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[[custom-modules]]
=== Custom Modules

In addition to the modules that come packaged with the Jetty distribution, users are able to create and define their own custom modules for use with their Jetty implementation.
Custom modules can be used for a number of reasons - they can extend features in Jetty, add new features, manage additional libraries available to the server...etc.

At the heart of a Jetty module is the `{name}.mod` file itself.
A jetty `.mod` file defines the following:

[NOTE]
--
It is important to note that when creating your own module, none of these sections are required - simply use those which are applicable to your implementation.
--

Module Description - `[description]`::
The description of the module.
This will be showing when viewing the `.mod` file itself or using the `--list-modules` command.
List of Dependent Modules - `[depend]`::
All modules can declare that they depend on other modules with the `[depend]` section.
The list of dependencies is used to transitively resolve other modules that are deemed to be required based on the modules that you activate.
The order of modules defined in the graph of active modules is used to determine various execution order for configuration, such as Jetty IoC XML configurations, and to resolve conflicting property declarations.
Optional Modules - `[optional]`;;
Of note: there is a special section `[optional]` used to describe structurally dependent modules that are not technically required, but might be of use to your specific configuration.
List of Libraries - `[lib]`::
Modules can optionally declare that they have libraries that they need to function properly.
The `[lib]` section declares a set of pathnames that follow the link:#base-vs-home-resolution[Jetty Base and Jetty Home path resolution rules].
List of Jetty IoC XML Configurations - `[xml]`::
A Module can optionally declare a list of Jetty IoC XML configurations used to wire up the functionality that this module defines.
The `[xml]` section declares a set of pathnames that follow the link:#base-vs-home-resolution[Jetty Base and Jetty Home path resolution rules].
Ideally, all XML files are parameterized to accept properties to configure the various elements of the standard configuration.
Allowing for a simplified configuration of Jetty for the vast majority of deployments.
The execution order of the Jetty IoC XML configurations is determined by the graph of active module dependencies resolved via the `[depend]` sections.
If the default XML is not sufficient to satisfy your needs, you can override this XML by making your own in the `${jetty.base}/etc/` directory, with the same name.
The resolution steps for Jetty Base and Jetty Home will ensure that your copy from `${jetty.base}` will be picked up over the default one in `${jetty.home}`.
List of Module Tags - `[tags]`::
For ease of sorting, modules can be assigned tags.
When using the `--list-modules` command, modules will be groups by the first tag that exists in this section.
Modules can also be listed specifically by these tags using `--list-modules=<tag name>` on the command line.
Ini Variables - `[ini]`::
The `[ini]` section is used to add or change server parameters at startup.
The `[ini]` section can also include a the path of a file or several files which should be made available to the server only.
This is helpful when you want to control what jars are available to deployed webapps.
Jetty INI Template - `[ini-template]`::
Each module can optionally declare a startup ini template that is used to insert/append/inject sample configuration elements into the `start.ini` or `start.d/*.ini` files when using the `--add-to-start=<name>` command line argument in `start.jar`.
Commonly used to present some of the parameterized property options from the Jetty IoC XML configuration files also referenced in the same module.
Required Files and Directories - `[files]`::
If the activation of a module requires some paths to exist, the `[files]` section defines them.
There are 2 modes of operation of the entries in this section.
Ensure Directory Exists;;
If you add a pathname that ends in `"/"` (slash), such as `"webapps/"`, then that directory will be created if it does not yet exist in `${jetty.base}/<pathname>` (eg: `"webapps/"` will result in `${jetty.base}/webapps/` being created).
Download File;;
There is a special syntax to allow you to download a file into a specific location if it doesn't exist yet: `<url>:<pathname>`.
Currently, the `<url>` must be a `http://` scheme URL (please link:#bugs[let us know] if you need more schemes supported).
The `<pathname>` portion follows the link:#base-vs-home-resolution[Jetty Base and Jetty Home path resolution rules].
Example: `http://repo.corp.com/maven/corp-security-policy-1.0.jar:lib/corp-security-policy.jar`
This will check for the existence of `lib/corp-security-policy.jar`, and if it doesn't exist, it will download the jar file from `http://repo.corp.com/maven/corp-security-policy-1.0.jar`
Licenses - `[license]`::
If you are implementing a software/technology that has a license, it's text can be placed here.
When a user attempts to activate the module they will be asked if they accept the license agreement.
If a user does not accept the license agreement, the module will not be activated.
Additional Startup Commands - `[exec]`::
The `[exec]` section is used to define additional parameters specific to the module.
These commands are added to the server startup.
JPMS Module-Path Definitions - `[jpms]`::
The `[jpms]` section is used to add link:#startup-jpms[JPMS modules] to the module-path for startup when using the `--jpms` command.

[[custom-module-properties]]
==== Module Properties

Properties are used to parameterize:

* XML files using the `<Property name="pname"/>` element
* Module files using the `${pname}` syntax

Properties and System Properties may be set on the command line, in a ini file or in a `[ini]` section of a module using the following syntax.

`name=value`;;
Set a property that can be expanded in XML files with the <Property> element.

`name+=value`;;
Append value to an existing property value.

`name+=,value`;;
Append value to an existing property value, using a comma separator if needed.

`name?=value`;;
Set a property only if it is not already set.

If any of the previous formats is preceded by `-D`, then a system property is set as well as a start property.

[[custom-module-location]]
==== Location of Modules
Jetty comes with dozens of modules as part of the distribution package.
By default these are located in the `${JETTY_HOME}/modules` directory.
These modules should not be modified.
In the unlikely circumstance you need to make changes to a stock module, copy it to your `${JETTY_BASE}` in a `modules` directory.

Custom modules should also be maintained separately as part of the `${JETTY_BASE}/modules` directory, though you can optionally place them in `${JETTY_HOME}/modules` for convenience if you have several `{$JETTY_BASE}` locations in your implementation.

[[custom-module-examples]]
==== Creating Custom Modules
As shown above, there are several options that can be utilized when creating custom module files.
This may seem daunting, but the good news is that creating custom modules is actually quite easy.

For example, here is a look at the `http.mod` file which defines parameters for enabling HTTP features for the server:

[source, screen]
----
include::{SRCDIR}/jetty-server/src/main/config/modules/http.mod[]
----

You'll notice that the `http.mod` file only includes a handful of the possible sections available - `[description]`, `[tags]`, `[depend]`, `[xml]`, and `[ini-template]`.
When configuring your own modules, you are free to pick and choose what you include.

As an example, below is a module file that defines a custom XML and lib, and activates a number of additional modules.
A module like this could be used to enable a set of standard modules and resources for a new JETTY_BASE without having to define them all manually.

[source, screen]
----
[description]
Enables the standard set of modules and resources for ACME Corp servers.

[tags]
core

[depend]
server
client
http
http2
jsp
console-capture
requestlog
stats
gzip
deploy
jmx

[files]
basehome:modules/acme/acme.xml|etc/acme.xml

[lib]
lib/acme/ACMECustom.jar
----

Activating this module will activate all the dependent modules, create any required directories and copy in any required files:

[source, screen]
----
java -jar ../start.jar --add-to-start=acme

ALERT: There are enabled module(s) with licenses.
The following 1 module(s):
 + contains software not provided by the Eclipse Foundation!
 + contains software not covered by the Eclipse Public License!
 + has not been audited for compliance with its license

 Module: alpn-impl/alpn-8
  + ALPN is a hosted at github under the GPL v2 with ClassPath Exception.
  + ALPN replaces/modifies OpenJDK classes in the sun.security.ssl package.
  + http://github.com/jetty-project/jetty-alpn
  + http://openjdk.java.net/legal/gplv2+ce.html

Proceed (y/N)? y
INFO  : webapp          transitively enabled, ini template available with --add-to-start=webapp
INFO  : server          transitively enabled, ini template available with --add-to-start=server
INFO  : requestlog      transitively enabled, ini template available with --add-to-start=requestlog
INFO  : alpn            transitively enabled, ini template available with --add-to-start=alpn
INFO  : jsp             transitively enabled
INFO  : servlet         transitively enabled
INFO  : alpn-impl/alpn-8 dynamic dependency of alpn
INFO  : annotations     transitively enabled
INFO  : gzip            transitively enabled, ini template available with --add-to-start=gzip
INFO  : ssl             transitively enabled, ini template available with --add-to-start=ssl
INFO  : plus            transitively enabled
INFO  : deploy          transitively enabled, ini template available with --add-to-start=deploy
INFO  : alpn-impl/alpn-1.8.0_92 dynamic dependency of alpn-impl/alpn-8
INFO  : security        transitively enabled
INFO  : jmx             transitively enabled
INFO  : apache-jsp      transitively enabled
INFO  : stats           transitively enabled, ini template available with --add-to-start=stats
INFO  : acme            initialized in ${jetty.base}/start.d/acme.ini
INFO  : jndi            transitively enabled
INFO  : console-capture transitively enabled, ini template available with --add-to-start=console-capture
INFO  : client          transitively enabled
INFO  : http            transitively enabled, ini template available with --add-to-start=http
INFO  : http2           transitively enabled, ini template available with --add-to-start=http2
MKDIR : ${jetty.base}/logs
MKDIR : ${jetty.base}/lib
MKDIR : ${jetty.base}/lib/alpn
MKDIR : ${jetty.base}/etc
COPY  : ${jetty.home}/modules/ssl/keystore to ${jetty.base}/etc/keystore
MKDIR : ${jetty.base}/webapps
DOWNLD: https://repo1.maven.org/maven2/org/mortbay/jetty/alpn/alpn-boot/8.1.8.v20160420/alpn-boot-8.1.8.v20160420.jar to ${jetty.base}/lib/alpn/alpn-boot-8.1.8.v20160420.jar
COPY  : ${jetty.home}/modules/acme/acme.xml to ${jetty.base}/etc/acme.xml
INFO  : Base directory was modified
----

==== Dependencies
When dependent modules are enabled, they are done so transitively by default.
This means that any `ini` files for dependent modules are not created in the `${JETTY_BASE}/start.d` directory (or added to `${JETTY_BASE}/start.ini`) and are as such not configurable.

For Jetty to create/add the `ini-template` parameters to `start.d` or `start.ini` the associated module must be enabled explicitly.

For example, if I activate the `http` module, it will be enabled, and the `server` module will be enabled transitively:

[source, screen]
----
$ java -jar ../start.jar --add-to-start=http
INFO  : server          transitively enabled, ini template available with --add-to-start=server
INFO  : http            initialized in ${jetty.base}/start.d/http.ini
INFO  : Base directory was modified
----

You'll notice that Jetty informs you of what modules were enabled, and where there associated ini files are located (when applicable).
It also tells the user what command they would need to run to enable any missing or desired ini files for the selected modules, in this case `--add-to-start=server`.

[source, screen]
----
$ java -jar ../start.jar --add-to-start=server
INFO  : server          initialized in ${jetty.base}/start.d/server.ini
INFO  : Base directory was modified
----

____
[NOTE]
It is important to keep in mind that when activating a dependency, Jetty does not just go one layer down.
If a dependent module also has dependencies they too will be enabled.
____
