多线程下载文件！Demo

package mutithreaddown;

import java.io.InputStream;
import java.io.RandomAccessFile;
import java.net.HttpURLConnection;
import java.net.URL;
import java.util.concurrent.CountDownLatch;

public class MutiThreadDownLoad {
	/**
     * 同时下载的线程数
     */
    private int threadCount;
    /**
     * 服务器请求路径
     */
    private String serverPath;
    /**
     * 本地路径
     */
    private String localPath;
    /**
     * 线程计数同步辅助
     */
    private CountDownLatch latch;
    
    public MutiThreadDownLoad(int threadCount, String serverPath, String localPath, CountDownLatch latch) {
        this.threadCount = threadCount;
        this.serverPath = serverPath;
        this.localPath = localPath;
        this.latch = latch;
    }

    public void executeDownLoad() {

        try {
            URL url = new URL(serverPath);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setConnectTimeout(5000);
            conn.setRequestMethod("GET");
            int code = conn.getResponseCode();
            if (code == 200) {
                //服务器返回的数据的长度，实际上就是文件的长度,单位是字节
                int length = conn.getContentLength();
                System.out.println("文件总长度:" + length + "字节(B)");
                RandomAccessFile raf = new RandomAccessFile(localPath, "rwd");
                //指定创建的文件的长度
                raf.setLength(length);
                raf.close();
                //分割文件
                int blockSize = length / threadCount;
                for (int threadId = 1; threadId <= threadCount; threadId++) {
                    //第一个线程下载的开始位置
                    int startIndex = (threadId - 1) * blockSize;
                    int endIndex = startIndex + blockSize - 1;
                    if (threadId == threadCount) {
                        //最后一个线程下载的长度稍微长一点
                        endIndex = length;
                    }
                    System.out.println("线程" + threadId + "下载:" + startIndex + "字节~" + endIndex + "字节");
                    new DownLoadThread(threadId, startIndex, endIndex).start();
                }

            }

        } catch (Exception e) {
            e.printStackTrace();
        }
    }
    
    /**
     * 内部类用于实现下载
     */
    public class DownLoadThread extends Thread {
        /**
         * 线程ID
         */
        private int threadId;
        /**
         * 下载起始位置
         */
        private int startIndex;
        /**
         * 下载结束位置
         */
        private int endIndex;

        public DownLoadThread(int threadId, int startIndex, int endIndex) {
            this.threadId = threadId;
            this.startIndex = startIndex;
            this.endIndex = endIndex;
        }


        @Override
        public void run() {

            try {
                System.out.println("线程" + threadId + "正在下载...");
                URL url = new URL(serverPath);
                HttpURLConnection conn = (HttpURLConnection) url.openConnection();
                conn.setRequestMethod("GET");
                //请求服务器下载部分的文件的指定位置
                conn.setRequestProperty("Range", "bytes=" + startIndex + "-" + endIndex);
                conn.setConnectTimeout(5000);
                int code = conn.getResponseCode();

                System.out.println("线程" + threadId + "请求返回code=" + code);

                InputStream is = conn.getInputStream();//返回资源
                RandomAccessFile raf = new RandomAccessFile(localPath, "rwd");
                //随机写文件的时候从哪个位置开始写
                raf.seek(startIndex);//定位文件

                int len = 0;
                byte[] buffer = new byte[1024];
                while ((len = is.read(buffer)) != -1) {
                    raf.write(buffer, 0, len);
                }
                is.close();
                raf.close();
                System.out.println("线程" + threadId + "下载完毕");
                //计数值减一
                latch.countDown();

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

        }
    }

    
    public static void main(String[] args) {
		int threadSize=4;
		String serverPath = "http://file.ws.126.net/3g/client/netease_newsreader_android.apk";
		String localPath = "NewsReader.apk";
		CountDownLatch latch = new CountDownLatch(threadSize);
		MutiThreadDownLoad m = new MutiThreadDownLoad(threadSize, serverPath, localPath, latch);
		long startTime = System.currentTimeMillis();
		try {
            m.executeDownLoad();
            latch.await();
        } catch (InterruptedException e) {
            e.printStackTrace();
        }
        long endTime = System.currentTimeMillis();
        System.out.println("全部下载结束,共耗时" + (endTime - startTime) / 1000 + "s");

	}
    
}