Instance of each servlet

Question

I have a web page ,JSP, which calls different servlets for DB stuff. My question is this : will there be a new instance of each servlet for each user when connecting to the the website or should I do something about it.

Answer 1

hello again
could you also tell me whats the correct way to do
this. As far as I know I can mix the Model and
controller inside a servlet, but with this fixture
everything will go nuts! do you think I should use
the servlet as a controller and a javabean as a
model. OR something else.
Thanks again for the help.I was in your situation 1 or 2 years ago, and I would just like to share what I've found and try to explain in an easy manner what technologies you could use, and how they work.
The problem with servlets is that each connection to the same servlet will create a new thread. This is different from the classical CGI approach, where each connection starts a new process.
The consequence is that you end up coding in a multithreaded environment, with all the hassles with deadlocks, race conditions, problems with shared variables (as mentioned in previous posts) etc.. These problems are difficult to solve, and time consuming. The reason for the multithreaded solution is that it provides a significant performance boost over the "new process" solution. However, it is obvious that a developer doesn't want to worry about multithreading when he/she has a deadline to comply with ;) So, you ask, should I just scrap Servlets or is there a better solution?
Well, to get rid of this problem for developers, Sun proposed a standard to reduce the complexity of web development. It's called Java EE (http://java.sun.com/javaee/5/docs/tutorial/doc/)
By maintaining persistent (long lasting) state in a database, and temporary state (session state etc) in a controlled manner, Java EE proposed a way of getting rid of the classical problems in Java web development, and also added several cool new features.
When using Java EE, you have a special type of beans to save persistent state, they're called entity beans. If you have defined a entity bean (wrote an ordinary java class, and supplied some settings), you can create a new entity bean, set its properties, and tell the Java EE system to save it to your database. You can also search among saved ones (something like "select p from Person as p where p.username='foo' and p.cousin.name='bar'"), let them reference each other and all sorts of neat things.
There are also beans which are supposed to perform logic using your entity beans, and they're called session beans. They have methods containing your business logic, namely methods like 'getLatestForumPost(), or 'createUser(String name)'...
The session beans can also save temporary state. If they do so, they're called "stateful session beans". The state they keep is stored like ordinary class attributes, but Java EE makes sure that you won't get thread problems (even though there still are multiple threads). For example, you can use this state to keep track of items in a shopping cart (based on the current user). If several users want their own cart, Java EE just creates a new stateful session bean for each user, you don't have to modify your bean in any way!
Until now, I've used the word "Java EE", as if it's an actual product, but as I mentioned, it's just a standard. Since it is just a standard, telling how things should work, there are several companies that have made solutions (programs) that actually make things work like this. An application that "make thing work" is called a Java EE Application Server. You need to start the application server in order to run you own Java EE program (you program will be almost like a plugin to the server, or you could say that you program runs within the server). Behind the scenes, the application server usually uses ordinary Servlets to process each request, but it handles all the multithread problems (and others) for you. If you for some reason would like to, you can always gain finer control by using Servlets directly.
When a user connects to your computer, the server can automaticly identify the user, give him some cookies identifying him, ask for login (if you told the server to do so), and then hand over control to you, telling you this persons login name, what he asked to do, amongst other things, which saves you a lot of hassle. You can alway tell the server how to behave in different situations (for example wich pages or requests that require login etc), by writing some xml config files for each application.
The application server also lets you send mails from your session beans, send messages between computers, run your application on clusters, parse XML and several of other neat things. It's a dream for a web application developer.
I hope this will inspire you to have a look at these things, because it really simplifies things, and if you learn it correctly, you can also make some serious money as a Java EE developer ;)
The only downside is that it seems pretty complex in the beginning, and that it's pretty hard to understand all the vague definitions in the docs etc... I would recommend that you download the Application server from Sun (http://www.sun.com/software/products/appsrvr/index.xml), read the tutorial at http://java.sun.com/javaee/5/docs/tutorial/doc/ and start to write some own programs.
Don't be passive, just try everything out and you'll figure out that it's less complicated than it seems at the first glance.
I really hope that this helps!, and sorry for my bad English.
Best regards
/Alexander T