Well, today I’m going to tell something about Apache Tomcat.  Apache Tomcat, often referred to as Tomcat Server, is an open-source Java Servlet Container developed by the Apache Software Foundation (ASF). Tomcat implements several Java EE specifications including Java Servlet, JavaServer Pages (JSP), Java EL, and WebSocket, and provides a “pure Java” HTTP web server environment in which Java code can run.
When you develop a web application it would be great if we could embed Apache Tomcat instead of having a separate server. Please note that I learnt this by referring this[3] git repository.

Prerequisites

• Basic Java knowledge, including an installed version of the JVM and Maven.
• Basic knowledge on Spring framework.

Create your pom.xml

Create a folder to hold  your app and create a file called pom.xml  in the root of that folder with  the following contents:

<project xmlns="http://maven.apache.org/POM/4.0.0" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
xsi:schemaLocation="http://maven.apache.org/POM/4.0.0 http://maven.apache.org/maven-v4_0_0.xsd">
<modelVersion>4.0.0</modelVersion>
<groupId>com.rnavagamuwa.sample</groupId>
<artifactId>embeddedTomcatSample</artifactId>
<version>1.0-SNAPSHOT</version>
<packaging>jar</packaging>
<name>embeddedTomcatSample Maven Webapp</name>
<url>http://maven.apache.org</url>
<properties>
<tomcat.version>8.0.28</tomcat.version>
</properties>
<dependencies>
<dependency>
<groupId>org.apache.tomcat.embed</groupId>
<artifactId>tomcat-embed-core</artifactId>
<version>${tomcat.version}</version>
</dependency>
<dependency>
<groupId>org.apache.tomcat.embed</groupId>
<artifactId>tomcat-embed-logging-juli</artifactId>
<version>${tomcat.version}</version>
</dependency>
<dependency>
<groupId>org.apache.tomcat.embed</groupId>
<artifactId>tomcat-embed-jasper</artifactId>
<version>${tomcat.version}</version>
</dependency>
<dependency>
<groupId>org.apache.tomcat</groupId>
<artifactId>tomcat-jasper</artifactId>
<version>${tomcat.version}</version>
</dependency>
<dependency>
<groupId>org.apache.tomcat</groupId>
<artifactId>tomcat-jasper-el</artifactId>
<version>${tomcat.version}</version>
</dependency>
<dependency>
<groupId>org.apache.tomcat</groupId>
<artifactId>tomcat-jsp-api</artifactId>
<version>${tomcat.version}</version>
</dependency>
</dependencies>
<build>
<finalName>embeddedTomcatSample</finalName>
<plugins>
<plugin>
<groupId>org.codehaus.mojo</groupId>
<artifactId>appassembler-maven-plugin</artifactId>
<version>1.1.1</version>
<configuration>
<assembleDirectory>target</assembleDirectory>
<programs>
<program>
<mainClass>launch.Main</mainClass>
<name>webapp</name>
</program>
</programs>
</configuration>
<executions>
<execution>
<phase>package</phase>
<goals>
<goal>assemble</goal>
</goals>
</execution>
</executions>
</plugin>
</plugins>
</build>
</project>

There is also a single plugin defined. The appassembler plugin generates a launch script that automatically sets up your classpath and calls your main method (created below) to launch your application.

Add a launcher class

package launch;
import java.io.File;
import org.apache.catalina.WebResourceRoot;
import org.apache.catalina.core.StandardContext;
import org.apache.catalina.startup.Tomcat;
import org.apache.catalina.webresources.DirResourceSet;
import org.apache.catalina.webresources.StandardRoot;
public class Main {
public static void main(String[] args) throws Exception {
String webappDirLocation = "src/main/webapp/";
Tomcat tomcat = new Tomcat();
//The port that we should run on can be set into an environment variable
//Look for that variable and default to 8080 if it isn't there.
String webPort = System.getenv("PORT");
if(webPort == null || webPort.isEmpty()) {
webPort = "8080";
}
tomcat.setPort(Integer.valueOf(webPort));
StandardContext ctx = (StandardContext) tomcat.addWebapp("/", new File(webappDirLocation).getAbsolutePath());
System.out.println("configuring app with basedir: " + new File("./" + webappDirLocation).getAbsolutePath());
// Declare an alternative location for your "WEB-INF/classes" dir
// Servlet 3.0 annotation will work
File additionWebInfClasses = new File("target/classes");
WebResourceRoot resources = new StandardRoot(ctx);
resources.addPreResources(new DirResourceSet(resources, "/WEB-INF/classes",
additionWebInfClasses.getAbsolutePath(), "/"));
ctx.setResources(resources);
tomcat.start();
tomcat.getServer().await();
}
}

Add a Servlet

package servlet;
import java.io.IOException;
import javax.servlet.ServletException;
import javax.servlet.ServletOutputStream;
import javax.servlet.annotation.WebServlet;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
@WebServlet(
name = "MyServlet",
urlPatterns = {"/hello"}
)
public class HelloServlet extends HttpServlet {
@Override
protected void doGet(HttpServletRequest req, HttpServletResponse resp)
throws ServletException, IOException {
ServletOutputStream out = resp.getOutputStream();
out.write("hello heroku".getBytes());
out.flush();
out.close();
}
}

Add a JSP

Create a file called index.jsp in the src/main/webapp directory:

<html>
<body>
<h2>Hello Heroku!</h2>
</body>
</html>

Run your application

On Mac and Linux, the command is:

$ mvn package
$ sh target/bin/webapp

That is all. 🙂 If you have any problem feel free to comment bellow.

References

1. https://en.wikipedia.org/wiki/Apache_Tomcat
2. http://tomcat.apache.org/
3. https://github.com/heroku/devcenter-embedded-tomcat
4. https://devcenter.heroku.com/articles/create-a-java-web-application-using-embedded-tomcat
5. http://www.mojohaus.org/appassembler/appassembler-maven-plugin/