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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。
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。
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。