多线程断点下载的代码实现

import java.io.BufferedReader;
import java.io.File;
import java.io.FileInputStream;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

public class MutileThreadDownload {
	/**
	 * 线程的数量
	 */
	private static int threadCount = 3;

	/**
	 * 每个下载区块的大小
	 */
	private static long blocksize;

	/**
	 * 正在运行的线程的数量
	 */
	private static int runningThreadCount;

	/**
	 * @param args
	 * @throws Exception
	 */
	public static void main(String[] args) throws Exception {
		// 服务器文件的路径
		String path = "http://127.0.0.1:8080/demo.exe";
		URL url = new URL(path);
		HttpURLConnection conn = (HttpURLConnection) url.openConnection();
		conn.setRequestMethod("GET");
		conn.setConnectTimeout(5000);
		int code = conn.getResponseCode();
		if (code == 200) {
			long size = conn.getContentLength();// 得到服务端返回的文件的大小
			blocksize = size / threadCount;
			// 1.首先在本地创建一个大小跟服务器一模一样的空白文件。
			File file = new File("temp.exe");
			<span style="color:#ff0000;">RandomAccessFile raf = new RandomAccessFile(file, "rw");
</span>			raf.setLength(size);
			// 2.开启若干个子线程分别去下载对应的资源。
			runningThreadCount = threadCount;
			for (int i = 1; i <= threadCount; i++) {
				long startIndex = (i - 1) * blocksize;
				long endIndex = i * blocksize - 1;
				if (i == threadCount) {
					// 最后一个线程
					endIndex = size - 1;
				}
				new DownloadThread(path, i, startIndex, endIndex).start();
			}
		}
		conn.disconnect();
	}

	private static class DownloadThread extends Thread {
		private int threadId;
		private long startIndex;
		private long endIndex;
		private String path;

		public DownloadThread(String path, int threadId, long startIndex,
				long endIndex) {
			this.path = path;
			this.threadId = threadId;
			this.startIndex = startIndex;
			this.endIndex = endIndex;
		}

		@Override
		public void run() {
			try {
				// 当前线程下载的总大小
				int total = 0;
				File positionFile = new File(threadId + ".txt");
				URL url = new URL(path);
				HttpURLConnection conn = (HttpURLConnection) url.openConnection();
				conn.setRequestMethod("GET");
				// 接着从上一次的位置继续下载数据
				if (positionFile.exists() && positionFile.length() > 0) {// 判断是否有记录
					FileInputStream fis = new FileInputStream(positionFile);
					BufferedReader br = new BufferedReader(new InputStreamReader(fis));
					// 获取当前线程上次下载的总大小是多少
					String lasttotalstr = br.readLine();
					int lastTotal = Integer.valueOf(lasttotalstr);
					startIndex += lastTotal;
					total += lastTotal;// 加上上次下载的总大小。
					fis.close();
				}

				conn.setRequestProperty("Range", "bytes=" + startIndex + "-"+ endIndex);
				conn.setConnectTimeout(5000);
				int code = conn.getResponseCode();
				InputStream is = conn.getInputStream();
				File file = new File("temp.exe");
				RandomAccessFile raf = new RandomAccessFile(file, "rw");
				// 指定文件开始写的位置。
				raf.seek(startIndex);
				int len = 0;
				byte[] buffer = new byte[512];
				while ((len = is.read(buffer)) != -1) {
					RandomAccessFile rf = new RandomAccessFile(positionFile,
							"rwd");
					raf.write(buffer, 0, len);
					total += len;
					rf.write(String.valueOf(total).getBytes());
					rf.close();
				}
				is.close();
				raf.close();

			} catch (Exception e) {
				e.printStackTrace();
			} finally {
				// 只有所有的线程都下载完毕后 才可以删除记录文件。
				synchronized (MutileThreadDownload.class) {
					System.out.println("线程" + threadId + "下载完毕了");
					runningThreadCount--;
					if (runningThreadCount < 1) {
						System.out.println("所有的线程都工作完毕了。删除临时记录的文件");
						for (int i = 1; i <= threadCount; i++) {
							File f = new File(i + ".txt");
							System.out.println(f.delete());
						}
					}
				}

			}
		}
	}
}