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Apache httpd and Apache Tomcat miscellany

When you start a newly installed Apache httpd web server, you may get the following message:

httpd: Could not reliably determine the server's fully qualified domain name, using for ServerName

It's only a warning! It means that the web server software can't work out what the computer is called; it will run as a web server anyway, but when it reports on the name if pages that it is returning to the user it will guess that they are pages. Only in a few circumstances does the browser actually use this returned information, however!

If you see a web page that just says It Works! and nothing else, you have probably got a brand new installation of Apache httpd 2.2.summat; this is the default page for that server. Why doesn't the Apache httpd team provide a nice front page? Well - they use to under 2.0.summat, but I understand that they got so many emails complaining about the web site not being what users expected that they took their name off it.

If you are building a mod_jk connector to add to your Apache httpd (mod_jk allows a Tomcat connection via ajp1.3 protocol), you MUST ensure that you build it to go with exactly the right version of httpd, If you try and install a 2.0.43 version on a 2.0.44 server (as small a difference as that!) you can have problems.

The way to do this is to configure the build as follows:
./configure --prefix=/usr/local/apache2 --with-apxs=/usr/local/apache2/bin/apxs
which uses the apxs utility that was built / loaded with your web server to provide the vital information about buffer sizes, etc, to your jk connector build.

Apache Tomcat does a lot of caching - and it's right that it should, as it's an application container and not a file server. Sometimes that can be a nuisance, and occasionally it can be more than a nuisance - it can be a major headache (if you have stepped back from an upgrade, for example).

In order to clear out work in progress and more recent temporary files from you Apache Tomcat setup, stop your Tomcat and delete the entire tmp and work directories which is where these deep hidden caches are to be found, then restart Tomcat. The first few hits to the web site after this operation may take a little longer as the directories are recreated, but they'll be recreated clean of information which has been removed from elsewhere and had remained only in the cache.

If you stop a web server and callup a page that it should have been serving on your browser "just to check that it really has stopped", you may sometimes be surprised to see the page pop up again.

Could it be that you have failed to stop the server? It could be, but it's far more likely that your browser tried to contact the server, failed, and so displayed a page it had cached locally. Hold the shift key down and reload again ... on most modern browsers, this will force a true reload, and tell you is it fails without giving you any cached versions.

The javap program tells you what symbols are defined in a java class file. And that lets you tell if a class can be run as an applet, a servlet, a stand alone program, or is something else.

[trainee@easterton classes]$ javap Smallest
Compiled from "Smallest.java"
public class Smallest extends javax.servlet.http.HttpServlet{
  public static int counter;
  public Smallest();
  public void doGet(javax.servlet.http.HttpServletRequest,
    throws java.io.IOException, javax.servlet.ServletException;
  static {};
[trainee@easterton classes]$

This is a Servlet - it extends HttpServlet, and it includes a method called doGet which means that it will respond to "href" links, to having its URL typed in to the command line, and to forms with method="GET". It does not have a POST method, nor specific code to allow it to be initialised or have its data saved when it is flushed out of memory.

There is no main method, so it cannot run as a stand alone program, and since it's a Servlet it can't be an Applet (Java does not allow multiple inheritance which would be needed for a class to double up in this way)

Above notes from today's Deploying Apache httpd and Tomcat course. We also offer a course that covers just Apache httpd if you're wanting to set up a more typical web server rather than being into the Java thing!
(written 2009-01-30)

Associated topics are indexed as below, or enter http://melksh.am/nnnn for individual articles
A601 - Web Application Deployment - Apache httpd - an overview
  [4434] Public training courses - upcoming dates - (2015-02-21)
  [4064] Apache httpd - a robust, open source web server - (2013-04-16)
  [2186] An FAQ on the Apache httpd and Apache Tomcat web servers, and on using them together - (2009-05-17)
  [2077] Why put Apache httpd in front of Apache Tomcat - (2009-03-12)
  [2063] Internal Dummy Connections on Apache httpd - (2009-03-02)
  [2054] Tuning httpd / the supermarket checkout comparison - (2009-02-26)
  [2038] Sticky Sessions with mod_jk (httpd to Tomcat) - (2009-02-12)
  [1897] Keeping on an even keel - (2008-11-21)
  [1593] Keep the client experience easy - single server contact point - (2008-03-27)
  [1265] Apache, Tomcat, Jakarta, httpd, web server - what are they? - (2007-07-13)
  [924] The LAMP Cookbook - Linux, Apache, MySQL, PHP / Perl - (2006-11-13)
  [659] Web Application Components - (2006-03-28)
  [576] Why run two different web servers - (2006-01-25)

A651 - Web Application Deployment - Tomcat Overview
  [3997] Servlet v JSP (Java Server Page). What is the difference? - (2013-02-06)
  [2753] You do not just fly - that is just part of the journey - (2010-05-06)
  [1941] Server - Service - Engine - Host, Tomcat - (2008-12-14)
  [1771] More HowTo diagrams - MySQL, Tomcat and Java - (2008-08-24)
  [498] Why is Tomcat called Tomcat? - (2005-11-17)
  [49] Business is the predominant user of Tomcat, Perl and Tcl - (2004-09-15)

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