Java - 如何查找网址的重定向网址？

Question

我通过java访问网页如下:

URLConnection con = url.openConnection();

但在某些情况下,网址会重定向到另一个网址.所以我想知道前一个网址重定向的网址.

以下是我作为回复得到的标题字段:

null-->[HTTP/1.1 200 OK]
Cache-control-->[public,max-age=3600]
last-modified-->[Sat, 17 Apr 2010 13:45:35 GMT]
Transfer-Encoding-->[chunked]
Date-->[Sat, 17 Apr 2010 13:45:35 GMT]
Vary-->[Accept-Encoding]
Expires-->[Sat, 17 Apr 2010 14:45:35 GMT]
Set-Cookie-->[cl_def_hp=copenhagen; domain=.craigslist.org; path=/; expires=Sun, 17     Apr 2011 13:45:35 GMT, cl_def_lang=en; domain=.craigslist.org; path=/; expires=Sun, 17 Apr 2011 13:45:35 GMT]
Connection-->[close]
Content-Type-->[text/html; charset=iso-8859-1;]
Server-->[Apache]

所以目前,我正在从头Set-Cookie字段的值构建重定向的URL .在上面的例子中,重定向的URL是copenhagen.craigslist.org

是否有任何标准方法可以确定特定网址将重定向到哪个网址.

我知道当一个url重定向到其他url时,服务器会发送一个包含Location头字段的中间响应,该头字段告诉重定向的url但我没有通过该url.openConnection();方法接收到该中间响应.

Answer 1

在调用getInputStream()之后,只需在URLConnection实例上调用getUrl():

URLConnection con = new URL( url ).openConnection();
System.out.println( "orignal url: " + con.getURL() );
con.connect();
System.out.println( "connected url: " + con.getURL() );
InputStream is = con.getInputStream();
System.out.println( "redirected url: " + con.getURL() );
is.close();

如果你需要知道重定向是否在实际获取它的内容之前发生,这里是示例代码:

HttpURLConnection con = (HttpURLConnection)(new URL( url ).openConnection());
con.setInstanceFollowRedirects( false );
con.connect();
int responseCode = con.getResponseCode();
System.out.println( responseCode );
String location = con.getHeaderField( "Location" );
System.out.println( location );

Answer 2

您需要转换URLConnection为HttpURLConnection并指示它不通过设置HttpURLConnection#setInstanceFollowRedirects()为重定向false.您也可以通过全局设置它HttpURLConnection#setFollowRedirects().

您只需要自己处理重定向.检查响应代码HttpURLConnection#getResponseCode(),获取Location标题URLConnection#getHeaderField(),然后在其上触发新的HTTP请求.

Answer 3

public static URL getFinalURL(URL url) {
    try {
        HttpURLConnection con = (HttpURLConnection) url.openConnection();
        con.setInstanceFollowRedirects(false);
        con.setRequestProperty("User-Agent", "Mozilla/5.0 (Windows NT 10.0; Win64; x64) AppleWebKit/537.36 (KHTML, like Gecko) Chrome/62.0.3202.94 Safari/537.36");
        con.addRequestProperty("Accept-Language", "en-US,en;q=0.8");
        con.addRequestProperty("Referer", "https://www.google.com/");
        con.connect();
        //con.getInputStream();
        int resCode = con.getResponseCode();
        if (resCode == HttpURLConnection.HTTP_SEE_OTHER
                || resCode == HttpURLConnection.HTTP_MOVED_PERM
                || resCode == HttpURLConnection.HTTP_MOVED_TEMP) {
            String Location = con.getHeaderField("Location");
            if (Location.startsWith("/")) {
                Location = url.getProtocol() + "://" + url.getHost() + Location;
            }
            return getFinalURL(new URL(Location));
        }
    } catch (Exception e) {
        System.out.println(e.getMessage());
    }
    return url;
}

要自己获得" User-Agent "和" Referer ",只需转到已安装浏览器的开发者模式(例如,在Google Chrome上按F12).然后转到"网络"标签,然后单击其中一个请求.你应该看到它的细节.只需按"标题"子标签(下图)