Before executing an HttpMethod
call, setFollowRedirects(true)
on the method; HttpClient will take care of following any
redirects a server may return in a response. The following example shows
what happens when a method requests a CGI script that returns a 302 (moved temporarily) response code:
import org.apache.commons.httpclient.HttpClient;
import org.apache.commons.httpclient.HttpException;
import org.apache.commons.httpclient.HttpMethod;
import org.apache.commons.httpclient.methods.GetMethod;
HttpClient client = new HttpClient( );
String url = "http://www.discursive.com/cgi-bin/jccook/redirect.cgi";
System.out.println( "Executing Method not following redirects: ");
HttpMethod method = new GetMethod( url );
method.setFollowRedirects( false );
executeMethod(client, method);
System.out.println( "Executing Method following redirects: ");
method = new GetMethod( url );
method.setFollowRedirects( true );
executeMethod(client, method);
private
static
void executeMethod(HttpClient client, HttpMethod method)
throws IOException, HttpException {
client.executeMethod( method );
System.out.println( "Response Code: " + method.getStatusCode( ) );
String response = method.getResponseBodyAsString( );
System.out.println( response );
method.releaseConnection( );
method.recycle( );
}This example executes two GetMethod instances; the first method is
configured not to follow redirects, and
the second is configured to follow redirects. The first method is
executed, and the server sends a 302
response code. Since this method is not configured to follow redirects,
HttpClient does not make another
request. When the second method is executed, HttpClient follows the initial redirect to a
redirect2.cgi script, which sends
another redirect to /jccook/index.html:
Executing Method not following redirects: 0 INFO [main] org.apache.commons.httpclient.HttpMethodBase - Redirect requested but followRedirects is disabled Response Code: 302 Executing Method following redirects: Response Code: 200 <html> <head> <title>JCCook Example</title> </head> <body> <h1>Hello World!</h1> </body> </html>
HttpClient can handle any of
the following response codes specifying a redirect:
-
Status Code 302:
HttpStatus.SC_MOVED_TEMPORARILY -
Status Code 301:
HttpStatus.SC_MOVED_PERMANENTLY -
Status Code 303:
HttpStatus.SC_SEE_OTHER -
Status Code 307:
HttpStatus.SC_TEMPORARY_REDIRECT
When a response code is retrieved, HttpClient sends another GET request for the
resource specified in the Location
header. The following code is the first request sent by a method
configured to follow redirects:
GET /cgi-bin/jccook/redirect.cgi HTTP/1.1 User-Agent: Jakarta Commons-HttpClient/3.0final Host: www.discursive.com
The redirect.cgi script will then send a 302
Moved response, supplying a Location header that points to redirect2.cgi:
HTTP/1.1 302 Moved Date: Sat, 15 May 2004 19:30:49 GMT Server: Apache/2.0.48 (Fedora) Location: /cgi-bin/jccook/redirect2.cgi Content-Length: 0 Content-Type: text/plain; charset=UTF-8
HttpClient then sends another
GET request for the resource specified in the previous response:
GET /cgi-bin/jccook/redirect2.cgi HTTP/1.1 User-Agent: Jakarta Commons-HttpClient/3.0final Host: www.discursive.com
The redirect2.cgi is
configured to send a redirect for /jccook/index.html, and the response to the
previous request does just that:
HTTP/1.1 302 Moved Date: Sat, 15 May 2004 19:30:49 GMT Server: Apache/2.0.48 (Fedora) Location: /jccook/index.html Content-Length: 0 Content-Type: text/plain; charset=UTF-8
How HttpClient handles redirect
responses can be further customized by three configurable parameters on
HttpClient. REJECT_RELATIVE_REDIRECT causes HttpClient to throw an exception if a server
sends a Location header with a
relative URL; for instance, if the redirect.cgi script returns a Location header of ../index.html, the redirection causes an
exception if REJECT_RELATIVE_REDIRECT
is set to true. If ALLOW_CIRCULAR_REDIRECTS is set to true, HttpClient throws an exception if a series of
redirects includes the same resources more than once. MAX_REDIRECTS allows you to specify a maximum
number of redirects to follow. The following example sets all three
parameters on an instance of HttpClientParams associated with an instance
of HttpClient:
HttpClient client = new HttpClient( ); HttpClientParams params = client.getParams( ); params.setBooleanParameter( HttpClientParams.REJECT_RELATIVE_REDIRECT, false ); params.setBooleanParameter( HttpClientParams.ALLOW_CIRCULAR_REDIRECTS, false ); params.setIntParameter( HttpClientParams.MAX_REDIRECTS, 10 );
For more information on how HttpClient handles redirection, take
a look at the source for the HttpMethodDirector.
The isRedirectNeeded() and processRedirectResponse() methods handle redirection, and the source for this class
can be viewed using ViewCVS (http://cvs.apache.org/viewcvs.cgi/jakarta-commons/httpclient/src/java/).
Navigate to the org.apache.commons.httpclient package and
click on HttpMethodDirector.
Common Java Cookbook by Tim O'Brien is licensed under a Creative Commons Attribution-Noncommercial-No Derivative Works 3.0 United States License.
Permissions beyond the scope of this license may be available at http://www.discursive.com/books/cjcook/reference/jakartackbk-PREFACE-1.html. Copyright 2009. Common Java Cookbook Chunked HTML Output. Some Rights Reserved.