Java多线程下载文件

public class DownLoad {
	private int threadSize = 3;

	public void URLLoad(String source, String fileName) {
		RandomAccessFile access = null;
		try {
			// 打开连接，获取内容长度，计算每个线程下载长度
			URL url = new URL(source);
			int sourceSize = url.openConnection().getContentLength();
			int threadLoadSize = sourceSize / threadSize;
			// 初始化接受文件
			File file = new File(fileName);
			if (!file.exists()) {
				file.createNewFile();
			}
			access = new RandomAccessFile(file, "rw");
			for (int i = 0; i < sourceSize; i++) {
				access.write(0);
			}
			// 开启线程下载
			for (int i = 0; i < threadSize; i++) {

				if (i != threadSize - 1) {
					new FileLoadThread(threadLoadSize * i, threadLoadSize
							* (i + 1), url.openStream(), new RandomAccessFile(
							file, "rw")).start();
				} else {
					// 最后一个线程下载
					int rem = sourceSize % threadSize;
					new FileLoadThread(threadLoadSize * i, threadLoadSize
							* (i + 1) + rem, url.openStream(), access).start();
				}
			}

		} catch (MalformedURLException e) {
			e.printStackTrace();
		} catch (IOException e) {
			e.printStackTrace();
		}
	}
	//下载线程
	class FileLoadThread extends Thread {
		private int bufferSize = 32;
		private int start;
		private int end;
		private InputStream input;
		private RandomAccessFile access;

		public FileLoadThread(int start, int end, InputStream input,
				RandomAccessFile access) {
			this.start = start;
			this.end = end;
			this.input = input;
			this.access = access;
		}

		@Override
		public void run() {
			System.out.println(Thread.currentThread().getName() + "进入下载");
			System.out.println("start:" + start + ";end:" + end);
			try {
				// 初始化下载，存储进度
				input.skip(start);
				access.seek(start);
				// 要读取总长度
				int readSize = end - start;
				// 已读取总长度
				int useReadSize = 0;
				// 缓冲数组
				byte[] buffer = new byte[bufferSize];
				int count = 0;
				// 开始下载
				while ((count = input.read(buffer)) != -1) {
					useReadSize += count;
					// 如果已读取总长度大于要读取长度，则在缓冲数组中截取需要读取的数据
					if (useReadSize > readSize) {
						int size = count - (useReadSize - readSize);
						access.write(buffer, 0, size);
						break;
					} else {
						access.write(buffer);
					}
				}
			} catch (FileNotFoundException e) {
				e.printStackTrace();
			} catch (IOException e) {
				e.printStackTrace();
			} finally {
				try {
					if (input != null) {
						input.close();
					}
					if (access != null) {
						access.close();
					}
				} catch (IOException e) {
					e.printStackTrace();
				}
			}
			System.out.println(Thread.currentThread().getName() + "下载完成");
		}
	}

	public static void main(String[] args) {
		DownLoad load = new DownLoad();
		load.URLLoad("http://www.baidu.com/", "D:/juhua.html");
	}
}