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GWT client and spring server with rest APIs

I am a GWT user and what I appreciate of it is the opportunity to develop web interfaces in Java.
Currently, a team is developing the server side of a web application that offers rest Apis and that is based on Spring.
My idea was to use GWT to create the client side application and, rather than invoking some gwt rest apis, I want to invoke, from such client, the rest apis exposed by the spring-based server.
My questions are: does this sound reasonable? Can i just code and cross compile a gwt client invoking the external rest Apps or do I still need GWT server handing off the requests to the spring-based server? What's a potential deployment scenario? Thanks.

You don't need GWT server capabilities at all (gwt-server.jar). You only need a web server to host your compiled GWT app like nginx.
What works for me is coding simple REST calls (Spring MVC) from the server and GWT using GWTP Rest support.
Jackson and GwtJackson are used in both sides to serialize/deserialize DTO's.
To avoid SOP problems use a reverse proxy or implement CORS.

How to connect WSDL file in my application using SOAP service?

I am doing a project using Java and BPEL. I successfully created webservices in Java and integrated them using BPEL. All i generated a single output WSDL file. Now, I have to use this output WSDL file in my application using SOAP communication. How can i do that? Is there any help out side for such scenarios? Walkthroughs are really appreciated..

Depending on the architecture of your application (Standard Java, Spring-based, ...) there might or not be a documented procedure to consume a SOAP-based webservice.
On the other hand, you're always free to pick a webservice development framework to handle that. For instance, you could pick either CXF or AXIS2 (I believe these are the two most popular frameworks for Java WebServices). Each of these frameworks provides a tool called "wsdl2java" that helps you generate client-side/server-side/both Java classes. Then, you can easily add those classes and the requireds libraries to your application.
Having used CXF in the past, It even does provide several way to consume a webservice
Generating the client-side classes
Using CXF dynamic client factory : basically, you'll retrieve an endpoint proxy from a factory object.
Hope that'll help

I start with SoapUI (or downloadable from sourceforge), that will let you consume the WSDL and fire off requests against your server. Typically I'm hitting someone else's webservice, and trying to figure out what the data looks like before I start wiring my code together, but in your case its just a verification that the services would be/are working.
Then, as #KHY said, you can automatically convert the wsdl into java with a wsdl2java and start coding (look under the Related list on the right panel of this SO screen)

If it is a Java application, then the easiest way to consume a service is using JAX-WS. It's really easy to create a Web service client from WSDL.
See this link
Once you deploy the BPEL project on server, then refer the WSDL with http://server:port/application/YourBPELProjectService?WSDL in the consuming application. You will need to write different client code based on the BPEL type - Synchronous, Asynchronous etc.

Java EE Web Applications vs Web Services

Can someone confirm or clarify for me:
From what I can tell, Java EE web apps consist of a Servlet and/or JSP driven dynamic web page being fed back in the HTTP response, triggered by the Java EE server receiving a HTTP GET or POST request.
From what I can tell, Java EE web services also make use of Servlets as the web tier components, however a WS Servlet receives a SOAP message and validates the contents of those messages with whatever WSDL the Servlet is WARed with. The response is also packaged in SOAP and sent back to the requestor.
So, from what I can tell, both Java EE web apps and WSes use Servlets as the web components, with the only real difference being the protocol used (raw HTTP vs SOAP, which is an extension of HTTP).
This is the best I could come up with - am I right? Totally wrong? Close?

The biggest difference is not in how they are delivered, but in how they are used. Web applications are targeted to web browsers and usually serve full HTML-based applications . Web services, on the other hand, are purposed to serve raw data for another application to consume.

You are right, and servlets are just a means to connect the code at a given webapp context to a URI path. Whether the servlet is serving HTML or WS is inconsequential to the webapp container, it's just data with a MIME type being sent back.
You could also manage the same thing with a filter under Servlet 2.3 and later specifications. Spring does this, and has a very robust means of adding to the filter chain to support it.

Java-ws tomcat invoke methods through http?

I made a web project in Java, using Java-WS.
How can I invoke service methods through HTTP only.
I don't want to generate (or worse write) any java web clients, and similar stuff.
I'd just like to invoke the method with a HTTP request.
And parse the result (manually) from response.
In .NET web services I invoke methods just with:
http://serviceUrl/serviceName.asmx/operationName?parametars=...
How to do the same thing in java + tomcat?
Edit: Let me rephrase my question.
So this is what I have done so far:
Created a web application (btw. using NetBeans IDE)
Added all the necessary source files
Added web service classes with WebMethods defined
I deploy the app on tomcat and it deploys fine.
Now, what do I need to do to be able to invoke my WebMethods via HTTP?
Typing:
http://localhost:8084/MyService/MyMethod
doesn't work.
Sorry if this is a stupid question, but I'm not really a Java guru, I've been working mostly on .NET.

Multiple possibilities:
use new URL(url).openConnection().getInputStream()
use apache http components
use a REST client (if you invoke restful services), like http://code.google.com/p/rest-client/">this, or these. Or spring's RestTemplate

In this case, if you want to do an HTTP Web Service that returns an HTTP 200 Web Response, why not look at doing a RESTFul application?
JavaWorld briefly explains the role/use of REST. There's been similar questions on REST tutorials in SO. I hope this helps you.

Apache CXF has a 'plain http binding', but we recommend that people write JAX-RS services, instead. They are very, very, simple. However, the plain HTTP binding is there and supports GET.

I generate a RESTful Web Service in NetBeans by clicking on "Generate SOAP-over-HTTP Wrapper" in my service context menu.
It generated successfully, compiles and deploys fine.
But I still can't figure out how to make a HTTP invoke

JAX-WS client: JAXB required?

I need to "dive into JAX-WS programming".
So, I played around with Netbeans, after 20 or so erroneous attempts,
finally managed to let a web service client execute a web service.
I noticed, that a lot of code is generated, especially JAXB classes
for the web service response.
My current task is, to write a web service and web client completely
by hand.
Is JAXB required at all? Is is part of the standard anyway? What would happen without it?
EDIT:
Seems that the answer is given here

You can see Developing Web Services with Java 2 Platform, Enterprise Edition (J2EE) 1.4 Platform

I took a wsdl and used eclipse to generate the service, serviceLocator, ws interface, ws binding stub, ws proxy, and then the client code. I added a main method and was able to hit the webservice successfully.
I used this tutorial as a guide with the url below and no, there are no jaxb objects as I'm not pulling any objects back in the simple example I used. I was just hitting some methods that pulled back a boolean value.
http://px.pats.no/px/Eclipse_tutorial.html