servlet life cycle while loadOnStartup

servlet life cycle while loadOnStartup - java

When we mention any servlet as loadOnStartup in web.xml then its init method is called for sure. Just wanted to confirm does service and destroy methods are also get called or not? If not when they get called?
Another question is as it is mentioned every where init method is called only once. Does that mean when the servlet gets the request first time, init method will be called and then service will be called.
But destroy will not be called. When the second request it will simply call the service method again but not init method. Is this understanding correct? But when container decides to call the destroy method?

loadOnStartup means that the servlet will be loaded (and its init(..) method called during container startup. Otherwise, as you suspect, it will be loaded when the first request comes
containers have only one instance of each servlet. When they create it, they call init(..). When the context is undeployed (for example - the container is stopped), destroy() is called

loadOnStartup gives priority on which threads loads before other threads. The Servlet container will load the servlet and call the init method. If loadOnStartup isn't mentioned in web.xml, the serlvet will be instantiated when there's a request call that requires the servlet.
Initialization of the servlet happens only once. Once it's initialized, the servlet container keeps it.
For request handling, the servlet container calls the serlvet's service() method and when the servlet container is shutdown it will call all the servlet's destroy() method and remove the servlet from the container.
For more info view some explanation here, or from Java EE.

When we enable load-on-start-up enable in web.xml then our servlet program will be instantiated during server start up. Means every request will take same amount of time for their response.

Related

request.setAttribute("name",name); is not working when project is run in 2nd time [duplicate]

Suppose, I have a webserver which holds numerous servlets. For information passing among those servlets I am setting session and instance variables.
Now, if 2 or more users send request to this server then what happens to the session variables?
Will they all be common for all the users or they will be different for each user?
If they are different, then how was the server able to differentiate between different users?
One more similar question, if there are n users accessing a particular servlet, then this servlet gets instantiated only the first time the first user accessed it or does it get instantiated for all the users separately?
In other words, what happens to the instance variables?

ServletContext
When the servlet container (like Apache Tomcat) starts up, it will deploy and load all its web applications. When a web application is loaded, the servlet container creates the ServletContext once and keeps it in the server's memory. The web app's web.xml and all of included web-fragment.xml files is parsed, and each <servlet>, <filter> and <listener> found (or each class annotated with #WebServlet, #WebFilter and #WebListener respectively) will be instantiated once and be kept in the server's memory as well, registred via the ServletContext. For each instantiated filter, its init() method is invoked with a new FilterConfig argument which in turn contains the involved ServletContext.
When a Servlet has a <servlet><load-on-startup> or #WebServlet(loadOnStartup) value greater than 0, then its init() method is also invoked during startup with a new ServletConfig argument which in turn contains the involved ServletContext. Those servlets are initialized in the same order specified by that value (1 is 1st, 2 is 2nd, etc). If the same value is specified for more than one servlet, then each of those servlets is loaded in the same order as they appear in the web.xml, web-fragment.xml, or #WebServlet classloading. In the event the "load-on-startup" value is absent, the init() method will be invoked whenever the HTTP request hits that servlet for the very first time.
When the servlet container is finished with all of the above described initialization steps, then the ServletContextListener#contextInitialized() will be invoked with a ServletContextEvent argument which in turn contains the involved ServletContext. This will allow the developer the opportunity to programmatically register yet another Servlet, Filter or Listener.
When the servlet container shuts down, it unloads all web applications, invokes the destroy() method of all its initialized servlets and filters, and all Servlet, Filter and Listener instances registered via the ServletContext are trashed. Finally the ServletContextListener#contextDestroyed() will be invoked and the ServletContext itself will be trashed.
HttpServletRequest and HttpServletResponse
The servlet container is attached to a web server that listens for HTTP requests on a certain port number (port 8080 is usually used during development and port 80 in production). When a client (e.g. user with a web browser, or programmatically using URLConnection) sends an HTTP request, the servlet container creates new HttpServletRequest and HttpServletResponse objects and passes them through any defined Filter in the chain and, eventually, the Servlet instance.
In the case of filters, the doFilter() method is invoked. When the servlet container's code calls chain.doFilter(request, response), the request and response continue on to the next filter, or hit the servlet if there are no remaining filters.
In the case of servlets, the service() method is invoked. By default, this method determines which one of the doXxx() methods to invoke based off of request.getMethod(). If the determined method is absent from the servlet, then an HTTP 405 error is returned in the response.
The request object provides access to all of the information about the HTTP request, such as its URL, headers, query string and body. The response object provides the ability to control and send the HTTP response the way you want by, for instance, allowing you to set the headers and the body (usually with generated HTML content from a JSP file). When the HTTP response is committed and finished, both the request and response objects are recycled and made available for reuse.
HttpSession
When a client visits the webapp for the first time and/or the HttpSession is obtained for the first time via request.getSession(), the servlet container creates a new HttpSession object, generates a long and unique ID (which you can get by session.getId()), and stores it in the server's memory. The servlet container also sets a Cookie in the Set-Cookie header of the HTTP response with JSESSIONID as its name and the unique session ID as its value.
As per the HTTP cookie specification (a contract any decent web browser and web server must adhere to), the client (the web browser) is required to send this cookie back in subsequent requests in the Cookie header for as long as the cookie is valid (i.e. the unique ID must refer to an unexpired session and the domain and path are correct). Using your browser's built-in HTTP traffic monitor, you can verify that the cookie is valid (press F12 in Chrome / Firefox 23+ / IE9+, and check the Net/Network tab). The servlet container will check the Cookie header of every incoming HTTP request for the presence of the cookie with the name JSESSIONID and use its value (the session ID) to get the associated HttpSession from server's memory.
The HttpSession stays alive until it has been idle (i.e. not used in a request) for more than the timeout value specified in <session-timeout>, a setting in web.xml. The timeout value defaults to 30 minutes. So, when the client doesn't visit the web app for longer than the time specified, the servlet container trashes the session. Every subsequent request, even with the cookie specified, will not have access to the same session anymore; the servlet container will create a new session.
On the client side, the session cookie stays alive for as long as the browser instance is running. So, if the client closes the browser instance (all tabs/windows), then the session is trashed on the client's side. In a new browser instance, the cookie associated with the session wouldn't exist, so it would no longer be sent. This causes an entirely new HttpSession to be created, with an entirely new session cookie being used.
In a nutshell
The ServletContext lives for as long as the web app lives. It is shared among all requests in all sessions.
The HttpSession lives for as long as the client is interacting with the web app with the same browser instance, and the session hasn't timed out at the server side. It is shared among all requests in the same session.
The HttpServletRequest and HttpServletResponse live from the time the servlet receives an HTTP request from the client, until the complete response (the web page) has arrived. It is not shared elsewhere.
All Servlet, Filter and Listener instances live as long as the web app lives. They are shared among all requests in all sessions.
Any attribute that is defined in ServletContext, HttpServletRequest and HttpSession will live as long as the object in question lives. The object itself represents the "scope" in bean management frameworks such as JSF, CDI, Spring, etc. Those frameworks store their scoped beans as an attribute of its closest matching scope.
Thread Safety
That said, your major concern is possibly thread safety. You should now know that servlets and filters are shared among all requests. That's the nice thing about Java, it's multithreaded and different threads (read: HTTP requests) can make use of the same instance. It would otherwise be too expensive to recreate, init() and destroy() them for every single request.
You should also realize that you should never assign any request or session scoped data as an instance variable of a servlet or filter. It will be shared among all other requests in other sessions. That's not thread-safe! The below example illustrates this:
public class ExampleServlet extends HttpServlet {
private Object thisIsNOTThreadSafe;
protected void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException {
Object thisIsThreadSafe;
thisIsNOTThreadSafe = request.getParameter("foo"); // BAD!! Shared among all requests!
thisIsThreadSafe = request.getParameter("foo"); // OK, this is thread safe.
}
}
See also:
What is the difference between JSF, Servlet and JSP?
Best option for Session management in Java
Difference between / and /* in servlet mapping url pattern
doGet and doPost in Servlets
Servlet seems to handle multiple concurrent browser requests synchronously
Why Servlets are not thread Safe?

Sessions
In short: the web server issues a unique identifier to each visitor on his first visit. The visitor must bring back that ID for him to be recognised next time around. This identifier also allows the server to properly segregate objects owned by one session against that of another.
Servlet Instantiation
If load-on-startup is false:
If load-on-startup is true:
Once he's on the service mode and on the groove, the same servlet will work on the requests from all other clients.
Why isn't it a good idea to have one instance per client? Think about this: Will you hire one pizza guy for every order that came? Do that and you'd be out of business in no time.
It comes with a small risk though. Remember: this single guy holds all the order information in his pocket: so if you're not cautious about thread safety on servlets, he may end up giving the wrong order to a certain client.

Session in Java servlets is the same as session in other languages such as PHP. It is unique to the user. The server can keep track of it in different ways such as cookies, url rewriting etc. This Java doc article explains it in the context of Java servlets and indicates that exactly how session is maintained is an implementation detail left to the designers of the server. The specification only stipulates that it must be maintained as unique to a user across multiple connections to the server. Check out this article from Oracle for more information about both of your questions.
Edit There is an excellent tutorial here on how to work with session inside of servlets. And here is a chapter from Sun about Java Servlets, what they are and how to use them. Between those two articles, you should be able to answer all of your questions.

When the servlet container (like Apache Tomcat) starts up, it will read from the web.xml file (only one per application) if anything goes wrong or shows up an error at container side console, otherwise, it will deploy and load all web applications by using web.xml (so named it as deployment descriptor).
During instantiation phase of the servlet, servlet instance is ready but it cannot serve the client request because it is missing with two pieces of information:
1: context information
2: initial configuration information
Servlet engine creates servletConfig interface object encapsulating the above missing information into it
servlet engine calls init() of the servlet by supplying servletConfig object references as an argument. Once init() is completely executed servlet is ready to serve the client request.
Q) In the lifetime of servlet how many times instantiation and initialization happens ??
A)only once (for every client request a new thread is created)
only one instance of the servlet serves any number of the client request ie, after serving one client request server does not die. It waits for other client requests ie what CGI (for every client request a new process is created) limitation is overcome with the servlet (internally servlet engine creates the thread).
Q)How session concept works?
A)whenever getSession() is called on HttpServletRequest object
Step 1: request object is evaluated for incoming session ID.
Step 2: if ID not available a brand new HttpSession object is created and its corresponding session ID is generated (ie of HashTable) session ID is stored into httpservlet response object and the reference of HttpSession object is returned to the servlet (doGet/doPost).
Step 3: if ID available brand new session object is not created session ID is picked up from the request object search is made in the collection of sessions by using session ID as the key.
Once the search is successful session ID is stored into HttpServletResponse and the existing session object references are returned to the doGet() or doPost() of UserDefineservlet.
Note:
1)when control leaves from servlet code to client don't forget that session object is being held by servlet container ie, the servlet engine
2)multithreading is left to servlet developers people for implementing ie., handle the multiple requests of client nothing to bother about multithread code
Inshort form:
A servlet is created when the application starts (it is deployed on the servlet container) or when it is first accessed (depending on the load-on-startup setting)
when the servlet is instantiated, the init() method of the servlet is called
then the servlet (its one and only instance) handles all requests (its service() method being called by multiple threads). That's why it is not advisable to have any synchronization in it, and you should avoid instance variables of the servlet
when the application is undeployed (the servlet container stops), the destroy() method is called.

Sessions - what Chris Thompson said.
Instantiation - a servlet is instantiated when the container receives the first request mapped to the servlet (unless the servlet is configured to load on startup with the <load-on-startup> element in web.xml). The same instance is used to serve subsequent requests.

The Servlet Specification JSR-315 clearly defines the web container behavior in the service (and doGet, doPost, doPut etc.) methods (2.3.3.1 Multithreading Issues, Page 9):
A servlet container may send concurrent requests through the service
method of the servlet. To handle the requests, the Servlet Developer
must make adequate provisions for concurrent processing with multiple
threads in the service method.
Although it is not recommended, an alternative for the Developer is to
implement the SingleThreadModel interface which requires the container
to guarantee that there is only one request thread at a time in the
service method. A servlet container may satisfy this requirement by
serializing requests on a servlet, or by maintaining a pool of servlet
instances. If the servlet is part of a Web application that has been
marked as distributable, the container may maintain a pool of servlet
instances in each JVM that the application is distributed across.
For servlets not implementing the SingleThreadModel interface, if the
service method (or methods such as doGet or doPost which are
dispatched to the service method of the HttpServlet abstract class)
has been defined with the synchronized keyword, the servlet container
cannot use the instance pool approach, but must serialize requests
through it. It is strongly recommended that Developers not synchronize
the service method (or methods dispatched to it) in these
circumstances because of detrimental effects on performance

No. Servlets are not Thread safe
This is allows accessing more than one threads at a time
if u want to make it Servlet as Thread safe ., U can go for
Implement SingleThreadInterface(i)
which is a blank Interface there is no
methods
or we can go for synchronize methods
we can make whole service method as synchronized by using synchronized
keyword in front of method
Example::
public Synchronized class service(ServletRequest request,ServletResponse response)throws ServletException,IOException
or we can the put block of the code in the Synchronized block
Example::
Synchronized(Object)
{
----Instructions-----
}
I feel that Synchronized block is better than making the whole method
Synchronized

As is clear from above explanations, by implementing the SingleThreadModel, a servlet can be assured thread-safety by the servlet container. The container implementation can do this in 2 ways:
1) Serializing requests (queuing) to a single instance - this is similar to a servlet NOT implementing SingleThreadModel BUT synchronizing the service/ doXXX methods; OR
2) Creating a pool of instances - which's a better option and a trade-off between the boot-up/initialization effort/time of the servlet as against the restrictive parameters (memory/ CPU time) of the environment hosting the servlet.

Life cycle of Servlet and its methods

I know that Servlets consist of the init, service and destroy methods. I also know that there are the doPost and doGet methods available. The question is how the service method relates to the doPost and doGet methods. Are they called from within the service method after the request is being identified? Is the service omitted when the do methods are implemented? I need some clarifications here.
For example in a life cycle of a Servlet that receives a single POST request, I would have guessed that the order would be:
init() is executed
when init() is finished the service() is called
service() identifies the request and calls the doPost() method
when both doPost() and service() finish the destroy() method is executed
Would that be right?

No, it isn't right.
init() and destroy() are called only once. The servlet is instantiated by the container, and its init() method is called. That usually happens at startup, or when the first request for the servlet comes in.
Then all the requests are handled by the service() method, which calls the appropriate doXxx() method based on the request type (as documented).
Then, when the application is undeployed (or the server stopped), the destroy() method is called.
The javadoc is your friend. Read it. It contains all the answers to your questions. The specifications are also freely available.

From the documentation, service is responsible for dispatching to the relevant servlet method, based on the HTTP method called (POST, GET...)
Receives standard HTTP requests from the public service method and
dispatches them to the doXXX methods defined in this class. This
method is an HTTP-specific version of the
Servlet.service(javax.servlet.ServletRequest,
javax.servlet.ServletResponse) method. There's no need to override
this method.
HTTPServlet.service

This is the basic flow,
- The servlet is initialized by calling the init () method.
The servlet calls service() method to process a client's request.
The service method invokes the doGet or doPost based on the request
type came from the client if get request came doGet is invoked if
post request doPost is invoked
The servlet is terminated by calling the destroy() method.
Finally, servlet is garbage collected by the garbage collector of the
JVM.
The service() method is the main method to perform the actual task. The servlet container (i.e. web server) calls the service() method to handle requests coming from the client( browsers) and to write the formatted response back to the client.
Each time the server receives a request for a servlet, the server spawns a new thread and calls service. The service() method checks the HTTP request type (GET, POST, PUT, DELETE, etc.) and calls doGet, doPost, doPut, doDelete, etc. methods as appropriate.
Servlet Life Cycle

servlet container managed bean [duplicate]

Suppose, I have a webserver which holds numerous servlets. For information passing among those servlets I am setting session and instance variables.
Now, if 2 or more users send request to this server then what happens to the session variables?
Will they all be common for all the users or they will be different for each user?
If they are different, then how was the server able to differentiate between different users?
One more similar question, if there are n users accessing a particular servlet, then this servlet gets instantiated only the first time the first user accessed it or does it get instantiated for all the users separately?
In other words, what happens to the instance variables?

Sessions
In short: the web server issues a unique identifier to each visitor on his first visit. The visitor must bring back that ID for him to be recognised next time around. This identifier also allows the server to properly segregate objects owned by one session against that of another.
Servlet Instantiation
If load-on-startup is false:
If load-on-startup is true:
Once he's on the service mode and on the groove, the same servlet will work on the requests from all other clients.
Why isn't it a good idea to have one instance per client? Think about this: Will you hire one pizza guy for every order that came? Do that and you'd be out of business in no time.
It comes with a small risk though. Remember: this single guy holds all the order information in his pocket: so if you're not cautious about thread safety on servlets, he may end up giving the wrong order to a certain client.

Session in Java servlets is the same as session in other languages such as PHP. It is unique to the user. The server can keep track of it in different ways such as cookies, url rewriting etc. This Java doc article explains it in the context of Java servlets and indicates that exactly how session is maintained is an implementation detail left to the designers of the server. The specification only stipulates that it must be maintained as unique to a user across multiple connections to the server. Check out this article from Oracle for more information about both of your questions.
Edit There is an excellent tutorial here on how to work with session inside of servlets. And here is a chapter from Sun about Java Servlets, what they are and how to use them. Between those two articles, you should be able to answer all of your questions.

When the servlet container (like Apache Tomcat) starts up, it will read from the web.xml file (only one per application) if anything goes wrong or shows up an error at container side console, otherwise, it will deploy and load all web applications by using web.xml (so named it as deployment descriptor).
During instantiation phase of the servlet, servlet instance is ready but it cannot serve the client request because it is missing with two pieces of information:
1: context information
2: initial configuration information
Servlet engine creates servletConfig interface object encapsulating the above missing information into it
servlet engine calls init() of the servlet by supplying servletConfig object references as an argument. Once init() is completely executed servlet is ready to serve the client request.
Q) In the lifetime of servlet how many times instantiation and initialization happens ??
A)only once (for every client request a new thread is created)
only one instance of the servlet serves any number of the client request ie, after serving one client request server does not die. It waits for other client requests ie what CGI (for every client request a new process is created) limitation is overcome with the servlet (internally servlet engine creates the thread).
Q)How session concept works?
A)whenever getSession() is called on HttpServletRequest object
Step 1: request object is evaluated for incoming session ID.
Step 2: if ID not available a brand new HttpSession object is created and its corresponding session ID is generated (ie of HashTable) session ID is stored into httpservlet response object and the reference of HttpSession object is returned to the servlet (doGet/doPost).
Step 3: if ID available brand new session object is not created session ID is picked up from the request object search is made in the collection of sessions by using session ID as the key.
Once the search is successful session ID is stored into HttpServletResponse and the existing session object references are returned to the doGet() or doPost() of UserDefineservlet.
Note:
1)when control leaves from servlet code to client don't forget that session object is being held by servlet container ie, the servlet engine
2)multithreading is left to servlet developers people for implementing ie., handle the multiple requests of client nothing to bother about multithread code
Inshort form:
A servlet is created when the application starts (it is deployed on the servlet container) or when it is first accessed (depending on the load-on-startup setting)
when the servlet is instantiated, the init() method of the servlet is called
then the servlet (its one and only instance) handles all requests (its service() method being called by multiple threads). That's why it is not advisable to have any synchronization in it, and you should avoid instance variables of the servlet
when the application is undeployed (the servlet container stops), the destroy() method is called.

Sessions - what Chris Thompson said.
Instantiation - a servlet is instantiated when the container receives the first request mapped to the servlet (unless the servlet is configured to load on startup with the <load-on-startup> element in web.xml). The same instance is used to serve subsequent requests.

The Servlet Specification JSR-315 clearly defines the web container behavior in the service (and doGet, doPost, doPut etc.) methods (2.3.3.1 Multithreading Issues, Page 9):
A servlet container may send concurrent requests through the service
method of the servlet. To handle the requests, the Servlet Developer
must make adequate provisions for concurrent processing with multiple
threads in the service method.
Although it is not recommended, an alternative for the Developer is to
implement the SingleThreadModel interface which requires the container
to guarantee that there is only one request thread at a time in the
service method. A servlet container may satisfy this requirement by
serializing requests on a servlet, or by maintaining a pool of servlet
instances. If the servlet is part of a Web application that has been
marked as distributable, the container may maintain a pool of servlet
instances in each JVM that the application is distributed across.
For servlets not implementing the SingleThreadModel interface, if the
service method (or methods such as doGet or doPost which are
dispatched to the service method of the HttpServlet abstract class)
has been defined with the synchronized keyword, the servlet container
cannot use the instance pool approach, but must serialize requests
through it. It is strongly recommended that Developers not synchronize
the service method (or methods dispatched to it) in these
circumstances because of detrimental effects on performance

No. Servlets are not Thread safe
This is allows accessing more than one threads at a time
if u want to make it Servlet as Thread safe ., U can go for
Implement SingleThreadInterface(i)
which is a blank Interface there is no
methods
or we can go for synchronize methods
we can make whole service method as synchronized by using synchronized
keyword in front of method
Example::
public Synchronized class service(ServletRequest request,ServletResponse response)throws ServletException,IOException
or we can the put block of the code in the Synchronized block
Example::
Synchronized(Object)
{
----Instructions-----
}
I feel that Synchronized block is better than making the whole method
Synchronized

As is clear from above explanations, by implementing the SingleThreadModel, a servlet can be assured thread-safety by the servlet container. The container implementation can do this in 2 ways:
1) Serializing requests (queuing) to a single instance - this is similar to a servlet NOT implementing SingleThreadModel BUT synchronizing the service/ doXXX methods; OR
2) Creating a pool of instances - which's a better option and a trade-off between the boot-up/initialization effort/time of the servlet as against the restrictive parameters (memory/ CPU time) of the environment hosting the servlet.

Are Servlet Init parameters accessible in whole servlet container

I have this question as i did not find any correct results when searching in Web.
Does Servlet Init Parameters y default accessible in whole container without making any configaration.I mean servlet A has some property x,If another servlet B ,in the same container(may be or may not be in same Application) access it? If so can anyone please show me with an example.
Thanks in Avance.

No, init parameters are servlet-scoped, context-param are application scoped.
There's no other wider scope (e.g. between two applications)

As servlet config, The init parameters of a servlet can only be accessed by that servlet
here is link http://tutorials.jenkov.com/java-servlets/web-xml.html#initParams

Can we replace the purpose of init method to the servlet constructor?

Can we replace the pupose of init method to the servlet constructor?
My question is why does servlet need a separate method int(ServletConfig config) for the pupose of initialization we could have achieve the same thing by having a parameterised constructor let XServlet(ServletConfig config) and the container can invoke the same .

Probably because you cannot specify the parameter signature for constructors in an interface.

It is generally considered a bad practice to perform logic in constructor (it should only initialize the field and create object in consistent state). It also makes testing harder.
Also it is much harder to perform injection - when using init the container can create a servlet, inject dependencies and run init. With constructor you would expect to have all the dependencies already set. Wicket works around this problem by injecting Spring beans into page class from superclass constructor - because superclass constructor runs first. However modifying subclass fields from superclass constructor seems wrong.
That being said using separate init method some things simpler and easier to maintain. Also note that EJB promotes #PostConstruct annotation as well.

I guess this choice was made to simplify the coding of servlets. In the current situation, a servlet having no need for specific initialization can be coded like this:
public class MyServlet extends HttpServlet {
// doGet, doPost, etc.
}
If the servlet was initialized using the constructor, it would need to be coded like this:
public class MyServlet extends HttpServlet {
public MyServlet(ServletConfig config) {
super(config);
}
// doGet, doPost, etc.
}
BTW, they even introduced a no-arg init method that can be used to avoid being forced to call super.init(config).

Short answer: NO.
Long answer:
If a servlet contains a constructor and a init method, you will see the constructor is called first, and then the init method (try it out with sysout statements).
A servlet is a java class, so it must follow the java process, so constructor is called first and then the init method (note that the init method is called by your servlet container like tomcat because that's how servlet's are supposed to be initialized; read the servlet specification).
So, if you need to do some operation that is specific to your business need (say create an DB connection or read a set of user details etc); constructors are not the best place to put these things in.
Usually constructors should never have any complex logic/business processing. They should initialize bare minimum member variables and leave the complex things to later; in case of servlets the heavy lifting can be done by init method.
Also note that by the time the init() is called the servlet container is up and ready with it's system resources, i.e it would have initialized the JNDi name bindings, data sources so on and so forth.
So in a way it guarantees you that when you call the init(), it will have the system resources ready for you to work on.
The servlet 3 spec says this:
2.3.1 Loading and Instantiation The servlet container is responsible
for loading and instantiating servlets. The loading and instantiation
can occur when the container is started, or delayed until the
container determines the servlet is needed to service a request. When
the servlet engine is started, needed servlet classes must be located
by the servlet container. The servlet container loads the servlet
class using normal Java class loading facilities. The loading may be
from a local file system, a remote file system, or other network
services. After loading the Servlet class, the container instantiates
it for use.
2.3.2 Initialization After the servlet object is instantiated, the
container must initialize the servlet before it can handle requests
from clients. Initialization is provided so that a servlet can read
persistent configuration data, initialize costly resources (such as
JDBC APIbased connections), and perform other one-time activities. The
container initializes the servlet instance by calling the init method
of the Servlet interface with a unique (per servlet declaration)
object implementing the ServletConfig interface. This configuration
object allows the servlet to access name-value initialization
parameters from the Web application’s configuration information. The
configuration object also gives the servlet access to an object
(implementing the ServletContext interface) that describes the
servlet’s runtime environment. See Chapter 4, Servlet Context for more
information about the ServletContext interface.
Read this part:
The servlet container loads the servlet class using normal Java class loading facilities.
so, the container will have to use standard java class loading mechanism. Which means it can not do what Duncan have just suggested.

int(ServletConfig config) is a life cycle method of Servlet.and it is being called by servlet container.
We can replace the init method code in parameter constructor of super class servlet.
but we can't force the developer to call super class constructor
for example
/*If generic servlet has a constructor like this*/
public GenericServlet(ServletConfig config){
....
}
/*And Developers servlet is calling only default servlet.*/
public MyServlet(){
super();
}
So in this scenario initialization never happens for that servlet. it cause issues problem. because ServletConfig obj is not initialized.
And we cant force the user to call specifict super class constructor.

The simple answer is that you have to have a no-arg ctor and an init method because that is what the spec requires...
Why the spec is written like that is probably a matter of history. These days I think that we might defer construction until the ServletContext is available and call a ctor with that argument. But looking back there was a ServletContext.getServlet(name) method, so that one servlet could look up another on initialisation and communicate with it. For this to work they would all have had to be created up-front and then init'd.
getServlet was deprecated and now returns null (I think it was a security hole for containers that were running multiple apps).

Develop Reference

Java is a programming language and computing platform first released by Sun Microsystems in 1995.