多线程下载

网上关于多线程下载的教程基本都是以请求某个具体的文件url路径为例子， 这种例子与实际应用不太相符。
一般我们都是通过请求一个Controller路径或servlet路径来下载文件内容：
这里就涉及到根据请求头rang返回客户端需要的字节数：

服务端代码：

package org.wenchj.servlet;

import java.io.File;
import java.io.FileInputStream;
import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.OutputStream;
import java.io.UnsupportedEncodingException;

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;

import org.apache.commons.io.FileUtils;
import org.apache.commons.lang3.ArrayUtils;

public class DownloadServlet extends HttpServlet {
	private static final long serialVersionUID = 1L;

	public void download(HttpServletRequest request, HttpServletResponse response) {
		try {
			String filePath = request.getSession().getServletContext().getRealPath("/115com_v2.1.1.exe");
			System.out.println(filePath);

			File f = new File(filePath);
			FileInputStream fis = new FileInputStream(f);
			response.reset();
			// 告诉客户端允许断点续传多线程连接下载
			// 响应的格式是:
			// Accept-Ranges: bytes
			response.setHeader("Accept-Ranges", "bytes");
			long p = 0;
			long l = 0;
			l = f.length();
			// 如果是第一次下,还没有断点续传,状态是默认的 200,无需显式设置
			// 响应的格式是:
			// HTTP/1.1 200 OK
			if (request.getHeader("Range") != null) // 客户端请求的下载的文件块的开始字节
			{
				// 如果是下载文件的范围而不是全部,向客户端声明支持并开始文件块下载
				// 要设置状态
				// 响应的格式是:
				// HTTP/1.1 206 Partial Content
				response.setStatus(javax.servlet.http.HttpServletResponse.SC_PARTIAL_CONTENT);// 206

				// 从请求中得到开始的字节
				// 请求的格式是:
				// Range: bytes=[文件块的开始字节]-
				p = Long.parseLong(request.getHeader("Range").replaceAll("bytes=", "").replaceAll("-", ""));
			}

			// 下载的文件(或块)长度
			// 响应的格式是:
			// Content-Length: [文件的总大小] - [客户端请求的下载的文件块的开始字节]
			response.setHeader("Content-Length", new Long(l - p).toString());

			if (p != 0) {
				// 不是从最开始下载,
				// 响应的格式是:
				// Content-Range: bytes [文件块的开始字节]-[文件的总大小 - 1]/[文件的总大小]
				response.setHeader(
						"Content-Range",
						"bytes " + new Long(p).toString() + "-" + new Long(l - 1).toString() + "/"
								+ new Long(l).toString());
			}

			// response.setHeader("Connection", "Close"); //如果有此句话不能用 IE 直接下载

			// 使客户端直接下载
			// 响应的格式是:
			// Content-Type: application/octet-stream
			response.setContentType("application/octet-stream");

			// 为客户端下载指定默认的下载文件名称
			// 响应的格式是:
			// Content-Disposition: attachment;filename="[文件名]"
			// response.setHeader("Content-Disposition",
			// "attachment;filename=\"" + s.substring(s.lastIndexOf("\\") + 1) +
			// "\""); //经测试 RandomAccessFile 也可以实现,有兴趣可将注释去掉,并注释掉
			// FileInputStream 版本的语句
			// response.setHeader("Content-Disposition", "attachment;filename=\"" + app.getName() + ".apk\"");

			// 可选
			setFileAttachmentHeader(response, f.getName());

			// raf.seek(p);
			fis.skip(p);

			byte[] b = new byte[1024];
			int i;

			// while ( (i = raf.read(b)) != -1 ) //经测试 RandomAccessFile
			// 也可以实现,有兴趣可将注释去掉,并注释掉 FileInputStream 版本的语句
			while ((i = fis.read(b)) != -1) {
				response.getOutputStream().write(b, 0, i);
			}
			// raf.close();//经测试 RandomAccessFile 也可以实现,有兴趣可将注释去掉,并注释掉
			// FileInputStream 版本的语句
			fis.close();

		} catch (UnsupportedEncodingException e) {
			e.printStackTrace();
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		} catch (Exception e) {
			e.printStackTrace();
		}

	}

	@Override
	protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
		download2(request, response);
		// download2(request, response);
	}

	private void download2(HttpServletRequest request, HttpServletResponse response) throws IOException {
		// String fileName = "/smack_3_3_1.zip";
		String fileName = "/115com_v2.1.1.exe";
		String filePath = request.getSession().getServletContext().getRealPath(fileName);
		System.out.println("[" + Thread.currentThread().getName() + "]" + filePath);

		File f = new File(filePath);

		response.setHeader("Content-Length", String.valueOf(f.length()));
		// 可选
		setFileAttachmentHeader(response, f.getName());

		System.out.println("Range--->" + request.getHeader("Range"));

		byte[] content = FileUtils.readFileToByteArray(f);
		System.out.println("文件总长度content.length-->" + content.length);

		String rangeHeader = request.getHeader("Range");
		if (rangeHeader != null) {

			String[] split = rangeHeader.split("=");
			String[] range = split[1].split("-");
			int start = Integer.valueOf(range[0]);
			int end = Integer.valueOf(range[1]);
			System.out.println("start:" + start + ", end:" + end);

			byte[] temp = new byte[end - start];

			// 截取数组
			temp = ArrayUtils.subarray(content, start, end + 1);

			response.setHeader("Content-Length", String.valueOf(end - start));
			responseFileContent(response, temp);
		} else {
			response.setHeader("Content-Length", String.valueOf(f.length()));
			responseFileContent(response, content);
		}
	}

	@Override
	protected void doPost(HttpServletRequest request, HttpServletResponse response) throws ServletException,
			IOException {
		this.doGet(request, response);
	}

	/*
	 * 返回文件内容
	 */
	private void responseFileContent(HttpServletResponse response, byte[] content) {
		try {
			OutputStream outStream = response.getOutputStream();
			outStream.write(content);
			outStream.flush();
			outStream.close();
		} catch (Exception e) {
			// e.printStackTrace();
		}
	}

	/*
	 * 以附件形式下载(用浏览器下载会弹出对话框)
	 */
	private static void setFileAttachmentHeader(HttpServletResponse response, String fileName) {
		try {
			String encodedFileName = new String(fileName.getBytes(), "ISO8859-1");
			response.setHeader("Content-Disposition", "attachment; filename=\"" + encodedFileName + "\"");
		} catch (UnsupportedEncodingException e) {
			e.printStackTrace();
		}
	}

}

客户端代码：

package org.wenchj.servlet;

import java.io.File;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

public class MulThreadDownloader {

	public static void main(String[] args) throws Exception {
		String path = "http://localhost:8088/download/DownloadServlet";
		int threadsize = 3;
		new MulThreadDownloader().download(path, threadsize);
	}

	private void download(String path, int threadsize) throws Exception {
		URL downpath = new URL(path);
		HttpURLConnection conn = (HttpURLConnection) downpath.openConnection();
		conn.setConnectTimeout(5000);
		conn.setRequestMethod("GET");
		if (conn.getResponseCode() == 200) {
			int length = conn.getContentLength();// 获取网络文件的长度
			System.out.println("获取网络文件的长度" + length);
			File file = new File("115com_v2.1.1.exe");
			RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");// 生成本地文件
			accessFile.setLength(length);
			accessFile.close();
			// 计算每条线程负责下载的数据量
			int block = (length % threadsize) == 0 ? length / threadsize : (length / threadsize) + 1;
			System.out.println("block:" + block);
			for (int threadid = 0; threadid < threadsize; threadid++) {
				new DownloadThread(threadid, downpath, block, file).start();
			}
		}
	}

	// 负责下载操作
	private final class DownloadThread extends Thread {
		private int threadid;
		private URL downpath;
		private int block;
		private File file;

		public DownloadThread(int threadid, URL downpath, int block, File file) {
			this.threadid = threadid;
			this.downpath = downpath;
			this.block = block;
			this.file = file;
		}

		@Override
		public void run() {
			// if (threadid == 0) {
			// try {
			// System.out.println("线程" + threadid + "睡眠20s...");
			// Thread.sleep(20 * 1000);
			// } catch (InterruptedException e) {
			// e.printStackTrace();
			// }
			// }

			int startposition = threadid * block;// 从网络文件的什么位置开始下载数据
			int endposition = ((threadid + 1) * block) - 1;// 下载到网络文件的什么位置结束
			// 指示该线程要从网络文件的startposition位置开始下载，下载到endposition位置结束
			// Range:bytes=startposition-endposition
			try {
				RandomAccessFile accessFile = new RandomAccessFile(file, "rwd");
				accessFile.seek(startposition);// 移动指针到文件的某个位置

				HttpURLConnection conn = (HttpURLConnection) downpath.openConnection();
				conn.setConnectTimeout(5000);
				conn.setRequestMethod("GET");
				conn.setRequestProperty("Range", "bytes=" + startposition + "-" + endposition);

				InputStream inStream = conn.getInputStream();
				System.out.println(Thread.currentThread().getName() + ", ContentLength:" + conn.getContentLength());

				byte[] buffer = new byte[1024];
				int len = 0;
				while ((len = inStream.read(buffer)) != -1) {
					accessFile.write(buffer, 0, len);
				}
				accessFile.close();
				inStream.close();

				System.out.println("线程" + (threadid) + "下载完成");
			} catch (Exception e) {
				e.printStackTrace();
			}
		}
	}

}