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Learn Servlet Basics: Building Your First Servlet

Dive into servlet development with this comprehensive guide. Explore the Servlet API, create generic templates, and output text, HTML, and time. Get step-by-step instructions for compiling, installing, and packaging servlets.

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Learn Servlet Basics: Building Your First Servlet

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  1. Servlets:Building Your First Servlet Ethan Cerami New York University First Servlet

  2. Road Map • Generic Template for Creating Servlets • Servlet 2.2 API • Hello World Examples • Outputting Text, HTML, and the current time. • Compiling your own Servlets • Instructions for installing/compiling servlets on I5 • Packaging Servlets • HTML Utilities First Servlet

  3. Generic Servlet Template First Servlet

  4. Servlet Template • First, let’s take a look at a generic servlet template. • The code does not actually do anything, but all your future servlets will follow this general structure. • The most important pieces are noted in yellow. First Servlet

  5. import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class ServletTemplate extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { // Use "request" to read incoming HTTP headers // (e.g. cookies) and HTML form data (e.g. data the user // entered and submitted). // Use "response" to specify the HTTP response status // code and headers (e.g. the content type, cookies). PrintWriter out = response.getWriter(); // Use "out" to send content to browser } } First Servlet

  6. Generic Template • Import the Servlet API: import javax.servlet.*; import javax.servlet.http.*; • To create servlets, you must remember to always use these two import statements. First Servlet

  7. Generic Template • All your servlets must extend HTTPServlet. • HTTPServlet represents the base class for creating Servlets within the Servlet API. • The Full Servlet API is available at: • http://www.java.sun.com/products/servlet/2.2/javadoc/index.html • Once you have extended HTTPServlet, you must override one or both: • doGet(): to capture HTTP Get Requests • doPost(): to capture HTTP Post Requests First Servlet

  8. doGet and doPost • The doGet() and doPost() methods each take two parameters: • HTTPServletRequest: encapsulates all information regarding the browser request. • Form data, client host name, HTTP request headers. • HTTPServletResponse: encapsulate all information regarding the servlet response. • HTTP Return status, outgoing cookies, HTML response. • If you want the same servlet to handle both GET and POST, you can have doGet call doPost or vice versa. First Servlet

  9. Getting an OutputStream • The HTTPResponse object has a getWriter() method. • This method returns a java.io.PrintWriter object for writing data out to the Web Browser. PrintWriter out = response.getWriter(); First Servlet

  10. Hello World! First Servlet

  11. Hello World! • We are finally ready to see our first real servlet. • This servlet outputs “Hello World!” as plain text, not HTML. • Note: All examples are available on the web site. • Let’s take a look at the code, and then see the servlet in action. First Servlet

  12. import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloWorld extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { PrintWriter out = response.getWriter(); out.println("Hello World"); } } First Servlet

  13. Output Stream • Once you have an OutputStream object, you just call the println() method to output to the browser. • Anything you print will display directly within the web browser. • As we will now see, you can also output any HTML tags. First Servlet

  14. Generating HTML • To generate HTML, you need to add two steps: • Tell the browser that you are sending back HTML. • Modify the println() statements to return valid HTML. First Servlet

  15. HelloWWW.java import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloWWW extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("<HTML>\n" + "<HEAD><TITLE>Hello WWW</TITLE></HEAD>\n" + "<BODY>\n" + "<H1>Hello WWW</H1>\n" + "</BODY></HTML>"); } } First Servlet

  16. Generating HTML • To return HTML, you must set the content MIME type to text/html: • response.setContentType("text/html"); • Remember that you must set the content type before you output any content. • Once you have set the MIME type, you can return any HTML document you want. First Servlet

  17. Time Servlet • Let’s try one more simple servlet… • Using the java.util.Date object, you can obtain the current time. • Let’s create a simple Servlet that outputs the current time. First Servlet

  18. import java.io.*; import java.util.*; import javax.servlet.*; import javax.servlet.http.*; public class TimeServlet extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); Date now = new Date(); PrintWriter out = response.getWriter(); out.println("<HTML>\n" + "<HEAD><TITLE>Hello WWW</TITLE></HEAD>\n" + "<BODY>\n" + "<H1>Hello WWW</H1>\n"+ "<H2>Time is now: "+now+"</H2>"+ "</BODY></HTML>"); } } First Servlet

  19. Installing / Compiling Servlets First Servlet

  20. Basic Setup • I5 now has the Jakarta-Tomcat Servlet Engine installed. • Tomcat is an open source servlet engine. • Supports both Servlets and JSPs. • Maintained by the same group that develops the Apache Web Server. First Servlet

  21. Tomcat Set-up Send request to port 9712 Web Browser Tomcat Servlet Engine Hello World Servlet First Servlet

  22. Getting Started • Let’s take a look at today’s handout…. • Handout is also available online at: • http://ecerami.com/applied_fall_2001/handouts/i5_servlets.php4 • Change to handout: • CLASSPATH="/usr/local/jakarta-tomcat-3.2.3/lib/servlet.jar:." Add :. to Classpath! First Servlet

  23. Packaging Servlets First Servlet

  24. What is a Package? • Package: Group of related classes. • For example: • package coreservlets; • In real web sites, multiple programmers may be creating multiple servlets. • By dividing your code base into packages, it helps to modularize the code. • A very common practice in the real world, and used throughout our textbook. First Servlet

  25. Creating Packages • To create your own package, you need to follow three steps: • Move your .java files to a subdirectory that matches your package name. • For example, the text book uses the package name coreservlets. • You therefore need to create a coreservlets directory within ~/public_html/tomcat/WEB-INF/classes and place your code here. First Servlet

  26. Creating Packages • Insert a package statement in the first line of your class file • For example: package coreservlets; • Compile your Java code like this: type: servlets type: javac coreservlets/HelloWWW2.java First Servlet

  27. package coreservlets; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloWWW2 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println("<HTML>\n" + "<HEAD><TITLE>Hello WWW</TITLE></HEAD>\n" + "<BODY>\n" + "<H1>Hello WWW</H1>\n" + "<H2>This is a servlet within a package.</H2>"+ "</BODY></HTML>"); } } First Servlet

  28. Invoking a Packaged Servlet • To invoke a packaged servlet, you need to specify the package name and the servlet name: • http://host/servlet/packageName.servletName • For example, to access the HelloWWW2 servlet on ecerami.com: • http://ecerami.com/servlet/coreservlets.HelloWWW2 • To access the servlet on I5: • http://i5.nyu.edu:9712/eqc3844/servlet/coreservlets.HelloWWW2 First Servlet

  29. HTML Utilities First Servlet

  30. Servlet Utilities • Author of our text book has created a class called ServletUtilities. • This class contains some simple HTML utilities that you can use. • As we read further in the book, the author adds more utilities to this class. • Let’s first examine the ServletUtilities class, and then examine how to use it. First Servlet

  31. ServletUtilities.java • For now, let us examine just one method: headWithTitle(). • This method outputs: • HTML DOCTYPE, used to specify which version of HTML we are using. • The title of the page via the HTML <TITLE> tag. First Servlet

  32. package coreservlets; import javax.servlet.*; import javax.servlet.http.*; /** Some simple time savers. Note that most are static methods. */ public class ServletUtilities { public static final String DOCTYPE = "<!DOCTYPE HTML PUBLIC \"-//W3C//DTD HTML 4.0 " + "Transitional//EN\">"; public static String headWithTitle(String title) { return(DOCTYPE + "\n" + "<HTML>\n" + "<HEAD><TITLE>" + title + "</TITLE></HEAD>\n"); } ... } First Servlet

  33. Using Servlet Utilities • To use the Servlet Utilities class, you just need to call the headWithTitle() method: ServletUtilities.headWithTitle("Hello WWW"); • If you are placing your servlet in a different package, you also need to import coreservlets; First Servlet

  34. package coreservlets; import java.io.*; import javax.servlet.*; import javax.servlet.http.*; public class HelloWWW3 extends HttpServlet { public void doGet(HttpServletRequest request, HttpServletResponse response) throws ServletException, IOException { response.setContentType("text/html"); PrintWriter out = response.getWriter(); out.println(ServletUtilities.headWithTitle("Hello WWW") + "<BODY>\n" + "<H1>Hello WWW</H1>\n" + "</BODY></HTML>"); } } First Servlet

  35. Compiling • To compile this servlet: • Download ServletUtilities.java and copy to your coreservlets directory. • Download HelloWorldWWW3.java and copy to your coreservlets directory. • Type: • javac coreservlets/HelloWWW3.java • Then, open browser and go to: • http://i5.nyu.edu:9712/[NET-ID]/servlet/coreservlets.HelloWWW3 First Servlet

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