多线程断点下载

  1. 多线程下载

     public class MultiThreadDownloader {

     private URL url;        // 目标地址
     private File file;        // 本地文件
     private long threadLen;    // 每个线程下载多少

     private static final int THREAD_AMOUNT = 3;                // 线程数
     private static final String DIR_PATH = "F:/Download";    // 下载目录

     public MultiThreadDownloader(String address) throws IOException {    
         // 记住下载地址
         url = new URL(address);                                                            
         // 截取地址中的文件名, 创建本地文件
         file = new File(DIR_PATH, address.substring(address.lastIndexOf("/") + 1));        
     }

     public void download() throws IOException {
         HttpURLConnection conn = (HttpURLConnection) url.openConnection();
         conn.setConnectTimeout(3000);

         // 获取文件总长度
         long totalLen = conn.getContentLength();                    
         // 计算每个线程要下载的长度
         // 总长度 如果能整除 线程数, 每条线程下载的长度就是 总长度 / 线程数
         // 总长度 如果不能整除 线程数, 那么每条线程下载长度就是 总长度 / 线程数 + 1
         threadLen = (totalLen + THREAD_AMOUNT - 1) / THREAD_AMOUNT;    

         // 在本地创建一个和服务端大小相同的文件
         RandomAccessFile raf = new RandomAccessFile(file, "rw");    
         // 设置文件的大小, 写入了若干个0
         raf.setLength(totalLen);                                    
         raf.close();

         // 按照线程数循环
         for (int i = 0; i < THREAD_AMOUNT; i++) {
             // 开启线程, 每个线程将会下载一部分数据到本地文件中 
             new DownloadThread(i).start();       
         }     
     }

     private class DownloadThread extends Thread {
         // 用来标记当前线程是下载任务中的第几个线程
         private int id;     

         public DownloadThread(int id) {
             this.id = id;
         }

         public void run() {
             // 从临时文件读取当前线程已完成的进度

             long start = id * threadLen;     
             long end = id * threadLen + threadLen - 1;
             System.out.println("线程" + id + ": " + start + "-" + end);

             try {
                 HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                 conn.setConnectTimeout(3000);
                 // 设置当前线程下载的范围(start和end都包含)
                 conn.setRequestProperty("Range", "bytes=" + start + "-" + end);    

                 InputStream in = conn.getInputStream();
                 // 随机读写文件, 用来向本地文件写出
                 RandomAccessFile raf = new RandomAccessFile(file, "rw");        
                 // 设置保存数据的位置
                 raf.seek(start);                                                

                 // 每次拷贝100KB
                 byte[] buffer = new byte[1024 * 100];    
                 int len;
                 while ((len = in.read(buffer)) != -1) {
                     // 从服务端读取数据, 写到本地文件
                     raf.write(buffer, 0, len);            
                 }
                 raf.close();

                 System.out.println("线程" + id + "下载完毕");
             } catch (IOException e) {
                 e.printStackTrace();
             }
         }
     }

     public static void main(String[] args) throws IOException {
         new MultiThreadDownloader("http://192.168.1.240:8080/14.Web/android-sdk_r17-windows.zip").download();
     }
 }

  2. 断点下载

     public class BreakpointDownloader {
     // 下载目录
     private static final String DIR_PATH = "F:/Download";    
     // 总线程数
     private static final int THREAD_AMOUNT = 3;                

     // 目标下载地址
     private URL url;            
     // 本地文件
     private File dataFile;        
     // 用来存储每个线程下载的进度的临时文件
     private File tempFile;        
     // 每个线程要下载的长度
     private long threadLen;      
     // 总共完成了多少  
     private long totalFinish;    
     // 服务端文件总长度
     private long totalLen;        
     // 用来记录开始下载时的时间
     private long begin;            

     public BreakpointDownloader(String address) throws IOException {    
         url = new URL(address);                                                            
         dataFile = new File(DIR_PATH, address.substring(address.lastIndexOf("/") + 1));    
         tempFile = new File(dataFile.getAbsolutePath() + ".temp");                        
     }

     public void download() throws IOException {
         HttpURLConnection conn = (HttpURLConnection) url.openConnection();
         conn.setConnectTimeout(3000);

         totalLen = conn.getContentLength();                                    
         threadLen = (totalLen + THREAD_AMOUNT - 1) / THREAD_AMOUNT;            

         if (!dataFile.exists()) {                                            
             RandomAccessFile raf = new RandomAccessFile(dataFile, "rws");    
             raf.setLength(totalLen);                                        
             raf.close();
         }

         if (!tempFile.exists()) {                                            
             RandomAccessFile raf = new RandomAccessFile(tempFile, "rws");    
             for (int i = 0; i < THREAD_AMOUNT; i++)                           
                 raf.writeLong(0);                                            
             raf.close();
         }

         for (int i = 0; i < THREAD_AMOUNT; i++) { 
             new DownloadThread(i).start();       
         } 

         // 记录开始时间
         begin = System.currentTimeMillis();        
     }

     private class DownloadThread extends Thread {
         // 用来标记当前线程是下载任务中的第几个线程
         private int id;     

         public DownloadThread(int id) {
             this.id = id;
         }

         public void run() {
             try {
                 RandomAccessFile tempRaf = new RandomAccessFile(tempFile, "rws");        
                 tempRaf.seek(id * 8);                        
                 long threadFinish = tempRaf.readLong();        
                 synchronized(BreakpointDownloader.this) {    
                     totalFinish += threadFinish;            
                 }

                 long start = id * threadLen + threadFinish;      
                 long end = id * threadLen + threadLen - 1;       
                 System.out.println("线程" + id + ": " + start + "-" + end);

                 HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                 conn.setConnectTimeout(3000);
                 conn.setRequestProperty("Range", "bytes=" + start + "-" + end);        

                 InputStream in = conn.getInputStream();                              
                 RandomAccessFile dataRaf = new RandomAccessFile(dataFile, "rws");    
                 dataRaf.seek(start);                                                

                 byte[] buffer = new byte[1024 * 100];            
                 int len;
                 while ((len = in.read(buffer)) != -1) {
                     dataRaf.write(buffer, 0, len);               
                     threadFinish += len;                        
                     tempRaf.seek(id * 8);                        
                     tempRaf.writeLong(threadFinish);           
                     synchronized(BreakpointDownloader.this) {   
                         totalFinish += len;                        
                     }
                 }
                 dataRaf.close();
                 tempRaf.close();

                 System.out.println("线程" + id + "下载完毕");
                 // 如果已完成长度等于服务端文件长度(代表下载完成)
                 if (totalFinish == totalLen) {                    
                     System.out.println("下载完成, 耗时: " + (System.currentTimeMillis() - begin));
                     // 删除临时文件
                     tempFile.delete();                            
                 }
             } catch (IOException e) {
                 e.printStackTrace();
             }
         }
     }

     public static void main(String[] args) throws IOException {
         new BreakpointDownloader("http://192.168.1.240:8080/14.Web/android-sdk_r17-windows.zip").download();
     }
 }

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