Java / Android 基于Http的多線程下載的實現

jopen 10年前發布 | 13K 次閱讀 Android開發移動開發

轉載請標明出處：http://blog.csdn.net/lmj623565791/article/details/26994463

有個朋友需要個多線程現在的例子，就幫忙實現了，在此分享下~

先說下原理，原理明白了，其實很簡單：

a、對于網絡上的一個資源，首先發送一個請求，從返回的Content-Length中回去需要下載文件的大小，然后根據文件大小創建一個文件。

this.fileSize = conn.getContentLength();// 根據響應獲取文件大小
File dir = new File(dirStr);
this.localFile = new File(dir, filename);
RandomAccessFile raf = new RandomAccessFile(this.localFile, "rw");
raf.setLength(fileSize);
raf.close();

b、根據線程數和文件大小，為每個線程分配下載的字節區間，然后每個線程向服務器發送請求，獲取這段字節區間的文件內容。

conn.setRequestProperty("Range", "bytes=" + startPos + "-"
                        + endPos);// 設置獲取實體數據的范圍

c、利用RandomAccessFile的seek方法，多線程同時往一個文件中寫入字節。

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

分析完了原理就很簡單了，我封裝了一個類，利用這個類的實例進行下載，所需參數：下載資源的URI，本地文件路徑，線程的數量。

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
{

    /**
     * 需要下載資源的地址
     */
    private String urlStr;
    /**
     * 下載的文件
     */
    private File localFile;
    /**
     * 需要下載文件的存放的本地文件夾路徑
     */
    private String dirStr;
    /**
     * 存儲到本地的文件名
     */
    private String filename;

    /**
     * 開啟的線程數量
     */
    private int threadCount;
    /**
     * 下載文件的大小
     */
    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();

        /**
         * 計算每個線程需要下載的數據長度
         */
        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，以及在本地創建文件
     */
    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();// 根據響應獲取文件大小
            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("需要下載的文件大小為 :" + this.fileSize + " , 存儲位置為： "
                    + dirStr + "/" + filename);

        } else
        {
            throw new RuntimeException("url that you conneted has error ...");
        }
    }

    private class DownloadThread extends Thread
    {
        /**
         * 下載文件的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);// 設置獲取實體數據的范圍

                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();

                /**
                 * 代表服務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();
            }
        }

    }



}

createFileByUrl方法，就是我們上述的原理的步驟1，得到文件大小和創建本地文件。我在程序使用了一個內部類DownloadThread繼承Thread，專門負責下載。download()方法，根據線程數量和文件大小計算每個線程需要下載的字節區間，然后開啟線程去下載。

服務器端：我就扔了幾個文件在Tomcat根目錄做實驗，下面是測試代碼：

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();
        }

    }
}

輸出結果：

需要下載的文件大小為 :31143237 , 存儲位置為： f:/backup/nexus/nexus.zip
Thread-1開始下載...
Thread-2開始下載...
Thread-3開始下載...
Thread-4開始下載...
Thread-4完成下載  ： 23357430 -- 31143237
Thread-2完成下載  ： 7785810 -- 15571619
Thread-1完成下載  ： 0 -- 7785809
Thread-3完成下載  ： 15571620 -- 23357429

截圖：