Java之多线程下载工具类

1、多线程下载工具类

import java.net.URL;import java.io.InputStream;import java.io.RandomAccessFile;import java.net.HttpURLConnection;public class DownUtil {    //定义下载路径    private String path;    //指定所下载的文件的保存位置    private String targetFile;    //定义下载线程的数量    private int threadNum;    //定义下载线程的对象    private DownThread[] threads;    //下载文件的总大小    private int fileSize;    public DownUtil(String path,String targetFile,int threadNum){        this.path = path;        this.targetFile = targetFile;        this.threadNum = threadNum;        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);        //设置 URL 请求的方法， GET POST HEAD OPTIONS PUT DELETE TRACE 以上方法之一是合法的，具体取决于协议的限制。        conn.setRequestMethod("GET");        //设置一般请求属性。如果已存在具有该关键字的属性，则用新值改写其值。         conn.setRequestProperty("Accpt",         "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");        //设置本地文件大小        file.setLength(fileSize);        file.close();        for(int i = 0;i < threadNum;i++){            //计算每个线程的下载位置            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);            threads[i].start();        }    }    public double getCompleteRate(){        int sumSize = 0;        for(int i = 0;i < threadNum;i++){            sumSize += threads[i].length;        }        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,此类的实例支持对随机访问文件的读取和写入。        private RandomAccessFile currentPart;        //定义该线程已下载的字节数        public int length;        private DownThread(int startPos,int currentPartSize,RandomAccessFile currentPart){            this.startPos = startPos;            this.currentPartSize = currentPartSize;            this.currentPart = currentPart;        }        public void run(){            try{                URL url = new URL(path);                //url.openConnection():返回一个 URLConnection 对象，它表示到 URL 所引用的远程对象的连接。                HttpURLConnection conn = (HttpURLConnection)url.openConnection();                 conn.setConnectTimeout(5 * 1000);                conn.setRequestMethod("GET");                conn.setRequestProperty("Accpt", "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 inputStream = conn.getInputStream();                //inputStream.skip(n);跳过和丢弃此输入流中数据的 n 个字节                inputStream.skip(this.startPos);                 byte[] buffer = new byte[1024];                int hasRead = 0;                //读取网络数据写入本地                while(length < currentPartSize && (hasRead = inputStream.read(buffer)) != -1){                    currentPart.write(buffer, 0, hasRead);                    length += hasRead;                }                currentPart.close();                inputStream.close();            }            catch(Exception e){                e.printStackTrace();            }        }    }}

2、主程序

package charpter1;public class MultiThreadDown {       public static void main(String[] args) throws Exception {        final DownUtil downUtil = new DownUtil("https://ss0.baidu.com/"                + "6ONWsjip0QIZ8tyhnq/it/u=1927822194,1885130936&fm=80&w=179&h=119&img.JPEG","11.JPEG",4);        downUtil.downLoad();        new Thread(() -> {            while(downUtil.getCompleteRate() < 1)            {                System.out.println("已完成: "+ downUtil.getCompleteRate());                try{//每隔0.1秒查询一次任务                    Thread.sleep(1000);                }                catch(Exception e){                    e.printStackTrace();                }            }            System.out.println("已完成: "+ downUtil.getCompleteRate());        }).start();    }}

3、下载的图片：