Could somebody tell what are the unique features of this server? What are the advantages of using it over the popular application servers like IBM WebSphere, BEA's WebLogic, Borland Enterprise Server... Firstly, I don't know much about Borland Enterprise Server. (That's the first time that I've ever heard Borland Enterprise Server called popular. :-) )
Secondly, I'll answer from the point of view of S1AS 7.0. Atlhough there might be some reasons why I would recommend 6.x, I think that 7.0 will be the release that most people are interested in.
That said, some of the big "bullet point" reasons for using S1AS would be:
1. Performance. Especially on web path and for web services.
2. Price. Free for the base platform. This generally means development,
testing, and for production where you don't need to do any remote monitoring or clustering. And even if you upgrade to the Standard Edition (adding additional management functionality) or the Enterprise Edition (adding clustering), you are looking at a price point about half of the competition.
3. Standards support. S1AS brings Sun's appserver back to the forefront of standards support.
J2EE 1.3, JDK 1.4.x, and all of the technologies that that implies. As far as I know, S1AS is the first appserver shipping with JDK 1.4.x. And, personally, I love the new JDK features with 1.4.
4. It's from Sun. This has lots of little side benefits. It means that its part of the Solaris install. It means that the application server developers are tightly integrated with the JDK developers. It means that you have a single support desk. It means SVR4 packaging. It probably means that you will be able to buy boxes preconfigured as S1AS application servers. Lots of little things.
There's lots of other cool things that don't necessarily apply to everyone. For example, someone else mentioned Process Manager, a workflow product that runs on S1AS. (Which is amazingly cool.) The JMS implementation in S1AS is also top notch. Others might be interested in the application framework (S1AF) that ships with S1AS. (Also amazingly cool.)
In answer to your direct question about whether you can "redeploy existing applications without packaging again?, i.e. can we modify the WAR EAR or application files after the application has been deployed?", the answer is yes you can. There is a lot of debate about whether this is a good practice. If you go out to your production application server and update a single .class file or servlet, then you raise a whole managementability spectre about tracking exactly what versions of code are deployed where. The whole point of EARs/WARs was to create single repeatable deployment mechanism.
During development the ability to drop update files without redeploying is pretty useful. I even built an
Ant target that does exactly that. But in production servers I personally believe that it creates more problems than it solves.
