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What is the advantage of using Direct Web Remoting over a REST web services design?

I have an enterprise Java web application deployed in tomcat.
I am looking for ways to integrate a HTML & JS UI with the mid tier java code. So far, from looking around I have read that there are two ways to do this which fit my requirements :
1) DWR - Direct web remoting
2) Use REST web services.
I am not sure which approach to use.
Edit: What is the advantage of using DWR over a rest WS design?
Is there a better approach available? Some people asked me to have a look at GWT, but I am not sure if these are scalable.
Edit 2: I will most likely be using dojo for the UI development(Works well with REST/DWR??), and I want to keep the java code chages minimal. I understand that by using web services, I would have to write an additional WS client layer on top of the existing java code.
Also, the usual enterprise needs, performance, scalability, etc.

Ok so you're building a web client frontend to your java backend, there are a few ways to go about this:
You want to use java standards as much as possible, preferably living in the java EE world: JSF. You will need some knowledge of HTML, XML and perhaps even javascript (though limited). Personally I have had a pretty buggy experience with JSF+richfaces to the extent that I don't use it anymore.
You are very well acquainted with java desktop application development but don't know much about javascript: vaadin: it allows you to write plain java using desktop application terminology which will be compiled to javascript. Currently I am working on a big vaadin project and it is very nice to be working with pure java in eclipse, the downside is however that you are far removed from the actual frontend, so tweaking can be tricky. Additionally everything is stored in sessions (afaik) and scales poorly.
You are primarily a frontend person with unrivaled javascript skills: use a REST interface in java and a pure js/html/css frontend. Personally I think this is the cleanest design and I have designed a few of my own applications like this. The downside is that managing large javascript projects tends to be hard because...well...javascript sucks. The upside is that this will always be the fastest most lightweight option available to you.
Ideologically I would definitely suggest the last approach but this can be hard for large projects. If you have the hardware to throw at it, vaadin is a nice option. My JSF 2 experience was a bit disappointing due to the bugs in (necessary) third party libraries like richfaces.
PS: I have never heard of DWR but the last stable release seems 2 years old and all it seems to do is expose java code as javascript methods which can be better handled with a REST interface.

Open interface standards like REST and SOAP make it easy to build code to consume these services if you are using frameworks to build a REST Client and a SOAP client code respectively. But the point is that you need to have this client code to make calls to these services.
DWR on the other hand, generates this client code. Your java classes are ready to be called (like you would do in Java).

What approach for java web service to use?

I'm developing a web application with multiple frameworks (spring, hibernate, spring-security, ZK for GUI), and using Tomcat as app server. I must say I have absolutely no experience with java web services technologies. Thing is, I will almost certainly have to expose number of services for some external applications in the near future, and I was wondering what would be the way to go (considering the frameworks I'm using)...
I saw and read various tutorials and some questions (link) regarding Axis, Axis2, JAX-WS... Thing that confuses me a little bit is that I don't know what is the common practice (if any) to integrate services within existing web application (mainly in the terms of project organization). As I see it now, these services that I need to implement will rely partially on the existing source code, so I don't know whether I should use completely separate project, or I can put it inside my existing web app folder (which I tried with Axis2, but don't know if it's a good practice).
Thanks.

How to organize the projects?
In general I agree with #ericacm, but there is one thing you should keep in mind... You said you're going to develop a number of services in the near future. You may come to a point at which you want to host the services on a separate server, e.g. for performance, availability or maintainability reasons. This may influence your decision of separating the projects. Furthermore, separation "enforces" loose coupling, but therefore introduces other challenges like session sharing across multiple WARs. It's a case-by-case decision.
If I were in your situation I'd first ask myself whether the service(s) logically belongs to the web application or not.
Implementation
When in comes to WS-* implementations you have to make 2 decisions:
Decide for an API to use; today, I can't see any reason for not going with JAX-WS together with JAXB as API, they work well and they are standardized.
Decide for a Framework; I've experience using Axis2 as well as METRO (keep in mind that JSE 1.6+ provides basic JAX-WS support). Both work well. It's fairly easy to change the frameworks if you use the JAX-WS APIs.

I have good experience with Spring-WS 2+ and manual Castor mapping . Is is easy but powerful combination.
Spring-ws 2:
provides contract-first development (specially good for the web app with number of services).
provides WS annotation
supports XML mapping (Castor, JaxB, etc)
Castor:
mapping based on xml configuration
allows map multiple messages (requests/responses) to one java object (based on xml configuration)
If you are using some Java EE 6 server, consider also JAXB for manual mapping:
mapping based on annotation
should be faster than Castor
allows map multiple messages (requests/response) to one java object (when you use java inheritance)

You can go ahead and put them into the same project. Each web service will be an additional interface and implementation class along with some configuration.
Since you are using Spring CXF is a good choice as a for JAX-WS as it integrates well with Spring. See this page as a starter.

Spring-WS is complex framework for simple web services. If you want to understand web services completely and to know the nuts and bolts of web services, learn Spring-WS. It is extremely flexible and provides lot of options.
Else, if you want simpler alternative use JAX-WS. Spring supports JAX-WS annotations. Refer to the section 17.5.7. Exporting web services using the JAX-WS RI's Spring support.
http://static.springsource.org/spring/docs/2.5.x/reference/remoting.html

Is it possible to create WS Server and WS Client manually (without generators) by JAX-WS? Specially if you are developing a big application you want to re-use objects but generators are generating a lot of classes that can be in 99% the same (for example if your app is WS Client and you have to connect to badly designed external WS Server). Is there some tutorial how co create ws manually?
There is a lot of reasons why I don't like generators and completely agree with http://ogrigas.eu/spring/2010/04/spring-ws-and-jaxb-without-a-code-generator

Easy way to generate web service clients in Java WITHOUT using Axis

I've always used the web service generation in Eclipse WTP to build my web service clients due to it's relative easy of use.
However, in the last couple years, as I've learned more about it I've seen the error of my ways.
I've read that one should use one of the plethora of other java web service client platforms such as CXF, Metro, etc.
At this point, I don't particularly care which platform I use - so long as it is not Axis.
My question is - the WTP tooling makes it easy to generate web service clients in java. Are there any similar tools, scripts, programs or tutorials that are of similar ease of use?
I don't really have time to learn all the intricacies of SOAP, RPC, etc. so please keep in mind that I'm not really trying to be lazy so much as I am pragmatic.
Thanks in advance.

SoapUI can generate the soap clients for many multiple java soap stacks like axis 1 & 2, xfire, GStep, JAX-WS, JAX-RPC, JBoss WS, Oracle Proxy, cxf, .Net, GStep. Here is the tutorial on how to do it.

Jetty vs CXF / Which one to choose?

I am fairly new to the Java world and pretty overwhelmed by the beauty of choice between dozens of libraries doing more or less the same thing, or not.
So with Jetty and CXF. I am looking for a web services stack that has built-in support for a wide range of transports and protocols.
I was under the impression that CXF is the more recent project and would be the preferred choice for new software projects. Especially when it comes to applications that have to talk a variety of different protocols and standards like WS-*.
Can you give your opinion on how those frameworks differ from each other?
Which one has broader support for different protocols and standards?
Which one would you prefer regarding its design e.g. how well they hide transport, authentication, authorization, serialization aspects from application logic?
Any kind of answer very welcome!
Cheers,
Alex

Jetty is a web server and servlet container. CXF is a library for web services. If you want to serve out content like is done with a web server, choose Jetty. If you need to connect to or provide web services, choose CXF. I'm not sure there's any blurred area between the two.

There is a misunderstanding here, Jetty and CXF are like apples and oranges, it doesn't make sense to compare them. One is a light servlet engine, the other one is a web services stack.
If you're looking for a web services stack (which is my understanding), consider JAX-WS RI or CXF if you are looking for a JAX-WS compliant stack (both support contract-first or java-first), or SpringWS ("only" contract-first), or maybe Axis2 (I don't like it personally it because of its development and deployment model and its performances).
If you don't need fancy WS-* stuff, I'd suggest to use JAX-WS RI which is included in Java 6 and is compliant with the WS-I Basic Profile 1.1 (so it covers a decent bunch of WS-* standards). If you need more advanced things (like WS-SecureConversation, WS-SecurityPolicy, WS-ReliableMessaging, WS-Trust, WS-AtomicTransactions/Coordination, WS-MetadataExchange, SOAP over TCP which are provided by WSIT/Tango), consider using Metro (Metro = JAX-WS RI + WSIT/Tango).
(EDIT: Answering a comment from the OP about WS-SecureConversation support.
Actually, I think that the stack that supports the most WS-* standards is Metro. You might want to check Apache Axis2, CXF and Sun JAX-WS RI in comparison for more details. But, it is very unlikely that you'll need all of them and things might have slightly changed since the article has been published. So CXF might indeed be an alternative. For example, regarding WS-SecureConversation, CXF does support it too according to its documentation but only with "wsdl-first" projects. Without more details about what you're going to do, it's hard to give you a more accurate answer about which one is the best for you.)

Jetty is a server(a servlet container), so is Tomcat and few others. Jetty is decent, Tomcat has been around for a while and has more documentation and tutorials.
CXF looks like an interesting choice. I've only use directly implementations and CXF was fairly new, I think it's the continuation of XFire project.
CXF looks like a wise choice. When in doubt, I would say go with standards and frameworks built upon them. I would say go through the documentation and for complicated matters, you might want to subscribe to some mailing lists.

Spring-ws or Axis2 or Something else for "Contract-First" approach to WS

Everyone's saying "Contract-First" approach to design WS is more inclined to SOA style design. Now, if we take the available open-source frameworks available to achieve that we have Spring-ws and also Axis2(which supports both styles). I have a task to design SOA based e-commerce app. where loose coupling, quick response, security and scalability are the key points. So it is very important to choose the right framework from the start.
Based on past experiences, which of them or something else do you guys think to be a more appropriate option for my requirements.

For contract first I'd recommend using JAX-WS. Either CXF, JAX-WS RI or Metro (Metro = JAX-WS RI + WSIT) seem to be the best implementations around that can take any WSDL contract and generate the POJOs (or vice versa).

That is a tough question.
I have used Axis2 in the past but am relatively new to Spring WS. What I do like about spring WS is the options I get with respect to what API's I use to handle my incoming and outgoing requests (XmlBeans, JDOM, Castor etc.) and the excellent integration with a Spring based stack.
You mentioned the Contract First approach. I am not sure if Axis 2 has something like this but Spring WS has a schema to wsdl generator. You can see an example of this here:
http://static.springsource.org/spring-ws/sites/1.5/reference/html/tutorial.html
Both the frameworks offer all that you ask for in terms of features such as loose coupling, response, scalability etc. Spring-ws may also offer good integration with Acegi as far as I think but I have really not dived deep into that topic.

If you're going to create spring based application then use spring-ws. If not, you can look at jax-ws referense implementation and metro service stack. And here is another good web-service stack you may want to look at - CXF.

Axis2 is what I work with and is pretty solid solution. Also, I'm kind of allergic to Spring.