Android 基于Http的多线程下载的实现

Dear 丶 2021-09-29 09:20 260阅读 0赞

a、对于网络上的一个资源，首先发送一个请求，从返回的Content-Length中回去需要下载文件的大小，然后根据文件大小创建一个文件。

this.fileSize = conn.getContentLength();// 根据响应获取文件大小
    File dir = new File(dirStr);
    this.localFile = new File(dir, filename);
    RandomAccessFile raf = new RandomAccessFile(this.localFile, "rw");
    raf.setLength(fileSize);
    raf.close();

b、根据线程数和文件大小，为每个线程分配下载的字节区间，然后每个线程向服务器发送请求，获取这段字节区间的文件内容。

conn.setRequestProperty("Range", "bytes=" + startPos + "-"
                            + endPos);// 设置获取实体数据的范围

c、利用RandomAccessFile的seek方法，多线程同时往一个文件中写入字节。

raf.seek(startPos);
    while ((len = is.read(buf)) != -1)
    {
        raf.write(buf, 0, len);
    }

分析完了原理就很简单了，我封装了一个类，利用这个类的实例进行下载，所需参数：下载资源的URI， 本地文件路径，线程的数量。

package com.zhy.mutilthread_download;
    
    import java.io.File;
    import java.io.IOException;
    import java.io.InputStream;
    import java.io.RandomAccessFile;
    import java.net.HttpURLConnection;
    import java.net.URL;
    
    public class MultipartThreadDownloador
    {
    
        /**
         * 需要下载资源的地址
         */
        private String urlStr;
        /**
         * 下载的文件
         */
        private File localFile;
        /**
         * 需要下载文件的存放的本地文件夹路径
         */
        private String dirStr;
        /**
         * 存储到本地的文件名
         */
        private String filename;
    
        /**
         * 开启的线程数量
         */
        private int threadCount;
        /**
         * 下载文件的大小
         */
        private long fileSize;
    
        public MultipartThreadDownloador(String urlStr, String dirStr,
                String filename, int threadCount)
        {
            this.urlStr = urlStr;
            this.dirStr = dirStr;
            this.filename = filename;
            this.threadCount = threadCount;
        }
    
        public void download() throws IOException
        {
            createFileByUrl();
    
            /**
             * 计算每个线程需要下载的数据长度
             */
            long block = fileSize % threadCount == 0 ? fileSize / threadCount
                    : fileSize / threadCount + 1;
    
            for (int i = 0; i < threadCount; i++)
            {
                long start = i * block;
                long end = start + block >= fileSize ? fileSize : start + block - 1;
    
                new DownloadThread(new URL(urlStr), localFile, start, end).start();
            }
    
        }
    
        /**
         * 根据资源的URL获取资源的大小，以及在本地创建文件
         */
        public void createFileByUrl() throws IOException
        {
            URL url = new URL(urlStr);
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setConnectTimeout(15 * 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("Referer", urlStr);
            conn.setRequestProperty("Charset", "UTF-8");
            conn.setRequestProperty(
                    "User-Agent",
                    "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");
            conn.setRequestProperty("Connection", "Keep-Alive");
            conn.connect();
    
            if (conn.getResponseCode() == 200)
            {
                this.fileSize = conn.getContentLength();// 根据响应获取文件大小
                if (fileSize <= 0)
                    throw new RuntimeException(
                            "the file that you download has a wrong size ... ");
                File dir = new File(dirStr);
                if (!dir.exists())
                    dir.mkdirs();
                this.localFile = new File(dir, filename);
                RandomAccessFile raf = new RandomAccessFile(this.localFile, "rw");
                raf.setLength(fileSize);
                raf.close();
    
                System.out.println("需要下载的文件大小为 :" + this.fileSize + " , 存储位置为： "
                        + dirStr + "/" + filename);
    
            } else
            {
                throw new RuntimeException("url that you conneted has error ...");
            }
        }
    
        private class DownloadThread extends Thread
        {
            /**
             * 下载文件的URI
             */
            private URL url;
            /**
             * 存的本地路径
             */
            private File localFile;
            /**
             * 是否结束
             */
            private boolean isFinish;
            /**
             * 开始的位置
             */
            private Long startPos;
            /**
             * 结束位置
             */
            private Long endPos;
    
            public DownloadThread(URL url, File savefile, Long startPos, Long endPos)
            {
                this.url = url;
                this.localFile = savefile;
                this.startPos = startPos;
                this.endPos = endPos;
            }
    
            @Override
            public void run()
            {
                System.out.println(Thread.currentThread().getName() + "开始下载...");
                try
                {
                    HttpURLConnection conn = (HttpURLConnection) url
                            .openConnection();
                    conn.setConnectTimeout(15 * 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("Referer", url.toString());
                    conn.setRequestProperty("Charset", "UTF-8");
                    conn.setRequestProperty("Range", "bytes=" + startPos + "-"
                            + endPos);// 设置获取实体数据的范围
    
                    conn.setRequestProperty(
                            "User-Agent",
                            "Mozilla/4.0 (compatible; MSIE 8.0; Windows NT 5.2; Trident/4.0; .NET CLR 1.1.4322; .NET CLR 2.0.50727; .NET CLR 3.0.04506.30; .NET CLR 3.0.4506.2152; .NET CLR 3.5.30729)");
                    conn.setRequestProperty("Connection", "Keep-Alive");
                    conn.connect();
    
                    /**
                     * 代表服务器已经成功处理了部分GET请求
                     */
                    if (conn.getResponseCode() == 206)
                    {
                        InputStream is = conn.getInputStream();
                        int len = 0;
                        byte[] buf = new byte[1024];
    
                        RandomAccessFile raf = new RandomAccessFile(localFile,
                                "rwd");
                        raf.seek(startPos);
                        while ((len = is.read(buf)) != -1)
                        {
                            raf.write(buf, 0, len);
                        }
                        raf.close();
                        is.close();
                        System.out.println(Thread.currentThread().getName()
                                + "完成下载  ： " + startPos + " -- " + endPos);
                        this.isFinish = true;
                    } else
                    {
                        throw new RuntimeException(
                                "url that you conneted has error ...");
                    }
                } catch (IOException e)
                {
                    e.printStackTrace();
                }
            }
    
        }
    
        
    
    }

createFileByUrl方法，就是我们上述的原理的步骤1，得到文件大小和创建本地文件。我在程序使用了一个内部类DownloadThread继承Thread，专门负责下载。download()方法，根据线程数量和文件大小计算每个线程需要下载的字节区间，然后开启线程去下载。

服务器端：我就扔了几个文件在Tomcat根目录做实验，下面是[测试][Link 1]代码：

package com.zhy.mutilthread_download;
    
    import java.io.IOException;
    
    public class Test
    {
    
        public static void main(String[] args)
        {
            try
            {
                new MultipartThreadDownloador("http://localhost:8080/nexus.zip",
                        "f:/backup/nexus", "nexus.zip", 2).download();
            } catch (IOException e)
            {
                e.printStackTrace();
            }
    
        }
    }

输出结果：

需要下载的文件大小为 :31143237 , 存储位置为： f:/backup/nexus/nexus.zip
    Thread-1开始下载...
    Thread-2开始下载...
    Thread-3开始下载...
    Thread-4开始下载...
    Thread-4完成下载  ： 23357430 -- 31143237
    Thread-2完成下载  ： 7785810 -- 15571619
    Thread-1完成下载  ： 0 -- 7785809
    Thread-3完成下载  ： 15571620 -- 23357429

截图：

![Center][]

[源码点击下载][Link 2]

转载于:https://www.cnblogs.com/zhujiabin/p/7052732.html

[Link 1]: http://lib.csdn.net/base/softwaretest
[Center]: /images/20210725/ef0a6a831aa14c978fd5dac850073e99.png
[Link 2]: http://download.csdn.net/detail/lmj623565791/7400443