Servlet

What?

• a technology that includes APIs and Java classes.

  

  

  

  💡

  Note: the Servlet technology is not limited to the HTTP protocol. “Servlet interface” and “GenericSevlet Class” follow the idea that a servlet could be used for any type of service (”not protocol dependent”), such as HTTP, FTP,…etc,… → 2 parameters request and respond are protocol-agnostic.

  • Are Java classes that dynamically process HTTP requests and construct responses.

    

• a web component that runs on the server side (middle tier) and can act as a controller:

  • Are Java codes that are used to add dynamic content to Web server.

  • receive requests, handle and reply back with a response.
    • There is only a single instance of Servlet created on the Web server.
    • A Servlet’s initializing code is used only for initializing in the 1st time
    • To service multiple clients’ requests, the Web server creates multiple threads for the same Servlet instance (Overcome CGI’s consumed more memory)
    • Gets auto refreshed on receiving a request each time

  • Servlets are under the control of another Java application called a Servlet Container.

  

• Merits

  • Enhanced efficiency (initializing only once, auto refresh)

  • Ease to use (using Java combining HTML)

  • Powerful (using Java)

  • Portable

  • Safe and cheap

• Demerits

  • Low-level HTML documentation (Static well-formed-ness is not maintained)

  • Unclear-session management (flow of control within the codes is very unclear)

🎮Servlet in action

Servlet Class

• a Java class, has only a single instance

• must be declared in the web.xml file / by annotations

  • Servlets Declaration in web.xml and be created cell memory by container

    

  • Servlet Mapping to define the URL for accessing Servlet via url-pattern

    

  

  • annotations

    

    

    

• Servlet URL Pattern:

  

  

  

Servlet Object

• be initialized
  • by Container
  • in 3 steps:
    1. load Servlet Class
    2. Instantiation: Create Servlet Instance
    3. Initialization: call init(ServletConfig)
  • at either of the stages (depending on the custom setup):
    • app deploy
    • the first request for the servlet is received by the Web Container.

• Exist until right before undeployed or server is crashed

• Output HTML String into ResObj

• must be mapped with an URL pattern for accessing.

life-cycle:

• who: container.

• when: at either of the stages (depending on custom setup):
  • app deploy
  • in the first request for the servlet is received by the Web Container.

• how:
  1. load Servlet Class
  2. Instantiation: Create Servlet Instance
  3. Initialization: init(ServletConfig)
     • is called only one by the container after Servlet Obj is initialized successfully.
     • support to replace the default constructor
  4. services()
     • doHTTPMethods → process every/each request
  5. destroy() :
     • called only one right before undeployed or the web server is stopped.
  6. Garbage Collection: finalize()

• when: undeployed or the web server is stopped/crashed.

• Read more:

  Difference between each instance of servlet and each thread of servlet in servlets?

  When the Servlet container starts, it: reads web.xml; finds the declared Servlets in the classpath; and loads and instantiates each Servlet only once. Roughly, like this: String urlPattern = parseWebXmlAndRetrieveServletUrlPattern(); String servletClass = parseWebXmlAndRetrieveServletClass(); HttpServlet servlet = (HttpServlet) Class.forName(servletClass).newInstance(); servlet.init(); servlets.put(urlPattern, servlet); // Similar to a map interface.

  https://stackoverflow.com/questions/2183974/difference-between-each-instance-of-servlet-and-each-thread-of-servlet-in-servle

  

  Servlet (Java EE 6 )

  All Known Implementing Classes: FacesServlet, GenericServlet, HttpServlet 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, the HyperText Transfer Protocol.

  https://docs.oracle.com/javaee/6/api/javax/servlet/Servlet.html

  Difference between init() & init(ServletConfig)

  Question- What's the difference between init() & init(ServletConfig) and Which is better ? Before start we have to understand the servlet flow. Answer- For example you have servlet LoginServlet which extends HttpServlet public class LoginServlet extends HttpServlet{ } And your HttpServlet internally extends GenericServlet.

  https://www.asknsay.com/difference-between-init-and-init-servletconfig.html

  

  Life Cycle of a Servlet - GeeksforGeeks

  The entire life cycle of a Servlet is managed by the Servlet container which uses the javax.servlet.Servlet interface to understand the Servlet object and manage it. So, before creating a Servlet object, let's first understand the life cycle of the Servlet object which is actually understanding how the Servlet container manages the Servlet object.

  https://www.geeksforgeeks.org/life-cycle-of-a-servlet/#:~:text=There%20are%20three%20life%20cycle,destroy()

  

  Servlet Life Cycle

  Loading Servlet Class : A Servlet class is loaded when first request for the servlet is received by the Web Container. Servlet instance creation :After the Servlet class is loaded, Web Container creates the instance of it. Servlet instance is created only once in the life cycle.

  https://www.studytonight.com/servlet/servlet-life-cycle.php