import java.io.BufferedInputStream;
import java.io.File;
import java.io.FileInputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;

public class ResumeDownload {
	public static final String DOWNLOAD_URL = "http://7xs0af.com1.z0.glb.clouddn.com/High-Wake.mp3";
	public static final String DOWNLOAD_PARENT_PATH = "D:\\test_resume_download\\hi";
	public static final int THREAD_COUNT = 3;

	public static void main(String[] args) {
		try {
			// 获取到下载地址的连接
			URL mUrl = new URL(DOWNLOAD_URL);
			HttpURLConnection conn = (HttpURLConnection) mUrl.openConnection();
			// 获取下载文件的大小
			int fileLen = conn.getContentLength();
			// 通过下载链接获取下载文件的文件名
			String filePathUrl = conn.getURL().getFile();
			String fileName = filePathUrl.substring(filePathUrl.lastIndexOf(File.separator) + 1);
			// 生成下载路径
			String fileDownloadPath = DOWNLOAD_PARENT_PATH + File.separator + fileName;
			// 判断父路径是否存在，不存在就生成
			File file = new File(fileDownloadPath);
			if (!file.getParentFile().exists()) {
				file.getParentFile().mkdirs();
			}
			// 关闭连接
			conn.disconnect();

			/**
			 * 以下为多线程下载，主要原理就是将文件大小均分多块（根据线程数） 每一个线程从不同的起始位置，下载相等大小的文件 主要通过
			 * HttpUrlConnection里面设置Range参数，设置每一个线程下载的范围
			 * setRequestProperty("Range", "bytes=" + startPos + "-" + endPos);
			 */

			int blockSize = fileLen / THREAD_COUNT;
			for (int threadId = 1; threadId <= THREAD_COUNT; threadId++) {
				// 获取每一个线程下载的起始位置和结束位置
				long startPos = (threadId - 1) * blockSize;
				long endPos = threadId * blockSize - 1;
				if (threadId == THREAD_COUNT) {
					endPos = fileLen;
				}

				// 然后通过再不同线程里面实现下载逻辑
				// 具体实现在DownloadThread这个Runnable里面
				new Thread(new DownLoadTask(threadId, startPos, endPos, fileDownloadPath, DOWNLOAD_URL)).start();
			}

		} catch (Exception e) {
			e.printStackTrace();
		}

	}
}

/**
 * 具体下载逻辑
 * 
 * @author Administrator
 *
 */
class DownLoadTask implements Runnable {
	public static final String TEMP_NAME = "_tempfile";
	private int threadId; // 当前线程id
	private long startPos; // 下载的起始位置
	private long endPos; // 下载的结束位置
	private String fileDownloadPath; // 下载文件存放的文件位置
	private String downloadUrl; // 下载链接

	private String tempFilePath; // 记录进度的临时文件路径

	public DownLoadTask(int threadId, long startPos, long endPos, String fileDownloadPath, String downloadUrl) {
		super();
		this.threadId = threadId;
		this.startPos = startPos;
		this.endPos = endPos;
		this.fileDownloadPath = fileDownloadPath;
		this.downloadUrl = downloadUrl;

		this.tempFilePath = fileDownloadPath + TEMP_NAME + threadId;
	}

	@Override
	public void run() {
		try {
			// 记录下载的开始时间
			long startTime = System.currentTimeMillis();

			URL mUrl = new URL(downloadUrl);

			// 为了实现断点下载，在重新下载时从缓存文件里面获取下载的起始位置
			if (getProgress(threadId) != 0) {
				startPos = getProgress(threadId);
			}

			System.out.println("线程" + threadId + "继续下载，开始位置：" + startPos + "结束位置是：" + endPos);

			// HttpUrlConnection的常规操作
			// 要实现断点下载的话，必须要设置mConnection.setRequestProperty("Range", "bytes=" +
			// startPos + "-" + endPos);
			HttpURLConnection mConnection = (HttpURLConnection) mUrl.openConnection();
			mConnection.setRequestMethod("POST");
			mConnection.setReadTimeout(5000);
			mConnection.setRequestProperty("Charset", "UTF-8");
			mConnection.setRequestProperty("Range", "bytes=" + startPos + "-" + endPos);
			mConnection.connect();

			// 如果下载路径不存在的话，则创建文件路径
			File file = new File(fileDownloadPath);
			if (!file.getParentFile().exists()) {
				file.getParentFile().mkdirs();
			}

			// 通过RandomAccessFile对要下载的文件进行读写
			RandomAccessFile downloadFile = new RandomAccessFile(fileDownloadPath, "rw");
			// 写的时候，将光标移到要下载的起始位置
			downloadFile.seek(startPos);

			BufferedInputStream bis = new BufferedInputStream(mConnection.getInputStream());
			int size = 0; // 获取缓存区存放的字节大小
			long len = 0; // 记录本次下载的大小，以便计算本次下载的起始位置移动到了哪里
			byte[] buf = new byte[1024];
			while ((size = bis.read(buf)) != -1) {
				// 累加
				len += size;
				// 然后将缓冲区的内容写到下载文件中
				downloadFile.write(buf, 0, size);
				// 然后将下载的起始位置移动到已经下载完的末尾，写到缓存文件里面去
				setProgress(threadId, startPos + len);
			}

			// 获取下载结束时间，输出
			long curTime = System.currentTimeMillis();
			System.out.println("线程" + threadId + "已经下载完成，耗时：" + (curTime - startTime) + "ms.");

			// 关闭流、文件和连接
			downloadFile.close();
			mConnection.disconnect();
			bis.close();

		} catch (Exception e) {
			e.printStackTrace();
		}
	}

	/**
	 * 从temp文件获取下载进度
	 * 
	 * @param threadId
	 * @return
	 */
	private long getProgress(int threadId) {
		try {
			File markFile = new File(tempFilePath);
			if (!markFile.exists()) {
				return 0;
			}
			FileInputStream fis = new FileInputStream(markFile);
			BufferedInputStream bis = new BufferedInputStream(fis);
			byte[] buf = new byte[1024];
			String startPos = "";
			int len = -1;
			while ((len = bis.read(buf)) != -1) {
				startPos += new String(buf, 0, len);
			}

			// 不关闭流的话，不能删除文件
			fis.close();
			bis.close();

			return Long.parseLong(startPos);

		} catch (Exception e) {
			e.printStackTrace();
		}
		return 0;
	}

	/**
	 * 在temp文件记录下载进度
	 * 
	 * @param threadId
	 * @param startPos
	 */
	private void setProgress(int threadId, long startPos) {
		try {
			File markFile = new File(tempFilePath);
			if (!markFile.getParentFile().exists()) {
				markFile.getParentFile().mkdirs();
			}
			
			RandomAccessFile rr = new RandomAccessFile(markFile, "rw");// 存储下载标记的文件
			String strStartPos = String.valueOf(startPos);
			rr.write(strStartPos.getBytes(), 0, strStartPos.length());
			
			rr.close();
		} catch (Exception e) {
			e.printStackTrace();
		} finally {
			// 当文件下载完成时，即开始位置和结束位置重合时，删除记录进度的缓存文件
			if (startPos >= endPos) {
				File markFile = new File(tempFilePath);
				if (markFile.exists()) {
					System.out.println("markFile delete");
					markFile.delete();
				}
			}
		}

	}
}