java HttpURLConnection 得到 Redirect 转向的例子

做网站的人，都知道可以在apache,iis里配置 302,302转向，这样对搜索引擎也是有好的。如果用java开发程序得到这个链接的话，通常是得不到真实的转向地址的，这需要手工处理。

URL obj = new URL(url);
HttpURLConnection conn = (HttpURLConnection) obj.openConnection();
conn.setInstanceFollowRedirects(true);  //you still need to handle redirect manully.
HttpURLConnection.setFollowRedirects(true);

如果在服务端，从原始的url 转向另一个url 那么得到的response 应该是:
301:moved permanently
302:temporary redirect
其实，可以通过 HTTP response 的 头部 :"Location" 得到这个转向的url。比如有一个例子，从http://www.twitter.com 自动转向https 的站点:https://www.twitter.com

一个例子如下：

import java.io.BufferedReader;
import java.io.InputStreamReader;
import java.net.HttpURLConnection;
import java.net.URL;
 
public class HttpRedirectExample {
 
  public static void main(String[] args) {
 
    try {
 
	String url = "http://www.twitter.com";
 
	URL obj = new URL(url);
	HttpURLConnection conn = (HttpURLConnection) obj.openConnection();
	conn.setReadTimeout(5000);
	conn.addRequestProperty("Accept-Language", "en-US,en;q=0.8");
	conn.addRequestProperty("User-Agent", "Mozilla");
	conn.addRequestProperty("Referer", "google.com");
 
	System.out.println("Request URL ... " + url);
 
	boolean redirect = false;
 
	// normally, 3xx is redirect
	int status = conn.getResponseCode();
	if (status != HttpURLConnection.HTTP_OK) {
		if (status == HttpURLConnection.HTTP_MOVED_TEMP
			|| status == HttpURLConnection.HTTP_MOVED_PERM
				|| status == HttpURLConnection.HTTP_SEE_OTHER)
		redirect = true;
	}
 
	System.out.println("Response Code ... " + status);
 
	if (redirect) {
 
		// get redirect url from "location" header field
		String newUrl = conn.getHeaderField("Location");
 
		// get the cookie if need, for login
		String cookies = conn.getHeaderField("Set-Cookie");
 
		// open the new connnection again
		conn = (HttpURLConnection) new URL(newUrl).openConnection();
		conn.setRequestProperty("Cookie", cookies);
		conn.addRequestProperty("Accept-Language", "en-US,en;q=0.8");
		conn.addRequestProperty("User-Agent", "Mozilla");
		conn.addRequestProperty("Referer", "google.com");
 
		System.out.println("Redirect to URL : " + newUrl);
 
	}
 
	BufferedReader in = new BufferedReader(
                              new InputStreamReader(conn.getInputStream()));
	String inputLine;
	StringBuffer html = new StringBuffer();
 
	while ((inputLine = in.readLine()) != null) {
		html.append(inputLine);
	}
	in.close();
 
	System.out.println("URL Content... \n" + html.toString());
	System.out.println("Done");
 
    } catch (Exception e) {
	e.printStackTrace();
    }
 
  }
 
}

输出的结果为:

Request URL ... http://www.twitter.com
Response Code ... 301
Redirect to URL : https://twitter.com/
URL Content... 





	
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