());
                inforaf.close();
            }
            is.close();
            raf.close();
            System.out.println("线程"+threadid+"下载完毕");

        }
    }catch(Exception e){
        e.printStackTrace();
    }finally{
        //同步代码块，保证同时间仅有一个线程执行此区块代码
        synchronized (MainActivity.class) {
            runningThreadCount--;
            if (runningThreadCount<=0) {
                System.out.println("多线程下载完毕");
                for (int i = 0; i <mTotalCount ; i++) {
                    File f=new File(Environment.getExternalStorageDirectory()
                    .getAbsolutePath()
                    +"/"+mTotalCount 
                    +getDownlaodName(mPath)
                    +i+".txt");
                    System.out.println(f.delete());
                    endTime=System.currentTimeMillis();
                }
                System.out.println("结束时间："+endTime);
                System.out.println("总共花费："
                +(endTime-startTime)/1000+"秒");
                runOnUiThread(new Runnable() {
                    @Override
                    public void run() {
                        Toast.makeText(getApplicationContext(), 
                        "总共花费："+(endTime-startTime)/1000+"秒",
                        Toast.LENGTH_SHORT).show();
                    }
                });
            }
        }
    }
}

备注：如果不使用断点下载，只需要将判断和存储历史下载信息的逻辑删除即可。