Alternative Java Clients - RESTful Java with JAX-RS 2.0 - page 246

Second, the framework does not do preemptive authentication for Basic or Digest Authentic-
ation. This means that HttpURLConnection will first try to invoke a request without any au-
thentication headers set. If the server requires authentication, the initial request will fail with
a 401, “Unauthorized,” response code. The HttpURLConnection implementation then looks
at the WWW-Authenticate header to see whether Basic or Digest Authentication should be
used and retries the request. This can have an impact on the performance of your system be-
cause each authenticated request will actually be two requests between the client and server.

Third, the framework can't do something as simple as form parameters. All you have to work
with are java.io.OutputStream and java.io.InputStream to perform your input and out-
put.
Finally, the framework only allows you to invoke the HTTP methods GET, POST, DELETE,
PUT, TRACE, OPTIONS, and HEAD. If you try to invoke any HTTP method other than
those, an exception is thrown and your invocation will abort. In general, this is not that im-
portant unless you want to invoke newer HTTP methods like those defined in the WebDAV
specification.

Apache HttpClient

The Apache foundation has written a nice, extensible, HTTP client library called HttpCli-
ent. [ 20 ] It is currently on version 4.x as of the writing of this topic. Although it is not JAX-
RS-aware, it does have facilities for preemptive authentication and APIs for dealing with a
few different media types like forms and multipart. Some of its other features are a full inter-
ceptor model, automatic cookie handling between requests, and pluggable authentication.
Let's look at a simple example:

import import org.apache.http.* org.apache.http.* ;

import import org.apache.http.client.* org.apache.http.client.* ;

public public class class MyClient MyClient {

public public static static void void main ( String [] args ) throws throws Exception {

DefaultHttpClient client = new new DefaultHttpClient ();

HttpGet get = new new HttpGet ( "http://example.com/customers/1" );

get . addHeader ( "accept" , "application/xml" );

HttpResponse response = client . execute ( get );
iif ( response . getStatusLine (). getStatusCode () != 200 ) {
throw throw new new RuntimeException ( "Operation failed: " +

response . getStatusLine (). getStatusCode ());

}