Java多线程下载实现

package com.test;

import java.io.FileNotFoundException;
import java.io.IOException;
import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.MalformedURLException;
import java.net.ProtocolException;
import java.net.URL;

public class DownLoadUtil {

	private String urlStr;
	private String targetFile;

	private int threadNum;
	private DownloadThread[] threads;

	private int fileSize;
	
	
	
	public DownLoadUtil(String urlStr, String targetFile, int threadNum) {
		this.urlStr = urlStr;
		this.targetFile = targetFile;
		this.threadNum = threadNum;
		this.threads = new DownloadThread[threadNum];
	}

	public void download() {
		try {
			HttpURLConnection conn = this.getHttpURLConnection();
			conn.setRequestProperty("Connection", "Keep-Alive");
			fileSize = conn.getContentLength();
			conn.disconnect();
			int currentPartSize = fileSize/threadNum + 1;
			RandomAccessFile file = new RandomAccessFile(targetFile, "rw");
			file.setLength(fileSize);
			file.close();
			
			for(int i=0; i<threadNum; i++) {
				int startPosition = currentPartSize * i;
				RandomAccessFile currentFile = new RandomAccessFile(targetFile, "rw");
				currentFile.seek(startPosition);
				threads[i] = new DownloadThread(startPosition, currentFile, currentPartSize);
				new Thread(threads[i]).start();
			}
			
		} catch (FileNotFoundException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
	}

	private class DownloadThread implements Runnable {

		private int startPosition;
		private RandomAccessFile downloadFile;
		private int currentPartSize;
		private int hasDownSize;

		public DownloadThread(int startPosition, RandomAccessFile downloadFile,
				int currentPartSize) {
			this.startPosition = startPosition;
			this.downloadFile = downloadFile;
			this.currentPartSize = currentPartSize;
		}

		@Override
		public void run() {
			try {
				HttpURLConnection conn = getHttpURLConnection();
				InputStream in = conn.getInputStream();
				in.skip(this.startPosition);
				byte[] buffer = new byte[1024];
				int hasRead = 0;
				while (hasDownSize < currentPartSize &&
						(hasRead = in.read(buffer)) != -1) {
					downloadFile.write(buffer, 0, hasRead);
					hasDownSize += hasRead;
				}
				downloadFile.close();
				in.close();
			} catch (IOException e) {
				e.printStackTrace();
			}
		}

	}

	private HttpURLConnection getHttpURLConnection() {
		HttpURLConnection conn = null;
		try {
			URL url = new URL(urlStr);
			conn = (HttpURLConnection) url.openConnection();
			conn.setConnectTimeout(5 * 1000);
			conn.setRequestMethod("GET");
			conn.setRequestProperty(
					"Accept",
					"image/gif, image/jpeg, image/pjpeg, image/pjpeg, "
							+ "application/x-shockwave-flash, application/xaml+xml, "
							+ "application/vnd.ms-xpsdocument, application/x-ms-xbap, "
							+ "application/x-ms-application, application/vnd.ms-excel, "
							+ "application/vnd.ms-powerpoint, application/msword, */*");
			conn.setRequestProperty("Accept-Language", "zh-CN");
			conn.setRequestProperty("Charset", "UTF-8");
		} catch (MalformedURLException e) {
			e.printStackTrace();
		} catch (ProtocolException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
		return conn;
	}
}