Java 文件多线程下载详解编程语言

最近在做文件下载这块的东西，研究了一下多线程文件下载这块的知识。这里只说一下原理，具体实现请看代码，已经写了注释了。

主要原理

         为了加快下载速度，每个文件固定N个线程来下载，然后每个线程负责下载该文件的某一部分，比如文件大小90M，用3个线程来下载，那么第一个线程负责下载文件的长度范围：0-30*1024*1024-1，第二个线程负责下载文件的长度范围：30*1024*1024-60*1024*1024-1，第三个线程负责下载文件的长度范围：60*1024*1024-90*1024*1024-1，3个线程下载完后就合成了整个文件。这里需要用到Http中的ContentLength和Range请求头，ContentLength对应文件的总长度，Range头用来请求文件某一子块的内容，例如：Range 0-10000，表示请求该文件0-10000字节的内容。

具体代码如下：

import java.io.File;   
import java.io.IOException;   
import java.io.InputStream;   
import java.io.RandomAccessFile;   
import java.util.zip.GZIPInputStream;   
   
import org.apache.http.Header;   
import org.apache.http.HttpResponse;   
import org.apache.http.HttpStatus;   
import org.apache.http.client.ClientProtocolException;   
import org.apache.http.client.HttpClient;   
import org.apache.http.client.methods.HttpGet;   
import org.apache.http.client.params.CookiePolicy;   
import org.apache.http.params.CoreConnectionPNames;   
   
import com.ricky.java.common.download.file.http.HttpClientManager;   
import com.ricky.java.common.download.file.util.Constants;   
   
public class Downloader {   
       
    private String url; // 目标地址   
    private File file;  // 本地文件   
    private static final int THREAD_AMOUNT = 3;                 // 线程数   
    private static final String DIR_PATH = "D:/download/file";      // 下载目录   
    private long threadLen;                                     // 每个线程下载多少   
   
    private HttpClient mHttpClient = HttpClientManager.getHttpClient();   
       
    public Downloader(String address) throws IOException {      // 通过构造函数传入下载地址   
        url = address;   
        file = new File(DIR_PATH, address.substring(address.lastIndexOf("?") + 1)+".xml");   
    }   
   
    public void download() throws IOException {   
           
        long totalLen = getContentLength(url);                          // 获取文件长度   
        threadLen = (totalLen + THREAD_AMOUNT - 1) / THREAD_AMOUNT;         // 计算每个线程要下载的长度   
           
        System.out.println("totalLen="+totalLen+"***threadLen="+threadLen);   
           
        RandomAccessFile raf = new RandomAccessFile(file, "rws");           // 在本地创建一个和服务端大小相同的文件   
        raf.setLength(totalLen);                                            // 设置文件的大小   
        raf.close();   
           
        for (int i = 0; i < THREAD_AMOUNT; i++)                              // 开启3条线程, 每个线程下载一部分数据到本地文件中   
            new DownloadThread(i).start();   
    }   
       
    public long getContentLength(String address) {   
        HttpGet httpget = null;   
        try {   
            httpget = new HttpGet(address);   
            httpget.setHeader("User-Agent","Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.1 (KHTML, like Gecko) Chrome/21.0.1180.79 Safari/537.1");   
            httpget.setHeader("Accept","text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8");   
               
            httpget.getParams().setParameter(CoreConnectionPNames.SO_TIMEOUT,Constants.SO_TIMEOUT);   
            httpget.getParams().setParameter(CoreConnectionPNames.CONNECTION_TIMEOUT,Constants.CONNECTION_TIMEOUT);   
            httpget.getParams().setParameter("http.protocol.cookie-policy",CookiePolicy.BROWSER_COMPATIBILITY);   
               
            HttpResponse response = mHttpClient.execute(httpget);   
               
            int status = response.getStatusLine().getStatusCode();   
               
            if (status == HttpStatus.SC_OK) {   
                   
                return response.getEntity().getContentLength();   
            }   
        } catch (ClientProtocolException e) {   
            e.printStackTrace();   
        } catch (IOException e) {   
            e.printStackTrace();   
        }finally{   
            if(httpget!=null){   
                httpget.abort();   
            }   
        }   
        return 0;   
    }   
   
    private class DownloadThread extends Thread {   
        private int id;    
        public DownloadThread(int id) {   
            this.id = id;   
        }   
        public void run() {   
            long start = id * threadLen;                        // 起始位置   
            long end = id * threadLen + threadLen - 1;      // 结束位置   
            System.out.println("线程" + id + ": " + start + "-" + end);   
               
            HttpGet httpget = null;   
            try {   
                httpget = new HttpGet(url);   
                httpget.setHeader("User-Agent","Mozilla/5.0 (Windows NT 6.1; WOW64) AppleWebKit/537.1 (KHTML, like Gecko) Chrome/21.0.1180.79 Safari/537.1");   
                httpget.setHeader("Accept","text/html,application/xhtml+xml,application/xml;q=0.9,*/*;q=0.8");   
                   
                httpget.getParams().setParameter(CoreConnectionPNames.SO_TIMEOUT,Constants.SO_TIMEOUT);   
                httpget.getParams().setParameter(CoreConnectionPNames.CONNECTION_TIMEOUT,Constants.CONNECTION_TIMEOUT);   
                httpget.getParams().setParameter("http.protocol.cookie-policy",CookiePolicy.BROWSER_COMPATIBILITY);   
                   
                HttpResponse response = mHttpClient.execute(httpget);   
                   
                int status = response.getStatusLine().getStatusCode();   
                   
                if (status == HttpStatus.SC_OK) {   
                    InputStream in = response.getEntity().getContent();   
                    Header contentEncoding = response.getFirstHeader("Content-Encoding");   
                    if (contentEncoding != null && contentEncoding.getValue().equalsIgnoreCase("gzip")) {   
                        System.out.println("gzip InputStream in post");   
                        in = new GZIPInputStream(in);   
                    }   
                       
                    RandomAccessFile raf = new RandomAccessFile(file, "rws");   
                    raf.seek(start);   
                       
                    byte[] buffer = new byte[1024];   
                    int len;   
                    while ((len = in.read(buffer)) != -1)   
                        raf.write(buffer, 0, len);   
                    raf.close();   
                       
                    System.out.println("线程" + id + "下载完毕");   
                }else{   
                    System.out.println("线程" + id + "请求失败,响应码="+status);   
                }   
                   
            } catch (ClientProtocolException e) {   
                e.printStackTrace();   
            } catch (IOException e) {   
                e.printStackTrace();   
            }finally{   
                if(httpget!=null){   
                    httpget.abort();   
                }   
            }   
        }   
    }   
       
    public static void main(String[] args) throws IOException {   
//      new Downloader("http://dldir1.qq.com/qqfile/qq/QQ6.2/12179/QQ6.2.exe").download();   
        new Downloader("http://api.t.dianping.com/n/api.xml?cityId=1").download();   
    }   
}  

HttpClientManager.java

   
import org.apache.http.client.HttpClient;   
import org.apache.http.conn.scheme.PlainSocketFactory;   
import org.apache.http.conn.scheme.Scheme;   
import org.apache.http.conn.scheme.SchemeRegistry;   
import org.apache.http.conn.ssl.SSLSocketFactory;   
import org.apache.http.impl.client.DefaultHttpClient;   
import org.apache.http.impl.conn.PoolingClientConnectionManager;   
import org.apache.http.params.BasicHttpParams;   
import org.apache.http.params.CoreConnectionPNames;   
import org.apache.http.params.HttpParams;   
   
import com.ricky.java.common.download.file.util.Constants;   
   
public class HttpClientManager {   
       
    private static HttpParams httpParams;   
    private static PoolingClientConnectionManager cm;   
   
    /**  
     * 最大连接数  
     */   
    public final static int MAX_TOTAL_CONNECTIONS = 200;   
    /**  
     * 每个路由最大连接数  
     */   
    public final static int MAX_ROUTE_CONNECTIONS = 300;   
       
   
    static {   
        SchemeRegistry schemeRegistry = new SchemeRegistry();   
        schemeRegistry.register(   
                new Scheme("http",80,PlainSocketFactory.getSocketFactory()));   
        schemeRegistry.register(   
                new Scheme("https", 443, SSLSocketFactory.getSocketFactory()));   
           
        cm = new PoolingClientConnectionManager(schemeRegistry);   
        cm.setMaxTotal(MAX_TOTAL_CONNECTIONS);   
        cm.setDefaultMaxPerRoute(MAX_ROUTE_CONNECTIONS);   
           
        HttpParams params = new BasicHttpParams();   
        params.setParameter(CoreConnectionPNames.CONNECTION_TIMEOUT,Constants.CONNECTION_TIMEOUT);   
        params.setParameter(CoreConnectionPNames.SO_TIMEOUT, Constants.SO_TIMEOUT);   
    }   
   
    public static HttpClient getHttpClient() {   
        return new DefaultHttpClient(cm, httpParams);   
    }   
   
}  

如果想实现文件断点下载的话，只需要在记录每个线程当前下载了多少长度的内容即可，可以将其持久化到文件或数据库中保存起来，然后线程开始下载的时候都先读取一下它当前下载了多少就OK了。