Android-多线程下载

当我们在下载网络资源时，开启多个线程下载会比一条线程下载速度快（一个人能干过一群人？）。

原理：服务器CPU分配给每条线程的时间片相同，服务器带宽平均分配给每条线程，所以客户端开启的线程越多，就能抢占到更多的服务器资源。

多线程下载的关键点：
1) 获取下载文件的大小
2）可以请求网络资源任意位置的数据
3) 确定每条线程下载多少数据
4) 计算每条线程下载数据的开始位置和结束位置
5）下载临时文件的创建与使用（临时文件应满足可以随机存取数据）
6） 临时进度文件的创建（这可文件用来实现断点续传）

多线程下载代码实现

下面代码可以实现多线程下载和断点续传。并且对开启的每个线程创建了一个线程下载进度文件，以保存线程下载的进度，不过这些进度文件
在下载完成后会删除。

public class MultiDownload {    static int ThreadCount = 3;    static int finishedThread = 0;    //确定下载地址    static String path = "http://192.168.13.13:8080/QQPlayer.exe";    public static void main(String[] args) {        //发送get请求，请求这个地址的资源        try {            URL url = new URL(path);            HttpURLConnection conn = (HttpURLConnection) url.openConnection();            conn.setRequestMethod("GET");            conn.setConnectTimeout(5000);            conn.setReadTimeout(5000);            if(conn.getResponseCode() == 200){                //拿到所请求资源文件的长度                int length = conn.getContentLength();                File file = new File("QQPlayer.exe");                //生成临时文件                RandomAccessFile raf = new RandomAccessFile(file, "rwd");                //设置临时文件的大小                raf.setLength(length);                raf.close();                //计算出每个线程应该下载多少字节                int size = length / ThreadCount;                for (int i = 0; i < ThreadCount; i++) {                    //计算线程下载的开始位置和结束位置                    int startIndex = i * size;                    int endIndex = (i + 1) * size - 1;                    //如果是最后一个线程，那么结束位置写死                    if(i == ThreadCount - 1){                        endIndex = length - 1;                    }                    //开启下载线程                    new DownLoadThread(startIndex, endIndex, i).start();                }            }        } catch (Exception e) {            // TODO Auto-generated catch block            e.printStackTrace();        }    }}class DownLoadThread extends Thread{    int startIndex;    int endIndex;    int threadId;    public DownLoadThread(int startIndex, int endIndex, int threadId) {        super();        this.startIndex = startIndex;        this.endIndex = endIndex;        this.threadId = threadId;    }    @Override    public void run() {        //再次发送http请求，下载原文件        try {            File progressFile = new File(threadId + ".txt");            //判断进度临时文件是否存在            if(progressFile.exists()){                FileInputStream fis = new FileInputStream(progressFile);                BufferedReader br = new BufferedReader(new InputStreamReader(fis));                //从进度临时文件中读取出上一次下载的总进度，然后与原本的开始位置相加，得到新的开始位置                startIndex += Integer.parseInt(br.readLine());                fis.close();            }            System.out.println("线程" + threadId + "的下载区间是：" + startIndex + "---" + endIndex);            HttpURLConnection conn;            URL url = new URL(MultiDownload.path);            conn = (HttpURLConnection) url.openConnection();            conn.setRequestMethod("GET");            conn.setConnectTimeout(5000);            conn.setReadTimeout(5000);            //设置本次http请求所请求的数据的区间            conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);            //请求部分数据，相应码是206            if(conn.getResponseCode() == 206){                //流里此时只有1/3原文件的数据                InputStream is = conn.getInputStream();                byte[] b = new byte[1024];                int len = 0;                int total = 0;                //拿到临时文件的输出流                File file = new File("QQPlayer.exe");                RandomAccessFile raf = new RandomAccessFile(file, "rwd");                //把文件的写入位置移动至startIndex                raf.seek(startIndex);                while((len = is.read(b)) != -1){                    //每次读取流里数据之后，同步把数据写入临时文件                    raf.write(b, 0, len);                    total += len;//                  System.out.println("线程" + threadId + "下载了" + total);                    //生成一个专门用来记录下载进度的临时文件                    RandomAccessFile progressRaf = new RandomAccessFile(progressFile, "rwd");                    //每次读取流里数据之后，同步把当前线程下载的总进度写入进度临时文件中                    progressRaf.write((total + "").getBytes());                    progressRaf.close();                }                System.out.println("线程" + threadId + "下载完毕");                raf.close();                MultiDownload.finishedThread++;                synchronized (MultiDownload.path) {                    if(MultiDownload.finishedThread == MultiDownload.ThreadCount){                        for (int i = 0; i < MultiDownload.ThreadCount; i++) {                            File f = new File(i + ".txt");                            f.delete();                        }                        MultiDownload.finishedThread = 0;                    }                }            }        } catch (Exception e) {            // TODO Auto-generated catch block            e.printStackTrace();        }    }}

手机多线程代码的实现

关键点：
相比上面的多线程下载实现，这里添加了进度条。
由于子线程中不能刷新UI，这里使用UI控件
这个进度条是android实现，我们可以把刷新UI的代码写在子线程中。

public class MainActivity extends Activity {    static int ThreadCount = 3;    static int finishedThread = 0;    int currentProgress;    String fileName = "QQPlayer.exe";    //确定下载地址    String path = "http://192.168.1.101:8080/" + fileName;    private ProgressBar pb;    TextView tv;    Handler handler = new Handler(){        public void handleMessage(android.os.Message msg) {            //把变量改成long，在long下运算            tv.setText((long)pb.getProgress() * 100 / pb.getMax() + "%");        }    };    @Override    protected void onCreate(Bundle savedInstanceState) {        super.onCreate(savedInstanceState);        setContentView(R.layout.activity_main);        pb = (ProgressBar) findViewById(R.id.pb);        tv = (TextView) findViewById(R.id.tv);    }    public void click(View v){        Thread t = new Thread(){            @Override            public void run() {                //发送get请求，请求这个地址的资源                try {                    URL url = new URL(path);                    HttpURLConnection conn = (HttpURLConnection) url.openConnection();                    conn.setRequestMethod("GET");                    conn.setConnectTimeout(5000);                    conn.setReadTimeout(5000);                    if(conn.getResponseCode() == 200){                        //拿到所请求资源文件的长度                        int length = conn.getContentLength();                        //设置进度条的最大值就是原文件的总长度                        pb.setMax(length);                        File file = new File(Environment.getExternalStorageDirectory(), fileName);                        //生成临时文件                        RandomAccessFile raf = new RandomAccessFile(file, "rwd");                        //设置临时文件的大小                        raf.setLength(length);                        raf.close();                        //计算出每个线程应该下载多少字节                        int size = length / ThreadCount;                        for (int i = 0; i < ThreadCount; i++) {                            //计算线程下载的开始位置和结束位置                            int startIndex = i * size;                            int endIndex = (i + 1) * size - 1;                            //如果是最后一个线程，那么结束位置写死                            if(i == ThreadCount - 1){                                endIndex = length - 1;                            }//                          System.out.println("线程" + i + "的下载区间是：" + startIndex + "---" + endIndex);                            new DownLoadThread(startIndex, endIndex, i).start();                        }                    }                } catch (Exception e) {                    // TODO Auto-generated catch block                    e.printStackTrace();                }            }        };        t.start();    }    class DownLoadThread extends Thread{        int startIndex;        int endIndex;        int threadId;        int currentTotal;        public DownLoadThread(int startIndex, int endIndex, int threadId) {            super();            this.startIndex = startIndex;            this.endIndex = endIndex;            this.threadId = threadId;        }        @Override        public void run() {            //再次发送http请求，下载原文件            try {                File progressFile = new File(Environment.getExternalStorageDirectory(), threadId + ".txt");                //判断进度临时文件是否存在                if(progressFile.exists()){                    FileInputStream fis = new FileInputStream(progressFile);                    BufferedReader br = new BufferedReader(new InputStreamReader(fis));                    //从进度临时文件中读取出上一次下载的总进度，然后与原本的开始位置相加，得到新的开始位置                    int lastProgress = Integer.parseInt(br.readLine());                    currentTotal = Integer.parseInt(br.readLine());                    startIndex += lastProgress;                    //把上次下载的进度显示至进度条//                  currentProgress += lastProgress;  //3个线程都加了！！！！！                    currentProgress += currentTotal;                    pb.setProgress(currentProgress);                    //发送消息，让主线程刷新文本进度                    handler.sendEmptyMessage(1);                    fis.close();                }                System.out.println("线程" + threadId + "的下载区间是：" + startIndex + "---" + endIndex);                HttpURLConnection conn;                URL url = new URL(path);                conn = (HttpURLConnection) url.openConnection();                conn.setRequestMethod("GET");                conn.setConnectTimeout(5000);                conn.setReadTimeout(5000);                //设置本次http请求所请求的数据的区间                conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);                //请求部分数据，相应码是206                if(conn.getResponseCode() == 206){                    //流里此时只有1/3原文件的数据                    InputStream is = conn.getInputStream();                    byte[] b = new byte[1024];                    int len = 0;                    int total = 0;                    //拿到临时文件的输出流                    File file = new File(Environment.getExternalStorageDirectory(), fileName);                    RandomAccessFile raf = new RandomAccessFile(file, "rwd");                    //把文件的写入位置移动至startIndex                    raf.seek(startIndex);                    while((len = is.read(b)) != -1){                        //每次读取流里数据之后，同步把数据写入临时文件                        raf.write(b, 0, len);                        total += len;                        currentTotal += len;                        System.out.println("线程" + threadId + "下载了" + total);                        //每次读取流里数据之后，把本次读取的数据的长度显示至进度条                        currentProgress += len;                        pb.setProgress(currentProgress);                        //发送消息，让主线程刷新文本进度                        handler.sendEmptyMessage(1);                        //生成一个专门用来记录下载进度的临时文件                        RandomAccessFile progressRaf = new RandomAccessFile(progressFile, "rwd");                        //每次读取流里数据之后，同步把当前线程下载的总进度写入进度临时文件中                        progressRaf.write((total + "\r\n"+currentTotal).getBytes());                        progressRaf.close();                    }                    System.out.println("线程" + threadId + "下载完毕-");                    raf.close();                    finishedThread++;                    synchronized (path) {                        if(finishedThread == ThreadCount){                            for (int i = 0; i < ThreadCount; i++) {                                File f = new File(Environment.getExternalStorageDirectory(), i + ".txt");                                f.delete();                            }                            finishedThread = 0;                        }                    }                }            } catch (Exception e) {                // TODO Auto-generated catch block                e.printStackTrace();            }        }    }}