实现一个多线程下载

来源：互联网发布：有耽美小说的软件编辑：程序博客网时间：2024/05/01 03:29

运用我们学过的HttpURLConnection网络编程，可以实现多线程下载：
代码如下：

public class DownUtil {    // 定义下载资源的路径    private String path;    // 指定所下载的文件的保存位置    private String targetFile;    // 定义需要使用多少线程下载资源    private int threadNum;    // 定义下载的线程对象    private DownThread[] threads;    // 定义下载的文件的总大小    private int fileSize;    public DownUtil(String path, String targetFile, int threadNum) {        this.path = path;        this.threadNum = threadNum;        // 初始化threads数组        threads = new DownThread[threadNum];        this.targetFile = targetFile;    }    public void download() throws Exception {        URL url = new URL(path);        HttpURLConnection 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");        conn.setRequestProperty("Connection", "Keep-Alive");        // 得到文件大小        fileSize = conn.getContentLength();        conn.disconnect();        int currentPartSize = fileSize / threadNum + 1;        RandomAccessFile file = new RandomAccessFile(targetFile, "rw");        // 设置本地文件的大小        file.setLength(fileSize);        file.close();        for (int i = 0; i < threadNum; i++) {            // 计算每条线程的下载的开始位置            int startPos = i * currentPartSize;            // 每个线程使用一个RandomAccessFile进行下载            RandomAccessFile currentPart = new RandomAccessFile(targetFile,                    "rw");            // 定位该线程的下载位置            currentPart.seek(startPos);            // 创建下载线程            threads[i] = new DownThread(startPos, currentPartSize, currentPart);            // 启动下载线程            threads[i].start();        }    }    // 获取下载的完成百分比    public double getCompleteRate() {        // 统计多条线程已经下载的总大小        int sumSize = 0;        for (int i = 0; i < threadNum; i++) {            sumSize += threads[i].length;        }        // 返回已经完成的百分比        return sumSize * 1.0 / fileSize;    }    private class DownThread extends Thread {        // 当前线程的下载位置        private int startPos;        // 定义当前线程负责下载的文件大小        private int currentPartSize;        // 当前线程需要下载的文件块        private RandomAccessFile currentPart;        // 定义已经该线程已下载的字节数        public int length;        public DownThread(int startPos, int currentPartSize,                RandomAccessFile currentPart) {            this.startPos = startPos;            this.currentPartSize = currentPartSize;            this.currentPart = currentPart;        }        @Override        public void run() {            try {                URL url = new URL(path);                HttpURLConnection 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");                InputStream inStream = conn.getInputStream();                // 跳过startPos个字节，表明该线程只下载自己负责哪部分文件。                inStream.skip(this.startPos);                byte[] buffer = new byte[1024];                int hasRead = 0;                // 读取网络数据，并写入本地文件                while (length < currentPartSize                        && (hasRead = inStream.read(buffer)) != -1) {                    currentPart.write(buffer, 0, hasRead);                    // 累计该线程下载的总大小                    length += hasRead;                }                currentPart.close();                inStream.close();            } catch (Exception e) {                e.printStackTrace();            }        }    }}

我们在MultiThreadDown中实现调用下载：

public class MultiThreadDown {    public static void main(String[] args) throws Exception {        // 初始化DownUtil对象        final DownUtil downUtil = new DownUtil(                "http://www.qingkai.com/tp/bbs/200708/20070830162156277.jpg",                "d:/new.jpg", 4);        // 开始下载        downUtil.download();        new Thread(() -> {            while (downUtil.getCompleteRate() < 1) {                // 每隔0.1秒查询一次任务的完成进度，                // GUI程序中可根据该进度来绘制进度条                System.out.println("已完成：" + downUtil.getCompleteRate());                try {                    Thread.sleep(1000);                } catch (Exception ex) {                }            }        }).start();    }}

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