Sagar Vasule (Director Arcelor Technology Pvt. Ltd.): How to Install Apache Tomcat 7 and Get Start with Java Servlet Programming

1.1 Web Application (Webapp)

A web application (or webapp), unlike standalone application, runs over the Internet. Examples of webapps are google, amazon, ebay, facebook and twitter. A webapp is typically a 3-tier (ormulti-tier) client-server application, typically involving a database. Webapps run on HTTP Application Protocol over TCP/IP. Users access an webapp via a web browser (HTTP client).

A web database application requires five components, as illustrated below:

HTTP Server: E.g., Apache HTTP Server, Apache Tomcat HTTP Server, Microsoft IIS, and etc.
HTTP Client (or Web Browser): E.g., MSIE, FireFox, Chrome, and etc.
Database: E.g., Open-source MySQL, Apache Derby, mSQL, SQLite, PostgreSQL, OpenOffice's Base; Commercial Oracle, IBM DB2, SAP SyBase, MS SQL Server, MS Access.
Client-Side Programs: could be written in HTML Form, JavaScript, VBScript, Flash, and etc.
Server-Side Programs: could be written in Java Servlet/JSP, ASP, PHP, CGI, and etc.

The procedures are:

A user, via a web browser, issue a URL to an HTTP server to start a webapp.
A client-side program (such as an HTML form) is loaded into the browser of the client.
The user fills up the query criteria in the form.
The client-side program sends the query parameters to a server-side program.
The server-side program queries the database and returns the query result to the client.
The client-side program displays the result on the browser.

1.2 Hypertext Transfer Protocol (HTTP)

HTTP is an asynchronous request-response application-layer protocol. A client sends a request message to the server. The server returns a response message to the client. The syntax of the message is defined in the HTTP specification.

1.3 Apache Tomcat HTTP Server

Apache Tomcat is a Java-capable HTTP server, which could execute special Java programs known as Java Servlet and Java Server Pages (JSP). It is the official Reference Implementation (RI) for Java Servlets and JavaServer Pages (JSP) technologies. Tomcat is an open-source project, under the "Apache Software Foundation" (which also provides the most use, open-source, industrial-strength Apache HTTP Server). The mother site for Tomcat is http://tomcat.apache.org. Alternatively, you can find tomcat via the Apache mother site @ http://www.apache.org.

Tomcat was originally written by James Duncan Davison (then working in Sun), in 1998, based on an earlier Sun's server called Java Web Server (JWS). It started at version 3.0 after JSWDK 2.1 it replaced. Sun subsequently made Tomcat open-source and gave it to Apache.

The various Tomcat releases are:

Tomcat 3.x (1999): RI for Servlet 2.2, JSP 1.1
Tomcat 4.x (2001): RI for Servlet 2.3, JSP 1.2
Tomcat 5.x (2002): RI for Servlet 2.4, JSP 2.0
Tomcat 6.x (2006): RI for Servlet 2.5, JSP 2.1
Tomcat 7.x (2010): RI for Servlet 3.0, JSP 2.2, EL 2.2

2. How to Install Tomcat 7 and Get Started with Java Servlet Programming

2.1 STEP 1: Download and Install Tomcat

(For Mac Users) Read "How to Install Tomcat 7 on Mac" Step 1.

From http://tomcat.apache.org ⇒ Select "Downloads" ⇒ "Tomcat 7.0" ⇒ "7.0.{xx}" (where {xx} is the latest upgrade number) ⇒ "Binary Distributions" ⇒ "Core" ⇒ "zip" ⇒ "apache-tomcat-7.0.{xx}.zip".
UNZIP into a directory of your choice. DO NOT unzip onto the Desktop (because its path is hard to locate). I suggest using "d:\myproject". Tomcat will be unzipped into directory "d:\myproject\apache-tomcat-7.0.{xx}". For ease of use, we shall shorten and rename this directory to "d:\myproject\tomcat". Take note of Your Tomcat Installed Directory. Hereafter, I shall refer to the Tomcat installed directory as <TOMCAT_HOME> (or <CATALINA_HOME> - "Catalina" is the codename for Tomcat 5 and above).

For academic learning, I recommend "zip" version, as you could simply delete the entire directory when Tomcat is no longer needed (without running any un-installer). You are free to move or rename the Tomcat's installed directory. You can install (unzip) multiple copies of Tomcat in the same machine. For production, it is easier to use the installer to properly configure the Tomcat.

Take a quick look at the Tomcat installed directory. It contains the following sub-directories:

bin: Contains the startup and shutdown scripts.
conf: Contains the system-wide configuration files, such as server.xml, web.xml, context.xml, and tomcat-users.xml.
lib: Contains the Tomcat's system-wide JAR files, accessible by all webapps. You could also place external JAR file (such as MySQL JDBC Driver) here.
logs: Contains Tomcat's log files. You may need to check for error messages here.
webapps: contains the webapps to be deployed. You can also place the WAR file for deployment here.
work: Tomcat's working directory used by JSP, for JSP-to-Servlet conversion.
temp: Temporary files.

(For Advanced Windows 7/Vista Users) A better approach is to keep the original folder name, such as apache-tomcat-7.0.{xx}, but create a symlink called tomcat for convenience via command "mklink /D tomcat apache-tomcat-7.0.30". Symlink is available in Windows 7/Vista only.

2.2 STEP 2: Create an Environment Variable JAVA_HOME

(For Mac Users) Skip this step.

You need to create an environment variable called "JAVA_HOME" and set it to your JDK installed directory.

First, take note of your JDK installed directory. The default is "c:\program files\java\jdk1.7.0_{xx}", where {xx} is the latest upgrade number. It is important to verify your JDK installed directory before you proceed further.
Start a CMD shell, and issue the command "set JAVA_HOME" to check if variable JAVA_HOME has been set:
```
prompt> set JAVA_HOME
Environment variable JAVA_HOME not defined
```
If JAVA_HOME is set, check if it is set to your JDK installed directory correctly. Otherwsie, goto next step.
To set the environment variable JAVA_HOME in Windows 2000/XP/Vista/7: Goto "Start" button ⇒ Control Panel ⇒ System ⇒ (Vista/7) Advanced system settings ⇒ Switch to "Advanced" tab ⇒ Environment Variables ⇒ System Variables ⇒ "New" (or "Edit" for modification) ⇒ In "Variable Name", enter "JAVA_HOME" ⇒ In "Variable Value", enter your JDK installed directory (e.g., "c:\program file\java\jdk1.7.0_{xx}") - I strongly suggest that you copy/paste the directory name to avoid typo error!

To verify, RE-START a CMD shell (to refresh the environment) and issue:

prompt> set JAVA_HOME
JAVA_HOME=c:\program file\java\jdk1.7.0_{xx}   <== Verify that this is YOUR JDK installed directory

2.3 STEP 3: Configure Tomcat Server

(For Mac Users) Your Tomcat is located at "/Applications/tomcat" if you follow Step 1.

(Note for Windows Users) Programmers need to view the file extension (such as .txt, .ini). To display the file extension in Windows, goto "Control Panel" ⇒ Folder Options ⇒ Select "View" tab ⇒ Uncheck "Hide extensions for known file types".

The Tomcat configuration files are located in the "conf" sub-directory of your Tomcat installed directory (e.g. "d:\myproject\tomcat\conf"). There are 4 configuration XML files:

server.xml
web.xml
context.xml
tomcat-users.xml

Make a backup of the configuration files before you proceed.

Step 3(a) "conf\server.xml" - Set the TCP Port Number

Use a programming text editor (e.g., NotePad++, TextPad) to open the configuration file "server.xml", under the "conf" sub-directory of Tomcat installed directory (e.g. "d:\myproject\tomcat\conf").

The default TCP port number configured in Tomcat is 8080, you may choose any number between 1024 and 65535, which is not used by an existing application. We shall choose 9999 in this article. (For production server, you should use port 80, which is pre-assigned to HTTP server as the default port number.)

<!-- Define a non-SSL HTTP/1.1 Connector on port 8080 -->
<Connector port="9999" protocol="HTTP/1.1" connectionTimeout="20000" redirectPort="8443" />

Step 3(b) "conf\web.xml" - Enabling Directory Listing

Again, use a programming text editor to open the configuration file "web.xml", under the "conf" sub-directory of Tomcat installed directory.

We shall enable directory listing by changing "listings" from "false" to "true" for the "default" servlet. This is handy for test system, but not for production for security reasons.

<!-- The default servlet for all web applications, that serves static     -->
<!-- resources.  It processes all requests that are not mapped to other   -->
<!-- servlets with servlet mappings.                                      -->
<servlet>
  <servlet-name>default</servlet-name>
  <servlet-class>org.apache.catalina.servlets.DefaultServlet</servlet-class>
  <init-param>
    <param-name>debug</param-name>
    <param-value>0</param-value>
  </init-param>
  <init-param>
    <param-name>listings</param-name>
    <param-value>true</param-value>
  </init-param>
  <load-on-startup>1</load-on-startup>
</servlet>

Step 3(c) "conf\context.xml" - Enabling Automatic Reload

We shall add the attribute reloadable="true" to the <Context> element to enable automatic reload after code changes. Again, this is handy for test system but not for production, due to the overhead of detecting changes.

<Context reloadable="true">
   ......
</Context>

Step 3(d) "conf\tomcat-users.xml"

Enable the Tomcat's manager by adding the highlighted lines:

<tomcat-users>
  <role rolename="manager-gui"/>
  <user username="manager" password="xxxx" roles="manager-gui"/>
</tomcat-users>

2.4 STEP 4: Start Tomcat Server

The Tomcat's executable programs are kept in the "bin" sub-directory of the Tomcat installed directory (e.g., "d:\myproject\tomcat\bin").

Step 4(a) Start Server

Launch a CMD shell. Set the current directory to "<TOMCAT_HOME>\bin", and run "startup.bat" as follows:

// Change the current directory to Tomcat's "bin"
// Assume that Tomcat is installed in "d:\myproject\tomcat"
> d:                        // Change the current drive
> cd \myproject\tomcat\bin  // Change Directory to YOUR Tomcat's "bin" directory
 
// Start Tomcat Server
D:\myproject\tomcat\bin> startup

A new Tomcat console window appears. Study the messages on the console. Look out for the Tomcat's port number (double check that Tomcat is running on port 9999). Future error messages will be send to this console. System.out.println() issued by your Java servlets will also be sent to this console.

......
INFO: Initializing Coyote HTTP/1.1 on http-9999
......
INFO: Starting Coyote HTTP/1.1 on http-9999
......
INFO: Server startup in 699 ms

(For Mac Users) Tomcat is installed in "/Applications/tomcat". To start the Tomcat server, open a new "Terminal" and issue:

$ cd /Applications/tomcat/bin
$ ./catalina.sh run

(Skip Unless ...) Cannot Start Tomcat: Read "How to Debug".

Step 4(b) Start a Client to Access the Server

Start a browser (as a HTTP client), and issue URL "http://localhost:9999" to access the Tomcat server's welcome page. The hostname "localhost" (with IP address of 127.0.0.1) is meant for local loop-back testing. For users on the other machines over the net, they have to use the server's IP address or DNS domain name or hostname in the format of "http://serverHostnameOrIPAddress:9999".

Try issuing URL http://localhost:9999/examples to view the servlet and JSP examples. Try running some of the servlet examples.

Try issuing URL http://localhost:9999/manager/html to run the Tomcat Web Manager. Enter the username and password configured earlier in tomcat-users.xml.

Step 4(c) Shutdown Server

You can shutdown the server by either:

Press control-C on the Tomcat console, or
Run "<TOMCAT_HOME>\bin\shutdown.bat".

(For Mac Users) To shutdown the Tomcat server, press control-c (NOT command-c), or open a new "Terminal" and issue:

$ cd /Applications/tomcat/bin
$ ./shutdown.sh

WARNING: You MUST properly shutdown the Tomcat. DO NOT kill the cat by pushing the Close button. Otherwise, you might have problem restarting it.

2.5 STEP 5: Develop and Deploy a WebApp

Step 5(a) Create the Directory Structure for your WebApp

First of all, choose a name for your webapp. Let us call it "hello". Goto Tomcat's "webapps" sub-directory, and create the following directory structure for you webapp "hello" (as illustrated):

Under Tomcat's "webapps", create your web application root directory "hello" (i.e., "<TOMCAT_HOME>\webapps\hello").
Under "hello", create a sub-directory "WEB-INF" (case sensitive, a "dash" not an underscore) (i.e., "<TOMCAT_HOME>\webapps\hello\WEB-INF").
Under "WEB-INF", create a sub-directory "classes" (case sensitive, plural) (i.e., "<TOMCAT_HOME>\webapps\hello\WEB-INF\classes").

You need to keep your web resources (e.g., HTMLs, CSSs, images, scripts, servlets, JSPs) in the proper directories:

"hello": The is called the context root (or document base directory) of your web application. You should keep all your HTML files and resources visible to the web users (e.g., CSSs, images, scripts, JSPs) under this context root.
"hello\WEB-INF": This directory, although under the context root, is not visible to the web users. This is where you keep your application's web descriptor file "web.xml".
"hello\WEB-INF\classes": This is where you keep all the Java classes such as servlet class-files.

You should RE-START your Tomcat server. Check the Tomcat's console to confirm that "hello" application has been properly depolyed:

......
INFO: Deploying web application directory ...\hello
......

You can issue the following URL to access the web application "hello":

http://localhost:9999/hello

You should see the directory listing of the directory "<TOMCAT_HOME>\webapps\hello", which shall be empty - provided you have enabled directory listing in web.xml earlier.

Step 5(b) Write a Welcome Page

Create the following HTML page and save as "HelloHome.html" in your application's root directory "hello".

<html>
  <head><title>My Home Page</title></head>
  <body>
    <h1>My Name is so and so. This is my HOME.</h1>
  </body>
</html>

You can browse this page by issuing this URL:

http://localhost:9999/hello/HelloHome.html

Alternatively, you can issue an URL to your web application root "hello":

http://localhost:9999/hello

The server will return the directory listing of your base directory. You can then click on "HelloHome.html".

Rename "HelloHome.html" to "index.html", and issue a directory request again:

http://localhost:9999/hello

Now, the server will redirect the directory request to "index.html", if the root directory contains an "index.html", instead of serving the directory listing.

You can check out the home page of your peers by issuing (or access your own server from another machine):

http://YourPeerHostnameOrIPAddress:9999/hello
http://YourPeerHostnameOrIPAddress:9999/hello/HelloHome.html
http://YourPeerHostnameOrIPAddress:9999/hello/index.html

with a valid "YourPeerHostnameOrIPAddress", provided that your peer has started his/her web server. You can use command such as "ipconfig", "winipcfg", "ping" to find your IP address.

(Skip Unless...) The likely errors are "Unable to Connect", "Internet Explorer cannot display the web page", and "404 File Not Found". Read "How to Debug" section.

2.6 STEP 6: Write a "Hello-Word" Java Servlet

A servlet is Java program that runs inside a Java-capable HTTP Server, such as Apache Tomcat. A web user invokes a servlet by issuing an appropriate URL from a web browser (or HTTP client).

Before you proceed, I shall assume that you are familiar with Java Programming and have installed the followings:

JDK (Read "How to install JDK and Get Started").
A programming text editor, such as TextPad or Notepad++ (Read "Programming Text Editor"); or a Java IDE such as Eclipse or NetBeans (Read "How to Install Eclipse" or "How to Install NetBeans").

Step 6(a) Install Servlet API Library

Before we can write our first servlet, we need to install the Servlet API. Servlet API is not part of JDK (but belongs to Java EE). Tomcat also includes a copy of Servlets API.

COPY the Tomcat's Servlet API JAR-file located at "<TOMCAT_HOME>\lib\servlet-api.jar", (e.g., "d:\myproject\tomcat\lib\servlet-api.jar") into JDK's extension directory at "<JAVA_HOME>\jre\lib\ext", (e.g., "c:\program files\java\jdk1.7.0\jre\lib\ext").

(For Mac Users) COPY the Servlet API JAR file ("servlet-api.jar") from "/Applications/tomcat/lib" to the JDK's extension directory at "/Library/Java/Extension".

(For Advanced Users Only) You could include the Servlet API JAR-file in the CLASSPATH: or the JDK's extension directory: or in the javac's command-line option -cp <paths>.

Step 6(b) Write a "Hello-world" Java Servlet

A Java servlet is a Java program that runs inside a HTTP server. A web user invokes a servlet by issuing a URL from a browser (or HTTP client).

In this example, we are going to write a Java servlet called HelloServlet, which says "Hello, world!". We will then write a configuration such that web users can invoke this servlet by issuing URLhttp://hostname:port/hello/sayhello from the browser, as illustrated:

Write the following source codes called "HelloServlet.java" and save it under your application "classes" directory (i.e., "<TOMCAT_HOME>\webapps\hello\WEB-INF\classes\HelloServlet.java"). Compile the source into "HelloServlet.class". This servlet says "Hello", echos some request information, and prints a random number upon each request.

// To save as "<TOMCAT_HOME>\webapps\hello\WEB-INF\classes\HelloServlet.java"
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
 
public class HelloServlet extends HttpServlet {
   @Override
   public void doGet(HttpServletRequest request, HttpServletResponse response)
         throws IOException, ServletException {
 
      // Set the response MIME type of the response message
      response.setContentType("text/html");
      // Allocate a output writer to write the response message into the network socket
      PrintWriter out = response.getWriter();
 
      // Write the response message, in an HTML page
      try {
         out.println("<html>");
         out.println("<head><title>Hello, World</title></head>");
         out.println("<body>");
         out.println("<h1>Hello, world!</h1>");  // says Hello
         // Echo client's request information
         out.println("<p>Request URI: " + request.getRequestURI() + "</p>");
         out.println("<p>Protocol: " + request.getProtocol() + "</p>");
         out.println("<p>PathInfo: " + request.getPathInfo() + "</p>");
         out.println("<p>Remote Address: " + request.getRemoteAddr() + "</p>");
         // Generate a random number upon each request
         out.println("<p>A Random Number: <strong>" + Math.random() + "</strong></p>");
         out.println("</body></html>");
      } finally {
         out.close();  // Always close the output writer
      }
   }
}

(Skip Unless...) Read "Common Errors in Compiling Java Servlet".

Step 6(c) Configure Servlet's Request URL in "webapps\hello\WEB-INF\web.xml"

A web user invokes a servlet, which is kept in the web server, by issuing a request URL from the browser. We need to configure this request URL for our HelloServlet.

Create the following configuration file called "web.xml", and save it under "webapps\hello\WEB-INF" (i.e., "<TOMCAT_HOME>\webapps\hello\WEB-INF\web.xml").

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app version="3.0"
  xmlns="http://java.sun.com/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
 
   <!-- To save as "hello\WEB-INF\web.xml" -->
 
   <servlet>
      <servlet-name>HelloWorld</servlet-name>
      <servlet-class>HelloServlet</servlet-class>
   </servlet>
 
   <!-- Note: All <servlet> elements MUST be grouped together and
         placed IN FRONT of the <servlet-mapping> elements -->
 
   <servlet-mapping>
      <servlet-name>HelloWorld</servlet-name>
      <url-pattern>/sayhello</url-pattern>
   </servlet-mapping>
</web-app>

In the above configuration, a servlet having a class file "HelloServlet.class" is mapped to request URL "/sayhello" (via an arbitrary servlet-name "HelloWorld"), under this web application "hello". In other words, the complete request URL for this servlet is "http://hostname:port/hello/sayhello".

This configuration file, saved under your web application "hello", is applicable only to this particular web application "hello".

Restart your Tomcat server.

IMPORTANT: For EACH servlet, you need to write a pair of <servlet> and <servlet-mapping> elements with a common but arbitrary <servlet-name>. Take note that all the <servlet>elements MUST be grouped together and placed IN FRONT of the <servlet-mapping> elements.

Step 6(d) Invoke the Servlet

To run this servlet, start a browser, and issue the request URL configured earlier:

http://localhost:9999/hello/sayhello

You shall see the output of the servlet displayed in your web browser.

Refresh the browser, you shall see a new random number upon each refresh. In other word, the doGet() method of the servlet runs once per request.

Try "View Source" to look at the output received by the web users. Take note that the web users receive only the output of the servlet (generated via the out.println() statements). They have no access to the servlet programs (which may contain confidential information).

<html>
<head><title>Hello, World</title></head>
<body>
<h1>Hello, world!</h1>
<p>Request URI: /hello/sayhello</p>
<p>Protocol: HTTP/1.1</p>
<p>PathInfo: null</p>
<p>Remote Address: 127.0.0.1</p>
<p>A Random Number: <strong>0.3523682325749493</strong></p>
</body>
</html>

(Skip Unless...) The likely errors are "404 File Not Found" and "500 Internal Server Error". Read "How to debug" Section.

2.7 STEP 7: Write a Database Servlet

This section assumes that you are familiar with Java database programming and MySQL database server. Otherwise, read "How to Install MySQL and Get Started".

Step 7(a) Setup a Database on MySQL

Start your MySQL server. Take note of the server's port number. I shall assume that the MySQL server is running on port 8888 (whereas the Tomcat is running on port 9999).

// For Windows
> d:
> cd \myproject\mysql\bin
> mysqld --console
 
// For Mac
$ cd /usr/local/mysql/bin
$ sudo ./mysqld_safe --console

Start a MySQL client. I shall assume that there is a user called "myuser" with password "xxxx".

// For Windows
> d:
> cd \myproject\mysql\bin
> mysql -u myuser -p
 
// For Mac
$ cd /usr/local/mysql/bin
$ ./mysql -u myuser -p

Run the following SQL statements to create a database called "ebookshop", with a table called "books" with 5 columns: id, title, author, price, qty.

create database if not exists ebookshop;

use ebookshop;

drop table if exists books;
create table books (
   id     int,
   title  varchar(50),
   author varchar(50),
   price  float,
   qty    int,
   primary key (id));

insert into books values (1001, 'Java for dummies', 'JIYA', 11.11, 11);
insert into books values (1002, 'More Java for dummies', 'Vivek', 22.22, 22);
insert into books values (1003, 'More Java for more dummies', 'NILESH', 33.33, 33);
insert into books values (1004, 'A Cup of Java', 'SAGAR', 55.55, 55);
insert into books values (1005, 'A Teaspoon of Java', 'Kevin Jones', 66.66, 66);

select * from books;

Step 7(b) Install MySQL JDBC Driver

You need to download MySQL JDBC driver if you have not done so. Read "Installing the MySQL JDBC Driver".

(For Advanced Users Only) You could also place the MySQL driver jar-file "mysql-connector-java-5.1.{xx}-bin.jar" in Tomcat's "lib" directory.

Step 7(c) Write a Client-side HTML Form

Let's write an HTML script to create a query form with 3 checkboxes and a submit button, as illustrated below. Save the HTML file as “querybook.html” in your application root directory “<TOMCAT_HOME>\webapps\hello”.

<html>
<head>
  <title>Yet Another Bookshop</title>
</head>
<body>
  <h2>Yet Another Bookshop</h2>
  <form method="get" action="http://localhost:9999/hello/query">
    <b>Choose an author:</b>
    <input type="checkbox" name="author" value="Tan Ah Teck">Ah Teck
    <input type="checkbox" name="author" value="Mohammad Ali">Ali
    <input type="checkbox" name="author" value="Kumar">Kumar
    <input type="submit" value="Search">
  </form>
</body>
</html>

You can browse the HTML page by issuing the following URL:

http://localhost:9999/hello/querybook.html

Check a box (e.g., "Tan Ah Teck") and click the "Search" button. An HTTP GET request will be issued to the URL specified in the <form>'s "action" attribute. Observe the URL of the HTTP GET request:

http://localhost:9999/hello/query?author=Tan+Ah+Teck

The request consists of two part: a URL corresponding to the "action" attribute of the <form> tag, and the "name=value" pair extracted from the <input> tag, separated by a '?'. Take note that blanks are replaced by '+' (or %20), because blanks are not allowed in the URL.

If you check two boxes (e.g., "Tan Ah Teck" and "Mohammad Ali"), you will get this URL, which has two "name=value" pairs separated by an '&'.

http://localhost:9999/hello/query?author=Tan+Ah+Teck&author=Mohammad+Ali

You are expected to get an error "404 File Not Found", as you have yet to write the server-side program.

Step 7(d) Write the Server-side Database Query Servlet

The next step is to write a Java servlet, which responses to the client’s request by querying the database and returns the query results.

// To save as "<TOMCAT_HOME>\webapps\hello\WEB-INF\classes\QueryServlet.java".
import java.io.*;
import java.sql.*;
import javax.servlet.*;
import javax.servlet.http.*;
 
public class QueryServlet extends HttpServlet {  // JDK 6 and above only
 
   // The doGet() runs once per HTTP GET request to this servlet.
   @Override
   public void doGet(HttpServletRequest request, HttpServletResponse response)
               throws ServletException, IOException {
      // Set the MIME type for the response message
      response.setContentType("text/html");
      // Get a output writer to write the response message into the network socket
      PrintWriter out = response.getWriter();
 
      Connection conn = null;
      Statement stmt = null;
      try {
         // Step 1: Allocate a database Connection object
         conn = DriverManager.getConnection(
            "jdbc:mysql://localhost:8888/ebookshop", "myuser", "xxxx"); // <== Check!
            // database-URL(hostname, port, default database), username, password
 
         // Step 2: Allocate a Statement object within the Connection
         stmt = conn.createStatement();
 
         // Step 3: Execute a SQL SELECT query
         String sqlStr = "select * from books where author = "
              + "'" + request.getParameter("author") + "'"
              + " and qty > 0 order by price desc";
 
         // Print an HTML page as the output of the query
         out.println("<html><head><title>Query Response</title></head><body>");
         out.println("<h3>Thank you for your query.</h3>");
         out.println("<p>You query is: " + sqlStr + "</p>"); // Echo for debugging
         ResultSet rset = stmt.executeQuery(sqlStr);  // Send the query to the server
 
         // Step 4: Process the query result set
         int count = 0;
         while(rset.next()) {
            // Print a paragraph <p>...</p> for each record
            out.println("<p>" + rset.getString("author")
                 + ", " + rset.getString("title")
                 + ", $" + rset.getDouble("price") + "</p>");
            count++;
         }
         out.println("<p>==== " + count + " records found =====</p>");
         out.println("</body></html>");
     } catch (SQLException ex) {
        ex.printStackTrace();
     } finally {
        out.close();  // Close the output writer
        try {
           // Step 5: Close the resources
           if (stmt != null) stmt.close();
           if (conn != null) conn.close();
        } catch (SQLException ex) {
           ex.printStackTrace();
        }
     }
   }
}

Step 7(e) Configure the Request URL for the Servlet

Open the configuration file "web.xml" of your application "hello" that you have created earlier for the HelloServlet, i.e., "<TOMCAT_HOME>\webapps\hello\WEB-INF\web.xml". Add the lines that are shown in red at the correct locations.

<?xml version="1.0" encoding="ISO-8859-1"?>
<web-app version="3.0"
  xmlns="http://java.sun.com/xml/ns/javaee"
  xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
  xsi:schemaLocation="http://java.sun.com/xml/ns/javaee http://java.sun.com/xml/ns/javaee/web-app_3_0.xsd">
 
   <!-- To save as "hello\WEB-INF\web.xml" -->
 
   <servlet>
      <servlet-name>HelloWorld</servlet-name>
      <servlet-class>HelloServlet</servlet-class>
   </servlet>
 
   <servlet>
      <servlet-name>UserQuery</servlet-name>
      <servlet-class>QueryServlet</servlet-class>
   </servlet>
 
   <!-- Note: All <servlet> elements MUST be grouped together and
         placed IN FRONT of the <servlet-mapping> elements -->
 
   <servlet-mapping>
      <servlet-name>HelloWorld</servlet-name>
      <url-pattern>/sayhello</url-pattern>
   </servlet-mapping>
 
   <servlet-mapping>
      <servlet-name>UserQuery</servlet-name>
      <url-pattern>/query</url-pattern>
   </servlet-mapping>
</web-app>

The above lines configure the following URL to invoke QueryServlet:

http://localhost:9999/hello/query

Step 7(f) Invoke the Servlet from the Client-Side Form

Issue the following URL to browse the HMTL form "querybook.html" that you have created earlier:

http://localhost:9999/hello/querybook.html

Select an author (e.g., "Tan Ah Teck") and click the submit button, which activates the following URL coded in the <form>'s "action" attribute, together with the name=value pair:

http://localhost:9999/hello/query?author=Tan+Ah+Teck

This URL "/query" triggers QueryServlet. The QueryServlet retrieves the name=value pair of "author=Tan+Ah+Teck". Inside the QueryServlet, the methodrequest.getParameter("author") returns "Tan Ah Teck", which is inserted into the SQL SELECT command to query the database. The processed query result is then written to the client as an HTML document.

(Skip Unless...) The likely errors are "404 File Not Found" and "500 Internal Server Error". Read "How to debug" Section.

2.8 (Advanced) Deploying Servlet using @WebServlet (Servlet 3.0 on Tomcat 7)

Servlet 3.0, which is supported by Tomcat 7, introduces the @WebServlet annotation, which greatly simplifies the deployment of servlets. You no longer need to write the deployment descriptor in "web.xml". Instead, you can use the @WebServlet annotation to specify the url mapping.

For example, let us write a new servlet called AnotherHelloServlet.java, by modifying the HelloServlet.java written earlier, with url mapping of "sayhi".

// To save as "<TOMCAT_HOME>\webapps\hello\WEB-INF\classes\AnotherHelloServlet.java"
import java.io.*;
import javax.servlet.*;
import javax.servlet.http.*;
import javax.servlet.annotation.*;
 
@WebServlet("/sayhi")
public class AnotherHelloServlet extends HttpServlet {
   @Override
   public void doGet(HttpServletRequest request, HttpServletResponse response)
         throws IOException, ServletException {
 
      // Set the response MIME type
      response.setContentType("text/html;charset=UTF-8");
      // Allocate a output writer to write the response message into the network socket
      PrintWriter out = response.getWriter();
 
      // Write the response message, in an HTML page
      try {
         out.println("<html>");
         out.println("<head><title>Hello, World</title></head>");
         out.println("<body>");
         out.println("<h1>Hello world, again!</h1>");  // says Hello
         // Echo client's request information
         out.println("<p>Request URI: " + request.getRequestURI() + "</p>");
         out.println("<p>Protocol: " + request.getProtocol() + "</p>");
         out.println("<p>PathInfo: " + request.getPathInfo() + "</p>");
         out.println("<p>Remote Address: " + request.getRemoteAddr() + "</p>");
         // Generate a random number upon each request
         out.println("<p>A Random Number: <strong>" + Math.random() + "</strong></p>");
         out.println("</body></html>");
      } finally {
         out.close();  // Always close the output writer
      }
   }
}

In Line 7, the annotation @WebServlet("/sayhi") is used to declare the URL mapping for this servlet, i.e., http://localhost:9999/hello/sayhi. There is no need to provide any more configuration in "web.xml"!

3. How to Debug?

"Everything that can possibly go wrong will go wrong." The most important thing to do is to find the ERROR MESSAGE!!!

Always...

Refresh your browser using Cntl-F5 (instead of refresh button or simply F5) to get a fresh copy, instead of from the cache.
You may re-start your Tomcat server. You may also re-start your browser to clear the cache.
Check your spelling! Always assume that all programs are case-sensitive. Don't type, copy and paste if possible!
and MOST IMPORTANTLY - Find the ERROR MESSAGE!!!
1. Check the Error Messages on Tomcat's Console. Most of the error messages have a few screens of lines. You need to scroll up slowly from the last line to look for the FIRST LINE of the error messages.
2. Check the Tomcat's log files, located at "<TOMCAT_HOME>\logs". The "catalina.yyyy-mm-dd.log" shows the Tomcat's startup messages. Also check the "localhost.yyyy-mm-dd.log".
If things were running fine until the lightning strikes, ask yourself "What have I changed?"

Cannot Start Tomcat - Tomcat's Console Flashes and Disappears

Try running the script "configtest.bat" (for Windows) or "./configtest.sh" (for Mac/Linux) to check your configuration files.
Check the Tomcat's log files for error messages. The log files are located at "<TOMCAT_HOME>\logs". The "catalina.{yyyy-mm-dd}.log" shows the Tomcat's startup messages. Also check the "localhost.{yyyy-mm-dd}.log".
If the error messages indicate that another Tomcat instance is running (java.net.BindException: Address already in use: JVM_Bind), kill the Tomcat process (see below).
If the error messages indicate that another application is running on the Tomcat's port numbers, then you need to change the Tomcat's port number in server.xml. You can issue command "netstat -an" to check the status of all the ports.

Locating/Killing Tomcat's Process

In windows, start "Task Manager", Tomcat run as a "process" named "java.exe". You may need to kill the process.
In Lunix/Mac, you need to issue "ps -ef | grep tomcat" to locate the Tomcat process. Note the process ID (pid), and you can kill the Tomcat process via "kill -9 pid".

(Firefox) Unable to Connect
(IE) Internet Explorer cannot display the webpage
(Chrome) Oops! Google Chrome could not connect to ...
(Safari) Safari can't connect to the server

Cause: You are simply not connecting to your Tomcat.

Solution:

Check if your Tomcat server has been started?
Check the hostname and port number, separated by a colon ':', of your URL (http://localhost:9999/...).

Error 404 File Not Found

Cause: You have connected to your Tomcat. But Tomcat server cannot find the HTML file or Servlet that your requested.