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How can I see what Tomcat is doing with respect to Servlets and the network side

Currently, I am learning about Java web development.
A lot of it seems to simply be configuration to me, and I feel that my understanding is superficial because I only see the configuration (i.e. define your servlets and their mappings in the web.xml file, make custom Servlets by extending the HttpServlet class, instantiate Tomcat in the main method, etc.)
I want do know a bit more about what is actually going on under the hood - so I am in some need of some guidance.
To this end, I have done some cursory reading on Tomcat and servlets from the following links:
What is a servlet
Difference between embedded and not embedded
Tomcat docs
So what I think I understand from this is that the servlets sit inside the Tomcat instance (a servlet container) and Tomcat handles all of the receiving all of the requests of the client and relaying them to the servlets. The servlets process the requests, send back a response, which Tomcat then sends back to the client. I suppose in the local setup I have, my machine would both be acting as the client and server.
Given the above I want to know:
How I can directly see and monitor the client's sending the request to Tomcat and verify that Tomcat has received the request? Essentially, how can I verify that this networking side of things is happening due to some implementation by Tomcat?
How does Tomcat parse the request information and send it to the servlets?
Is Tomcat a servlet container or a web server? Are these the same thing?
In the answer given in the second link regarding embedded vs. non-embedded, the answer states that an embedded server looks like a regular java program. Does this mean that for an embedded server, the server is in the java application while the web application is inside the server in the non-embedded case? Like the containment relationship is reversed? What does containment mean in the first place here?
Apologies for the numerous questions and thank you for helping to clarify.

2. How does Tomcat parse the request information and send it to the servlets?
The Servlet specification explains that in detail. The spec is surprisingly easy to read; I suggest giving it a go.
As a simplified overview…
The job of a Servlet container is to process the incoming request, which is just a bunch of text. The Servlet container pulls out the various pieces and assembles them into a request object.
Likewise, the response produced by your servlet is packaged up as a response object. The Servlet container's job is to use all the info contained in that object to create a stream of text to be sent back to the client web browser.
The whole point of Servlet containers is to relieve the servlet-writing programmers of the need to know much of the details of HTTP and how to make a server. The Servlet container does all that work. In other words, the great thing about Servlet technology is that you the programmer need not ask this # 2 question of yours!
3. Is Tomcat a servlet container or a web server? Are these the same thing?
(a) both, (b) no.
No, servlet containers and web servers are two different kinds of software.
A web server handles:
listening for incoming connections from clients (web browsers, etc.)
sending response back to the client
A web server handles all the network traffic.
A Servlet container provides an environment in which relatively small chunks of code (servlets) can process a request and formulate a response. The small servlet does not have to handle network traffic, launching & shutting down, security, and all the other responsibilities of a full server. That explains the "-let" in "Servlet".
Your servlet you write plugs into a Servlet container. The container communicates with the web server, receiving each request passed by the web server, and passing to the web server the response produced by your servlet. When a request arrives, the container invokes your servlet.
Your servlet remains blissfully ignorant as to what particular Servlet container implementation is running, as long as it complies with the Jakarta Servlet specification. And your servlet remains blissfully ignorant as to the existence of web servers.
Some products, such as Tomcat & Jetty, can be composed of both a web server and a Servlet container.
Tomcat is composed mainly of three components: (1) Catalina, a servlet container, (2) Coyote, a web server, and (3) Jasper, a Jakarta Server Pages processor. See Wikipedia.
For most people's needs, the Coyote web server in Tomcat is a suitable web server. So you can use Tomcat as as all-in-one application server, handling both web traffic and servlets.
[web request] ➜ [Tomcat Coyote] ➜ [Tomcat Catalina] ➜ [your servlet]
Alternatively, some folks choose to use Tomcat only as a Servlet container, sitting behind a separate web server such as Apache HTTP Server. In such a case, Tomcat’s Coyote component goes unused. Instead, the separate web server handles client browser components, and processes incoming requests. If a request is asking for a static resource, the web server serves it out, without any involvement from Tomcat. If the request is asking for work that has been assigned to a servlet, then the separate web server passes the request on to Tomcat and its Catalina component. After your servlet produces a response, the response moves from Tomcat back to the external web server, which traffics the response onwards to the client web browser.
[web request] ➜ [Apache HTTP Server] ➜ [Tomcat Catalina] ➜ [your servlet]
4 … embedded vs. non-embedded …
Non-embedded is the classic situation, as originally envisioned when Servlet technology was first invented.
Back then, servers were few, expensive, and already in place permanently. The goal of Servlet technology was to make it easy for companies to keep those expensive servers busy by having many web applications running alongside each other.
Servlet technology allowed many different servlets to be running on one machine without stepping on each other, and without the programmers of each servlet having known anything about the other servlets being written. The Servlet container can stay up and running as servlets are deployed and un-deployed.
Fast forward, and we have cloud technology where servers are many, cheap, and convenient to create and destroy on-the-fly. So nowadays many people want to run their web applications separately, one web app per virtual machine or virtual service. Thus the need for embedded mode. We need an application that can be launched and shutdown on its own, to run one specific servlet (or multiple servlets meant to work together) without any other unrelated web apps.
One way to achieve this new goal is to package a web server and servlet container into a standalone Java app. A system administrator can launch and quit this standalone app like any other Java app, without knowing anything about how to configure an on-going web server and Servlet container.

Is it possible to access a JAX-WS from a web browser?

I was just wondering, whether is it possible to deploy a JAX-WS and access it from a browser without the need to install any software or proxy classes ..

Of course, JAX-WS is just a library built on top of SOAP, which is built on top of XML, which is built on top of HTTP (duh!)
If you have SOAP web services (the fact that it was developed using JAX-WS is irrelevant), any application capable of sending and receiving XML over HTTP can call it. We are successfully accessing our back-end web services using AJAX POST (the web services must be deployed on the same domain as your front-end due to same-origin-policy).

As far as I've seen it's not possible, since you need to have proxy classes in order to call the JAX-WS functions ... but maybe I'm wrong !

Which Web technologies allow for the development of static user interface/presentation layers

I have a Spring-WS web service that runs on a Jboss application server. I also have a Spring-MVC application running on a separate server running Jboss 7. The Spring-MVC application is used mainly for the user interface. The Spring-WS application contains all the services for the business logic.
The standard approach to use the components we have would be in the following order:
Client Browser ---(HTTP Request)---> Spring MVC ----(SOAP Request)-----> Spring-WS
Client Browser <---(HTTP Request)--- Spring MVC <----(SOAP Response)<----- Spring-WS
There is a requirement to change some of the requests coming from the Client Browser to go direct to the Web Services instead of via the Spring MVC application. The Spring MVC application will be used to load the initial presentation screens but the actions that involve any updates/writes will be going through the Spring-WS process.
To achieve this, we have a bespoke process which runs on the same machine as the client browser that traps all HTTP requests. Its purpose is to convert the request to a SOAP message and the response to a HTTP response. The path is shown below:
Initial Request (Retrieve presentation/user interface)
Client Browser ---(HTTP Request)---> Spring MVC
Subsequent requests
Client Browser ---(Http Request) ----> SOAP Converter (Local process) ------> (SOAP Request) ----> Spring-WS
Client Browser <---(Http Request) ---- SOAP Converter (Local process) <------ (SOAP Request) <---- Spring-WS
There are two paths that happen in the above scenario. The initial request to display the pages on the screen will be a request to the Spring-MVC process. Any subsequent requests that involve changing the data will be via the path shown above.
The problem that i have now is that all responses from Spring-WS (the webservice) are in XML format. This means that when the request comes from the browser, the data will have to come from the web service but the pages will need to be refreshed from the Spring-MVC application. This somehow feels a bit wrong as each request will involve to calls. One to get the data and one to get the presentation data.
To overcome this, i would like to implement the Spring-MVC layer using a technique where i only need to make one initial request. This means that the user interface will rendered on the screen. All subsequent requests to the Spring-WS service should not result in the browser to be rendered other than refreshing the data.
I am interested in knowing what kind of technologies i can use to achieve this. One way of doing this is by using Applets but this has been ruled out for security reasons. I have seen several websites which work exactly the way i have described above. i.e. the page never refreshes. A very good example is the Sonatype Nexus Maven repository manager user interface shown below:
It runs in a web browser and when the Nexus user interface loads on the browser, it is almost like a Swing type application. (Any one know what technology Nexus use for the user interface?)
I guess my question is which Web based user interface technologies (preferably open source) can i use which has a swing type look and feel but is not Swing and requires minimal requests to the server to refresh the screens?
Thanks in advance.

Nexus' interface is built using Sencha GXT3, which now also contains ExtJS.
Have a look at the GXT API: it contains a lot of web components which knows how to update their own state without doing the full request/response cycle (using Ajax), which is what you seem to be trying to get away from.

What is Java Servlet?

I read many articles to understand Java servlet but I did not succeed.
Can you please give brief introduction of Java servlets (in easy language). What is a servlet? What are the advantages?
I can't understand the difference between server-side programming languages (PHP, ASP) and servlets.

A servlet is simply a class which responds to a particular type of network request - most commonly an HTTP request. Basically servlets are usually used to implement web applications - but there are also various frameworks which operate on top of servlets (e.g. Struts) to give a higher-level abstraction than the "here's an HTTP request, write to this HTTP response" level which servlets provide.
Servlets run in a servlet container which handles the networking side (e.g. parsing an HTTP request, connection handling etc). One of the best-known open source servlet containers is Tomcat.

A servlet at its very core is a java class; which can handle HTTP requests.
Typically the internal nitty-gritty of reading a HTTP request and response over the wire is taken care of by the containers like Tomcat. This is done so that as a server side developer you can focus on what to do with the HTTP request and responses and not bother about dealing with code that deals with networking etc. The container will take care of things like wrapping the whole thing in a HTTP response object and send it over to the client (say a browser).
Now the next logical question to ask is who decides what is a container supposed to do? And the answer is; In Java world at least It is guided (note I did not use the word controlled) by specifications. For example Servlet specifications (See resource 2) dictates what a servlet must be able to do. So if you can write an implementation for the specification, congratulations you just created a container (Technically containers like Tomcat also implement other specifications and do tricky stuff like custom class loaders etc but you get the idea).
Assuming you have a container, your servlets are now java classes whose lifecycle will be maintained by the container but their reaction to incoming HTTP requests will be decided by you. You do that by writing what-you-want-to-do in the pre-defined methods like init(), doGet(), doPost() etc. Look at Resource 3.
Here is a fun exercise for you. Create a simple servlet like in Resource 3 and write a few System.out.println() statements in it's constructor method (Yes you can have a constructor of a servlet), init(), doGet(), doPost() methods and run the servlet in tomcat. See the console logs and tomcat logs.
Resources
Look how the HTTP servlet looks here(Tomcat example).
Servlet Specification.
Simple Servlet example.
Start reading the book online/PDF
It also provides you download of the whole book. May be this will help.
if you are just starting servlets may be it's a good idea to read the material along with the servlet API. it's a slower process of learning, but is way more helpful in getting the basics clear.

In addition to the above, and just to point out the bleedin' obvious...
To many this is hyper obvious, but to someone used to writing apps which are just run and then end: a servlet spends most of its time hanging around doing nothing... waiting to be sent something, a request, and then responding to it. For this reason a servlet has a lifetime: it is initalised and then waits around, responding to anything thrown at it, and is then destroyed. Which implies that it has to be created (and later destroyed) by something else (a framework), that it runs in its own thread or process, and that it does nothing unless asked to. And also that, by some means or other, a mechanism must be implemented whereby this "entity" can "listen" for requests.
I suggest that reading about threads, processes and sockets will throw some light on this: it's quite different to the way a basic "hello world" app functions.
It could be argued that the term "server" or "servlet" is a bit of an overkill. A more rational and simpler name might be "responder". The reason for the choice of the term "server" is historical: the first such arrangements were "file servers", where multiple user/client terminals would ask for a specific file from a central machine, and this file would then be "served up" like a book or a plate of fish and chips.

What is a Servlet?
A servlet is simply a class which responds to a particular type of network request - most commonly an HTTP request.
Basically servlets are usually used to implement web applications - but there are also various frameworks which operate on top of servlets (e.g. Struts) to give a higher-level abstraction than the "here's an HTTP request, write to this HTTP response" level which servlets provide.
Servlets run in a servlet container which handles the networking side (e.g. parsing an HTTP request, connection handling etc). One of the best-known open source servlet containers is Tomcat.
In a request/response paradigm, a web server can serve only static pages to the client
To serve dynamic pages, a we require Servlets.
Servlet is nothing but a Java program
This Java program doesn’t have a main method. It only has some callback methods.
How does the web server communicate to the servlet? Via container or Servlet engine.
Servlet lives and dies within a web container.
Web container is responsible for invoking methods in a servlets. It knows what callback methods the Servlet has.
Flow of Request
Client sends HTTP request to Web server
Web server forwards that HTTP request to web container.
Since Servlet can not understand HTTP, its a Java program, it only understands objects, so web container converts that request into valid request object
Web container spins a thread for each request
All the business logic goes inside doGet() or doPost() callback methods inside the servlets
Servlet builds a Java response object and sends it to the container. It converts that to HTTP response again to send it to the client
How does the Container know which Servlet client has requested for?
There’s a file called web.xml
This is the master file for a web container
You have information about servlet in this file-
servlets
Servlet-name
Servlet-class
servlet-mappings- the path like /Login or /Notifications is mapped here in
Servlet-name
url-pattern
and so on
Every servlet in the web app should have an entry into this file
So this lookup happens like- url-pattern -> servlet-name -> servlet-class
How to "install" Servlets?
* Well, the servlet objects are inherited from the library- javax.servlet.* . Tomcat and Spring can be used to utilize these objects to fit the use case.
Ref- Watch this on 1.5x- https://www.youtube.com/watch?v=tkFRGdUgCsE . This has an awesome explanation.

Servlet is server side technology which is used to create dynamic web page in web application. Actually servlet is an api which consist of group of classes and interfaces, which has some functionality. When we use Servlet API we can use predefined functionality of servlet classes and interfaces.
Lifecycle of Servlet:
Web container maintains the lifecycle of servlet instance.
1 . Servlet class loaded
2 . Servlet instance created
3 . init() method is invoked
4 . service() method invoked
5 . destroy() method invoked
When request raise by client(browser) then web-container checks whether the servlet is running or not if yes then it invoke the service() method and give the response to browser..
When servlet is not running then web-container follow the following steps..
1. classloader load the servlet class
2. Instantiates the servlet
3. Initializes the servlet
4.invoke the service() method
after serving the request web-container wait for specific time, in this time if request comes then it call only service() method otherwise it call destroy() method..

If you are beginner, I think this tutorial may give basic idea about What Servlet is ...
Some valuable points are below from the given link.
Servlet technology is used to create web application which resides at server side and generates dynamic web page.
Servlet can be described in many ways, depending on the context.
Servlet is a technology i.e. used to create web application.
Servlet is an API that provides many interfaces and classes including
documentations.
Servlet is an interface that must be implemented for creating any
servlet.
Servlet is a class that extend the capabilities of the servers and
respond to the incoming request. It can respond to any type of
requests.
Servlet is a web component that is deployed on the server to create
dynamic web page.
Reference:Here.

Servlets are Java classes that run certain functions when a website user requests a URL from a server. These functions can complete tasks like saving data to a database, executing logic, and returning information (like JSON data) needed to load a page.
Most Java programs use a main() method that executes code when the program in run. Java servlets contain doGet() and doPost() methods that act just like the main() method. These functions are executed when the user makes a GET or POST request to the URL mapped to that servlet. So the user can load a page for a GET request, or store data from a POST request.
When the user sends a GET or POST request, the server reads the #WebServlet at the top of each servlet class in your directory to decide which servlet class to call. For example, let's say you have a ChatBox class and there's this at the top:
#WebServlet("/chat")
public class ChatBox extends HttpServlet {
When a user requests the /chat URL, your ChatBox class with be executed.

Java Servlets are server-side Java program modules that procedure and answer customer demands and actualize the servlet interface. It helps in improving Web server usefulness with negligible overhead, upkeep and support.
A servlet goes about as a mediator between the customer and the server. As servlet modules keep running on the server, they can get and react to demands made by the customer. Demand and reaction objects of the servlet offer a helpful method to deal with HTTP asks for and send content information back to the customer.
Since a servlet is coordinated with the Java dialect, it additionally has all the Java highlights, for example, high movability, stage autonomy, security and Java database availability.

Servlet is a java class to respond a HTTP request and produce a HTTP response...... when we make a page with the use of HTML then it would be a static page so to make it dynamic we use SERVLET {in simple words one can understand}
To make use of servlet is overcomed by JSP it uses the code and HTML tag both in itself..

As this article describes, a Servlet is a standardized way of extending a Java server, and accessing its capabilities.
Each Servlet can be seen as a tiny server (hence the name), that gets access to the request and response modelled in Java code, along with other context data, like the Session.
With these in hand, the Java code of the servlet can interface with whatever it needs to to render a response, including handing off to a JSP page for generating a HTML view.

I think servlet is basically a java class which acts as a middle way between HTTP request and HTTP response.Servlet is also used to make your web page dynamic. Suppose for example if you want to redirect to another web page on server then you have to use servlets. Another important thing is that servlet can run on localhost as well as a web browser.

You just got the answer for a normally servlet. However, I want to share you about something about Servlet 3.0
What is first a Servlet?
A servlet is a Web component that is managed by a container and
generates dynamic content. Servlets are Java classes that are compiled
to byte code that can be loaded dynamically into and run by a Java
technology-enabled Web server or Servlet container.
Servlet 3.0 is an update to the existing Servlet 2.5 specification.
Servlet 3.0 required API of the Java Platform, Enterprise Edition 6.
Servlet 3.0 is focussed on extensibility and web framework
pluggability. Servlet 3.0 bring you up some extensions such as Ease of
Development (EoD), Pluggability, Async Support and Security
Enhancements
Ease of Development
You can declare Servlets, Filter, Listeners, Init Params, and almost
everything can be configured by using annotations
Pluggability
You can create a sub-project or a module with a web-fragment.xml. It
means that it allows to implement pluggable functional requirements
independently.
Async Support
Servlet 3.0 provides the ability of asynchronous processing, for
example: Waiting for a resource to become available, Generating
response asynchronously.
Security Enhancements
Support for the authenticate, login and logout servlet security
methods
I found it from Java Servlet Tutorial

Difference between web server and application server [duplicate]

This question already has answers here:
What is the difference between application server and web server?
(28 answers)
Closed 4 years ago.
As a layman, how do I understand the difference between web server and application server ? If you could give an example using a Java based web app in very "simple" terms that would be really great..
Also when we say Weblogic, is it a web server only ?

A web server is something that handles HTTP requests and responses.
An application server (like WebLogic, WebSphere, JBoss AS, Glassfish, etc) usually includes a web server, but also adds a lot more features. The most important is that it manages objects. Whether they will be servlets (Servlet container), EJBs (ejb container), JMS listeners, etc.

Webserver can execute only web applications i,e servlets and JSPs and has only a single container known as Web container which is used to interpret/execute web applications
Application server can execute Enterprise application, i,e (servlets, jsps, and EJBs) it is having two containers 1. Web Container(for interpreting/executing servlets and jsps) 2. EJB container(for executing EJBs). it can perform operations like load balancing , transaction demarcation etc etc

I would say definitions vary. In the generalized context, a Web Server is a server that can receive incoming web-requests and have knowledge about how they should be handled and responded to. Some requests are static (html files, images etc), some are dynamic. In the case of dynamic requests, the web server will know where to route handling of the request, could be a JSP page or a java servlet, a PHP script, a perl CGI script etc etc.
While the "web server" in this context executes the dynamic handler, it is not considered to include any supporting middleware features for the dynamic handler.
An Application Server, by contrast, is a general execution environment that offers some type of middleware tier support. Examples are EJB containers or the .NET framework built into Windows (in where Windows in itself is an "application server"). There is no inherent requirement that an application-server have anything to do with web requests (although many do), it's just a general execution context and container for any type of application that offers some sort of additional middleware support.
In a purely web-centric context, many people will draw the line at static vs dynamic content. In this definition, a "web server" can only handle requests for static information itself and it will pass on requests for dynamic content to the "application server". For example, Apache httpd is a web server and Tomcat is an application server. IIS is a combination of both. In the Java web world, an application server can be either a servlet container (like Tomcat), or a full blown Java EE container (like JBoss, WebLogic or WebSphere) that provides the Java EE middleware support (EJB) container in addition to the servlet container.

And adding to previous answers, Weblogic is app server and not only web server.

Basically if we say the major difference between Web Server & Application Server, is the protocols on which these servers work.
Web Server -- it works on protocols like HTTP & HTTPS. Example of this server is Apache. For web server you use JSP, Servlet.
Application Server -- it works on any protocol. example is JBOSS. On application server we host EJB, web service or any business Logic.