Lab Notes for Fri Apr 10

/* <applet code=Unicorn.class width=400 height=400> 
   </applet>
 */ 

import java.applet.*; 
import java.awt.*; 

public class Unicorn extends Applet {
  int i = 0; 
  public void paint(Graphics g) {
    g.drawOval(30, 30, 50, 50); 
    i = i + 1; 
    System.out.println("paint has been called " + i + " times."); 
  } 
}


Analogy: 

A servlet is like an applet. 
The equivalent of paint is doGet.
(paint gets a Graphics argument, doGet gets an HttpServletRequest and
an HttpServletResponse. Somebody has defined these types already...)
appletviewer is like the servletRunner which is now deep into Tomcat's
context manager). 


import javax.servlet.*;
import javax.servlet.http.*; // for HttpServlet
import java.io.*; // for PrintWriter 

public class Unicorn extends HttpServlet {
  public void doGet(HttpServletRequest request, HttpServletResponse response) {
    response.setContentType("text/html");  
    PrintWriter out = response.getWriter();
    out.println(" How are you? ");  
    
  } 
}

So this is my servlet.

I put it into a context's WEB-INF/classes folder. 

So I create a chat context in $CATALINA_HOME/webapps. 

The structure of the context is: WEB-INF/classes
                                        /lib

I place my Unicorn.java in $CATALINA_HOME/webapps/chat/WEB-INF/classes/Unicorn.java

I compile and I get this:

-bash-3.2$ javac Unicorn.java 
Unicorn.java:8: unreported exception java.io.IOException; must be caught or declared to be thrown
    PrintWriter out = response.getWriter();
                                        ^
1 error
-bash-3.2$ pico Unicorn.java

Which means I need to change the servlet to this: 

import javax.servlet.*;
import javax.servlet.http.*; // for HttpServlet
import java.io.*; // for PrintWriter

public class Unicorn extends HttpServlet {
  public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException {
    response.setContentType("text/html");
    PrintWriter out = response.getWriter();
    out.println(" How are you? ");

  }
}

So I throw the exception that getWriter() might actually raise. 

I also prepare myself for other types of exceptions, and I have:

import javax.servlet.*; // for ServletException
import javax.servlet.http.*; // for HttpServlet
import java.io.*; // for PrintWriter

public class Unicorn extends HttpServlet {
  public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, 
                                                                                     ServletException { 
    response.setContentType("text/html");  
    PrintWriter out = response.getWriter();
    out.println(" How are you? ");
   
  }
}

Compile and deploy it, then call it. 

For this you need to describe to Tomcat in the context where the servlet is
how Tomcat should find and load/activate the sevrlet when the call comes from
the user. This is done by preparing two entries in the web.xml file of the WEB-INF
folder of the context where you have the servlet. For us chat/WEB-INF/web.xml:

<?xml version="1.0" encoding="ISO-8859-1"?>

<!DOCTYPE web-app
    PUBLIC "-//Sun Microsystems, Inc.//DTD Web Application 2.3//EN"
    "http://java.sun.com/dtd/web-app_2_3.dtd">

<web-app>

    <servlet>
        <servlet-name>masonv1.3</servlet-name>
        <servlet-class>Unicorn</servlet-class>
    </servlet>

    <servlet-mapping>
        <servlet-name>masonv1.3</servlet-name>
        <url-pattern>/servlet/Mason</url-pattern>
    </servlet-mapping>


</web-app>

Once I create and save this file I can access the servlet:

http://silo.cs.indiana.edu:47067/chat/servlet/Mason

I get an error, I think about it and fix it: 

-bash-3.2$ clear
-bash-3.2$ javac
-bash-3.2$ which javac
/usr/bin/javac
-bash-3.2$ javac -version
javac 1.6.0_07
-bash-3.2$ pwd
/u/dgerman/apache-tomcat-5.5.17/webapps/chat/WEB-INF
-bash-3.2$ cd classes/
-bash-3.2$ ls -l
total 16
-rw-r--r-- 1 dgerman faculty 676 Apr 10 11:31 Unicorn.class
-rw-r--r-- 1 dgerman faculty 520 Apr 10 11:31 Unicorn.java
-bash-3.2$ $JAVA_HOME/bin/javac Unicorn.java 
-bash-3.2$ 

Remember you need to reload the context using Tomcat manager. 

Now I write a new, more complex servlet: 

import javax.servlet.*; // for ServletException
import javax.servlet.http.*; // for HttpServlet
import java.io.*; // for PrintWriter

public class Unicorn extends HttpServlet {
  public void doGet(HttpServletRequest request, HttpServletResponse response) throws IOException, 
                                                                                     ServletException { 
    response.setContentType("text/html");  
    PrintWriter out = response.getWriter();

    String message = request.getParameter("message"), 
           n1      = request.getParameter("n1"), 
           n2      = request.getParameter("n2"), 
           correct = request.getParameter("correct"),
           total   = request.getParameter("total"); 

    if (message == null) {
      message = "Welcome."; 
      n1 = (int)(Math.random() * 100 - 50) + ""; 
      n2 = (int)(Math.random() * 100 - 50) + ""; 
      correct = "0"; 
      total = "0"; 
      message += " Your current score: " + correct + " / " + total; 
    } else {
      if (Integer.parseInt(n1) + Integer.parseInt(n2) == Integer.parseInt(request.getParameter("answer"))) {
        message = "Very good."; 
        correct = (Integer.parseInt(correct) + 1) + ""; 
      } else { 
        message = "No, that's not it."; 
      } 
      total = (Integer.parseInt(total) + 1) + ""; 
      n1 = (int)(Math.random() * 100 - 50) + ""; 
      n2 = (int)(Math.random() * 100 - 50) + ""; 
      message += " Your current score: " + correct + " / " + total; 
    } 

    out.println(

"<form> " + 
  message + "<p>" + 
  "What is " + n1 + " + " + n2 + "? Answer: <input type=text name=answer size=4><p>" + 
  "Press <input type=submit value=Proceed> to move on. " + 
  "<input type=hidden name=message value=\"" + message + "\">" + 
  "<input type=hidden name=correct value=\"" + correct + "\">" + 
  "<input type=hidden name=total   value=\"" + total   + "\">" + 
  "<input type=hidden name=n1      value=\"" + n1      + "\">" + 
  "<input type=hidden name=n2      value=\"" + n2      + "\">" + 
"</form> "

    ); 
   
  }
}


We transform this into $CATALINA_HOME/webapps/chat/unicorn.jsp

<%
    String message = request.getParameter("message"), 
           n1      = request.getParameter("n1"), 
           n2      = request.getParameter("n2"), 
           correct = request.getParameter("correct"),
           total   = request.getParameter("total"); 

    if (message == null) {
      message = "Welcome."; 
      n1 = (int)(Math.random() * 100 - 50) + ""; 
      n2 = (int)(Math.random() * 100 - 50) + ""; 
      correct = "0"; 
      total = "0"; 
      message += " Your current score: " + correct + " / " + total; 
    } else {
      if (Integer.parseInt(n1) + Integer.parseInt(n2) == Integer.parseInt(request.getParameter("answer"))) {
        message = "Very good."; 
        correct = (Integer.parseInt(correct) + 1) + ""; 
      } else { 
        message = "No, that's not it."; 
      } 
      total = (Integer.parseInt(total) + 1) + ""; 
      n1 = (int)(Math.random() * 100 - 50) + ""; 
      n2 = (int)(Math.random() * 100 - 50) + ""; 
      message += " Your current score: " + correct + " / " + total; 
    } 
 %>

<form> 
  <%=message%> <p>
  What is <%=n1%> + <%=n2%>? Answer: <input type=text name=answer size=4><p>
  Press <input type=submit value=Proceed> to move on. 
  <input type=hidden name=message value="<%=message%>">
  <input type=hidden name=correct value="<%=correct%>">
  <input type=hidden name=total   value="<%=total%>">
  <input type=hidden name=n1      value="<%=n1%>">
  <input type=hidden name=n2      value="<%=n2%>">
</form> 

As you can tell, the conversion is immediate. 

After we call it and it runs we notice that a servlet has been created and compiled
automatically for us, implementing our .jsp (see the code we wrote embedded inside): 

-bash-3.2$ more /u/dgerman/apache-tomcat-5.5.17/work/Catalina/localhost/chat/org/apache/jsp/unicorn_jsp.java 
package org.apache.jsp;

import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.jsp.*;

public final class unicorn_jsp extends org.apache.jasper.runtime.HttpJspBase
    implements org.apache.jasper.runtime.JspSourceDependent {

  private static java.util.List _jspx_dependants;

  public Object getDependants() {
    return _jspx_dependants;
  }

  public void _jspService(HttpServletRequest request, HttpServletResponse response)
        throws java.io.IOException, ServletException {

    JspFactory _jspxFactory = null;
    PageContext pageContext = null;
    HttpSession session = null;
    ServletContext application = null;
    ServletConfig config = null;
    JspWriter out = null;
    Object page = this;
    JspWriter _jspx_out = null;
    PageContext _jspx_page_context = null;


    try {
      _jspxFactory = JspFactory.getDefaultFactory();
      response.setContentType("text/html");
      pageContext = _jspxFactory.getPageContext(this, request, response,
                        null, true, 8192, true);
      _jspx_page_context = pageContext;
      application = pageContext.getServletContext();
      config = pageContext.getServletConfig();
      session = pageContext.getSession();
      out = pageContext.getOut();
      _jspx_out = out;


    String message = request.getParameter("message"), 
           n1      = request.getParameter("n1"), 
           n2      = request.getParameter("n2"), 
           correct = request.getParameter("correct"),
           total   = request.getParameter("total"); 

    if (message == null) {
      message = "Welcome."; 
      n1 = (int)(Math.random() * 100 - 50) + ""; 
      n2 = (int)(Math.random() * 100 - 50) + ""; 
      correct = "0"; 
      total = "0"; 
      message += " Your current score: " + correct + " / " + total; 
    } else {
      if (Integer.parseInt(n1) + Integer.parseInt(n2) == Integer.parseInt(request.getParameter("answer"))) {
        message = "Very good."; 
        correct = (Integer.parseInt(correct) + 1) + ""; 
      } else { 
        message = "No, that's not it."; 
      } 
      total = (Integer.parseInt(total) + 1) + ""; 
      n1 = (int)(Math.random() * 100 - 50) + ""; 
      n2 = (int)(Math.random() * 100 - 50) + ""; 
      message += " Your current score: " + correct + " / " + total; 
    } 
 
      out.write("\n");
      out.write("\n");
      out.write("<form> \n");
      out.write("  ");
      out.print(message);
      out.write(" <p>\n");
      out.write("  What is ");
      out.print(n1);
      out.write(' ');
      out.write('+');
      out.write(' ');
      out.print(n2);
      out.write("? Answer: <input type=text name=answer size=4><p>\n");
      out.write("  Press <input type=submit value=Proceed> to move on. \n");
      out.write("  <input type=hidden name=message value=\"");
      out.print(message);
      out.write("\">\n");
      out.write("  <input type=hidden name=correct value=\"");
      out.print(correct);
      out.write("\">\n");
      out.write("  <input type=hidden name=total   value=\"");
      out.print(total);
      out.write("\">\n");
      out.write("  <input type=hidden name=n1      value=\"");
      out.print(n1);
      out.write("\">\n");
      out.write("  <input type=hidden name=n2      value=\"");
      out.print(n2);
      out.write("\">\n");
      out.write("</form> \n");
      out.write("\n");
      out.write("\n");
      out.write("\n");
    } catch (Throwable t) {
      if (!(t instanceof SkipPageException)){
        out = _jspx_out;
        if (out != null && out.getBufferSize() != 0)
          out.clearBuffer();
        if (_jspx_page_context != null) _jspx_page_context.handlePageException(t);
      }
    } finally {
      if (_jspxFactory != null) _jspxFactory.releasePageContext(_jspx_page_context);
    }
  }
}
-bash-3.2$