Why do I get ClassCastException using HttpURLConnection in AppEngine? - google-app-engine

Question

I try to call an external web service (not mine) from my GWT application working with App Engine.
I know it's impossible to do it from the client due to the SOP (Same Origin Policy) and RequestBuilder is not a solution on the server. I followed the tutorial on the web site and using java.net as well
Here is the client
AsyncCallback<CustomObject> callback = new AsyncCallback<CustomObjectCustomObject>() {
#Override
public void onFailure(Throwable caught) {
caught.printStackTrace();
}
#Override
public void onSuccess(CustomObject result) {
// code omitted
}
};
service.callMethod(aString, callback);
And this is the server
try {
String xmlRequest = "xmlToSend";
URL url = new URL("https://www.externalWebService.com");
HttpURLConnection conn = (HttpURLConnection)url.openConnection();
conn.setRequestMethod("POST");
conn.setAllowUserInteraction(false);
conn.setDoOutput(true);
conn.setRequestProperty("Content-Type","application/soap+xml");
conn.setRequestProperty( "Content-length", Integer.toString(xmlRequest.length()));
conn.setRequestProperty("charset", "utf-8");
conn.setConnectTimeout(10000);
OutputStream rawOutStream = conn.getOutputStream();
PrintWriter pw = new PrintWriter(rawOutStream);
pw.print(xmlRequest);
pw.flush();
pw.close();
if(conn.getResponseCode() != 200){
// Something...
}
I keep having the same error at conn.getResponseCode() :
java.lang.ClassCastException: com.google.appengine.repackaged.org.apache.http.message.BasicHttpRequest cannot be cast to com.google.appengine.repackaged.org.apache.http.client.methods.HttpUriRequest
Without making a real request, the remote service works well : it's able to serialize and return objects to the client. The issue is not linked to the communication between the client and the server, it's more like AppEngine doesn't support HttpURLConnection. But it should on the server (isn't it?)
Any thoughts would be hightly appreciated! Thanks in advance

Answer 1

Your problem has nothing to do with GWT: as long as you are running on the server, you can use any 'normal' Java and it will work unless AppEngine has restrictions.
It seems you have imported the repackaged version of Apache HttpClient in your class. You should not do that: download your own HttpClient .jar, add it to the dependencies and use that one.
AppEngine also has some issues with HttpClient. There's an adapter available here that fixes most of the issues.

Answer 2

Thanks #Marcelo, you were right!
Here is the solution I found.
I added httpcore.jar and httpclient.jar to my build path and wrote the code below for the server (the client is the same) :
String xmlRequest = "xmlToSend";
CloseableHttpClient httpclient = HttpClients.custom().build();
//RequestConfig requestConfig = RequestConfig.custom()
// .setConnectionRequestTimeout(10000)
// .build();
try {
ByteArrayOutputStream out = new ByteArrayOutputStream();
Writer writer = new OutputStreamWriter(out);
writer.write(xmlToSend);
writer.flush();
writer.close();
HttpPost request = new HttpPost("https://www.externalWebService.com/path");
request.setEntity(new ByteArrayEntity(out.toByteArray()));
//request.setConfig(requestConfig);
CloseableHttpResponse response = httpclient.execute(request);
if(response.getStatusLine().getStatusCode() == 200){
// retrieve content with a BufferReader
// from response.getEntity().getContent()
...
}
The code works and is up to date.
Edit
Here is the rest of the solution when using a proxy. Mine only deals with NTCredentials but otherwise UsernamePasswordCredentials can be used instead.
HttpHost proxy = new HttpHost("addresse.proxy.com", port);
CredentialsProvider credsProvider = new BasicCredentialsProvider();
credsProvider.setCredentials(
new AuthScope(proxy),
new NTCredentials(System.getProperty("user.name") + ":" + password));
RequestConfig requestConfig = RequestConfig.custom()
.setProxy(proxy)
.build();
CloseableHttpClient httpclient = HttpClients.custom()
.setDefaultCredentialsProvider(credsProvider)
.setDefaultRequestConfig(requestConfig)
.build();
Thanks again for your help, I really appreciated!