Servlet Life Cycle

A servlet life cycle can be defined as the entire process from its creation till the destruction. The following are the paths followed by a servlet

• The servlet is initialized by calling the init () method.
• The servlet calls service() method to process a client's request.
• The servlet is terminated by calling the destroy() method.
• Finally, servlet is garbage collected by the garbage collector of the JVM.

Now let us discuss the life cycle methods in details.

The init() method :

The init method is designed to be called only once. It is called when the servlet is first created, and not called again for each user request. So, it is used for one-time initializations, just as with the init method of applets.

The servlet is normally created when a user first invokes a URL corresponding to the servlet, but you can also specify that the servlet be loaded when the server is first started.

When a user invokes a servlet, a single instance of each servlet gets created, with each user request resulting in a new thread that is handed off to doGet or doPost as appropriate. The init() method simply creates or loads some data that will be used throughout the life of the servlet.

The init method definition looks like this:

public void init() throws ServletException {
  // Initialization code...
}

The service() method :

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.

Here is the signature of this method:

public void service(ServletRequest request, 
                    ServletResponse response) 
      throws ServletException, IOException{
}

The service () method is called by the container and service method invokes doGe, doPost, doPut, doDelete, etc. methods as appropriate. So you have nothing to do with service() method but you override either doGet() or doPost() depending on what type of request you receive from the client.

The doGet() and doPost() are most frequently used methods with in each service request. Here are the signature of these two methods.

The doGet() Method

A GET request results from a normal request for a URL or from an HTML form that has no METHOD specified and it should be handled by doGet() method.

public void doGet(HttpServletRequest request,
                  HttpServletResponse response)
    throws ServletException, IOException {
    // Servlet code
}

The doPost() Method

A POST request results from an HTML form that specifically lists POST as the METHOD and it should be handled by doPost() method.

public void doPost(HttpServletRequest request,
                   HttpServletResponse response)
    throws ServletException, IOException {
    // Servlet code
}

The destroy() method :

The destroy() method is called only once at the end of the life cycle of a servlet. This method gives your servlet a chance to close database connections, halt background threads, write cookie lists or hit counts to disk, and perform other such cleanup activities.

After the destroy() method is called, the servlet object is marked for garbage collection. The destroy method definition looks like this:

  public void destroy() {
    // Finalization code...
  }

Architecture Digram:

The following figure depicts a typical servlet life-cycle scenario.

• First the HTTP requests coming to the server are delegated to the servlet container.
• The servlet container loads the servlet before invoking the service() method.
• Then the servlet container handles multiple requests by spawning multiple threads, each thread executing the service() method of a single instance of the servlet.

Shallow Cloning and Deep Cloning in Java

Cloning is of 2 Types:

1. Shallow Cloning  2. Deep Cloning

Whenever clone() method is used 2 different objects are created in 2 different memory locations. So
separate memory is occupied.

If a class contains  X, Y variable and clone is applied two different objects are created in two different situations.

In Shallow Cloning  main problem is instance variables are common for both objects , so if one object is modifed then automatically second object is also modified.

In deep cloning 2 different objects are created in 2 different memory locations , if in case original object is modified then also its duplicate object won't effect , why because there is  no relation between original and duplicate object.

KNOWLEDGE HUB: How many ways can you create an object in Java

KNOWLEDGE HUB: How many ways can you create an object in Java

different ways to create an object in Java

There are 4 ways of creating objects in Java.

1. Using new operator

Employee emp = new Employee();

Here , we are creating Employee class object  'emp' using new operator.

2. Using factrory methods:

NumberFormat nf  = NumberFormat.getNumberInstace();

Here, we are creatig NumberFormat object using the factory method getNumberInstacne().

3. Using newInstance() method .

(a) First,store the class name  'Employee' as string into an  object. For this purpose , factory method
forName() of the class 'Class' will be useful.

Class c = Class.forName("Employee");

Note: class with the name 'Class' in java.lang package.
(b) create another object to the class whose name is in the object c . For this purpose , we need
newInstance()  method of the class 'Class' . as:

Employee obj = (Employee)c.newInstance();

4. By cloning an already available object, we can create another object . Creating exact copy of an
existing object is called 'cloning':

Employee obj1 = new Employee();
Employee obj2 = (Employee)obj1.clone();

Constructor VS Method in Java

Constructor	Method
A constructor is used to initialize the instance variables of a class.	A Method is used for any general purpose processing or calculations.
A constructors name and class name should be same.	A method’s name and class name can be same or different.
A constructor is called at the time of creating the object.	A method can be called after creating the object.
A Constructor  is called only once per object.	A method can be called several times on the object.
A constructor is called and executed automatically.	A method is executed only when we call it.

Difference between Set and List in Java

Set:

1. A set represents a collection of elements . Order of the elements may change in the set.
2. Set will not allow duplicate values to be stored.
3. Accessing elements by their index(position number) is not possible in case of sets.
4.Sets will not allow null elements.

List:

1. A List represents ordered collection of elements . List preserves the order of elements in
which they are entered.
2. List will allow duplicate value.
3.Accessing elements by index is possible in lists.
4. Lists allow null elements to be stored.

Difference between HashMap and Hashtable in Java

HashMap:

1. HashMap object is not synchronized by default.
2. In case of a single thread , using HashMap is faster than the Hashtable.
3. HashMap allows null keys and null values to be stored.
4. Iterator in the HashMap is fail-fast . This means iterator will produce exception if concurrent
updates are made to the HashMap.
5. We can make HashMap object synchronied using synchronizedMap() method as:

Collections.synchronizedMap(new HashMap())

Hashtable:

1. Hashtable object is synchronized by default.
2.In case fo multiple threads , using Hashtable is advisable. With a single thread, Hashtable
becomes slow.
3.Hashtable does not allow null keys or values.
4.Enumeration for the Hashtable is not fail-fast. This means even if concurrent updations
are done to to Hashtable, there will not be any incorrect results produced by the Enumeration.