Java / Android 基于Http的多线程下载的实现

转载请标明出处：http://blog.csdn.net/lmj623565791/article/details/26994463

有个朋友需要个多线程现在的例子，就帮忙实现了，在此分享下~

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

截图：

ok，多线程下载介绍完毕，如果代码设计不合理，以及方法使用错误，欢迎各位留言，，，

源码点击下载