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What is the reason that the tomcat is not an application server

I know that Tomcat is a web server but why it is not an application server?
Any server needs to follow some specification. What is that spec?
Is it possible for apache to make the tomcat application server?
Also I have read in a blog that the tomcat do not have some lib to act as an application server. What are those libs?
Thanks

I've heard once the following explanation I tend to agree with:
There is a spec of JEE (Java enterprise edition).
Formally you can think about it as a bunch of pdf-s describing the behavior of various technologies that comprise the JEE stack (for example: JMS, EJB, JPA, JPA, JSF, CDI and so on and so forth) as well as deployment models (EARs for example).
Implementors of Application servers have to implement all those technologies and offer interfaces that can be used by the application developers. So teams that stand behind WildFly (former JBoss), Geronimo, WebSphere, WebLogic and so on have read these specs and implemented everything in there.
Now, Tomcat didn't do that, they've only concentrated on (primarily) Servlets/JSPs. These are web technologies, so Tomcat can't be considered as an Application server that implements the whole JEE stack.
In general Tomcat (as well as Jetty, incidentally) should be more lightweight than full JEE compliant Application servers, it should start-up faster and it memory footprint should be smaller. So Tomcat/Jetty call themselves web servers.
I understand that this answer can be considered as speculation, but for me it makes a lot of sense.

Bellow is my perceive:
know that Tomcat is a web server but why it is not an application server?
Here, Application Server is specially for Java EE Server，Java EE is a huge specific collections for Enterprice Application Development，So Application Server should implement most specifics of these, while Tomcat(or Jetty) is only a Web Server(More accurately，Servlet Container)，they only implements the specific about Web(such as Servlet Spec(JSR340), JSP(JSR245)). Therefore，Application Server is stronger than Web Server，but Web Server is more lightweight and enough to satisfy most web applications.
Any server needs to follow some specification. What is that spec?
Of course，it depends on that your Server want to provide what services(functions)，these specifications can view here.
Is it possible for apache to make the tomcat application server?
I don't think apache will make tomcat to be an application server. Now, there are some popular Java EE Server: Jboss，WebLogic, etc. Not all enterprices need a heavyweight Application Server, on the contrary, most only need a lightweight Web Server。
Also I have read in a blog that the tomcat do not have some lib to act as an application server. What are those libs?
Tomcat only need care the specs about Web，and implements them.
Hope for your help.

Spring application deployment on Java-ee servers

Can I deploy a Spring java application on all (or at least most) of JavaEE servers ? like Weblogic, JBoss, Webshpere, Tomcat, Jettty etc.
And is there is preferred Java EE server for Spring ?

Yes you can. And no - there isn´t. Errata: you can take a look at the SpringSource dm Server. Be aware that spring is not a java EE implementation. Read this post here.

Yes, you can deploy it on any server, and you can use it on standalone applications and even on mobile applications. It is in no way restricted to web deployment. It is not even restricted to java, there is Spring.NET and even a Python version. However, java and web servers are a common if not the most common use for it.
Currently recommended web server by VMWare/Pivotal (current owner of Spring) is VFabric tcServer, which is a Tomcat fork containing some extra tools and functionality. But server in itself does not matter, Spring is container agnostic.
There is no preferred EE server. VFabric tcServer is a servlet container, not an EE server. It contains "enterprise capabilities" instead, but that's not the same thing. As said however, you can use whichever EE server you want.

What are the differences when deploying on Tomcat vs. Websphere?

If I were to deploy an application on Tomcat vs. Websphere, what are things that I need to consider?
Do I have to develop my Java code differently if developing in one app server vs another?
Edit:
I will be funneling people from a website into a web app that does credit card processing and e-signatures (cc processing and e-sigs are through separate services). That is its sole job

You cannot use EJBs on Tomcat (unless you add OpenEJB). If your WebSphere deployment uses EJBs, you'll have to remove them to deploy on Tomcat.
If you use any Java EE features beyond servlet/JSP engine and JNDI naming service you'll have to eliminate them from your app.
Tomcat accepts WAR packages. If you package your app into an EAR on WebSphere, you'll have to change it to WAR for Tomcat.
Both use JNDI for data sources. There might be some nagging differences in naming conventions, but if you stick to the standard they should be portable.
If you use any WebSphere specific code in your app, you'll have to remove it to deploy on Tomcat.
If your app is servlets, JSPs, and JDBC you can deploy on either one without any problems.

You can think as Tomcat as a subset of Websphere, so theoretically everything that works on Tomcat will work in Websphere.
But...Deploying in Websphere, in my humble opinion, is a terrible pain, while deploying in Tomcat just works. (And if fails, just delete temporary folders)
Without knowing the technologies you are using, that's all I can say.

Depends, what are you trying to deploy?
Tomcat isn't a full EE server--are you trying to deploy an EE app?
If you're just deploying a web app, it's more important to consider which version of the servlet spec/etc. each server implements.

What is the difference between Tomcat, JBoss and Glassfish?

I am starting to look into Enterprise Java and the book I am following mentions that it will use JBoss. Netbeans ships with Glassfish. I have used Tomcat in the past.
What are the differences between these three programs?

Tomcat is just a servlet container, i.e. it implements only the servlets and JSP specification. Glassfish and JBoss are full Java EE servers (including stuff like EJB, JMS, ...), with Glassfish being the reference implementation of the latest Java EE 6 stack, but JBoss in 2010 was not fully supporting it yet.

Tomcat is merely an HTTP server and Java servlet container. JBoss and GlassFish are full-blown Java EE application servers, including an EJB container and all the other features of that stack. On the other hand, Tomcat has a lighter memory footprint (~60-70 MB), while those Java EE servers weigh in at hundreds of megs. Tomcat is very popular for simple web applications, or applications using frameworks such as Spring that do not require a full Java EE server. Administration of a Tomcat server is arguably easier, as there are fewer moving parts.
However, for applications that do require a full Java EE stack (or at least more pieces that could easily be bolted-on to Tomcat)... JBoss and GlassFish are two of the most popular open source offerings (the third one is Apache Geronimo, upon which the free version of IBM WebSphere is built). JBoss has a larger and deeper user community, and a more mature codebase. However, JBoss lags significantly behind GlassFish in implementing the current Java EE specs. Also, for those who prefer a GUI-based admin system... GlassFish's admin console is extremely slick, whereas most administration in JBoss is done with a command-line and text editor. GlassFish comes straight from Sun/Oracle, with all the advantages that can offer. JBoss is NOT under the control of Sun/Oracle, with all the advantages THAT can offer.

You should use GlassFish for Java EE enterprise applications.
Some things to consider:
A web Server means: Handling HTTP requests (usually from
browsers).
A Servlet Container (e.g. Tomcat) means: It can handle
servlets & JSP.
An Application Server (e.g. GlassFish) means: *It can manage
Java EE applications (usually both servlet/JSP and EJBs).
Tomcat - is run by Apache community - Open source and has two flavors:
Tomcat - Web profile - lightweight which is only servlet container and does not support Java EE features like EJB, JMS etc.
Tomcat EE - This is a certified Java EE container, this supports all Java EE technologies.
No commercial support available (only community support)
JBoss - Run by RedHat
This is a full-stack support for JavaEE and it is a certified Java EE container.
This includes Tomcat as web container internally.
This also has two flavors:
Community version called Application Server (AS) - this will have only community support.
Enterprise Application Server (EAP) - For this, you can have a subscription-based license (It's based on the number of Cores you have on your servers.)
Glassfish - Run by Oracle
This is also a full stack certified Java EE Container. This has its own web container (not Tomcat).
This comes from Oracle itself, so all new specs will be tested and implemented with Glassfish first. So, always it would support the latest spec. I am not aware of its support models.

jboss and glassfish include a servlet container(like tomcat), however the two application servers (jboss and glassfish) also provide a bean container (and a few other things aswell I imagine)

It seems a bit discouraging to use Tomcat when you read these answers. However what most fail to mention is that you can get to identical or almost identical use cases with tomcat but that requires you to add the libraries needed (through Maven or whatever include system you use).
I have been running tomcat with JPA, EJBs with very small configuration efforts.

JBoss and Glassfish are basically full Java EE Application Server whereas Tomcat is only a Servlet container.
The main difference between JBoss, Glassfish but also WebSphere, WebLogic and so on respect to Tomcat but also Jetty, was in the functionality that an full app server offer. When you had a full stack Java EE app server you can benefit of all the implementation of the vendor of your choice, and you can benefit of EJB, JTA, CDI(JAVA EE 6+), JPA, JSF, JSP/Servlet of course and so on. With Tomcat on the other hands you can benefit only of JSP/Servlet. However to day with advanced Framework such as Spring and Guice, many of the main advantage of using an a full stack application server can be mitigate, and with the assumption of a one of this framework manly with Spring Ecosystem, you can benefit of many sub project that in the my work experience let me to left the use of a full stack app server in favour of lightweight app server like tomcat.

Both JBoss and Tomcat are Java servlet application servers, but JBoss is a whole lot more. The substantial difference between the two is that JBoss provides a full Java Enterprise Edition (Java EE) stack, including Enterprise JavaBeans and many other technologies that are useful for developers working on enterprise Java applications.
Tomcat is much more limited. One way to think of it is that JBoss is a Java EE stack that includes a servlet container and web server, whereas Tomcat, for the most part, is a servlet container and web server.

Apache tomcat is just an only serverlet container it does not support for Enterprise Java application(JEE). JBoss and Glassfish are supporting for JEE application but Glassfish much heavy than JBOSS server : Reference Slide

Java EE without Application Server

Since EJB 3 we have embeddable EJB containers, JPA implementations can be used without an application server, there is Weld for contexts and dependency injection and so on. Since on many systems is only Tomcat available, I wonder, if Java EE could be used without an application server but with a Servlet container like Tomcat.
What would I have to do to set up an Java environment? What drawbacks do you see?

Note that Tomcat is an Application Server. That said, in October we released Apache TomEE which is Tomcat with the missing JavaEE parts added, then Java EE 6 certified using the official TCK from Oracle.
The stack evolved from what used to be simply called "OpenEJB/Tomcat", which was a useful stack with a bad name :) Commonly overlooked because of the "EJB" part, meanwhile it also delivered Transactions, JMS, WebServices and more to Tomcat. The new name is far better and now it's officially certified like JBoss or GlassFish. We're pretty excited about its future.

If I understand well, you want to use EJB3/JPA within a servlet container.
There are not only stand-alone implementations of JPA, but also embeddable EJB3 container, such as OpenEJB or Glassfish embeddable container. So nothing prevents you from starting such an embeddable container from the Servlet container to use EJB3.
(Note: I don't know all the details about transactions. In a full-blown app. server, you have JTA and a distributed transaction manager. You don't have that in a Servlet container such as Tomcat. JPA works with JTA and plain JDBC, but I don't know exactly how an embeddable container work without JTA. Still, I guess that would work given that such embeddable containers are also meant for unit-testing where I guess there is no JTA distributed transaction manager.)
Another approach would be to use Spring. Spring and EJB3 have indeed become very similar. You can start the Spring DI container within the Servlet container and benefit more or less from the same facilities as EJB3 (declarative transactions, etc.). See this post about Spring vs. EJB3.
All these technologies have become pretty modular, especially with Java EE profiles. You can use Sevlets, EJB3, JMS, JPA, even JTA somehow independently of one other. You can also create an environment where you cherry pick the ones you would like, either with Spring or with Java EE. The question is when does it stop to make sense and rather use an app. server with everything available and easily manageable. I think Servlet/EJB3/JPA is the limit, if more is needed go for an app. server.

You will generally require some kind of container, even if that container doesn't provide Java EE-related services. After all, you do need a long-lived JVM process to host the code that you're executing. Tomcat and Jetty will do the job nicely, and in addition to basic servlet services, provide a few useful extras that will be relevant, like connection pooling.

J2EE without application server was introduced years ago by me (Guy Pardon, from Atomikos), with this seminal article: http://www.onjava.com/pub/a/onjava/2006/02/08/j2ee-without-application-server.html - focused on JMS and JDBC at the time.
In general it's easy to setup thanks to Spring and Hibernate. Actually, I got inspired to do this after doing a Java EE project and being confronted with the XML hell associated with app servers and EJBs. Without application server things suddenly became a lot simpler and more testable.
If you need a Tomcat installation then can be a bit more of a hassle to configure, but recently Atomikos has introduced out-of-the-box Tomcat integration as part of its commercial offering at http://www.atomikos.com.
HTH