Invoking a Java/AXIS Web Service from .NET: the ‘return null’ issue

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When you try invoke a Java/Axis Web Service from a proxy class generated by Visual Studio 2005 or Visual Studio 2008 you often crash against the ‘return null’ issue.

The web service seems to get called correctly and it responds to your client in the right way (you have no exception of any sort), but your returned object is null, it happened to me to face this situation today for the first time, there are a couple of things you can do to debug and resolve this situation:

  • Let’s consider our function call:
       1: [Test]
       2: public void T1()
       3: {
       4:     Test.TestWs ws = new AxisWebService.Test.TestWs ();
       5:     Test.State[] arr = ws.getStates(1);
       6:     Assert.IsNotNull(arr);
       7: }

here we expect to have back an array of State objects, instead we obtain the hated ‘null’.

  • The first thing to do is to download and install ‘Fiddler’ (do it right now if you don’t have it already) and have a look at what the web service respond to us (the snippet is a trimmed response):
       1: HTTP/1.1 200 OK
       2: Connection: close
       3: Date: Wed, 15 Oct 2008 12:36:26 GMT
       4: Server: Microsoft-IIS/6.0
       5: X-Powered-By: ASP.NET
       6: Content-Type: text/xml;charset=utf-8
       7:  
       8: <?xml version="1.0" encoding="utf-8"?>
       9: <soapenv:Envelope xmlns:soapenv="http://schemas.xmlsoap.org/soap/envelope/" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
      10: <soapenv:Body>
      11: <getStatesResponse xmlns="">
      12: <item xsi:type="ns1:State" xmlns:ns1="http://mynamespace.it/">
      13: <Code>A001</Code>
      14: <Description>Test</Description>
      15: </item>
      16: </getStatesResponse>
      17: </soapenv:Body>
      18: </soapenv:Envelope>

Since the web service is responding correctly the problem is in the deserialization stage of the data stream sent back by the web service.

  • It’s time to show some hidden file of the solution and look inside the ‘reference.cs’ (this is the default file in which Visual Studio creates some proxy classes).

Looking at the proxy classes generated by Visual Studio, it seems that we have all that we need: a class to call the web service and series of classes that map the objects the service returns; where’s the problem then? it turns out that the web service client can’t understand the response stream, so the problem is in a mismatch somewhere.

Given the fact we have all the classes and all of them have the right properties, it’s time to look for the namespaces:

  • Visual Studio 2005
       1: [System.Web.Services.Protocols.SoapDocumentMethodAttribute("http://mynamespace.it/getStates",
       2:     Use = System.Web.Services.Description.SoapBindingUse.Literal,
       3:     ParameterStyle = System.Web.Services.Protocols.SoapParameterStyle.Bare)]
       4: [return: System.Xml.Serialization.XmlArrayAttribute("getStatesResponse", Namespace = "http://mynamespace.it/")]
       5: [return: System.Xml.Serialization.XmlArrayItemAttribute("item", Form = System.Xml.Schema.XmlSchemaForm.Unqualified, IsNullable = false)]
       6: public Stato[] getStates([System.Xml.Serialization.XmlElementAttribute(Namespace = "http://mynamespace.it/")] int getStatesRequest)
       7: {
       8:     object[] results = this.Invoke("getStates", new object[] {
       9:                 getStatesRequest});
      10:     return ((State[])(results[0]));
      11: }

pay attention to line number 4:

   1: [return: System.Xml.Serialization.XmlArrayAttribute("getStatesResponse", Namespace = "http://mynamespace.it/")]

here it states that the getResponseState element is qualified with the ‘http://mynamespace.it'/’ namespace...but look at what Fiddler captured for us (line 11 of the previous snippet): there we see that the namespace associated with the element is “” (empty string), so here it is our mismatch. To fix the problem you have to manually edit the attribute and correct the namespace to “” (empty string).

Be very careful: writing Namespace = “” or removing it at all are two completely different things.

Having made this fix our test passes and we are able to get our objects back from the web service.

  • Visual Studio 2008
    It produces a completely different set of classes to call the web service, we have an interface that describes the service, a series of classes that represent the request and response of each method exposed by the interface and finally we have the proxy classes for the objects returned. We know that the problem is at the ‘client’ side so checking the request classes is useless, we focus our attention on the response classes and on object classes to verify the namespace mappings:
       1: [System.Diagnostics.DebuggerStepThroughAttribute()]
       2: [System.CodeDom.Compiler.GeneratedCodeAttribute("System.ServiceModel", "3.0.0.0")]
       3: [System.ServiceModel.MessageContractAttribute(IsWrapped=false)]
       4: public partial class getStatesResponse {
       5:     
       6:     [System.ServiceModel.MessageBodyMemberAttribute(Name="getStatesResponse", Namespace="http://mynamespace.it/", Order=0)]
       7:     [System.Xml.Serialization.XmlArrayItemAttribute("item", Form=System.Xml.Schema.XmlSchemaForm.Unqualified, IsNullable=false)]
       8:     public State[] getStatesResponse1;
       9:     
      10:     public getStatesResponse() {
      11:     }
      12:     
      13:     public getStatesResponse(State[] getStatesResponse1) {
      14:         this.getStatesResponse1 = getStatesResponse1;
      15:     }
      16: }

look at line 6, you can see a namespace mismatch again, the fix is the same applied before.

In the end, if you are using a Java/Axis Web Service and you get null results from you service calls, don't trust the auto-generated proxy classes too much and check that the attribute that defines the namespace for each object match what you get from the wsdl and from the traced response.

 

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Axis, Java, Web service, .net, Wcf

8 comments

Related Post

  1. #1 da Yuval - Saturday March 2010 alle 01:15

    I think this issue was fixed in AXIS: http://issues.apache.org/jira/browse/AXIS-1935

  2. #2 da Guardian - Saturday March 2010 alle 01:15

    Thank you for pointing it, the fact is I do not have control over that web service cause it was hosted by another company...and as you may immagine sometimes it's hard for people to understand they have to keep their installations updated :D

  3. #3 da asdf - Saturday March 2010 alle 01:15

    Thank you for your useful tip and trick.

  4. #4 da Eduardo - Saturday March 2010 alle 01:15

    Thank you very much!! IT WORK´S!!!! EXCELLENT!!!!! There was three namespaces wrong, they were different from XML schema on webservice test return! PERFECT!!!! Regards!!!

  5. #5 da Tiger - Monday June 2011 alle 02:55

    Great article. Thanks helped solve the problem using VS2008

  6. #6 da Danilo - Tuesday October 2011 alle 09:31

    Hi,
    i've same problem but with VS2010, where adding a web reference doesn't create any proxy class. How can i solve this issue?
    Thanks.

  7. #7 da Danilo - Tuesday October 2011 alle 10:48

    Hi,
    i have the same problem with VS2010, but adding a web reference doesn't create any proxy class under my solution. So how can I solve this issue with VS2010?
    Thanks.
    Danilo

  8. #8 da alessandro giorgetti - Wednesday October 2011 alle 09:18

    Hi Danilo, if you are sure you can call that web service (and it works, try making some 'raw' call to it and read what it returns using a tool like fiddler or similar)...I have no clue what the problem can be in VS2010 because (thanks God) I didn't had the chances to use any AXIS web service lately.
    So I'm sorry but I cannot be of any help at this time.

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  1. #1 da http://www.trouble86.com/invoking-a-javaaxis-web-service-from-net-the-%e2%80%98return-null%e2%80%99-issue/

    Invoking a Java/AXIS Web Service from .NET: the ‘return null’ issue | trouble86.com

  2. #2 da http://asp-mvc.thekollectable.com/java-web-service-for-ios-and-asp-net-web-development/10826

    [RESOLVED]Java web service for iOS and ASP.Net Web development. | ASP.NET MVC