多线程断点下载.md

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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