Related
Can I write my web-site only in JavaScript being sure that my code is concealed from anyone? In this respect, is Node.js, like Apache, can be given access to through an Internet-provider?
The answer to both of your questions is yes.
Node.js can completely replace Apache (assuming you are willing to re-write all of your PHP as JavaScript). If you have your Apache running in reverse-proxy mode between your server and client, you can even handle some requests in Node.JS while handing others in PHP. This will allow you to maintain original functionality while changing the code out, and also allow PHP to handle more mundane tasks.
While you cannot prevent raw JavaScript from being read through any means of obfuscation, you can prevent people from reading your code by note making use of standard JavaScript at all. You can use a NativeExtension for Node to add an extension handler for encrypted JavaScript files:
require.extensions[".jse"] = function (m) {
m.exports = MyNativeExtension.decrypt(fs.readFileSync(m.filename));
};
require("YourCode.jse");
This will convert your JavaScript code to a .jse, which you would then package for production. Considering the encryption is done inside the native extension, the encryption key wouldn't be revealed.
Hope this helps! :)
Yes, you definitely can. It may, however, take a while to transition existing code, and if this is for a corporate institution, you'll have to ask your coworkers and your boss/supervisor. Good luck, and remember, always document your code in JavaScript (no types) all languages.
NodeJS is much faster: http://www.hostingadvice.com/blog/comparing-node-js-vs-php-performance/
Many more libraries: http://npmjs.org
Only need one language for everything
Comparing PHP with another technology like Node.js which is meant for an entirely different type of task, comparer must mention the difference of use-case/context in which one is suitable over others.
Let's talk about in terms of a different area of execution because we can not Disgrace any of them and both have its own priority.
If you talk about in terms of Application Domain.
PHP :
CMS (Content Management Systems) like WordPress, Drupal also use PHP which makes it possible to be used in creating blogs, websites, e-commerce sites, etc.
Used in developing CPU-intensive applications like meteorology applications and scientific applications.
should be used in applications in which the client does not have to interact with the server again and again
PHP 7 is based on the PHPNG engine that speeds up PHP applications more than the previous PHP interpreter (Zend Engine 2.0). Thanks to PHPNG, your apps see up to the 2x faster performance and 50% better memory consumption than PHP 5.6.
NodeJs:
Nodejs is ideal for developing highly scalable server-side solutions because of its non-blocking I/O, and event-driven model.
Used massively in Real-time applications like chat applications, blogs, video streaming applications.
Used in developing single-page applications like resume portfolios, individual websites.
Node.js should be used for the applications which require a lot of interaction between client and server.
For some tasks, Node.js may be faster than the “standard” web server with PHP because it runs as a single thread with non-blocking IO for each connection, hence there is no memory overrun. Due to this, Node.js is useful when there is a need to process data in real-time (chats, games, video, big data streams without logic)
PHP is Still alive and he has learned its lessons from Node.JS
ReactPHP enables developers to write PHP-based socket server to process requests constantly as well as Node.js does it (yes, Node.js is faster, but the fact is that PHP can also do it). The same thing with Workers (the classes responsible for running asynchronous jobs and synchronizing their results) or Amp (libraries that are used for writing non-blocking asynchronous code). Hence, it is easy to create long-running processes using PHP. Additionally, there are a lot of tools for supporting and managing these processes (such as supervisord).
So, the same tasks may be performed either with PHP or with Node.js. The question “what tool to use” is a question of personal preferences. you can use Node.js for tasks involving big data flows and PHP for tasks involving complex logic, high-load tasks, for dealing with external utilities, applications, and OS.
From the scalability perspective, there are no big differences between PHP and Node.js, it is more important to consider the project’s architecture.
Dayle Rees (a Laravel Framework contributor and developer): For a long time PHP was the butt of many language jokes, but I honestly feel that it’s becoming not only a popular language but a powerful one. PHP7 is great. The speed boost is one thing, but having optional support for full type hinting is a game changer. We’ve also got modern tools like Laravel and Composer, breathing new life into the language and its supporting community. With this in mind, I think it’s unlikely that Laravel will move from PHP. I think it’s more likely to gain further integration with front-end tools to provide a complete application building platform. That’s where I see it heading in terms of future expansion. I’m sure the Node will continue to excel when dealing with microservices and threaded applications.
The most important and most awaited News from PHP is, PHP is scheduled to receive a Just In Time (JIT) compiler in its next major version PHP-8 (most probably in sep-2021).This is going to boom the php and it breaks all his limitation due to JIT.
Wrap up
To wrap up Both have some pros, both have some cons but the amazing thing is both are created by intellects to make the web development better. While selecting the technology the question shouldn’t be which one is better but which one can serve your project needs in a better way. Understanding your project and business logic can give you a clear idea about selecting the right technology for your project.Moreover, one more important thing to consider is the skills and proficiency of the developers using the technology, how they use them and apply to the project.
I have the following situation. A customer uses JavaScript with jQuery to create a complex website. We would like to use JavaScript and jQuery on the server (IIS) for the following reasons:
Skills transfer - we would like to use JavaScript and jQuery on the server and not have to use eg VB Script. / classic asp. .Net framework/Java etc is ruled out because of this.
Improved options for search/accessibility. We would like to be able to use jQuery as a templating system, but this isn't viable for search engines and users with js turned off - unless we can selectively run this code on the server.
There is significant investment in IIS and Windows Server, so changing that is not an option.
I know you can run jScript on IIS using windows Script host, but am unsure of the scalability and the process surrounding this. I am also unsure whether this would have access to the DOM.
Here is a diagram that hopefully explains the situation. I was wondering if anyone has done anything similar?
EDIT: I am not looking for critic on web architecture, I am simply wanting to know if there are any options for manipulating the DOM of a page before it is sent to the client, using javascript. Jaxer is one such product (no IIS) Thanks.
Have a look at bringing the browser to the server, Rhino, and Use Microsoft's IIS as a Java servlet engine.
The first link is from John Resig's (jQuery's creator) blog.
Update August 2 2011
Node.js is coming to Windows.
The idea to reuse client JS on the server may sound tempting, but I am not sure that jQuery itself would be ready to run in server environment.
You will need to define global context for jQuery somehow by initializing window, document, self, location, etc.. I am not sure it is doable.
Besides, as Cheeso has mentioned, Active Server Pages is a very outdated technology, it was replaced with ASP.Net by Microsoft in the beginning of the century. I used to maintain a legacy system using ASP 3.0 for more than a year and that was pain. The most wonderful pastime was debugging: you will hardly find anything for the purpose today and will have to decript beautiful errors like in IIS log:
error '800a9c68'
Application-defined or object-defined error
Nevertheless, I can confirm that I managed to reuse client and server JScript. But this was code written by me who knew that it was going to be used on the server.
P.S. I would not recommend move that way. There are plenty templating frameworks which are familiar to those who write HTML and JavaScript.
JScript runs on IIS via something called ASP.
Active Server Pages.
It was first available in 1996.
Eventually ASP.NET was introduced as a successor. But ASP is still supported.
There is no DOM for the HTML page, though.
You might need to reconsider your architecture a bit.
I think the only viable solutions you're likely to find anywhere near ready to go involve putting IIS in front of Java. There are two browser-like environments I'm aware of coded for Java:
1) Env-js (see http://groups.google.com/group/envjs and http://github.com/thatcher/env-js )
I believe this one has contributions from jQuery's John Resig and was put together with jQuery testing/support in mind.
2) HTMLUnit (see http://htmlunit.sourceforge.net/ ) This one's older, and wasn't originally conceived around jQuery, but there are reports in the wild of using it to run jQuery's test suite successfully (http://daniel.gredler.net/2007/08/08/htmlunit-taming-jquery/ ).
If you want something pure-IIS/MS, I think your observation about windowsScript host and/or something like the semi-abandoned JScript.NET is probably about as close as you're going to come, along with a port (which you'll probably have to start) of something like Env-js or HTMLUnit.
Also, I don't know if you've seen the Wikipedia list of server-side JavaScript solutions:
http://en.wikipedia.org/wiki/Server-side_JavaScript
Finally... you could probably write a serviceable jQuery-like library in any language that already has some kind of DOM library and first-class functions (or, failing that an eval facility). See, for example pQuery for Perl (http://metacpan.org/pod/pQuery ). This would get you the benefits of the jQuery style of manipulating documents. Skill transfer is great and JavaScript has a wonderful confluence of very nice features, but on the other hand, having developers who care enough to learn multiple languages is also great, and js isn't the only nice language out there.
I think it's mainly a browser based script so probably you are better of using technologies based on VB or .NET to perform or generate HTML from templates. I'm sure there are because in the java world there are a few of these around (like velocity). You'd then use jQuery to create or add client side functionality and usability so it makes the website more usable than it would have been.
What exactly do you mean by
"A customer uses JavaScript with
jQuery to create a complex website"
Half the point of jQuery is to make it easy for the developer to manipulate the DOM, and therefore add interactive enhancements to a web site. By running the Javascript on the server and only rendering HTML you will lose the ability to add these enhancements, without doing a round trip to the server (think WebForms postback model...ugh).
Now if what you really mean is the customer uses a site builder based on jQuery, why not have that tool output flat HTML in the first place?
Take a look at this technology. You can invoke scripts to run at server, at client, or both. Plus, this really implements the firefox engine on the server. Take a look at it.
Aptana's Jaxer is the first AJAX web server so far. I have not tryed it yet, but I will. Looks promising and very powerful.
I'm currently looking for a new web stack to build a hobby project on and would like it to be powered by JavaScript. I've had a quick look at Nitro, NarwhalJS etc. but was wondering if anyone had any solid recommendations or experience of an entire end-to-end javascript/json architecture ( jquery, middleware, standard libraries, db etc.) that they could share.
I'd prefer it to be a stack that you think is going to grow and is actively being looked after, documentation, community of nice like-minded individuals etc.
Thanks in advance.
Caveat: This answer somewhat fails to meet your basic requirement of personal experience with the resources listed. :-)
Off-the-cuff, there's Jaxer from Aptana and Chrome Server (which, despite the name, isn't related to Google Chrome AFAIK). Speaking of Chrome, though, there's an Apache CGI module that supports server-side scripting using JavaScript via Google's V8 engine, but that doesn't help you much with DB connectivity and such.
I'll also plug Java and Rhino, which I have used a bit. Via Rhino, you can compile JavaScript into Java bytecode (which, if you're using the Sun runtime, is JIT-compiled to machine code by Hotspot). That means you can run JavaScript in any servlet container (Tomcat, Resin, etc.). The joy here is that all of the huge array of goodies available for Java is instantly available to you via JavaScript -- so, MySQL connectors, image manipulation libraries, just about anything you can imagine. It's also amazingly easy to access those resources from JavaScript via Rhino. To give you an idea:
importPackage(java.io);
var f = new FileWriter("test.txt");
f.write("Testing 1 2 3");
f.close();
...and we've just written a file on the server via JavaScript, using Java's java.io.FileWriter class. You can also execute dynamic scripts at runtime via the javax.script package, which (for JavaScript) uses Rhino under the covers, although I'm not immediately coming up with a use case for doing that. :-)
For the database part of the stack:
Couchdb uses JSON and REST to store data in a document format. It uses PUT,DELETE for storage - I'm not sure how that would work with Javascript.
Helma should work well as a web server layer. It streamlines the use of Rhino as the web tier logic language.
I'm building a new service called PageForest that helps developers write totally client-side javascript programs, with PageForest providing storage and user management. Here's a sample page:
http://pfsamples.googlecode.com/svn/trunk/SAMTable/index.html#mckoss_16
This is still a work in progress, but I'd love to get some feedback on the approach. You can find more samples at the pfsamples.googlecode.com site.
Check out JScript / Windows Scripting Host(wsh) and possibly HTA's. HTA's can actually be served via a webserver and act as a locally running application with extended rights. If you want pure web development you can use WSH and some IIS tweaking to process server requests on the back end with pure javascript code in a WSF file. WSH also provides access to the file system, ODBC compliant databases and a slew of other COM exposed applications via the ActiveX model. We're not talking blazing speed, but you're programming in javascript to begin with.
Here are some links on the "stack"
http://msdn.microsoft.com/en-us/library/ms536496(VS.85).aspx
http://msdn.microsoft.com/en-us/library/15x4407c(VS.85).aspx
How about using
GWT-Spring-Hibernate-MYSql
Given a set of Java source code files, how can I compile them into one or more JavaScript files that can be used with hand-crafted JavaScript?
GWT is one option, but every example I've seen so far is aimed at building fancy websites. The simple use case of just converting Java source to Javascript that can be used together with handcrafted JavaScript hasn't been well-documented.
I started a thread on the GWT mailing list on this subject, but opinions seem to be mixed on whether this is even feasible.
One person gave a very useful tip, which was to check out GWT-Exporter. The problem is that neither source code nor documentation is readily available, although there's this and this.
edit: GWT-Exporter source code is here
I've also seen Java2Script. But again, I wasn't able to find examples of how to solve my simple use case.
What's the best approach to this problem? Is there something better I'm missing?
When you use GWT, you're basically converting the UI portion into Javascript (and it assumes that you use the UI widgets provided when you write your Java). Only some of the Java libraries are accessible within Javascript. Typically in a GWT application anything that makes heavy use of Java libraries would run on the server side and connect to the Javascript as AJAX (which GWT handles for you). So GWT isn't necessarily converting your full application into Javascript (though it can if you're willing to limit your use of Java libraries and some functionality).
At any rate, if this approach (calling out to Java running on a server from within Javascript) appeals to you, one nice option is DWR, which basically allows your Javascript to directly call methods in Java classes running on the server (without you having to build a web service or other frontend). Not what you asked, I know.
More relevantly, it looks like there's source code for a sample app demonstrating the use of gwt-exporter.
I am not sure if it fits your use case, but if you agree to drop Java APIs and use JavaScript APIs from Java, then you can use JSweet, a Java to JavaScript transpiler built on the top of TypeScript. It gives you access to hundreds of well-typed JavaScript APIs (DOM, jQuery, underscore, angularjs, etc). It generates JavaScript code and you can mix it with legacy JavaScript and TypeScript code.
Note: JSweet will not work for legacy Java code and legacy Java APIs, but your use case did not mention reusing legacy code.
[UPDATE] Since version 1.1, JSweet now also supports some Java APIs such as Collections (java.util). So, it is possible to reuse legacy Java code to a certain extent. It is also quite straightforward to add your own support for Java APIs.
While the question is about compiling Java sources to JavaScript I think it's worth mentioning that there is TeaVM which compiles Java bytecode to JavaScript. I have never tried it, but it seems very promising.
Given a set of Java source code files, how can I compile them into one or more JavaScript files that can be used with hand-crafted JavaScript?
Although there are many solutions to convert Java applications to Javascript, you are interested on a solution where new javascript code may interact with the resulting code. This is an update (as 2018) of the other answers.
There are different types of tools. For instance, you may find tools that allow you (1) convert java code to javascript; (2) convert bytecode to javascript, asm.js or webassembly; (3) execute java applications directly in the browser and (4) create solutions that combine java and javascript. You must select the solution to use depending on your requirements.
Converting Java source code to Javascript
Some solutions take java source code and produce a javascript equivalent version. Usually, these solutions transforms the Java to Javascript, but do not support all the behaviours and libraries of the Java runtime. The resulting code may not support some java standard libraries. Typically, they are used to create HTML application using Java but not for migrating the code. Pros: The resulting solution may include very small files. You can use it to reuse your own business logic classes without considering GUI or platform specific libraries. Cons: it is possible that you cannot use some functionalities of the Java platform. It requires access to the source code.
JSweet converts Java to javascript. It includes API bindings for 1000+ javascript libraries. You can write java code that use these libraries.
It converts java to typescript and later uses the typescript compiler to create the corresponding javascript.
It can produce multiple types of javascript modules and typescript definitions. You can use the resulting code in javascript very easily.
j2s, is the compiler used by the Eclipse RAP platform to translate java code to javascript. It is used there to convert the SWT (GUI) widgets to javascript and HTML. It does not support all the Java standard libraries
It may convert simple Java classes to be used in javascript. You must use the classes considering the OO Java-Javascript simulation performed by j2s.
Converting Javascript bytecode to javascript
These solutions take compiled java code (.class files) and produces equivalent code in javascript, asm.js or webassembly. Considering that the java code may depend on java standard libraries (i.e., the JRE), these solutions typically includes ported and pre-compiled libraries. Pros: you do not need to change anything in your code. You do not need the source code neither. Cons: the resulting solution may require the load of a lot of files.
Bck2Brwsr, a Java VM that may compile ahead-of-time the java bytecode to javascript. It produces a javascript file for each .jar file.
You may use the vm javascript object to load a class into javascript and execute static methods (using vm.loadClass(.., function(class){..}}). There is an example in the documentation for the gradle plugin and the maven task.
TeaVM, is another Java VM that may convert ahead-of-time the code to javascript. In contrast to Bck2Brwsr, it supports threads, produces a single file for all your classes and provide better debugging support.
You may call java from javascript using the javaMethods.get(..).invoke(...) method.
DukeScript, transpile java code and bytecode to javascript using Bck2Brwsr or TeaVM.
It provides maven tasks to compile the java code. The resulting code (and technique to call java from javascript) depends on the used transpiler.
Dragome, transpile java bytecode to javascript.
You can annotate static methods in Java and use it in Javascript: e.g. you can annotate a method with #MethodAlias(alias="windows.method1") and invoke it from javascript using window.method1()
CheerpJ (a commercial product) may run complete java applications using Swing and AWT. It provides a very complete javascript environment that support operating system, thread and network functionalities.
It provides a complete runtime API. You can run a main method using cheerpjRunMain( <class>, <jar> ). You can create objects using cjNew( <class>, <params>...) and invoke static methods using cjCall( <class>,<method>,<params>...). There are many other methods you may consider.
Running Java code in Javascript
DoppioJVM is a complete JVM written in Typescript. Pros: It emulates a lot of elements of the operating system, including filesystems, TTY consoles and threads. Cons: Considering that it is an interpeter, it may result slower than other solutions. (I have not tested it)
DoppioJVM is a JVM written in Typescript
The documentation includes snippets of code to load and run the classes. You can run a static method using jvm.runClass( <class>, [ <args>...], function(response){..}). You can run a Jar file and perform many other tasks.
Create applications combining Java and Javascript
Some other solutions provide, not only the tools for compiling the code, but also frameworks and solutions to create java and javascript solutions. For instance, CheerpJ has complete versions of the Swing and AWT libraries for graphical user interfaces, but they may result very slow. You may replace the user interface by using new HTML versions that run faster on the browser. Pros: You can reuse existing code without changes, mainly some libraries and business logic. You may remove from your solutions libraries that run not efficiently in the browser. Cons: If you wanna keep maintaining your java desktop version, you must deal with different code for the browser.
GWT converts Java code to javascript but uses a different set of libraries for the user interface and client-to-server communications.
The documentation maintains information of differences with the JRE
You may export Java classes to be used by Javascript code.
Dukescript uses the conversion tools mentioned before.
It provides a Knockout4j library that may interact easily with HTM, DOM and additional javascript code.
Dragome not only convert java code but also include means to interact with the HTML/DOM
You may use annotations to bind java objects to HTML elements and to handle the DOM
HTML/Java API is an Apache project that standardize the access to HTML/DOM/javascript from transpiled code
It was donated by Dukescript and other tool providers.
Recommendation
If you wanna reuse few classes created by your own, you may try JSweet. You may create javascript modules (libraries) that you can use easily with javascript and typescript.
If you wanna reuse a medium to large codebase that rely on multiple java libraries, you may try CheerpJ, Dukescript or Dragome. You may reuse large parts of your code and create (gradually) the user interface and client-to-server communications using technologies that are more browser-friendly.
If you wanna run complete java applications without change, you may try CheerpJ. It can run Swing and AWT user interfaces. It also provide an Applet runner.
Here's two other options, things to look into and a third option not converting, just living together.
Java2Javascript
I have been wanting to try this out -- Looks closer to what's been asked. Quoting the web page:
an Eclipse Java to JavaScript compiler plugin and an implementation of JavaScript version of Eclipse Standard Widget Toolkit (SWT) with other common utilities, such as java.lang.* and java.util.*. You can convert your SWT-base Rich Client Platform (RCP) into Rich Internet Application (RIA) by Java2Script Pacemaker.
A limited Java in Javascript experience - You'd need to port your necessary dependencies or find alternatives via tools like jQuery, etc.
DukeScript
As I view DukeScript, it compiles some front-end Javascript and calls Java behind, from the browser's Javascript. It seems more or less a hybrid approach so you can use the Java wealth of libraries from Javascript. I will fall foul of a browser security policy for Java.
A more full-Javascript on Java experience leveraging the Java-runtime. If I wanted that outside the browser environment I'd use Javascript on Java.
Nashhorn
Consider this as an example of using Java's resources as foundation for Javascript: Nashorn and JavaFX, as an example for a rich Javascript operated client. Anyway with a Javascript engine inside Java you're not needing to translate between a Javascript-VM to object-code to a Java-VM quite so much.
Not entirely on-topic, but Kotlin is a 100% Java-compatible language that can compile to JavaScript.
IntelliJ IDEA can automatically convert Java to Kotlin and compile it to run on Node or the browser.
Given a set of Java source code files, how can I compile them into one
or more JavaScript files that can be used with hand-crafted
JavaScript?
There is no direct correlation between both the built-in Java API and Java language features, and those of JavaScript. So any attempt at creating a "converter" is going to be incomplete. As a fundamental example, Java classes don't have a direct corresponding JavaScript idiom.
Whether or not an incomplete conversion tool will work for your use case is impossible to know without the source code.
That said, my suggestion to solving your problem would be to first attempt to use GWT: set up a demo project, drop in the source of your library and call it from the JavaScript side and see what GWT outputs in it's .js file. Some of the other tools suggested by other posters here are definitely worth checking out as well.
If that is fruitful and gets you part of the way, great.
From there, you'll need/want to do the remainder of the conversion by hand. After all, assuming you want the code to actually function correctly, a manual review would definitely be in order. Some unit tests being converted along with it would be ideal as well.
You don't state how large the source of your project is, but if it's small (let's say less than a thousand lines of code), even a complete conversion by hand shouldn't be extremely difficult. If it's much larger than that, I would suggest reviewing if you actually want that as JavaScript code anyway.
This question is from 13 years ago; yes, I know. But I'll put my addition here as this is an excellent repository of answers on the topic, and as it comes up first in a Google search it is likely to have the most eyeballs on it.
I'm a former GWT developer. And agree it is an excellent solution & technology. But also yes as mentioned, the cross compilation part mostly (arguably, completely) focuses on the W in its name (Google WEB Toolkit): the client-side JavaScript part of full stack SPA Web applications, its serialization & communication, it's client/server logic partitioning, et al.
I just stumbled onto something that is relatively new -- a 2014-2015 offshoot of GWT placed into the public domain in 2018 -- that I think is more generally focused and what was being requested in the original post (13 years ago, lol). I'm eager to try it.
J2CL: https://github.com/google/j2cl
I read about it here, which to me was a good & thorough intro of the technology (have a look and let me know if you agree or not):
https://www.infoq.com/news/2019/05/j2cl-java-javascript-transpiler/
I would like to create a database backed interactive AJAX webapp which has a custom (specific kind of events, editing) calendaring system. This would involve quite a lot of JavaScript and AJAX, and I thought about Google Web Toolkit for the interface and Ruby on Rails for server side.
Is Google Web Toolkit reliable and good? What hidden risks might be if I choose Google Web Toolkit? Can one easily combine it with Ruby on Rails on server side? Or should I try to use directly a JavaScript library like jQuery?
I have no experience in web development except some HTML, but I am an experienced programmer (c++, java, c#), and I would like to use only free tools for this project.
RoR is actually one of the things the GWT is made to work well with, as long as you're using REST properly. It's in the Google Web Toolkit Applications book, and you can see a demo from the book using this kind of idea here. That's not to say that you won't have any problems, but I think the support is definitely out there for it.
There's a neat project for making RoR/GWT easy that you can find here (MIT license). I haven't had a chance to try it out yet, but it looks like a good amount of thought has been put into it. One catch is that it looks like it hasn't been fully tested with 2.1 Rails yet, just 2.0, so you may run into a few (probably minor and fixable) errors.
If you are looking to integrate GWT with non-Java backends such as ROR, PHP etc., you should bear in mind that GWT 1.5 now supports JavaScript Overlay types. This feature lets you write classes that can be mapped over the top of native JavaScript objects to easily provide accessor methods for properties of those objects and other extended functionality.
See this link for more details:
JavaScript Overlay Types
So you could return JSON encoded data from your backend via AJAX calls, parse it into a JavaScript Object and then access the data through your GWT Java code using the overlay classes you've created. Or when you render your page you can render static config data as JavaScript Objects and read it in via this mechanism, rather than having to do an AJAX call to grab the data.
If you know JAVA, and have somewhere you can host it (like a tomcat or glassfish container) I would recommend that much more than using Ruby for the back end. The main reason is that then you can share all of your objects, and use the built in RPC mechanism. I've done this for quite a lot of our projects and it's a huge timesaver, not to mention that the code is less error prone, because you don't convert your java objects to anything and then back again.
I have linked my GWT with Rails before, using the to_json function in Rails and then reading the JSON in GWT. It's all supported, but it is far more annoying than just doing the back end in JAVA.
Of course if you have cheap hosting, then Java containers are pretty much out of the question, in which case I would think Rails would be the next best thing.
GWT is very high quality with a great community. However you do need to know CSS if you want to adjust the look of things (you will) - CSS can do a lot of the layout, just like regular web if you want it to. Libraries like GWT-ext or ExtGWT can help a bit as they have stunning "out of the box" looks but for a price (extra size to your app).
You can code everything in Java using GWT, and you can integrate existing 3rd party javascript libraries with it. It's very good. I've never used RoR much though, so can't say anything about that.
If you're experienced in Java but not in Javascript/CSS, then GWT is going to be a lifesaver (unless you want to learn them, of course). CSS has so many little fiddly details. It is not uncommon to spend half a day fixing a 2 pixel misalignment that only occurs in IE6.
I am not sure about how easy it would be to use ROR for the back end... It is possible, I am sure, since GWT ajax communication is just servlets. But they provide some really nice functionality for passing Java objects back and forth which you won't be able to utilize if your server isn't also using Java.
I wrote about some of the disadvantages of GWT recently. Mainly, the disadvantages are: long deployment cycle for changes to some parts of the application and a rather steep learning curve. As a seasoned Java programmer, the second should be less of a problem and if you use a seperate backend, the first is also mitigated (as a complete redeploy is primarily required when you change the 'server' part of the application).
GWT is a wonderful framework with lots of potential. Keep in mind that it's still quite new, though. There are some unresolved bugs that can really annoy you, and they usually require ugly workarounds to get past. The community is great but you'll probably end up with a few problems sooner or later that Google can't answer yet.
But hey, I say go for it. The potential for GWT is awesome, and I bet it's future will be bright.
You should definitely use GWT for a new project (it's pretty easy to use in an old project too).
I my experience it's very fast to learn and use. The compiled javascript code is much better than anything you could ever write by hand and it works fast too.
Another benefit is the ability to debug you're code (which is hell with javascript alone)
This blog has inputs from many experienced users of GWT and have some great discussion points. I personally have huge experience with varied UI Frameworks. I will add my two cents. Lets look at fundamental advantages and disadvantages of GWT
Fundamental Advantage
GWT takes the web layer programming to JAVA. So, the obvious advantages of Java start getting into play. It will provide Object Oriented programming. It will also provide great debugging and compile time checks. Since it generates HTML and Javascript, it will also have ability to hide some complexity within its generator.
Fundamental Disadvantage
The disadvantage starts from the same statement. GWT takes the web layer programming to JAVA. If you know JAVA, probably you will never look out for an alternative language to write your business logic. It's self sufficient and great. But when it comes to writing configurations for a JAVA application. We use property files, database, XML etc. We never store configurations in a JAVA class file. Think hard, why is that?
This is because configuration is a static data. It often require hierarchy. It is supposed to be readable. It never requires compilation. It doesn't require knowledge of JAVA programming language. In short, it is a different ball game. Now the question is, how it relates to our discussion?
Now, lets think about a web page. Do you think when we write a web page we write a business logic? Absolutely not. Web page is just a configuration. It is a configuration of hierarchical containers and fields. We need to write business logic for the data that will be captured from and displayed on the web page and not to create the web page itself.
Previous paragraph makes a very very strong statement. This will explain why HTML and XML based web pages are still the most popular ones. XML is the best in business to write configurations. A framework must allow a clear separation of web page from business logic (the goal of MVC framework). By doing this a web designer will be able to apply his skills of visualization and artistry to create brilliant looking web pages just by configuring XMLs and without being bothered about the intricacies of a programming language. Developers will be able to use their best in business JAVA for writing business logic.
Finally, lets talk about the repercussions in direct terms. GWT breaks this principal so it is bound to fail. The cost for developing GWT application will be very high because you will need multiskill programmers to write web pages. The required look and feel will be very hard to achieve. The turn around time of modifying the web page will be very high because of unnecessary compilation. And lastly, since you are writing web pages in JAVA it is very easy to corrupt it with business logic. Unknowingly you will introduce complexities that must be avoided.
You could also consider Grails ("Groovy on Rails") which gives you the benefits of a Rails framework and the use of the Java VM.
Our team recently asked the same question, and we chose to go with GWT, especially since the designer plugin made working with GWT more accessible to non-java experts on the team. Whoever makes this choice, just beware you DON'T use the GWT Designer plugin !! It has not been updated (in at least a year, apparently) to create a GWT application that is compatible with IE8.
Our team had almost completed our application layouts, which were working perfectly in Chrome, FF and Safari. Then they blew up in IE. IE 7 would load partial pages (but not composite includes), and IE8 was not even able to load up the application. It just hung.
The designer plugin has buttons that allow the user to add CellTable widgets that are not IE compatible (CellTable, DeckPanel, Horizontal Panel, Vertical Panel, among others). These will cause intense pain when the layouts have to be re-done in java without assistance from the designer.
Experienced GWT users love it, but the designer plugin will kill you.