Manav Goyal

Java Servlet - Java

Basic
- Acts as middle layer between a request from client to database
Architecture
- Different threads can be created for each request instead of sending multiple requests
Life Cycle
- Servelet interface declares all life cycles methods
- GenericServelet is an abstract class which provides based Servelet implementations except service() method
- HttpServlet is abstract class which provides http specific methods like doGet(), etc
- init()
  - Called only once, Servlet is created
  - Code
```
    public void init() throws ServletException {
        // Statements
    }
```
- service()
  - Called whenever a request is made, Handles request
  - Checks the http request type and calls the required method
```
    public void service(ServletRequest req, ServletResponse res) throws ServletException, IOException {
        // Statements
    }
```
  - doGet()
```
    public void doGet(HTTPServletRequest request, HTTPServletResponse response) throws ServletException, IOException {
        // Statements
    }
```
- destroy()
  - Called only once, Servlet is destroyed
  - Code
```
    public void destroy() throws ServletException {
        // Statements
    }
```

Code

main.java

    import javax.servlet.*
    import javax.servlet.http.*
    public class NamasteDunia extends HttpServlet {
        private string message;
        public void init() throws ServeletException {
            message = "Namaste Dunia";
        }
        public void doGet(HTTPServeletRequest request, HTTPServeletResponse response) throws ServeletException, IOException {
            response.setContentType("text/html");
            printWriter out = response.getWriter();
            out.println("<h1>"+message+"</h1>");
        }
        public void destroy() throws ServeletException {
            // Statements
        }
    }

web.xml

    <servlet>
        <servlet-name>NamasteDunia</servlet-name>
        <servlet-class>NamasteDunia</servlet-class>
    </servlet>
    <servlet-mapping>
        <servlet-name>NamasteDunia</servlet-name>
        <url-pattern>/NamasteDunia</url-pattern>
    </servlet-mapping>

ServeletConfig object is created by web container for each Servelet during initialization
- Should be placed within tag
ServeletContext is created while deploying web application
- Should be placed outside tag

RequestDispatcher

Provides facility to dispatch client's request to another web resource and to include the response of another web resource

Forward():

    RequestDispatcher
    rd=request.getRequestDispatcher("resource")
    rd.forward(request, response);

Include():

    RequestDispatcher
    rd=request.getRequestDispatcher("resource")
    rd.include(request, response);

HTML Forms
- request.getParameter("name") => Get a value from a parameter
- request.getParameterValues() => When parameter returns multiple values or appearns more than once
- request.getParameterNames() => Gives complete list of all the parameters
Session Tracking
- Way to store client data throught the session
- Techniques
  - URL Techniques
  - Hidden Form Field
  - Cookies
    - Types
      - Non-persistent
        
        Valid only for single session => Removed each time browser is closed
      - Persistent
        
        Valid only for multiple session
    - Code
      - Cookie c = new Cookie("key", "value"); => Create Cookie
      - c.setMaxAge(N); => Set max age
      - response.addCookie(c); => Send into HTTP header
      - Cookie c[] = request.getCookies(); => Get all the cookies
  - HttpSession
    - Servelet creates the object and generates a unique id along with client information
    - Response will contain the generated session id
    - Methods
      - getId(), setAttribute(String name, Object value), getAttribute(String name), invalidate()

Servelet Filter

Does pre and post processing with Request and Response

Code

    <filter>
        <display-name>MyFilter</display-name>
        <filter-name>MyFilter</filter-name>
        <filter-class>com.MyFilter</filter-class>
    </filter>