1. public interface Servlet (cont.)

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1 1. public interface Servlet (cont.) The doget() and dopost() methods are two widely used methods provided by the HttpServlet class. There are two basic HTTP request types in HTTP protocol: GET or POST. The GET request is commonly used to retrieve data or images from a Web server and the POST request is commonly used to send form data to the Web server. GET and POST methods can be used alternatively in some cases such as submitting a HTTP form.

2 1. public interface Servlet (cont.) Although GET is basically used for reading it can also include some short data along with the request. The GET type request is the default type. It can also be made by clicking on a hyperlink that points to the Servlet. The SET type request is also URL book mark able if it is used as part of URL. It is better to use POST method if you don t want anybody see the contents of your request or the contents of the request are very large.

3 1. public interface Servlet (cont.) An example of a HTTP form which makes a request to an HTTP Servlet.. <html xmlns = " <head> <title>processing post requests with data</title> </head> <body> <form action = " /conv/conversion" method = "post"> <p><label> Convert from Feet to Meters. Enter the feet value in the field. <br /><input type = "text" name = "feet" /> <input type = "submit" value = "Submit" /> </p></label> </form> </body> </html>

4 1. public interface Servlet (cont.) You can see the POST method type is specified in this form request to a Servlet named conversion as the action attribute of the form tag. The Servlet class name is not necessarily the same as this name since the deployment descriptor may map a virtual name to the actual Servlet class name. The HTTP form which invokes the Servlet conversion is shown here. conv is this Web application s ROOT directory.

5 1. public interface Servlet (cont.)

6 Servlet Life Cycle A Servlet has a life cycle just like a Java applet. The life cycle is managed by the Servlet container. There are three methods in the Servlet interface which each Servlet class must implement. They are init(), service(), and destroy().

7 Servlet Life Cycle (cont.)

8 Servlet Life Cycle (cont.) The init() method is called only once. The Servlet object can be instantiated either by the web.xml Servlet configuration file ( <load-on-startup/> in its servlet tag in the web.xml file), or by its first request access. The init() method is called by the Servlet container right after the Servlet object is loaded. Some routine processes such as database connection can be specified in the init() method so that the connection time can be reduced significantly. Otherwise, these functions must be performed for each HTTP Servlet request, which may require substantial time.

9 Servlet Life Cycle (cont.) All subsequent requests are handled by service() method. If an application Servlet is a generic Servlet instead of HTTP Servlet the service() method must be specified. The service() method is called by the Servlet container once per request. If the request to the Servlet is an HTTP request then the service() method will forward the request to either dopost() method or doget() method respectively according to the HTTP request types. Here are the prototypes of these two methods.

10 Processing of HTTP Requests and Responses (cont.) The prototypes of these two methods. void dopost( HttpServletRequest request, HttpServletResponse response ) throws ServletException, IOException; void dopost( HttpServletRequest request, HttpServletResponse response ) throws ServletException, IOException;

11 public abstract interface HttpServletRequest extends ServletRequest This interface has many methods. The important ones of them are shown here: public Cookie[] getcookies() //Returns an array containing the entire Cookie //objects the browser sent with this request. This //method will be discussed again in the Cookie //section

12 public String getquerystring() //Returns the query string that is contained in the //HTTP request URL after the path. public HttpSession getsession(boolean create) //Returns the current HttpSession of this request //or, if necessary, creates a new session for the //request. Use true to create a new session, or //use false to return the current HttpSession.

13 public abstract interface HttpServletRequest extends ServletRequest (cont.) public String getrequestedsessionid() //Returns the session ID specified by the client. The following methods are inherited from ServletRequest interface: public void setattribute(java.lang.string key,java.lang.object o) //Stores an attribute in the context of this request. //Attributes are reset when the request is switched. public Object getattribute(string name) //Returns the value of the named attribute as an Object. public String getparameter(string name) //Returns the value of a request parameter or null if the //parameter does not exist.

14 p HttpServletRequest extends ServletRequest (cont.) Some of its important methods are listed here: public abstract void addcookie(cookie cookie) //Adds the specified cookie to the response. public abstract String encodeurl(string url) //Encodes the URL by including the session ID in it, //or, if encoding is not needed, returns URL //unchanged. public abstract String encoderedirecturl(string url) //Encodes the specified URL for use in the //sendredirect method.

15 a. public abstract interface HttpServletRequest extends ServletRequest (cont.) public abstract void senderror(int sc) throws IOException //Sends an error response to the client using the //specified status code and a default message. public abstract void sendredirect(string location) throws IOException //Sends a redirect response to the client using the //specified redirect location URL. The location may //point to a static or dynamic resource.

16 a. public abstract interface HttpServletRequest extends ServletRequest (cont.) The following methods are inherited from ServletResonse interface. public ServletOutputStream getoutputstream()throws java.io.ioexception //Returns a ServletOutputStream for writing binary //data in the response. The servlet container does not //encode the binary data. public PrintWriter getwriter()throws java.io.ioexception //Returns a PrintWriter object that can send character //text to the client.

17 a. public abstract interface HttpServletRequest extends ServletRequest (cont.) HttpServletRequest and HttpServletResponse are the two interfaces that provide the Servlet with full access to all information from the request and the response sent back to the client. When the doget() or dopost() is called the Servlet container passes in the HttpServletRequest and HttpServletResponse objects.

18 a. public abstract interface HttpServletRequest extends ServletRequest (cont.) The method String getparameter(string name) returns the value of a parameter as a part of GET or POST type requests. In the Servlet Conversion, request.getparameter( feet ) returns the value that the client input in the feet text field of the HTML form. The types of the component parameters can be a radio button, check box, combo box, etc. The method HttpSession getsession(boolean create) returns an HttpSession object which can be shared by all Web components within client s current session. There are many other methods such as getparameters() which returns all parameter names, getcookies() which returns Cookie objects used to identify the client of the Servlet.

19 a. public abstract interface HttpServletRequest extends ServletRequest (cont.) The HttpServletResponse interface comes with some useful methods. The setcontexttype(string type) method is used to tell the client browser the MIME type of the response. It can be text/html, text/xml, or others. The PrintWriter getwriter() method is widely used to return a text output stream for sending text data to clients. The ServletOutputStream getoutputstream() method is used to return a byte output stream for sending binary data such as music or image to clients. There are also some other methods such as addcookie(cookie cookie).

20 a. public abstract interface HttpServletRequest extends ServletRequest (cont.) A simple Servlet which takes a input of feet units from a HTML page and converts it to meter units in the metric system. import java.util.*; import java.text.*; import javax.servlet.*; import javax.servlet.http.*; import java.io.*;

21 public class Conversion extends HttpServlet { // process "post" request from client protected void dopost( HttpServletRequest request, HttpServletResponse response ) throws ServletException, IOException { String feet = request.getparameter( "feet" ); Double feetnum = Double.valueOf(feet); Double meters = ; Double metersheight = feetnum*meters; response.setcontenttype( "text/html" ); PrintWriter out = response.getwriter(); DecimalFormat decimalformatter = new DecimalFormat("0.000"); String mtrs = decimalformatter.format(metersheight);

22 a. public abstract interface HttpServletRequest extends ServletRequest (cont.) // send HTML document to client out.println( "<html>" ); // head section of document out.println( "<head>" ); out.println( "<title>processing get requests with data</title>" ); out.println( "</head>" ); // body section of document out.println( "<body>" ); out.println( "<h1>"+feet+" ft = " + mtrs + " meters<br />" ); out.println( "Thanks!</h1>" ); out.println( "</body>" ); } } // end HTML document out.println( "</html>" ); out.close(); // close stream to complete the page

23 a. public abstract interface HttpServletRequest extends ServletRequest (cont.) Let s take a look how the Servlet dynamically produces HTML content. The response object s method setcontenttype() specifies the type of content as text/html format rather than some other format such as image/gif etc. The response object s method getwriter() gets a reference out pointing to a PrintWriter object to let the Servlet send HTML contents to the client. The rest of the out.println() statements just appends the HTML contents to the outgoing page one line per statement. Here is the output HTTP page dynamically generated by this Servlet. You can see that the header title specified in the head tag on the top of the page and the output result in the body section is displayed in the side of h1

24 a. public abstract interface HttpServletRequest extends ServletRequest (cont.)