Gerrit Code Review - Plugin Development
=======================================

The Gerrit server functionality can be extended by installing plugins.
This page describes how plugins for Gerrit can be developed.

Depending on how tightly the extension code is coupled with the Gerrit
server code, there is a distinction between `plugins` and `extensions`.

A `plugin` in Gerrit is tightly coupled code that runs in the same
JVM as Gerrit. It has full access to all server internals. Plugins
are tightly coupled to a specific major.minor server version and
may require source code changes to compile against a different
server version.

An `extension` in Gerrit runs inside of the same JVM as Gerrit
in the same way as a plugin, but has limited visibility to the
server's internals. The limited visibility reduces the extension's
dependencies, enabling it to be compatible across a wider range
of server versions.

Most of this documentation refers to either type as a plugin.

Requirements
------------

To start development, clone the sample Maven project:

----
$ git clone https://gerrit.googlesource.com/plugins/helloworld
----

This project includes the dependencies file that matches the war file to
develop against. Dependencies are offered in two different formats:

gerrit-extension-api.jar::
  A stable but thin interface. Suitable for extensions that need
  to be notified of events, but do not require tight coupling to
  the internals of Gerrit. Extensions built against this API can
  expect to be binary compatible across a wide range of server
  versions.

gerrit-plugin-api.jar::
  The complete internals of the Gerrit server, permitting a
  plugin to tightly couple itself and provide additional
  functionality that is not possible as an extension. Plugins
  built against this API are expected to break at the source
  code level between every major.minor Gerrit release. A plugin
  that compiles against 2.5 will probably need source code level
  changes to work with 2.6, 2.7, and so on.

Manifest
--------

Plugins may provide optional description information with standard
manifest fields:

====
  Implementation-Title: Example plugin showing examples
  Implementation-Version: 1.0
  Implementation-Vendor: Example, Inc.
  Implementation-URL: http://example.com/opensource/plugin-foo/
====

ApiType
~~~~~~~

Plugins using the tightly coupled `gerrit-plugin-api.jar` must
declare this API dependency in the manifest to gain access to server
internals. If no `Gerrit-ApiType` is specified the stable `extension`
API will be assumed. This may cause ClassNotFoundExceptions when
loading a plugin that needs the plugin API.

====
  Gerrit-ApiType: plugin
====

Explicit Registration
~~~~~~~~~~~~~~~~~~~~~

Plugins that use explicit Guice registration must name the Guice
modules in the manifest. Up to three modules can be named in the
manifest. `Gerrit-Module` supplies bindings to the core server;
`Gerrit-SshModule` supplies SSH commands to the SSH server (if
enabled); `Gerrit-HttpModule` supplies servlets and filters to the HTTP
server (if enabled). If no modules are named automatic registration
will be performed by scanning all classes in the plugin JAR for
`@Listen` and `@Export("")` annotations.

====
  Gerrit-Module:     tld.example.project.CoreModuleClassName
  Gerrit-SshModule:  tld.example.project.SshModuleClassName
  Gerrit-HttpModule: tld.example.project.HttpModuleClassName
====

[[reload_method]]
Reload Method
~~~~~~~~~~~~~

If a plugin holds an exclusive resource that must be released before
loading the plugin again (for example listening on a network port or
acquiring a file lock) the manifest must declare `Gerrit-ReloadMode`
to be `restart`. Otherwise the preferred method of `reload` will
be used, as it enables the server to hot-patch an updated plugin
with no down time.

====
  Gerrit-ReloadMode: restart
====

In either mode ('restart' or 'reload') any plugin or extension can
be updated without restarting the Gerrit server. The difference is
how Gerrit handles the upgrade:

restart::
  The old plugin is completely stopped. All registrations of SSH
  commands and HTTP servlets are removed. All registrations of any
  extension points are removed. All registered LifecycleListeners
  have their `stop()` method invoked in reverse order. The new
  plugin is started, and registrations are made from the new
  plugin. There is a brief window where neither the old nor the
  new plugin is connected to the server. This means SSH commands
  and HTTP servlets will return not found errors, and the plugin
  will not be notified of events that occurred during the restart.

reload::
  The new plugin is started. Its LifecycleListeners are permitted
  to perform their `start()` methods. All SSH and HTTP registrations
  are atomically swapped out from the old plugin to the new plugin,
  ensuring the server never returns a not found error. All extension
  point listeners are atomically swapped out from the old plugin to
  the new plugin, ensuring no events are missed (however some events
  may still route to the old plugin if the swap wasn't complete yet).
  The old plugin is stopped.

To reload/restart a plugin the link:cmd-plugin-reload.html[plugin reload]
command can be used.

Classpath
---------

Each plugin is loaded into its own ClassLoader, isolating plugins
from each other. A plugin or extension inherits the Java runtime
and the Gerrit API chosen by `Gerrit-ApiType` (extension or plugin)
from the hosting server.

Plugins are loaded from a single JAR file. If a plugin needs
additional libraries, it must include those dependencies within
its own JAR. Plugins built using Maven may be able to use the
link:http://maven.apache.org/plugins/maven-shade-plugin/[shade plugin]
to package additional dependencies. Relocating (or renaming) classes
should not be necessary due to the ClassLoader isolation.

SSH Commands
------------

Plugins may provide commands that can be accessed through the SSH
interface (extensions do not have this option).

Command implementations must extend the base class SshCommand:

====
  import com.google.gerrit.sshd.SshCommand;

  class PrintHello extends SshCommand {
    protected abstract void run() {
      stdout.print("Hello\n");
    }
  }
====

If no Guice modules are declared in the manifest, SSH commands may
use auto-registration by providing an `@Export` annotation:

====
  import com.google.gerrit.extensions.annotations.Export;
  import com.google.gerrit.sshd.SshCommand;

  @Export("print")
  class PrintHello extends SshCommand {
    protected abstract void run() {
      stdout.print("Hello\n");
    }
  }
====

If explicit registration is being used, a Guice module must be
supplied to register the SSH command and declared in the manifest
with the `Gerrit-SshModule` attribute:

====
  import com.google.gerrit.sshd.PluginCommandModule;

  class MyCommands extends PluginCommandModule {
    protected void configureCommands() {
      command("print").to(PrintHello.class);
    }
  }
====

For a plugin installed as name `helloworld`, the command implemented
by PrintHello class will be available to users as:

----
$ ssh -p 29418 review.example.com helloworld print
----

HTTP Servlets
-------------

Plugins or extensions may register additional HTTP servlets, and
wrap them with HTTP filters.

Servlets may use auto-registration to declare the URL they handle:

====
  import com.google.gerrit.extensions.annotations.Export;
  import com.google.inject.Singleton;
  import javax.servlet.http.HttpServlet;
  import javax.servlet.http.HttpServletRequest;
  import javax.servlet.http.HttpServletResponse;

  @Export("/print")
  @Singleton
  class HelloServlet extends HttpServlet {
    protected void doGet(HttpServletRequest req, HttpServletResponse res) throws IOException {
      res.setContentType("text/plain");
      res.setCharacterEncoding("UTF-8");
      res.getWriter().write("Hello");
    }
  }
====

If explicit registration is being used, a Guice ServletModule must
be supplied to register the HTTP servlets, and the module must be
declared in the manifest with the `Gerrit-HttpModule` attribute:

====
  import com.google.inject.servlet.ServletModule;

  class MyWebUrls extends ServletModule {
    protected void configureServlets() {
      serve("/print").with(HelloServlet.class);
    }
  }
====

For a plugin installed as name `helloworld`, the servlet implemented
by HelloServlet class will be available to users as:

----
$ curl http://review.example.com/plugins/helloworld/print
----

Documentation
-------------

If a plugin does not register a filter or servlet to handle URLs
`/Documentation/*` or `/static/*`, the core Gerrit server will
automatically export these resources over HTTP from the plugin JAR.

Static resources under `static/` directory in the JAR will be
available as `/plugins/helloworld/static/resource`.

Documentation files under `Documentation/` directory in the JAR
will be available as `/plugins/helloworld/Documentation/resource`.

Documentation may be written in
link:http://daringfireball.net/projects/markdown/[Markdown] style
if the file name ends with `.md`. Gerrit will automatically convert
Markdown to HTML if accessed with extension `.html`.

Within the Markdown documentation files macros can be used that allow
to write documentation with reasonably accurate examples that adjust
automatically based on the installation.

The following macros are supported:

[width="40%",options="header"]
|===================================================
|Macro       | Replacement
|@PLUGIN@    | name of the plugin
|@URL@       | Gerrit Web URL
|@SSH_HOST@  | SSH Host
|@SSH_PORT@  | SSH Port
|===================================================

The macros will be replaced when the documentation files are rendered
from Markdown to HTML.

Macros that start with `\` such as `\@KEEP@` will render as `@KEEP@`
even if there is an expansion for `KEEP` in the future.

Automatic Index
~~~~~~~~~~~~~~~

If a plugin does not handle its `/` URL itself, Gerrit will
redirect clients to the plugin's `/Documentation/index.html`.
Requests for `/Documentation/` (bare directory) will also redirect
to `/Documentation/index.html`.

If neither resource `Documentation/index.html` or
`Documentation/index.md` exists in the plugin JAR, Gerrit will
automatically generate an index page for the plugin's documentation
tree by scanning every `*.md` and `*.html` file in the Documentation/
directory.

For any discovered Markdown (`*.md`) file, Gerrit will parse the
header of the file and extract the first level one title. This
title text will be used as display text for a link to the HTML
version of the page.

For any discovered HTML (`*.html`) file, Gerrit will use the name
of the file, minus the `*.html` extension, as the link text. Any
hyphens in the file name will be replaced with spaces.

If a discovered file name beings with `cmd-` it will be clustered
into a 'Commands' section of the generated index page. All other
files are clustered under a 'Documentation' section.

Some optional information from the manifest is extracted and
displayed as part of the index page, if present in the manifest:

[width="40%",options="header"]
|===================================================
|Field       | Source Attribute
|Name        | Implementation-Title
|Vendor      | Implementation-Vendor
|Version     | Implementation-Version
|URL         | Implementation-URL
|API Version | Gerrit-ApiVersion
|===================================================

Deployment
----------

Compiled plugins and extensions can be deployed to a running Gerrit
server using the link:cmd-plugin-install.html[plugin install] command.

Plugins can also be copied directly into the server's
directory at `$site_path/plugins/$name.jar`.  The name of
the JAR file, minus the `.jar` extension, will be used as the
plugin name. Unless disabled, servers periodically scan this
directory for updated plugins. The time can be adjusted by
link:config-gerrit.html#plugins.checkFrequency[plugins.checkFrequency].

For disabling plugins the link:cmd-plugin-remove.html[plugin remove]
command can be used.

GERRIT
------
Part of link:index.html[Gerrit Code Review]