Tomcat 7 has been improved a lot and along with everything else that it brings, a very nice feature is provided - an API for embedding Tomcat into your application. The API was provided in earlier versions of Tomcat but it was quite cumbersome to use.

To to start the embedded version of Tomcat one may need to build the required JARs.

 svn co https://svn.apache.org/repos/asf/tomcat/trunk tomcat
 cd tomcat
 ant embed-jars
 ls -l output/embed
total 5092
-rw-r--r-- 1 anton None 56802 2011-03-06 17:09 LICENSE
-rw-r--r-- 1 anton None 1194 2011-03-06 17:09 NOTICE
-rw-r--r-- 1 anton None 1690519 2011-03-06 17:09 ecj-3.6.jar
-rw-r--r-- 1 anton None 234625 2011-03-06 17:09 tomcat-dbcp.jar
-rw-r--r-- 1 anton None 2402517 2011-03-06 17:09 tomcat-embed-core.jar
-rw-r--r-- 1 anton None 781989 2011-03-06 17:09 tomcat-embed-jasper.jar
-rw-r--r-- 1 anton None 34106 2011-03-06 17:09 tomcat-embed-logging-juli.jar

The following snippet demonstrates the embedded Tomcat usage with a deployed servlet instance.

import javax.servlet.ServletException;
import javax.servlet.http.HttpServlet;
import javax.servlet.http.HttpServletRequest;
import javax.servlet.http.HttpServletResponse;
import java.io.File;
import java.io.IOException;
import java.io.Writer;

public class Main {

  public static void main(String[] args)
  throws LifecycleException, InterruptedException, ServletException {
    Tomcat tomcat = new Tomcat();
    tomcat.setPort(8080);

    Context ctx = tomcat.addContext("/", new File(".").getAbsolutePath());

    Tomcat.addServlet(ctx, "hello", new HttpServlet() {
      protected void service(HttpServletRequest req, HttpServletResponse resp) 
      throws ServletException, IOException {
        Writer w = resp.getWriter();
        w.write("Hello, World!");
        w.flush();
      }
    });
    ctx.addServletMapping("/*", "hello");

    tomcat.start();
    tomcat.getServer().await();
  }

}

The only two JARs required are tomcat-embed-core.jar and tomcat-embed-logging-juli.jar. It means that there will be no JSP support and pooling will also be disabled. But that's enough to start a servlet and in most cases that is what you probably need. 

From http://arhipov.blogspot.com/2011/03/embedded-tomcat-minimal-version.html