This document explains how to connect Tomcat to the popular
open source web server, Apache. It was originally part of
Tomcat: A Minimalistic User’s Guide by Gal Shachor, but
has been split off for organizational reasons. It should be
considered a work in progress. Since the Tomcat source
tree is constantly changing, the information herein may be out
of date. The only definitive reference at this point is the source
code
.

Other important documents:

Table of Contents


Introduction

Need for cooperation

As stated in the Tomcat User’s Guide,
Tomcat currently supports three
modes
of execution. While it is entirely possible to have
Tomcat serve both your static and dynamic document provision needs, there
are several reasons why you might not want want to do this.
With respect to the Apache web server,

1. Tomcat is not as fast as Apache when it comes to static pages.
2. Tomcat is not as configurable as Apache.
3. Tomcat is not as robust as Apache.
4. Tomcat may not address many
sites’ need for functionality found only in Apache modules (e.g. Perl,
PHP, etc.).

For all these reasons it is recommended that real-world sites use an
industrial-strength web server, such as Apache, for serving static
content, and use Tomcat as a Servlet/JSP add-on.

How will they work together?

In a nutshell a web server is waiting for requests. When
these requests arrive the server does whatever is needed to
serve the requests by providing the necessary content. Adding
Tomcat to the mix may somewhat change this behavior. Now the web
server needs to perform the following:

  • Before the first request can be served, Apache needs to load a web
    server adapter library (so Tomcat can communicate with Apache)
    and initialize it.
  • When a request arrives, Apache needs to check and see if it
    belongs to a servlet; if so it needs to let the adapter
    take the request and handle it.

We’d like Apache to handle our static content, such as
images and HTML documents, and forward all requests for
dynamic content to Tomcat. More specifically, we need answers to the following questions:

1. How will Apache know which request / type of
requests should be forwarded to Tomcat?
2. How will Apache forward these requests to
Tomcat?
3. How will Tomcat accept and handle these requests?

The majority of our time will be spent dealing with points 1 and 2;
3 should be a snap!

What’s required to pull this off?

Answers to the above three questions!

1. Modify Apache’s httpd.conf file.
2. Install a web server adapter.
3. Modify Tomcat’s server.xml file.

It is assumed that you are comfortable modifying the configuration of Tomcat
and Apache separately before you’ve attempted to integrate the
two. As such, we speak in Tomcat/Apache/Servlet lingo, not pausing
to explain what’s already been taught in their respective user
guides. Details on Tomcat setup can be found in the Tomcat
User’s Guide
, while Apache configuration information can be found in
the Apache User’s Guide.


Running Examples

Throughout this document, we’ll refer back to these examples,
as these run the gambit of the commonly-desired Apache installs.

Non-virtual Hosts

1. All JSP and Servlet
requests consisting of http://localhost/
are sent to the “rootExample” Context.
2. All JSP and Servlet
requests consisting of http://localhost/subdirExample/
are sent to the “slifkaExample” Context.

Virtual Hosts

3. All JSP and Servlet
requests consisting of http://virtualhost/
are sent to “vhostExample” Context.
4. All JSP and Servlet
requests consisting of http://virtualhost/vhostSubdir/
are sent to the “vhostSubdir” Context.

As part of all of these configurations, servlets will be mounted as
well. To reach a servlet registered via a particular Context’s
WEB-INF/web.xml file, we’ll configure
it so that appending /servlet/registeredServletName to the URL does the
trick. For Example (2), let’s say we had the
following as part of our web.xml for this context:

SlifkaWorld

foo.bar.baz.SomeClass

someParameter

A value

To reach this servlet, we would request: http://localhost/subdirExample/servlet/SlifkaWorld

If you’re unsure as to what web.xml, Contexts, servlets, or webapps are,
then you probably haven’t read through the Tomcat User’s
Guide
. Get to it!


Configuring Tomcat

We can configure Tomcat independantly of the other two items on our to-do
list. At this point,
we’re assuming that you’ve followed the directions in the User’s
Guide
and have Tomcat up and running in stand-alone
mode.

Setting up the Contexts

First off, let’s explicitly define our example Contexts in Tomcat’s server.xml.
The numbers preceding each configuration snippet refer to the
aforementioned Running Examples.

1.

docBase=”webapps/rootExample”/>

2.

docBase=”webapps/slifkaExample”/>

Example (3) must be placed inside a element in server.xml.
This is because a with the same path attribute exists
already; Example (1) is located at “/” as is (3). We place (4)
inside the element as well, because we only want it to be
accessible when people specify this virtual host in the
request. For more information on the
element, see the server.xml section of the User’s Guide, and the Virtual
Hosting
section below.

Remember
to omit the numbers 3 and 4 if you plan on pasting this in!

3.

docBase=”webapps/vhostExample”/>
4.

docBase=”webapps/vhostSubdir”/>

After you’ve made the appropriate changes to server.xml, remember to
restart Tomcat.

Making sure it all works

Where do we stand now? So
far it’s just a normal Tomcat install, which you should be well-familiar
with by now from the User’s Guide. Tomcat should be serving all the
documents, both static and dynamic, for all of your Contexts. An
easy way to see which Contexts are serving which requests is to watch
your Tomcat log output, either via stdout/err or the log file.

The AJP Connector

Before we leave this section, there’s one more Tomcat aspect to
discuss, the AJP element in server.xml. This is
the mechanism by which Tomcat will communicate with Apache.


value=”org.apache.tomcat.service.connector.Ajp12ConnectionHandler”/> value=”8007″/>

To ensure that it is indeed listening on that port, Telnet to it or
HTTP request it. The Ajp12ConnectionHandler will throw an
exception (visible in the tomcat log file), and you’ll know it’s listening. As far as the web
server adapter goes, this is all you really need to know on the Tomcat
side for now.

Tomcat also supports AJP v1.3 via the Ajp13ConnectionHandler.
We’ll get to this when we discuss mod_jk and mod_jserv later on.


The Web Server Adapter

The next step in integrating Apache with Tomcat is to install a web
server adapter. This is the piece of software that will relay
information between Tomcat and Apache.
It doesn’t really belong under Apache configuration, and it doesn’t
really belong under Tomcat configuration, but it’s required for both of them
to work together.

The web server adapter answers question 2 posed above,
“How will Apache forward these requests to
Tomcat?”

mod_jk versus mod_jserv

Currently there are two adapters available – jk and JServ.

Taken from our, “mod_jk HOWTO“,

mod_jk is a replacement to the elderly mod_jserv. It is a completely new
Tomcat-Apache plugin that passed adaptation to Tomcat. With some luck, working
with it is going to be simpler for everyone. Here are some things to
consider:

  • mod_jserv was too complex
    because it supported JServ specific requirements that Tomcat does not
    pose.
  • mod_jserv supported only
    Apache; on the other hand Tomcat supports many web servers through a
    compatibility layer named the jk library. Supporting two different modes
    of work became problematic in terms of support, documentation and bug
    fixes. mod_jk should fix that.
  • The layered approach provided
    by the jk library makes it easier to support both Apache1.3.x and
    Apache2.xx.
  • mod_jserv only supports AJP v1.2. JServ is feature complete, and
    won’t be enhanced aside from bug fixes.
  • mod_jk supports
    • AJP v1.3 – reuse the TCP connection between Apache and Tomcat, much
      faster.
    • JNI – the fastest if you have a multithreaded server like Apache 2.0.

Where do I get them?

Binaries are available for Linux and Win32 under the bin directory where
you obtained the Tomcat distribution file. For Linux, JServ is available
as mod_jserv.so and Jk is available as mod_jk.so. For Win32, JServ is
available as ApacheModuleJServ.dll and Jk is available as mod_jk.dll.

For UNIX, JServ and Jk must be build from source. Source is available
as part of the Tomcat source distribution. JServ source is found in the
src/native/apache/jServ directory, which contains make files for UNIX and
Win32. Jk source is found in src/native/jk plus src/native/apache1.3 or
src/native/apache2.0 depending on which Apache is the target. Jk make files
for FreeBSD, Linux, Netware, and Win32 (as a Visual C++ 6 project file)
are found in the apache1.3 and apache2.0 directories.

How do I build them?

The make files for JServ contain a small amount of documentation within.
A little more can be found in
User’s Guide
. What we have below serves as a brief treatment of
the mod_jk build steps for Win32 and *nix. This is by no means meant
to be a complete treatment of the build process in all environments, just a
quick hit to get you started.

Win32 (an excerpt from our mod_jk HOWTO):

The redirector was developed using Visual C++ Ver.6.0, so having this
environment is a prereq if you want to perform a custom build. The steps that you need to take are:

  1. Change to the desired Apache (as explained above) source directory.
  2. Set an APACHE1_HOME environment variable which points to where your Apache is
    installed.
  3. Execute the following
    command:

    MSDEV mod_jk.dsp /MAKE ALL

    NOTE: If msdev is not in your path, enter the full path to msdev.exe. Also, ApacheCore.lib
    is expected to exist in the APACHE1_HOME”src”CoreD and APACHE1_HOME”src”CoreR
    directories before linking will succeed. You will need to build enough of the
    Apache source to create these libraries.

  4. Copy mod_jk.dll to Apache’s modules directory.

This will build both release and debug versions of the redirector plugin (mod_jk).
An alternative will be to open mod_jk.dsp in msdev and build it using the build
menu.

UNIX:

  1. Change to the desired Apache (as explained above) source directory.
  2. Set the environment variable $APACHE_HOME to the root of the Apache source
    tree.
  3. Execute the following command:

    ./build.sh

That’s it!

Configuration implications

The effects we’re concerned with relate to the different
Apache directives each adapter makes available to us. Conceptually, we’ll be
adding Apache directives to get the same job done, but depending on which
adapter you’ve chosen, they will differ. We will take both
adapters into consideration while explaining the Apache configuration in
subsequent sections.


httpd.conf – Apache’s main configuration file

Now that we’ve answered questions (2) and (3),
we’re ready to dive into question (1) – Apache itself. We need to tell
Apache how to load and initialize our adapter, and that certain requests should be handled by
this adapter and
forwarded onto Tomcat. Which requests depends on your
configuration. Surprisingly enough, this part can be just as simple as
the previous two, considering that (surprise!) Tomcat does most of the work
for you.

Each time you start Tomcat, after it loads Contexts (both from the
server.xml and automatically from $TOMCAT_HOME/webapps), it automagically
generates a number of files for you. The two that we’re concerned with
are:

  • tomcat-apache.conf (should really be named mod_jserv.conf-auto)
  • mod_jk.conf-auto

Both of these files do exactly what we want – supplement Apache’s
httpd.conf file with the directives necessary to have Apache (Tomcat) serve
our static (dynamic) content needs. We’ll be examining each file in
the coming sections. For many users, directly including one of these files in httpd.conf,
depending on the adapter you’re using, suffices. For example, if
you’re using mod_jk (which we suggest!), you would insert the following:

include /tomcat/conf/mod_jk.conf-auto

…of course substituting in the directory you’ve installed Tomcat into,
in place of /tomcat. NOTE: These files are generated each time Tomcat
starts, and a Tomcat configuration change affecting whichever file you’re
including necessitates
an Apache restart. If you plan on customizing this file, do it
elsewhere, as it is overwritten at each restart.

mod_jk.conf-auto

For now, refer to the comments in the mod_jk.conf-auto file and
mod_jk HOWTO for details.

tomcat-apache.conf

This isn’t nearly as verbose as the
mod_jk file, and is therefore a bit trickier to read through. What
you will see below is a condensed version of the .conf file generated from a server.xml
containing our Running Examples. To
aid in our explanation of the file’s contents, we’ve inserted comments
directly, and funked with the formatting.

Before we get started, there is one JServ directive used extensively
that you should know about: ApJServMount. ApJServMount has the
following syntax:

ApJServMount

In English, this says, “Any request that starts with
should be passed on to for
execution.” Don’t worry if it’s not quite clear yet, examples
follow!

This file has been modified quite a bit as the auto-generated one doesn’t do
exactly what we want, and isn’t laden with comments 🙂 but it comes pretty close! With the knowledge
gained thus far, this shouldn’t be too much of a leap, so here goes:

Apache-Tomcat Configuration where Apache Serves the Static Content


















All of the above directives, with the exception of anything prefixed
by “ApJServ” are extensively documented in the Apache
User’s Guide
. If you find our coverage brief, please look
there for the desired information.

Below, we go into further detail about each commented step:

1. Instruct Apache to load the jserv
shared-object (or the NT world dll).

If the loading went well and the module came from a file named
mod_jserv.c (1a) we can start with the rest of the configuration.

2. This step sets various JServ internal
parameters, in order:

  • Instruct JServ not to start the Tomcat process. Automatically
    starting Tomcat is not implemented yet.
  • Disable the secret key challenge/response between Apache and Tomcat.
    Again, the secret key work is not implemented yet.
  • Instruct JServ to copy the base server mount points (see next
    section) in case of virtual hosting.
  • Instruct JServ to use the notice log level. Other log levels
    include emerg, alert, crit, error, warn, info and debug.

3. This step sets JServ’s default communication
parameters.

It
says that the default protocol used for the communication is ajpv12 (do
not mess with this one) and that the Tomcat process runs on the same
machine and listens on port 8007. If you run Tomcat on a machine other
than the one used for Apache you should either update your
ApJServDefaultHost or use a full URL when mounting contexts (see next).
Also, if you configured the Tomcat connectors to use a port other then
8007, you should update your ApJServDefaultPort or use a full URL when
mounting contexts.

4. These steps “register” (for lack of
a better word) our Example (1) context
w/Apache.

Most everything is explained up inside the example, but there
are a few things you should know. The first is that, as you
can see, contexts mounted at the root of your web (e.g. http://localhost/
versus http://localhost/myApp)
server require a little more work than they would otherwise.
Second, don’t take anything auto-generated for granted. It’s a
very nice feature and works in many, many cases. Still though,
you’re setting up two complicated pieces of software and should
probably be familiar with what it all means and why/how it all
works! Tomcat has no way of knowing how you’re going to
“register” the context in Apache, and does its best to
guess.

5. These steps register our Example
(2)
context w/Apache.

There are two differences between Example (1) and Example (2)’s
setup. First, since the DocumentRoot is already defined and
mounted to our rootExample content (Example (1)), we use an Alias to
tell Apache how to serve up our static documents. Second, we
modify the ApJServMount directive to reflect the path that the
context resides in (/subdirExample).

Your configuration is complete! All you have to do now is
restart Apache, and you’re cooking with gas.


Multiple Tomcat JVMs

Sometimes it is useful to have different contexts handled by
different JVMs, for example:

  • When each context serves a different, specific task and runs
    on a different machine.
  • When we want to have multiple developers work on a private
    Tomcat process but use the same web server.

Implementing such schemes where different contexts are served by different
JVMs is very easy and the following configuration file demonstrates this:

Tomcat Configuration

Here’s where we have to step back to Tomcat. First, you
have to be running two separate instances of Tomcat, hence the “mutliple
JVMs”. Of course, each instance of Tomcat will need its web
adapter Connector listening on a unique port. You’ll remember the
following snippet from server.xml in our AJP section:

As you can see, the key to integrating multiple instances with Apache is to specify the full URL when mounting via the ApJServMount directive. This is how you're able to tell JServ that the Tomcat processes are each listening on separate ports. If the Tomcat instances were running on separate machines, you would change the "localhost" to the appropriate machine name in the ApJServMount directive(s).

NOTE: Your $TOMCAT_HOME/conf/tomcat-apache.conf file is overwritten each time you restart Tomcat. This configuration requires a custom tomcat-apache.conf, so making your changes to, and subsequently including, one of the auto-generated ones is a *bad idea*. Your changes will be overwritten each time Tomcat restarts.


Virtual Hosting

Once you've got Apache configured correctly with the entries (discussed in part below), getting virtual hosting working under Tomcat isn't that difficult. There are two ways achieving this:

  1. Use a different Tomcat port (and therefore multiple instances of Tomcat) between Apache and Tomcat per virtual host.
    • Advantages
      • Works in Tomcat 3.1 and 3.2
      • Because it sends calls for different Virtual Hosts to different Tomcat JVMs, this can be used to spread the load over several machines
      • Development can occur in isolation (an instance per developer) but still use the same machine, and same overall installation of Tomcat
    • Disadvantages
      • Doesn't scale well (an instance of Tomcat per virtual host is required)
      • Difficult to maintain for more than a few hosts (requires a different server.xml for each virtual host)
  2. Use the directive in Tomcats' server.xml file
    • Advantages
      • Easier to set up
      • Uses less system resources => scales much better (only one Tomcat instance, single adapter connector port)
    • Disadvantages
      • Only works under Tomcat 3.2
      • If Tomcat needs to be restarted for one virtual host, it needs to be restarted for all of them

To set up Apache and Tomcat using the first method, you need to set up a different ports for Apache and Tomcat to communicate for each host. Here's a sample Apache configuration (which uses mod_jserv):

Option 1 - Different Tomcat for each Apache Virtual Host

NOTE: Remember to set the DocumentRoot so Apache knows where to serve the static files from. In our mod_jserv and mod_jk examples, this was done by specifying an Alias since the DocumentRoot was already defined. Keep in mind that the ApJServMount is relative from the DocumentRoot. What's a DocumentRoot and what's an Alias? See the Apache User's Guide!

As you can see from the above example, using Tomcat in a virtual hosting environment isn't that different insofar as Apache is concerned. Of course, you'll need to setup the appropriate entries, but other than that, there isn't much difference. Inside each virtual host entry, you have all of your context-specific information in the usual way, and that's it really.

Here is an example setup for a machine serving virtual hosts using the second method described above.

Option 2 - Same Tomcat for all Virtual Hosts (httpd.conf)

This creates two virtual hosts, host1 and host2, both running off of the same IP - 192.168.0.1. Any requests for paths beginning with "/servlet" are passed to Tomcat for processing, as are JSP requests, providing of course that you've added the aforementioned AddType and AddHandler directives.

Next, you need to configure Tomcat's server.xml file.

Option 2 - Same Tomcat for all Virtual Hosts (server.xml)

That's it! Place your servlets in the WEB-INF/classes directory for each virtual host, restart Tomcat and Apache, and you should be away. If not, be sure to check the Common Problems section.


Common Installation and Configuration Problems

This section isn't meant to be your one-stop shop for all troubles Tomcat-Apache, but a resource for stumbling blocks common to many first-time Tomcat'ers. See the help section of the User's Guide for additional links.

One thing that many users don't recall is that there is a wealth of information to be found in the log files! Primarily, the tomcat.log file in your $TOMCAT_HOME/logs directory, or wherever you've configured it. If you want more detail, see the User's Guide for instructions on how to edit your server.xml to have more verbose logging.

In addition, the web server adapter has a log file as well. It's usually .log (e.g. mod_jserv.log).

Requesting http://webserver:8007/ produces an HTTP 500 error

If this occurs, you should see a stack trace in your tomcat.log file, starting with:

HANDLER THREAD PROBLEM: java.io.IOException: Stream broken

By default, Tomcat listens for AJP connections on port 8007. AJP is a protocol used to communicate between the web server and Tomcat, not Tomcat and your browser. Many first-time users mistakenly assume that this is how you test your Tomcat installation or Tomcat-Apache integration, while this is not the case.

Apache doesn't start when Tomcat is running

This most likely means that Tomcat is trying to use a port that is already being used by someone else - usually Apache or another instance of Tomcat. By default, Tomcat's comes configured to run an HTTP server on port 8080. If you examine the supplied server.xml file, you'll see the following element:


value=”org.apache.tomcat.service.http.HttpConnectionHandler”/> value=”8080″/>

To disable this, just comment out the entire
element. Otherwise, just change it to a port that doesn’t conflict
with Apache. If you’re not going to use the Tomcat HTTP server, and
you probably won’t be if you are reading this document, disable it.

If you’re running Apache / JServ, JServ may be clashing with Tomcat on
port 8007 as well.

Can I run both Apache/JServ and
Apache/Tomcat on the same machine?

Apparently you can, but you’ll need to use mod_jserv rather than mod_jk.
As of Tomcat 3.2b2, the mod_jserv.so is the same as that used by
Apache/JServ. You will need to run Apache/JServ on a different port
to Tomcat, then you can control which servlet engine/container handles
which requests by specifying the appropriate port via the ApJServMount
directive in Apache’s httpd.conf.


and directives ignored

Apache forwards all requests underneath a mounting to Tomcat.
Let’s say you had the following at the root-level of your httpd.conf
file. You’re thinking, “I’ll forward all JSP and Servlet
requests to Tomcat”. What Apache thinks is, “I’ll forward
EVERYTHING to Tomcat”:

Counter-intuitive Tomcat-Apache configuration


What’s happened here is that the first ApJServMount is saying,
“Anything under the root ‘/’ should be forwarded to the following
Tomcat process/path for handling.” This results in requests for
static documents being forwarded as well, which isn’t what we want.
If I was told to ‘fix’ the above conf file, I would just change the ‘/’
after ApJServMount to “/servlet”. You would have
already defined earlier up that all JSP requests go to the jserv-servlet
handler (see our Apache-mod_jserv configuration
section).

mod_ssl – pages sometimes don’t finish loading
using Internet Explorer

Tomcat sends data back to the browser using the Transfer-Encoding:
Chunked method. This causes problems with Internet Explorer when
using SSL. The solution is to make sure the first line of the
returned file isn’t blank.

mod_ssl – getScheme() always returns HTTP!

The protocol used by mod_jserv can’t identify whether a page was
requested via HTTP or HTTPS. Yes, Apache/JServ did it, but that
was a hack (just checked for requests on port 443). The solution
is either to check for port 443 yourself, or to upgrade to mod_jk and
the ajpv13 protocol. Another symptom of this is finding requests
redirected to http://yourserver.com:443
rather than https://yourserver.com/.

Migrating from Apache/JServ JSSI
to Apache/Tomcat

Tomcat doesn’t support JSSI. Until someone writes an Interceptor to
handle them, the easiest way is to convert your .jhtml files to .jsp
files. Just replace:

with



mod_rewrite – Used to work fine with
Apache/JServ, what gives?

Tomcat implements the servlet specification v2.2, whereas JServ
implemented version 2.1. In the newer version, there are tighter
restrictions on the mapping from URL (what the user requests) to the
filename (what they get). Specifically, it insists that:

request URI = context path + servlet path + path info

This means that the arbitrary mappings that mod_rewrite is capable of
simply won’t work without breaking the Servlet specification. One
solutions is to use the [R] flag on the RewriteRule directive to
“externally redirect” the request.

Tomcat isn’t receiving requests under
/servlet

If you have a “” directive in your
httpd.conf file, this will take precedence over any ApJServMount
directives. The following example won’t forward “/servlet”
requests to Tomcat.

ApJServMount /servlet /ROOT

Options Indexes Multiviews

A workaround is to use instead of

How do I get Tomcat to start
automatically as “nobody”?

If your UNIX-style box has an “rc.d”-style boot directory
(Solaris, RedHat, etc.), then the simplest way is to create a file in the
appropriate boot directory which looks something like this:

#!/bin/sh
CLASSPATH=/your/classpath/here
export CLASSPATH
su – nobody -c “/tomcat/bin/tomcat.sh $@”

You can then invoke this as /etc/rc.d/init.d/tomcat (start | stop) from
your boot sequence in the same way that you start Apache (for instance).

mod_jk – I want to use it, but I can’t find
it. Where is it?

As of this writing (Sept. 30, 2000) mod_jk has to be built from the
Tomcat source distibution.

mod_jk – Apache locks up when
requesting a Servlet or JSP

mod_jk reuses the same port when talking to Tomcat, unlike mod_jserv.
You’ll need to restart Apache whenever you restart Tomcat.


Credits

This document was created by
Gal Shachor. It was split
off into a separate document and revised by Rob Slifka and Mike Bremford.
Contributors, listed in alphabetical order:

  • Jonathan Bnayahu
  • Mike Bremford
  • Alex Chaffee
  • Fiona Czuczman
  • Costin Manolache
  • Chris Pepper
  • Rob Slifka
  • …the countless many on the tomcat-dev and tomcat-user lists!

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