Java/Android 基于Http的多线程下载的实现

这篇文章简单介绍了一个文件分段下载,非常简单明了,适合初学者,我就转载了,分享给需要的小伙伴,代码如下:

先说下原理,原理明白了,其实很简单: 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

看完代码是不是觉得原理比较简单,如果要用于生产,得修改修改。

转自:http://blog.csdn.net/lmj623565791/article/details/26994463