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I am fairly new to Java and my experience is limited to Web Based Applications running on a Web Container (Jboss in my case).
Am I correct in saying that for Web Applications the web container takes care of multi-threading? If so, can I introduce new treads in a Web Based applications? Is there any advantage in doing so and in what scenario one would need to do that?
Am I correct in saying that for Web Applications the web container takes care of multi-threading?
Most webservers (Java and otherwise, including JBoss) follow a "one thread per request" model, i.e. each HTTP request is fully processed by exactly one thread. This thread will often spend most of the time waiting for things like DB requests. The web container will create new threads as necessary.
Some servers (in the Java ecosystem primarily Netty) do asynchronous request handling, either with a "one thread does everything" model, or something more complex. The basic idea there is that having lots of waiting threads wastes resources, so working asynchronously can be more efficient.
If so, can I introduce new treads in a Web Based applications?
It's possible, but should be done very carefully, since errors (like memory leaks or missing synchronization) can cause bugs that are very hard to reproduce, or bring down the whole server.
Is there any advantage in doing so and in what scenario one would need to do that?
Well, the advantage is that you can do stuff in parallel. Using threads to improve pure computational speed is something you should not do on a webserver, as it would slow down the handling of other requests. That kind of thing should be done on a separate server, probably using some sort of job queue.
A legitimate scenario for multithreading in the context of handling an HTTP request might be if you need to access other network resources, e.g. call several different web services. If you do it in a single thrad, you have to wait for each call to finish in turn. But if you use multiple threads, the total waiting time is only the delay of the single slowest call.
In response to your question:
How is multi-threading different in a Java based Web Application vs Stand-alone Java Application
It is not different. Usually the software web applications are built upon will provide some multi-threading already by making each new request a thread. By you can use/create threads just as you would any other application.
Indeed multi-threading can give you drastic performance increases if you use it right. For I/O intensive tasks such as network access and disk drive access the performance increase is nearly always guaranteed. For computationally intensive tasks you should stick to the rule of one thread per core in the server. For example if your core has an i7 processor you should stick to 7 threads to do computational tasks.
In response to your question:
Is there any advantage in doing so and in what scenario one would need to do that?
Michael Borgwart says you should not do this to improve computational speed. I disagree, as this article suggests multi-threading can help provide responsiveness when users need a computationally heavy task done. In plain English, users wont have to wait as long to complete computationally heavy tasks with multi-threading.
If you intend to use threads a lot, I suggest use of a thread pool. This will reduce the overhead of creating threads.
Well this is a nice question and I think most of the developers who work in web application development don't use multithreading explicitly. The reason is quite obvious since you are using a application server to deploy your application, the application server internally manages a thread pool for incoming requests.
Then why use multithreading explicitly? What is need a web application developer expose himself to multithreading?
When you work on a large scale application where you have to server many request concurrently it is difficult to serve every kind of request synchronously because particular kind of request could have been doing a lot processing which could bring down the performance your application.
Lets take an example where a web application after serving particular kind of request has to notify users through email and SMS. Doing it synchronously with the request thread could bring down the performance of your web application. So here comes the role of mutlithreading. In such cases it is advisable to develop a stand alone multithreaded application over the network which is responsible for sending email and SMS only.
Concurrency Utilities.