How to build a Java client to consume a RESTful Grails web service

Question posted on the Grails mailing list:

I'm trying to build a REST client for my REST server (which I build
with the help of the Definitive Guide To Grails) using RESTclient.
I'm using the following code at the server side:

def save = {
   def u = new Signup()
   u.properties = params
   if (u.save()) {
     render u as XML
   } else {
     render u.errors as XML
   }
}

and this piece of code at the client:

def signup = new RESTClient(signupsUrl)
def response = signup.post(
  contentType: XML,
  requestContentType: XML,
  body: {
    field(name:"surname", "Pascal")
    field(name:"infix", "de")
    field(name:"lastname", "Vink")
    field(name:"email", "pascal@example.com")
    field(name:"signupdate", params.signupdate)
  }
}
return response.data

Not only does it not put anything in the database, the response.data
is virtually unreadable, at least it's not XML as I would expect.
The following is a snippet of the output of println response.data:
emailSignupfalseorg.springframework.validation.FieldErrornullableSignup.email.nullable.error.Signup.emailSignup.email.nullable.error.emailSignup.email.nullable.error.java.lang.StringSignup.email.nullable.errorsignup.email.nullable.error.Signup.emailsignup.email.nullable.error.emailsignup.email.nullable.error.java.lang.Stringsignup.email.nullable.errorSignup.email.nullable.Signup.emailSignup.email.nullable.emailSignup.email.nullable.java.lang.StringSignup.email.nullablesignup.email.nullable.Signup.emailsignup.email.nullable.emailsignup.email.nullable.java.lang.Stringsignup.email.nullablenullable.Signup.emailnullable.emailnullable.java.lang.StringnullableProperty
[{0}] of class [{1}] cannot be
nullemailSignupinfixSignupfalseorg.springframework.validation.FieldErrornullable
and this goes on for a while (full print here http://gist.github.com/152217)
Does anyone know how to build a compatible client? And perhaps a
server that returns proper XML error messages?
Thanks in advance.
Pascal de Vink

My Answer:

Interestingly I spent a decent portion of this week fighting with similar problems! What I learned is that there are not many examples on the web about how to write more than the simplest restful clients, particularly for POST/PUT. Secondly that there's a difference between sending things as HTTP parameters and as content body. Most examples on the web use params - this is also how information is sent by your browser during an HTML form POST. In this case the content type application/x-www-form-urlencoded should be used. Below I have sent the data as XML inside the request content body, so I used text/xml as the content type. I’m not sure if there is a “best practice” but it seems to me that web services are better aligned to sending XML or JSON in the request content – that way you can post lists of objects or objects within objects.

The client which actually worked in the end, I wrote with Java and the Jersey library which was quite nice. You can do a post like this:

private static final String SERVICE_PREFIX = "http://localhost:8080/myapp/";

String myXml = "<?xml version=\"1.0\" encoding=\"UTF-8\"?><machine><model enumType=\"MachineModel\">Forklift</model><name>Big lifter</name></machine>";

Client client = Client.create();
WebResource webResource = client.resource(SERVICE_PREFIX + "machine/saveXml/3");
String response = webResource.type(MediaType.TEXT_XML).accept(MediaType.TEXT_XML).post(String.class, myXml);

System.out.println(response);

 Code in my Grails controller to serve the Web service:

def saveXml = {
  def xmlMachine = request.XML
  def machine = new Machine(name:xmlMachine.name.text(),
  model:MachineModel.valueOf(xmlMachine.model.text())).save()
  render machine as XML
}

Important things to note

1. My example sends xml as content, not as name/value pairs in the params. That’s why I can access the XML with the call request.XML. If you send it as params there’s another handy call, I think request.params or something similar. You can also println(request) in the controller which gives you a bit more idea what’s going on... the value of Content-Length tells you how much content you sent.
2. The value of request.XML – in my case xmlMachine – is confusing. If you print it or try to “cast” it as your target object, it doesn’t look like anything. But you can indeed access the xml with “.element.text()”.
3. Lastly I’m using an Enum, hence the valueOf syntax. I think there may actually be a more convenient (1 call) way of creating the target object from the parsed XML but I think it might not have worked for me due to my Enum, so I just parse it manually.

If you're still having problems, break it down. Put some XML directly into your controller as a string and try and parse it with the Groovy XML Slurper, and send a response to the client. Once you've got that going, you can remove the hard-coded String and try and actually send the XML from the client.