More on Servlet API [ Application Programming Interface ]

• Servlet API
• javax.servlet package
• javax.servlet.http package
• Examples

The Servlet API consists of:

javax.servlet package contains :

• Interfaces
• Classes
• Exceptions

javax.servlet.http package contains :

• Interfaces
• Classes

javax.servlet package contains following interfaces ,classes and Exceptions :-

Interfaces

• Filter
• RequestDispatcher
• Servlet
• ServletConfig
• ServletContext
• ServletRequest
• ServletResponse
• SingleThreadModel

Classes

• GenericServlet
• ServletInputStream
• ServletOutputStream
• ServletRequestWrapper
• ServletResponseWrapper

Exceptions

• ServletException :- Defines a general exception a servlet can throw when it encounters difficulty
• UnavailableException :- Defines an exception that a servlet or filter throws to indicate that it is permanently or temporarily unavailable.

Servlet

• Defines methods that all servlets must implement.
• A servlet is a small Java program that runs within a Web server. Servlets receive and respond to requests from Web clients, usually across HTTP.
• To implement this interface, you can write a generic servlet that extends javax.servlet.GenericServlet or an HTTP servlet that extends javax.servlet.http.HttpServlet.
• This interface defines methods to initialize a servlet, to service requests, and to remove a servlet from the server. These are known as life-cycle methods and are called in the following sequence:
  • The servlet is constructed, then initialized with the init method.
  • Any calls from clients to the service method are handled.
  • The servlet is taken out of service, then destroyed with the destroy method, then garbage collected and finalized.

GenericServlet :- It is abstract class.

• extends java.lang.Object
• implements Servlet, ServletConfig, java.io.Serializable
• Defines a generic, protocol-independent servlet. To write an HTTP servlet for use on the Web, extend HttpServlet instead.
• GenericServlet implements the Servlet and ServletConfig interfaces. GenericServlet may be directly extended by a servlet, although it’s more common to extend a protocol-specific subclass such as HttpServlet.

ServletRequest

• Defines an object to provide client request information to a servlet.
• The servlet container creates a ServletRequest object and passes it as an argument to the servlet’s service method.
• A ServletRequest object provides data including parameter name and values, attributes, and an input stream.

ServletResponse

• Defines an object to assist a servlet in sending a response to the client.
• The servlet container creates a ServletResponse object and passes it as an argument to the servlet’s service method.
• To send binary data in a MIME body response, use the ServletOutputStream returned by getOutputStream().
• To send character data, use the PrintWriter object returned by getWriter().

ServletContext

• Defines a set of methods that a servlet uses to communicate with its servlet container, for example, to get the MIME type of a file, dispatch requests, or write to a server log file.
• There is one context per “web application” per Java Virtual Machine.

ServletConfig

A servlet configuration object used by a servlet container used to pass information to a servlet during initialization.

RequestDispatcher

• Defines an object that receives requests from the client and sends them to any resource (such as a servlet, HTML file, or JSP file) on the server.
• The servlet container creates the RequestDispatcher object, which is used as a wrapper around a server resource located at a particular path or given by a particular name.
• It has two methods forward and include.

SingleThreadModel

• Ensures that servlets handle only one request at a time.
• This interface has no methods.
• If a servlet implements this interface, you are guaranteed that no two threads will execute concurrently in the servlet’s service method.
• The servlet container can make this guarantee by synchronizing access to a single instance of the servlet.

Filter

• A filter is an object than perform filtering tasks on either the request to a resource (a servlet or static content), or on the response from a resource, or both.
• Filters perform filtering in the doFilter method.
• Every Filter has access to a FilterConfig object from which it can obtain its initialization parameters, a reference to the ServletContext which it can use, for example, to load resources needed for filtering tasks.
• Filters are configured in the deployment descriptor of a web application.
• Examples that have been identified for this design are:
  1) Authentication Filters
  2) Logging and Auditing Filters
  3) Image conversion Filters
  4) Data compression Filters
  5) Encryption Filters
  6) Tokenizing Filters
  7) Filters that trigger resource access events
  8) XSL/T filters
  9) Mime-type chain Filter

javax.servlet.http package contains following interfaces and classes:

Interfaces

• HttpServletRequest
• HttpServletResponse
• HttpSession

Classes

• Cookie
• HttpServlet
• HttpServletRequestWrapper
• HttpServletResponseWrapper

HttpServlet :- It is abstract class.

• extends GenericServlet
• implements java.io.Serializable
• Any class should be subclassed to create an HTTP servlet suitable for a Web site.
• A subclass of HttpServlet must override at least one method, usually one of these:
  • doGet, if the servlet supports HTTP GET requests
  • doPost, for HTTP POST requests
  • doPut, for HTTP PUT requests
  • doDelete, for HTTP DELETE requests
  • init and destroy, to manage resources that are held for the life of the servlet
  • getServletInfo, which the servlet uses to provide information about itself
• There’s almost no reason to override the service method.
• service method handles standard HTTP requests by dispatching them to the handler methods for each HTTP request type (the doXXX methods listed above).

HttpServletRequest

• extends ServletRequest
• Extends the ServletRequest interface to provide request information for HTTP servlets.
• The servlet container creates an HttpServletRequest object and passes it as an argument to the servlet’s service methods (doGet, doPost, etc).

HttpServletResponse

• extends ServletResponse
• Extends the ServletResponse interface to provide HTTP-specific functionality in sending a response. For example, it has methods to access HTTP headers and cookies.
• The servlet container creates an HttpServletRequest object and passes it as an argument to the servlet’s service methods (doGet, doPost, etc).

HttpSession

• Provides a way to identify a user across more than one page request or visit to a Web site and to store information about that user.
• The servlet container uses this interface to create a session between an HTTP client and an HTTP server.
• The session persists for a specified time period, across more than one connection or page request from the user.
• A session usually corresponds to one user, who may visit a site many times.
• The server can maintain a session in many ways such as using cookies or rewriting URLs.
• This interface allows servlets to
  • View and manipulate information about a session, such as the session identifier, creation time, and last accessed time
  • Bind objects to sessions, allowing user information to persist across multiple user connections

Cookie

• extends java.lang.Object
• implements java.lang.Cloneable
• Creates a cookie, a small amount of information sent by a servlet to a Web browser, saved by the browser, and later sent back to the server.
• A cookie’s value can uniquely identify a client, so cookies are commonly used for session management.
• A cookie has a name, a single value, and optional attributes such as a comment, path and domain qualifiers, a maximum age, and a version number.

One Constructor of Cookie class as below
Cookie(java.lang.String name, java.lang.String value) Constructs a cookie with a specified name and value.

1. Example of Displaying Browser Details by using Servlet
2. Example of Storing Customer Transaction thru Servlet in database by getting customer information from user.
3. Example of Retrieving Customer Details thru Servlet from database by getting customer Id from user.
4. Example of ServletContext using setAttribute and getAttribute
5. Example of <context-param> using initParameter() from ServletContext
6. Example of <init-param> using initParameter() from ServletConfig
7. Example of ServletEvent which will log into server log file when added or removed parameter with context