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Java Servlets. Basics. What is Java Servlets? Java’s solution to CGI programs. Wrong way to use Java as CGI Write stand-alone Java program Invoke it via new CGI shell process. Right way to use Java as CGI Use servlets and a servlet-compatible server. How it works?.
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Basics • What is Java Servlets? • Java’s solution to CGI programs. • Wrong way to use Java as CGI • Write stand-alone Java program • Invoke it via new CGI shell process. • Right way to use Java as CGI • Use servlets and a servlet-compatible server.
How it works? • Browser makes a servlet request to webserver. • Webserver recognizes the URL as a servlet URL. (e.g., http://www.obvious.com/servlets/blah) • Webserver loads servlet if not already loaded. • Servlet executes and delivers output to webserver (HTML output). • Webserver delivers output to Browser.
What you need to write servlets? • Either JDK 1.1 and JSDK (Java Servlet Development Kit). • JDK 1.2 • A servlet-compatible webserver.
GET & POST • When Form data is sent by the browser, it can be sent in one of two ways: (1) using the GET method and (2) using the POST method. • In the GET method, the form data (parameters) is appended to the URL, as in: http://www.yahoo.com/search?music • Here, the text field contains music. • In the POST method, the browser simply sends the form data directly. • When you create an HTML form, you decide whether you want to use GET or POST. • When you use GET, the doGet() method of your servlet is called, otherwise the doPost() method is called. • The standard practice is to use POST, unless you need to use GET. • You need to use GET when you want a link to invoke a CGI program (servlet).
Your first servlet -- Hello World import javax.servlet.*; import javax.servlet.http.*; import java.io.*; public class helloworld extends HttpServlet { public void doGet (HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { // Set the content type of the response. resp.setContentType ("text/html"); // Create a PrintWriter to write the response. java.io.PrintWriter out = new PrintWriter (resp.getOutputStream()); // The first part of the response. out.println (""); out.println (""); out.println (""); // The greeting. out.println ("Yo, Hello World!"); // Last part. out.println (""); out.println (""); out.close(); // Screen I/O System.out.println ("Inside servlet ... servlet complete"); } public void doPost (HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doGet (req, resp); } }
Hello World Notes • The servlet class libraries need to be imported, along with java.io.PrintWriter. • There are two methods, one for each type of request: GET or POST. • You can decide not to "really" implement one method and instead have it call the other. • There are two object parameters to each method, the "request" and the "respponse". • The HttpServletResponse instance has an OutputStream that is used to write directly to the requesting browser. • To make writing easier, we wrap a PrintWriter around the OutputStream instance: // Create a PrintWriter to write the response. java.io.PrintWriter out = new PrintWriter (resp.getOutputStream()); • We set the content-type (as required by the HTTP protocol): // Set the content type of the response. resp.setContentType ("text/html"); • We write HTML to the output, e.g., out.println (""); out.println (""); • Don't forget to close the output stream: out.close(); • Optionally, for debugging, we can also write to the local screen where the webserver is running: System.out.println ("Inside servlet ... servlet complete");
Extracting Parameters in a Servlet • Idea -- create an HTML Form, and have a servlet pick up the Form data entered by the user. • HTML Form <html> <head><title>Test Post</title></head> <body> <form action="http://localhost:8502/servlets/Testform" method="post"> Enter a string: <input type="text" name="param1"> And then press "Go": <input type="submit" value="Go"> </form> </body> </html>
Extracting Parameters in a Servlet • Next the Java Servlet import javax.servlet.*; import javax.servlet.http.*; import java.io.*; import java.util.*; public class TestForm extends HttpServlet { public void doPost (HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { // Set the content type of the response. resp.setContentType ("text/html"); // Create a PrintWriter to write the response. java.io.PrintWriter out = new PrintWriter (resp.getOutputStream()); // The first part of the response. out.println (""); out.println (""); out.println (""); // Now get the parameters and output them back. out.println ("Request parameters: "); Enumeration e = req.getParameterNames(); while (e.hasMoreElements()) { String name = (String) e.nextElement(); String value = req.getParameter (name); if (value != null) out.println (" name=[" + name + "] value=[" + value + "]"); else out.println (" name=[" + name + "] did not have a value"); } // Last part. out.println (""); out.println (""); out.close(); } public void doGet (HttpServletRequest req, HttpServletResponse resp) throws ServletException, IOException { doPost (req, resp); } }
Notes for Extracting Parameters • We need to import java.util.Enumeration. Whatever parameters were provided are all in the HttpServletRequest instance. We can list these parameters by getting an Enumeration instance from the request by calling getParameterNames(): Enumeration e = req.getParameterNames(); • The "name" of a parameter is really the string in the name attribute of the tag for the particular Form element. For example, the name string is param1 below. Enter a string: <input type="text" name="param1"> • Now, if we want to retrieve the actual string typed in by the user, we use that name in getParameter(): String whatTheUserTyped = req.getParameter ("param1");
