Java多线程下载文件Demo

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原文：http://www.cnblogs.com/warrior4236/p/5686404.html

一：需求分析

1：下载远程资源文件，通过多线程下载，达到高效的目的。

2：使用5个线程分别下载文件的不同部分。

二：定义成员变量以及初始化变量

// 定义成员变量    private String path; // 远程资源路径    private String targetPath; // 本地存储路径    private DownFileThread[] threads; // 线程list    private int threadNum; // 线程数量    private long length; // 下载的文件大小    // 构造初始化    public DownloadUtil(String path, String targetPath, int threadNum) {        super();        this.path = path;        this.targetPath = targetPath;        this.threads = new DownFileThread[threadNum];        this.threadNum = threadNum;    }

三：多线程下载文件

// 多线程下载文件资源    public void download() {        URL url;        try {            url = new URL(path);            HttpURLConnection conn = (HttpURLConnection) url.openConnection();            conn.setReadTimeout(5 * 1000); // 设置超时时间为5秒            conn.setRequestMethod("GET");            conn.setRequestProperty("connection", "keep-alive");            conn.setRequestProperty("accept", "*/*");            // 获取远程文件的大小            length = conn.getContentLength();            conn.disconnect();            // 设置本地文件大小            RandomAccessFile targetFile = new RandomAccessFile(targetPath, "rw");            targetFile.setLength(length);            // 每个线程下载大小            long avgPart = length / threadNum + 1;            // 下载文件            for (int i = 0; i < threadNum; i++) {                long startPos = avgPart * i;                RandomAccessFile targetTmp = new RandomAccessFile(targetPath,                        "rw");                targetTmp.seek(startPos); // 分段下载                threads[i] = new DownFileThread(startPos, targetTmp, avgPart);                threads[i].start();            }        } catch (Exception e) {            e.printStackTrace();        }    }

启动多个线程，每个线程下载文件的一部分，并发执行。

四：监控文件下载进度

// 监控下载进度    public double getDownRate() {        int currentSize = 0;        for (int i = 0; i < threadNum; i++) {            currentSize += threads[i].length;        }        return currentSize * 1.0 / length;    }

五：定义线程类

// 定义线程类    class DownFileThread extends Thread {        private long startPos;        private RandomAccessFile raf;        private long size;        private long length;        public DownFileThread(long startPos, RandomAccessFile raf, long size) {            super();            this.startPos = startPos;            this.raf = raf;            this.size = size;        }        public void run() {            URL url;            try {                url = new URL(path);                HttpURLConnection conn = (HttpURLConnection) url                        .openConnection();                conn.setReadTimeout(5 * 1000); // 设置超时时间为5秒                conn.setRequestMethod("GET");                conn.setRequestProperty("connection", "keep-alive");                conn.setRequestProperty("accept", "*/*");                InputStream in = conn.getInputStream();                in.skip(this.startPos);                byte[] buf = new byte[1024];                int hasRead = 0;                while (length < size && (hasRead = in.read(buf)) != -1) {                    raf.write(buf, 0, hasRead);                    length += hasRead;                }                raf.close();                in.close();            } catch (Exception e) {                e.printStackTrace();            }        }    }

六：测试多线程下载文件

public static void main(String[] args) {        String path = "http://localhost:8080/healthcare/html/HealthCare.war";        String targetPath = "D:/healthcare.war";        final DownloadUtil download = new DownloadUtil(path, targetPath, 4);        download.download();        // 主线程负责下载文件，在启动一个线程负责监控下载的进度        new Thread(new Runnable() {            @Override            public void run() {                while (download.getDownRate() < 1) {                    System.out.println(download.getDownRate());                    try {                        Thread.sleep(200); // 200毫秒扫描一次                    } catch (InterruptedException e) {                        e.printStackTrace();                    }                }            }        }).start();    }

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