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多个线程下载同一个资源文件

一个线程下载一部分

package sqy;import java.io.InputStream;import java.io.RandomAccessFile;import java.net.HttpURLConnection;import java.net.URL;import java.util.concurrent.locks.ReentrantLock;/** * @author suqinyi * @Date 2021/7/1 */public class DownloadUnite {
       static ReentrantLock lock = new ReentrantLock();    //此方法用于获取一个HttpURLConnection对象    public static HttpURLConnection getConn() throws Exception {
           String path = "https://www.baidu.com/img/flexible/logo/pc/result.png";        URL url = new URL(path);        HttpURLConnection conn = (HttpURLConnection) url.openConnection();        conn.setRequestMethod("GET");        conn.setConnectTimeout(5000);        conn.setRequestProperty("Accept", "application/x-jpg");        return conn;    }    //此方法用于创建多线程并且进行下载    public void download() throws Exception {
           RandomAccessFile file = new RandomAccessFile("picture.jpg", "rw");        HttpURLConnection conn = DownloadUnite.getConn();        int size = conn.getContentLength();        conn.disconnect();        int parts = size / (4 * 1024);        DownloadThread[] thread = new DownloadThread[4];        int starts;        int ends;        System.out.println(size);        for (int i = 0; i < 4; i++) {
               starts = i * parts * 1024;            ends = (i + 1) * parts * 1024 - 1;            if (i == 3) {
                   ends = size;            }            thread[i] = new DownloadThread(starts, ends, file);            thread[i].start();        }    }    //将循环读取IO流的操作写入read方法中，用于加锁    public static int read(int starts, int ends, RandomAccessFile file, InputStream pip) throws Exception {
           lock.lock();        int hasRead = 0;        byte[] buffer = new byte[1024];        file.seek(starts);        hasRead = pip.read(buffer);        starts += hasRead;        file.write(buffer, 0, hasRead);        lock.unlock();        return starts;    }    //多线程下载类    class DownloadThread extends Thread {
           int starts;        int ends;        RandomAccessFile file;        public DownloadThread(int starts, int ends, RandomAccessFile file) {
               this.starts = starts;            this.ends = ends;            this.file = file;        }        public void run() {
               try {
                   HttpURLConnection conn = (HttpURLConnection) DownloadUnite.getConn();                InputStream pip = conn.getInputStream();                pip.skip(starts);                while ((ends - starts) > 0) {
   //此方法用于加锁                    starts = DownloadUnite.read(starts, ends, file, pip);                }                System.out.println(starts);            } catch (Exception e) {
                   e.printStackTrace();            }        }    }    //主方法用于运行程序    public static void main(String[] args) throws Exception {
           new DownloadUnite().download();    }}

效果：

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