Google Web Toolkit 虽然还是beta版本，和google的其他产品一样，刚出生就注定不凡，也许将影响后面几年的b/s开发。
gwt利用了java开发的一切成熟条件，包括Unit test, refactor, IDE(eclipse...)，传统的b/s framework必将受到重创，横扫过后，JSF/ECHO等Server side framework可能幸存。如果哪天google加上serverside支持（从包命名上看是留有余地的）。。。虽然gwt目前还是小样一个，但是背后站的是重量级的google，强大的资源和数不完的银子。。。。
看到这玩意首先想到的是echo2, 客户端技术都是ajax, 编码都是java. 不同的是gwt发行时编译成HTML+JS，Echo2则是完全的服务器端生成+更新。gwt跟server端交互依靠类似于ws的service把前后台完全区分开。
在echo的论坛里面已经有人在讨论这玩意了，并且八卦了一下gwt的前身似乎是Morfik的一部分（待证实）
从原理上，echo的开发者作出了对比，这个网站似乎被封锁，这里转贴一下:
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http://echotwo.blogspot.com/ 作者tod liebeck
Comparing the Google Web Toolkit to Echo2
The Google Web Toolkit (GWT) is being compared to Echo2 quite frequently. Some of these comparisons have been fairly accurate, while others contain bits of misinformation. This article, written by the lead developer of Echo2, discusses the similarities and differences between these two frameworks.
Overview
The Google Web Toolkit and Echo2 definitely make for an interesting comparison. Both of these frameworks take a non-traditional approach toward web application development, even considering the latest crop of "AJAX-based frameworks" available today.
The most obvious similarity between GWT and Echo2 is that they both enable the developer to create dynamic, AJAX-driven web user interfaces using only Java. In both projects, UIs are developed in a fashion similar to SWT or Swing: by assembling hierarchies of components and registering event handlers. Neither project requires the developer to work with HTML, JavaScript, or XML.
The most obvious difference between GWT and Echo2 is that all of your GWT code is executed on the client, whereas your Echo2 code is executed on the server. There are advantages and disadvantages to both of these approaches, which will be highlighted throughout the article.
GWT's defining attribute is the Java-to-JavaScript compiler. This compiler allows you to develop the web interface to your application in Java, then compile it to JavaScript. GWT limits the developer to a subset of the Java 1.4 libraries. GWT applications can be served by any web server, such as Apache, without the need for server-side processing.
Echo2 applications are compiled to Java byte code and run on a Java server. Their Java code is executed by Echo2's "Web Application Container" layer, which sits atop a Java Servlet. On the web browser, the Echo2 "Client Engine" communicates user input to the Web Application Container via AJAX requests, with the server responding with directives to perform incremental updates to the state of the client web browser.
User Interface Performance
With GWT, all of your user interface code exists on the client browser. In operations that do not require server communication--that is, that do not require retrieving data from the middle tier--this configuration results in response times that are not dependent on the server. When data must be retrieved from the application's middle tier or business logic layer, the response time is subject to the same criteria as any other AJAX application, i.e., network latency, bandwidth, and server performance.
Echo2 application code is run on the server, so for each user interaction that requires a call to the middle tier or immediate execution of the application's Java code, an AJAX connection is made to the server. Echo2 components are designed to minimize the client/server communication as much as is possible, limiting it to times when the server must be notified immediately of events. For example, simple events such as user input to a TextField component will not result in server contact. The server's response is the minimum set of instructions to incrementally update the client to reflect the new screen state.
GWT applications are served to the client as a single HTML/JavaScript file, containing the entirety of the user interface. The size of this file will be proportional to the size of your user interface code and the toolkit libraries used by your application.
Echo2 JavaScript modules are lazy-loaded to the client, and thereafter cached. A module will be retrieved when a component first appears on-screen that requires it. Application code is never sent to the client, only the state of the user interface.
Middle Tier / Data Retrieval
To access business data or perform a business process, a GWT user interface makes a remote procedure call (RPC) from the browser to a Servlet. GWT provides a mechanism to make the RPC invocation transparent to the developer, allowing the developer to build the application with "Plain Old Java Objects" (POJOs). However, any application that provides an RPC capability is a distributed application -- even when the RPC is accomplished transparently to the developer. Distributed applications in businesses and enterprises usually have security considerations and the remote objects serving the GWT clients must be designed with a focus on security to deflect attacks from imitated or hostile client applications.
Echo2 applications support, but do not require, the use of distributed application logic or a Service Oriented Architecture (SOA). Alternatively, Echo2 applications can be built to run entirely within a single JVM instance, backed by a POJO-based middle tier. This allows Echo2 developers to build applications without the security concerns of distributed application logic -- and leverage the many strong frameworks built around POJO development such as the Spring Framework and Hibernate. Echo2 accomplishes this by keeping the state of a user's web interface on the server so that no remote objects need to be exposed.
Run-time Environment
GWT has some limitations due to the fact that applications are run on the client browser. First, GWT applications are limited to using a subset of the core Java class libraries, consisting of 27 classes, 11 interfaces, and 18 exception types found in the java.util and java.lang packages (as of GWT 1.0.21). This limitation prevents GWT applications from linking to most existing Java libraries. Additionally, all Java code must be compliant with the Java 1.4 specification; 1.5 is not supported. Localization-related portions of the Java API are not provided.
Debugging
GWT provides an alternate deployment environment for applications to facilitate debugging. The environment, called "Hosted Mode", allows a GWT application to be run as Java byte code in a local JVM, to which an IDE's debugger can be connected. In this mode, the application's user interface is displayed in a special web browser (a Mozilla/Firefox derivative).
Echo2 applications may be debugged in the conventional manner, by connecting an IDE's debugger to a JVM running a Servlet container.
Licensing
The primary component of GWT, the Java-to-JavaScript cross-compiler, is proprietary, binary-only software. The Java API libraries are open source software, distributed under the Apache License. The API libraries have essentially no value without the proprietary compiler. The (non-critical) hosted-mode browser is also under the proprietary license. GWT is provided free of charge.
Echo2 is open source software, licensed under the Mozilla Public License, and provided free of charge.
Applicability
GWT can be used as a means of creating AJAX components to embed in traditional web applications (or even in static web pages) as well as for creating complete application user interfaces. There are some issues to using it for the creation of large applications, where downloading an entire application to a client web browser in one shot would not be practical. The lack of localization and full Java API support also presents a problem for larger solutions.
Echo2 is practical for creating web applications of any size. It is however not intended to scale downward to function as a platform for simply creating AJAX components in traditional web frameworks (or static web sites).
More Information
Google Web Toolkit:
Home Page, Example Applications, Getting Started Guide, Developer Guide
Echo2:
Home Page, Example Applications, Tutorial
posted by Tod Liebeck at 5:13 AM | 0 comments
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毕竟gwt还是小baby, 以后怎么发展还难说，现在下结论太早。这两天试用一下，有空从细节上对比一下。