In order to write and compile a Java programs, applets or servlets one must download the Java Development Kit (JDK) which provides a compiler, class libraries and tools to compile and run Java code. In this tutorial we will use the Oracle/Sun JDK but I'm sure any will do. See YoLinux Java for a list of available JDK's for Linux. Linux also ships with OpenJDK which will also work with Tomcat.

One must choose complimentary versions of Tomcat and Java as they must match appropriately:

• EL is the "Unified Expression Language" used for embedding expressions into JSP web pages and based on JSTL to employ XML based scripting.
• JASPIC provides an interface to authentication implementations with Tomcat

• Java Standard Edition (SE) JDK - Main page download and info

• The Java Runtime Environment (JRE) will be adequate to configure the server environment but the Software Development Kit (SDK) is required if one wants to write and compile Java programs. The Java SDK is available in RPM and tar format.
• Tomcat version require compatible Java versions. Thus use Java version 1.8 or 1.7 for use with Tomcat version 8, use Java 1.7 for use with Tomcat 7 and Java 1.6 for use with Tomcat 6.

• Java JDK 1.8: jdk-8u77-linux-x64.rpm (Red Hat, Fedora, CentOS or Suse systems) or jdk-8u77-linux-x64.tar.gz (all other versions of Linux).
• Java JDK 1.7: jdk-7u40-linux-x64.rpm

RHEL Install: rpm -ivh jdk-8u77-linux-x64.rpm

SDK installed in /usr/java/latest/.

Configuration:

#!/bin/bash
if [ -d /usr/java/latest ]
then
    export PATH=/usr/java/latest/bin:$PATH
    export JAVA_HOME=/usr/java/latest
    export CLASSPATH=/usr/java/latest/lib/tools.jar:/usr/java/latest/jre/lib/rt.jar
    export MANPATH=$JAVA_HOME/man:/opt/man:$MANPATH
fi

Test:

Run:

[prompt]$ java Test
Hello world

OpenJDK can be installed using package management:

• RHEL/CentOS/Fedora:
  • Java 1.7: yum install java-1.7.0-openjdk java-1.7.0-openjdk-devel java-1.7.0-openjdk-javadoc alternatives
  • Java 1.6: yum install java-1.6.0-openjdk java-1.6.0-openjdk-devel java-1.6.0-openjdk-javadoc alternatives
    (Note: JAVA_HOME=/usr/lib/jvm/java-1.6.0-openjdk-1.6.0.0/)
• Ubuntu: (Ubuntu 12.4: add repository: add-apt-repository ppa:openjdk-r/ppa, not required for 16.04+)
  • Java 1.9: apt-get install openjdk-9-jdk
  • Java 1.8: apt-get install openjdk-8-jdk
  • Java 1.7: apt-get install openjdk-7-jdk
  • Java 1.6: apt-get install openjdk-6-jdk
  To support multiple versions, select a default: update-alternatives --config java
  (Note: JAVA_HOME=/usr/lib/jvm/java-7-openjdk/)

Links:

• Oracle 1.9 install doc: Java installation instructions
• Oracle 1.9 docs:Java Documentation
• Oracle 1.8 install doc: Java installation instructions
• Oracle 1.8 docs:Java Documentation
• Oracle 1.7 install doc: Java installation instructions
• Oracle 1.7 docs:Java Documentation

Tomcat is the Java "container" or processor for Java Servlets and Java Server Pages (JSP). Note also that Java must be installed in order for Tomcat to operate. (See previous section above) This tutorial will focus on the use of Tomcat with Apache but it should be noted that the default Tomcat installation enables Tomcat to be a stand-alone http web server and servlet container.

Tomcat can be directly downloaded from the Apache Tomcat website or installed by the package manager from the package repository. By default RHEL6 comes with Tomcat 6.0.

Apache Tomcat Java Servlet and JSP container home page

Tomcat 9 requires Java 1.8. Tomcat 8 requires Java 1.7 or 1.8 JDK installation. Tomcat 8 will NOT run with Java version 1.6!

Ubuntu 18.04 users have the option of installing using the package manager: sudo apt-get install tomcat9

Ubuntu 16.06 users have the option of installing using the package manager: sudo apt-get install tomcat8

All versions of Linux can install using the tar file available from the Apache Tomcat website.

Download: apache-tomcat-8.0.0-RC1.tar.gz

• cd /opt
• tar xzf ~/Downloads/apache-tomcat-8.0.0-RC1.tar.gz
  (Result: /opt/apache-tomcat-8.0.0-RC1/)

Stop: /opt/apache-tomcat-8.0.30/bin/shutdown.sh

• cd /opt/apache-tomcat-8.0.0-RC1/bin
• tar xzf commons-daemon-native.tar.gz
• cd commons-daemon-1.0.15-native-src/unix
• ./configure --with-java=/usr/java/latest
• make
• cp jsvc ../..
• cd ../..

#!/bin/bash
if [ -d /usr/java/latest ]
then
  export PATH=/usr/java/latest/bin:$PATH
  export JAVA_HOME=/usr/java/latest
  export CLASSPATH=/usr/java/latest/lib/tools.jar:./
  export MANPATH=$JAVA_HOME/man:/opt/man:$MANPATH
fi

export CATALINA_HOME=/opt/apache-tomcat-8.0.0-RC1/
    CATALINA_BASE=$CATALINA_HOME
    cd $CATALINA_HOME
    ./bin/jsvc \
        -classpath $CATALINA_HOME/bin/bootstrap.jar:$CATALINA_HOME/bin/tomcat-juli.jar \
        -outfile $CATALINA_BASE/logs/catalina.out \
        -errfile $CATALINA_BASE/logs/catalina.err \
        -Dcatalina.home=$CATALINA_HOME \
        -Dcatalina.base=$CATALINA_BASE \
        -Djava.util.logging.manager=org.apache.juli.ClassLoaderLogManager \
        -Djava.util.logging.config.file=$CATALINA_BASE/conf/logging.properties \
        org.apache.catalina.startup.Bootstrap

[root@hostname apache-tomcat-8.0.0-RC1]# /opt/bin/tomcat8start.sh
Using CATALINA_BASE:   /opt/apache-tomcat-8.0.0-RC1/
Using CATALINA_HOME:   /opt/apache-tomcat-8.0.0-RC1/
Using CATALINA_TMPDIR: /opt/apache-tomcat-8.0.0-RC1//temp
Using JRE_HOME:        /usr/java/latest
Using CLASSPATH:
/opt/apache-tomcat-8.0.0-RC1//bin/bootstrap.jar:/opt/apache-tomcat-8.0.0-RC1//bin/tomcat-juli.jar

See all jsvc daemon options with "jsvc --help"

Pitfall: If you get a ClassNotFoundException or a NoClassDefFoundError for a Commons-Daemon class, add the Commons-Daemon JAR to your classpath or use -cp argument when launching jsvc.

Run:

Place WAR file applications in /opt/apache-tomcat-8.0.0-RC1/webapps/ and access via the URL http://localhost:8080/

Try the example applications in /opt/apache-tomcat-8.0.0-RC1/webapps/examples/ and access via the URL http://localhost:8080/examples/

If executing a single Java class file like the HelloWorld example below, place in the directory /opt/apache-tomcat-8.0.0-RC1/webapps/ROOT/WEB-INF/classes/ and access via the URL http://localhost:8080/HelloWorld

Tomcat Configuration:

This installation uses Tomcat RPM packages furnished with RHEL6.

• tomcat6-6.0.24-45.el6.noarch
• tomcat6-lib-6.0.24-45.el6.noarch
• tomcat6-el-2.1-api-6.0.24-45.el6.noarch
• tomcat6-servlet-2.5-api-6.0.24-45.el6.noarch
• tomcat6-jsp-2.1-api-6.0.24-45.el6.noarch
• apache-tomcat-apis-0.1-1.el6.noarch
• jakarta-commons-pool-tomcat5-1.3-12.7.el6.x86_64
• jakarta-commons-dbcp-tomcat5-1.2.1-13.8.el6.noarch

Documentation:

• Tomcat 6.0 Documentation
• Also look at local documentation: /usr/share/doc/tomcat6-6.0.24/ for release notes.
• Local app documentation files: /var/tomcat6/webapps/tomcat-docs/

Config files:

<?xml version='1.0' encoding='utf-8'?>
 <tomcat-users>
 <role rolename="manager"/>
 <user name="tomcat" password="supersecret" roles="manager" />
 </tomcat-users>
                                                

• cd /var/lib/tomcat6/webapps/
• mkdir -p WEB-INF/classes
• mkdir WEB-INF/lib

Start service: service tomcat6 start (or: /etc/init.d/tomcat6 start)

• Tomcat will execute as user tomcat
• To add to init boot process: /sbin/chkconfig --add tomcat6
• Tomcat Log files: /var/tomcat6/logs/
• JAR files and JDBC drivers: /var/tomcat6/webapps/examples/WEB-INF/lib/

Tomcat Manager:

Use URL to list web applications: http://localhost:8080/manager/list

Documentation: http://localhost:8080/tomcat-docs/manager-howto.html - [Web]

Hello World Tomcat servlet example WAR file:

• mkdir -p src/main/java/com/megacorp/projectx
• mkdir -p src/main/webapp/WEB-INF

File: src/main/java/com/megacorp/projectx/HelloWorld.java

[prompt]$ javac -cp /opt/apache-tomcat-8.0.30/lib/servlet-api.jar HelloWorld.java

• RHEL6: /var/lib/tomcat6/webapps/examples/WEB-INF/classes/com/megacorp/projectx/HelloWorld.class
• Tomcat 8: /opt/apache-tomcat-8.0.30/webapps/examples/WEB-INF/classes/com/megacorp/projectx/HelloWorld.class

• RHEL6: /var/lib/tomcat6/webapps/examples/WEB-INF/web.xml
• Tomcat 8: /opt/apache-tomcat-8.0.30/webapps/examples/WEB-INF/web.xml

Note:

• Java class com.mycompany.mypackage.MyServlet would be stored in /WEB-INF/classes/com/mycompany/mypackage/MyServlet.class
• Libraries (JAR files, JDBC drivers, etc) held in /WEB-INF/lib/ if one follows standard configuration.

Tomcat Test: http://localhost:8080/examples/servlets/servlet/HelloWorld
Don't expect a lot. It just generates a web page dynamically which states "Hello World".

Note:

• The mapping of path /var/tomcat6/webapps/examples/WEB-INF/classes/ to the URL /examples/servlet/ is defined in /var/tomcat6/conf/web.xml. Look for the XML tag "<servlet-mapping>".
• Many examples with source code are included with Tomcat. See: http://localhost:8080/examples/servlets/index.html

Typically Java web applications are deployed as a single deployable file known as a "WAR" file (Web ARchive). Java web applications typically are comprised of multiple servlet classes, HTML and JSP pages with accompanying images and libraries. All of these can be deployed as a single Java WAR file. We will use Apache ANT and Apache Maven to build and deploy a WAR file for the HelloWorld servlet web application.

build.xml
src/main/java/com/megacorp/projectx/HelloWorld.java
src/main/webapp/WEB-INF/web.xml
src/main/webapp/index.html

• WebApp/build.xml: This is the Apache ANT build script
• WebApp/src/main/java/com/megacorp/projectx/HelloWorld.java: The source code to generate WEB-INF/classes/com/megacorp/projectx/HelloWorld.class, the executable Java byte code
• WebApp/src/main/webapp/index.html: The default home page of the web application

• Apache ANT: ant install
  see build.xml below.
• Apache Maven: mvn install
  see pom.xml below.

This will generate the Java WAR file myservlet.war which when deployed to the Apache TomCat webapp deployment directory $TOMCAT_HOME/webapp/ will be deployed as the Java web application http://localhost:8080/myservlet/

Note that the name of the WAR file becomes a distinguishing path in the URL. Select the hyperlink on the page to execute the servlet.

For more on building a Java Web Archive (WAR) file and examples see the Java WAR files tutorial.

Build and deply the web application: ant install

Build and install: mvn install

For more on Apache Maven, see the Yolinux Apache Maven tutorial.

Apache httpd is a fast and configurable web server whic can act as a proxy front-end on port 80 to Tomcat dynamic content. This section covers using Apache httpd as the primary web server but using Tomcat to process JSP and Servlets using AJP. Connect to Apache httpd 2.4 and 2.2 with mod_proxy_ajp (comes with the httpd Linux package installations:

• Red Hat/CentOS: /usr/lib64/httpd/modules/mod_proxy_ajp.so
• Ubuntu: /etc/apache2/mods-available/proxy_ajp.load

...
<Connector port="8009" protocol="AJP/1.3" redirectPort="8443" />
...

• Red Hat/CentOS: /etc/httpd/conf.d/tomcat-proxy.conf
  view sourceprint?

  01	# this line already exists by default

  02	LoadModule proxy_ajp_module modules/mod_proxy_ajp.so

  03

  04

  <Proxy *>

  05	Header set Access-Control-Allow-Origin "*"

  06	AddDefaultCharset Off

  07	Order deny,allow

  08	Allow from all

  09

  </Proxy>

  10

  11	# This proy's everything from "/" (don't do this if you want to serve static content from the root path of the web server along side dynamic content via /apps).

  12	#ProxyPass / ajp://localhost:8009/

  13	#ProxyPassReverse / ajp://localhost:8009/

  14

  15	# Proxy a non-root path. This proy's everything from "/apps".

  16	ProxyPass /apps ajp://localhost:8009/apps

  17	ProxyPassReverse /apps ajp://localhost:8009/apps

• Ubuntu:
  Enable Apache proxy: a2enmod proxy_ajp
  add to /etc/apache2/apache2.conf
  view sourceprint?

  1	ProxyPass /apps ajp://localhost:8009/apps

  2	ProxyPassReverse /apps ajp://localhost:8009/apps

• Red Hat/CentOS: service httpd restart
• Ubuntu: systemctl restart apache2

This adds "/apps/" to the URL path.

Run:

• Start Tomcat first: service tomcat start
• Start Apache: service httpd start
  After Apache has started, one may perform a syntax check of the Apache configuration files with the following command:
  [root prompt]# /usr/sbin/apachectl configtest
  If sucessful it should return the statement: Syntax OK
• Test with the URL:
  • Test Tomcat WAR file: http://localhost/apps/myservlet/
  • Test static page with Apache: http://localhost/

Log Files:

• Apache:
  • /var/log/httpd/access_log
  • /var/log/httpd/error_log
• Tomcat: /var/tomcat6/logs/...

We will cover connectivity to two databases:

1. PostgreSQL
2. MySQL

Install and configure a database. See: YoLinux Tutorial: PostgreSQL and Linux

JDBC and PostgreSQL JAR files: The "CLASSPATH" variable can set the Java runtime environment so that it will find the appropriate Java libraries (JAR files). Environment variables for Tomcat can be set in /etc/tomcat6/conf/tomcat6.conf. One may also set the CLASSPATH variable to include PostgreSQL JDBC JAR files. I did not set the CLASSPATH environment variable in the configuration file but instead employed the default path by performing the following steps:

List of PostgreSQL JDBC drivers:

[prompt]# rpm -ql postgresql-jdbc-7.1.3-2
/usr/share/pgsql/jdbc7.0-1.1.jar
/usr/share/pgsql/jdbc7.1-1.2.jar

Place JDBC JAR libraries in path where they can be found:

cp /usr/share/pgsql/jdbc7.1-1.2.jar /var/tomcat6/lib

Java Servlet run under Tomcat, accessing PostgreSQL using JDBC:

Notes:

• String loginUrl = "jdbc:postgresql://localhost/bedrock";
  The format for this is jdbc:postgresql:host-name-of-server:port/database-name
  Examples:
  • jdbc:postgresql:bedrock - bedrock is the PostgreSQL database name. See PostgreSQL tutorial.
  • jdbc:postgresql://localhost/bedrock
  • jdbc:postgresql://localhost:5432/bedrock - Default PostgreSQL standard port number is 5432
• public void doGet(...
  Using http GET request. For http POST operations use doPost
• String m_name = rs.getString("name");
  One may also use database field names or field numbers. i.e.
  String m_name = rs.getString(1);
  String m_dept = rs.getString(2);

Set CLASSPATH environment variable:

export CLASSPATH=$CLASSPATH:/var/tomcat6/lib/jdbc7.1-1.2.jar

OR
export CLASSPATH=/usr/java/j2sdk1.4.0/lib/tools.jar:/usr/java/j2sdk1.4.0/jre/lib/rt.jar:/var/tomcat6/common/lib/servlet.jar:/var/tomcat6/lib/jdbc7.1-1.2.jar

Compile:

[prompt]# cd /var/tomcat6/webapps/examples/WEB-INF/classes
[prompt]# javac ShowBedrock.java

(OR /usr/java/j2sdk1.4.0/bin/javac ShowBedrock.java )

Set: tcpip_socket = true
This allows JDBC to connect to PostgreSQL.

Test:

• Tomcat only: http://localhost:8080/examples/servlet/ShowBedrock
• Apache Request: http://localhost/examples/servlet/ShowBedrock

Results:

JDBC/PostgreSQL Links:

• PostgreSQL.org: JDBC

Install and configure a database: YoLinux Tutorial: MySQL and Linux

Download JDBC MySQL JAR file:

• Install package mysql-connector-java-5.1.17-6.el6.noarch which includes /usr/share/java/mysql-connector-java.jar (sym linked to mysql-connector-java-5.1.17.jar)
  Add JAR file to CLASSPATH. Add mysql-connector-java-5.1.17.jar to your WAR file.

"GRANT ALL PRIVILEGES ON bedrock to 'user@hostname' identified by 'password';
FLUSH PRIVILEGES

where hostname is localhost.localdomain (not localhost on default Red Hat installation)

Java Servlet run under Tomcat, accessing MySQL using JDBC:

[prompt]# export CLASSPATH=/var/tomcat6/common/lib/mysql-connector-java-5.1.17.jar:$CLASSPATH
[prompt]# cd /var/tomcat6/webapps/examples/WEB-INF/classes
[prompt]# javac ShowBedrock.java

(ORexport CLASSPATH=/usr/java/latest/lib/tools.jar:/usr/java/latest/jre/lib/rt.jar:/var/tomcat6/common/lib/servlet.jar:/var/tomcat6/lib/mysql-connector-java-5.1.17.jar )
(OR /usr/java/j2sdk1.4.0/bin/javac ShowBedrock.java )

Test:

• Tomcat only: http://localhost:8080/examples/servlet/ShowBedrock
• Apache Request: http://localhost/examples/servlet/ShowBedrock

Notes:

• [Potential Pitfall]: The MySQL "user" table will define users and their host. I made the mistake of using "localhost" when the host name returned by the Unix command "hostname" was different. When the servlet tried to connect to the database it was refused.

  [prompt]$ hostname
  
  superserver
  [prompt]$ mysql -h localhost -u root -p
  ...
  mysql> use mysql
  mysql> select user,host from user;
  +-------+------------+
  | user  | host       |
  +-------+------------+
  | root  |            |
  | Dude1 | localhost  |
  +-------+------------+
  mysql> update user set Host='superserver' where User='Dude1';
  Query OK, 1 row affected (0.00 sec)
  Rows matched: 1  Changed: 1  Warnings: 0
              

Links:

• JDBC MySQL driver "mm"
• Nusphere.com: Discussion and explanation of simple JDBC mm.MySQL servlet
• MySQL JDBC Examples
• MySQL Links - Lots of JDBC links as well.
• mm.MySQL guide

Java JDBC programs require the package: java.sql. Contained within this package are:

• Classes:
  • Date
  • DriverManager
  • DriverPropertyInfo
  • Time
  • Timestamp
  • Types
• Interfaces:
  • CallableStatement
  • Connection
  • DatabaseMetaData
  • Driver
  • PreparedStatement
  • ResultSet
  • ResultMetaData
  • Statement

JDBC Links:

• JDBC tutorials
• JDBC API tutorial
• JDBC database support
• Sun's database of JDBC database drivers

Basic JSP elements:

• <jsp: ... %> : Define/execute Java statements.
  Example:

     <jsp:usebean id="clock" scope="page" class="dates.JspCalendar" type="dates.JspCalendar">
     Year: is  <jsp:getProperty name="clock" property="year"/>
     <BR>
     Month: is  <jsp:getProperty name="clock" property="month"/>
      </jsp:usebean>
            

• <% ... %> : Execute Java statements. Nothing displayed.
  Example: <% numguess.reset(); %>
• <%= ... %> : Execute Java expression and place results here.
  Example: Calendar:<%= table.getDate() %>
• <%@ ... %> : Declare Java variable or method.
  Example:

     <%@ page language="java" import="cal.*" %>
     <jsp:useBean id="table" scope="session" class="cal.TableBean" />
      

The samples delivered with Tomcat show numerous JSP examples and the source code:

• http://localhost:8080/examples/jsp/index.html

• Apache Project: Tomcat
• J2EE:
  • JBoss.org - J2EE Java Enterprise application server - Want to define Java Enterprise Bean services which a servlet may invoke, use JBoss (or WebSphere).
  • YoLinux: List of Java middleware servers
• YoLinux Tutorial: CGI programming with C++ and GNU GciCc - Perform the same functionality as this Java servlet tutorial but use CGI and C++.

Books:

	"Core Java 2, Volume 1: Fundamentals " by Cay S. Horstmann, Gary Cornell ISBN # 0132354764, Prentice Hall PTR 8th edition The industry standard. Need I say more?
	"Core Java 2: Volume 2 Advanced Features " by Cay S. Horstmann, Gary Cornell ISBN # 0132354799, Prentice Hall PTR 8th edition The industry standard. Need I say more?
	"Core Java Server Faces" by David Geary, Cay S. Horstmann ISBN # 0131738860, Prentice Hall PTR 2nd edition
	Tomcat: The Definitive Guide by Jason Brittain, Ian F. Darwin ISBN # 0596003188 O'Reilly & Associates; 1st Edition edition
	"Professional Apache Tomcat" by Chanoch Wiggers, Ben Galbraith, Vivek Chopra, Sing Li, Debashish Bhattacharjee, Amit Bakore, Romin Irani, Sandip Bhattacharya, Chad Fowler ISBN # 0764543725 Wrox
	Apache Jakarta-Tomcat by James Goodwill ISBN # 1893115364, APress
	Apache Tomcat Security Handbook by Vivek Chopra, Ben Galbriaths, Brian Rickabaugh, Gotham Pollysetty, John Turner ISBN # 1861008309, Wrox Press
	"JSP, Servlets, and MySQL" by David Harms ISBN # 0764547879, Hungry Minds, Inc
	"MySQL" by Paul DuBois, Michael Widenius ISBN # 0735709211, New Riders Publishing
	"Manageing and Using MySQL" by George Reese, Randy Jay Yarger, Tim King ISBN # 0596002114, O'Reilly
	PostgreSQL Essential Reference by Barry Stinson ISBN #0735711216, New Riders
	PostgreSQL: Developer's Handbook by Ewald Geschwinde, Hans-Juergen Schoenig, Hans-Jurgen Schonig ISBN #0672322609, SAMS
	Practical PostgreSQL John C. Worsley, Joshua D. Drake ISBN #1565928466, O'Reilly
	Beginning Databases with PostgreSQL by Richard Stones, Neil Matthew ISBN #1861005156, Wrox Press Inc