多线程下载

谁借莪１个温暖的怀抱￠ 2021-12-21 15:35 392阅读 0赞

## 思路： ##

将文件分成多个部分，多个线程进行读取和写入，如3个线程下载1个大小为902byte的文件，第一个线程下载0-299字节，第二个线程下载300-599字节，第三个线程下载剩下的即600-901字节

## 需要用的知识点： ##

### HTTP Range ###

HTTP1.1中，Range请求头可以指定下载的范围，从0开始计算

Range 请求头格式

Range: bytes=start-end  
例如：

Range: bytes=0-39：第0个字节到第39个字节之间的数据

Range: bytes=40-100：第40个字节到最后一个字节的数据.

注意，这个表示\[start,end\]，即是包含请求头的start及end字节的，所以，下一个请求，应该是上一个请求的\[end+1, nextEnd\]

响应头

Content-Range

Content-Range: bytes 0-10/3103

这个表示，服务器响应了前(0-10)个字节的数据，该资源一共有(3103)个字节大小。

### RandomAccessFile ###

RandomAccessFile支持随机访问文件，通过seek()方法移动到指定位置进行读写

package io;
    
    import java.io.File;
    import java.io.IOException;
    import java.net.HttpURLConnection;
    import java.net.URL;
    import java.util.concurrent.CountDownLatch;
    
    public class MultiThreadDownloadFile
    {
        public static void main(String []args) throws IOException, InterruptedException
        {
            URL url = new URL(
                    "https://timgsa.baidu.com/timg?image&quality=80&size=b9999_10000&sec=1489511816166&di=c47b4abe7e4dad502b0672b8065c0fb6&imgtype=0&src=http%3A%2F%2Fimg.taopic."
                    + "com%2Fuploads%2Fallimg%2F111130%2F573-1111301A01938.jpg");
            HttpURLConnection httpConn = (HttpURLConnection) url.openConnection();
            httpConn.setConnectTimeout(10000);
            httpConn.setReadTimeout(10000);
            //待下载的文件长度，用于计算各个线程的起始/结束位置
            int fileLen = httpConn.getContentLength();
            System.out.println("文件大小是：" +fileLen +" byte");
            
            //下载文件的线程个数
            int threadNums = 5;
            int oneLen = fileLen/threadNums;
            int []startPos = new int[threadNums];
            int []endPos = new int[threadNums];
            
            for(int i=0;i<threadNums;i++)
            {
                startPos[i] = i*oneLen;
                endPos[i] = (i+1)*oneLen-1;
            }
            //重新设置最后一个线程的endPos
            endPos[threadNums-1] = fileLen - 1;
        
            //下载文件本地保存路径/名称
            File downloadFile = new File("downloadFile.jpg");
            //用于线程同步
            CountDownLatch doneSignal = new CountDownLatch(threadNums);
            
            for(int i=0;i<threadNums;i++)
            {
                MultiDownloadThread mt = new MultiDownloadThread(url,doneSignal, downloadFile,startPos[i],endPos[i]);
                new Thread(mt).start();
            }
            
            //等待线程下载完成
            doneSignal.await();
            
            System.out.println("下载完成...文件下载大小是" + downloadFile.length() +" byte");
        }
    }

package io;
    
    import java.io.File;
    import java.io.FileNotFoundException;
    import java.io.IOException;
    import java.io.InputStream;
    import java.io.RandomAccessFile;
    import java.net.HttpURLConnection;
    import java.net.URL;
    import java.util.concurrent.CountDownLatch;
    
    public class MultiDownloadThread implements Runnable
    {
        private URL url = null;
        private CountDownLatch doneSignal = null;
        private File file = null;
        private long startPos = 0;
        private long endPos = 0;
        private long curPos = 0;// 用于支持断点下载，未实现
    
        public MultiDownloadThread(URL url, CountDownLatch doneSignal, File file, long startPos, long endPos)
        {
            this.url = url;
            this.doneSignal = doneSignal;
            this.file = file;
            this.startPos = startPos;
            this.endPos = endPos;
        }
    
        @Override
        public void run()
        {
            System.out.println("启动了一个下载线程....");
            try
            {
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setConnectTimeout(1000);
                conn.setReadTimeout(1000);
                // 创建一个新的连接，设置下载的开始和结束位置
                conn.setRequestProperty("Range", "bytes=" + startPos + "-" + endPos);
                // 如果请求部分资源 206 ok
                System.out.println("code=" + conn.getResponseCode()+", msg=" + conn.getResponseMessage());
                RandomAccessFile raf = new RandomAccessFile(file, "rw");
                raf.seek(startPos);
                InputStream os = conn.getInputStream();
    
                byte[] buff = new byte[1024];
                int len = 0;
                while ((len = os.read(buff)) != -1)
                {
                    raf.write(buff, 0, len);
                }
    
                os.close();
                raf.close();
    
                doneSignal.countDown();
    
            } catch (FileNotFoundException e)
            {
                // TODO Auto-generated catch block
                e.printStackTrace();
            } catch (IOException e)
            {
                // TODO Auto-generated catch block
                e.printStackTrace();
            }
    
            System.out.println("线程下载完成....");
        }
    
    }

参考：

[http://emacsist.github.io/2015/12/29/Http-%E5%8D%8F%E8%AE%AE%E4%B8%AD%E7%9A%84Range%E8%AF%B7%E6%B1%82%E5%A4%B4%E4%BE%8B%E5%AD%90/][http_emacsist.github.io_2015_12_29_Http-_E5_8D_8F_E8_AE_AE_E4_B8_AD_E7_9A_84Range_E8_AF_B7_E6_B1_82_E5_A4_B4_E4_BE_8B_E5_AD_90]

转载于:https://www.cnblogs.com/qqdcy/p/6623411.html

[http_emacsist.github.io_2015_12_29_Http-_E5_8D_8F_E8_AE_AE_E4_B8_AD_E7_9A_84Range_E8_AF_B7_E6_B1_82_E5_A4_B4_E4_BE_8B_E5_AD_90]: http://emacsist.github.io/2015/12/29/Http-%E5%8D%8F%E8%AE%AE%E4%B8%AD%E7%9A%84Range%E8%AF%B7%E6%B1%82%E5%A4%B4%E4%BE%8B%E5%AD%90/