Java / Android 基于Http的多线程下载的实现

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

截图：