Http的多執行緒下載的實現
a、對於網路上的一個資源,首先發送一個請求,從返回的Content-Length中回去需要下載檔案的大小,然後根據檔案大小建立一個檔案。
b、根據執行緒數和檔案大小,為每個執行緒分配下載的位元組區間,然後每個執行緒向伺服器傳送請求,獲取這段位元組區間的檔案內容。
c、利用RandomAccessFile的seek方法,多執行緒同時往一個檔案中寫入位元組。
【HTTP之Range】
1、什麼是Range?
當用戶在聽一首歌的時候,如果聽到一半(網路下載了一半),網路斷掉了,使用者需要繼續聽的時候,檔案伺服器不支援斷點的話,則使用者需要重新下載這個檔案。而Range支援的話,客戶端應該記錄了之前已經讀取的檔案範圍,網路恢復之後,則向伺服器傳送讀取剩餘Range的請求,服務端只需要傳送客戶端請求的那部分內容,而不用整個檔案傳送回客戶端,以此節省網路頻寬。
2、HTTP1.1規範的Range是怎樣一個約定呢?
如果Server支援Range,首先就要告訴客戶端,咱支援Range,之後客戶端才可能發起帶Range的請求。這裡套用唐僧的一句話,你不說我怎麼知道呢。response.setHeader(‘Accept-Ranges’, ‘bytes’);
Server通過請求頭中的Range: bytes=0-xxx來判斷是否是做Range請求,如果這個值存在而且有效,則只發回請求的那部分檔案內容,響應的狀態碼變成206,表示Partial Content,並設定Content-Range。如果無效,則返回416狀態碼,表明Request Range Not Satisfiable(
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
{
/**
* 需要下載資源的地址
*/
private String urlStr;
/**
* 下載的檔案
*/
private File localFile;
/**
* 需要下載檔案的存放的本地資料夾路徑
*/
private String dirStr;
/**
* 儲存到本地的檔名
*/
private String filename;
/**
* 開啟的執行緒數量
*/
private int threadCount;
/**
* 下載檔案的大小
*/
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();
/**
* 計算每個執行緒需要下載的資料長度
*/
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();
}
}
/**
* 根據資源的URL獲取資源的大小,以及在本地建立檔案
*/
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();// 根據響應獲取檔案大小
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("需要下載的檔案大小為 :" + this.fileSize + " , 儲存位置為: "
+ dirStr + "/" + filename);
} else
{
throw new RuntimeException("url that you conneted has error ...");
}
}
private class DownloadThread extends Thread
{
/**
* 下載檔案的URI
*/
private URL url;
/**
* 存的本地路徑
*/
private File localFile;
/**
* 是否結束
*/
private boolean isFinish;
/**
* 開始的位置
*/
private Long startPos;
/**
* 結束位置
*/
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() + "開始下載...");
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);// 設定獲取實體資料的範圍
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();
/**
* 代表伺服器已經成功處理了部分GET請求
*/
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()
+ "完成下載 : " + startPos + " -- " + endPos);
this.isFinish = true;
} else
{
throw new RuntimeException(
"url that you conneted has error ...");
}
} catch (IOException e)
{
e.printStackTrace();
}
}
}
public static void main(String[] args)
{
try
{
new MultipartThreadDownloador("http://img.firefoxchina.cn/2016/09/5/201609141103140.jpg",
"D:\\KwDownload", "dd.jpg", 2).download();
} catch (IOException e)
{
e.printStackTrace();
}
}
}