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`.

[[plugin]]
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.

[[extension]]
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.

[[getting-started]]
Getting started
---------------

To get started with the development of a plugin there are two
recommended ways:

. use the Gerrit Plugin Maven archetype to create a new plugin project:
+
With the Gerrit Plugin Maven archetype you can create a skeleton for a
plugin project.
+
----
mvn archetype:generate -DarchetypeGroupId=com.google.gerrit \
    -DarchetypeArtifactId=gerrit-plugin-archetype \
    -DarchetypeVersion=2.8-SNAPSHOT \
    -DgroupId=com.google.gerrit \
    -DartifactId=testPlugin
----
+
Maven will ask for additional properties and then create the plugin in
the current directory. To change the default property values answer 'n'
when Maven asks to confirm the properties configuration. It will then
ask again for all properties including those with predefined default
values.

. clone the sample plugin:
+
This is a project that demonstrates the various features of the
plugin API. It can be taken as an example to develop an own plugin.
+
----
$ git clone https://gerrit.googlesource.com/plugins/cookbook-plugin
----
+
When starting from this example one should take care to adapt the
`Gerrit-ApiVersion` in the `pom.xml` to the version of Gerrit for which
the plugin is developed. If the plugin is developed for a released
Gerrit version (no `SNAPSHOT` version) then the URL for the
`gerrit-api-repository` in the `pom.xml` needs to be changed to
`https://gerrit-api.storage.googleapis.com/release/`.

[[API]]
API
---

There are two different API formats offered against which plugins can
be developed:

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
====

[[plugin_name]]
Plugin Name
~~~~~~~~~~~

A plugin can optionally provide its own plugin name.

====
  Gerrit-PluginName: replication
====

This is useful for plugins that contribute plugin-owned capabilities that
are stored in the `project.config` file. Another use case is to be able to put
project specific plugin configuration section in `project.config`. In this
case it is advantageous to reserve the plugin name to access the configuration
section in the `project.config` file.

If `Gerrit-PluginName` is omitted, then the plugin's name is determined from
the plugin file name.

If a plugin provides its own name, then that plugin cannot be deployed
multiple times under different file names on one Gerrit site.

For Maven driven plugins, the following line must be included in the pom.xml
file:

[source,xml]
----
<manifestEntries>
  <Gerrit-PluginName>name</Gerrit-PluginName>
</manifestEntries>
----

For Buck driven plugins, the following line must be included in the BUCK
configuration file:

[source,python]
----
manifest_entries = [
   'Gerrit-PluginName: name',
]
----

A plugin can get its own name injected at runtime:

[source,java]
----
public class MyClass {

  private final String pluginName;

  @Inject
  public MyClass(@PluginName String pluginName) {
    this.pluginName = pluginName;
  }

  [...]
}
----

[[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.

[[init_step]]
Init step
~~~~~~~~~

Plugins can contribute their own "init step" during the Gerrit init
wizard. This is useful for guiding the Gerrit administrator through
the settings needed by the plugin to work propertly.

For instance plugins to integrate Jira issues to Gerrit changes may
contribute their own "init step" to allow configuring the Jira URL,
credentials and possibly verify connectivity to validate them.

====
  Gerrit-InitStep: tld.example.project.MyInitStep
====

MyInitStep needs to follow the standard Gerrit InitStep syntax
and behavior: writing to the console using the injected ConsoleUI
and accessing / changing configuration settings using Section.Factory.

In addition to the standard Gerrit init injections, plugins receive
the @PluginName String injection containing their own plugin name.

Bear in mind that the Plugin's InitStep class will be loaded but
the standard Gerrit runtime environment is not available and the plugin's
own Guice modules were not initialized.
This means the InitStep for a plugin is not executed in the same way that
the plugin executes within the server, and may mean a plugin author cannot
trivially reuse runtime code during init.

For instance a plugin that wants to verify connectivity may need to statically
call the constructor of their connection class, passing in values obtained
from the Section.Factory rather than from an injected Config object.

Plugins' InitSteps are executed during the "Gerrit Plugin init" phase, after
the extraction of the plugins embedded in the distribution .war file into
`$GERRIT_SITE/plugins` and before the DB Schema initialization or upgrade.

A plugin's InitStep cannot refer to Gerrit's DB Schema or any other Gerrit
runtime objects injected at startup.

[source,java]
----
public class MyInitStep implements InitStep {
  private final ConsoleUI ui;
  private final Section.Factory sections;
  private final String pluginName;

  @Inject
  public GitBlitInitStep(final ConsoleUI ui, Section.Factory sections, @PluginName String pluginName) {
    this.ui = ui;
    this.sections = sections;
    this.pluginName = pluginName;
  }

  @Override
  public void run() throws Exception {
    ui.header("\nMy plugin");

    Section mySection = getSection("myplugin", null);
    mySection.string("Link name", "linkname", "MyLink");
  }
}
----

[[classpath]]
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.

[[events]]
Listening to Events
-------------------

Certain operations in Gerrit trigger events. Plugins may receive
notifications of these events by implementing the corresponding
listeners.

* `com.google.gerrit.extensions.events.LifecycleListener`:
+
Gerrit server startup and shutdown

* `com.google.gerrit.extensions.events.NewProjectCreatedListener`:
+
Project creation

* `com.google.gerrit.extensions.events.ProjectDeletedListener`:
+
Project deletion

[[stream-events]]
Sending Events to the Events Stream
-----------------------------------

Plugins may send events to the events stream where consumers of
Gerrit's `stream-events` ssh command will receive them.

To send an event, the plugin must invoke one of the `postEvent`
methods in the `ChangeHookRunner` class, passing an instance of
its own custom event class derived from `ChangeEvent`.

[[ssh]]
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:

[source,java]
----
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:

[source,java]
----
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:

[source,java]
----
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
----

Multiple SSH commands can be bound to the same implementation class. For
example a Gerrit Shell plugin can bind different shell commands to the same
implementation class:

[source,java]
----
public class SshShellModule extends PluginCommandModule {
  @Override
  protected void configureCommands() {
    command("ls").to(ShellCommand.class);
    command("ps").to(ShellCommand.class);
    [...]
  }
}
----

With the possible implementation:

[source,java]
----
public class ShellCommand extends SshCommand {
  @Override
  protected void run() throws UnloggedFailure {
    String cmd = getName().substring(getPluginName().length() + 1);
    ProcessBuilder proc = new ProcessBuilder(cmd);
    Process cmd = proc.start();
    [...]
  }
}
----

And the call:

----
$ ssh -p 29418 review.example.com shell ls
$ ssh -p 29418 review.example.com shell ps
----

[[configuration]]
Configuration
-------------

In Gerrit, global configuration is stored in the `gerrit.config` file.
If a plugin needs global configuration, this configuration should be
stored in a `plugin` subsection in the `gerrit.config` file.

This approach of storing the plugin configuration is only suitable for
plugins that have a simple configuration that only consists of
key-value pairs. With this approach it is not possible to have
subsections in the plugin configuration. Plugins that require a complex
configuration need to store their configuration in their own
configuration file where they can make use of subsections. On the other
hand storing the plugin configuration in a 'plugin' subsection in the
`gerrit.config` file has the advantage that administrators have all
configuration parameters in one file, instead of having one
configuration file per plugin.

To avoid conflicts with other plugins, it is recommended that plugins
only use the `plugin` subsection with their own name. For example the
`helloworld` plugin should store its configuration in the
`plugin.helloworld` subsection:

----
[plugin "helloworld"]
  language = Latin
----

Via the `com.google.gerrit.server.config.PluginConfigFactory` class a
plugin can easily access its configuration and there is no need for a
plugin to parse the `gerrit.config` file on its own:

[source,java]
----
@Inject
private com.google.gerrit.server.config.PluginConfigFactory cfg;

[...]

String language = cfg.getFromGerritConfig("helloworld")
                     .getString("language", "English");
----

[[project-specific-configuration]]
Project Specific Configuration
------------------------------

In Gerrit, project specific configuration is stored in the project's
`project.config` file on the `refs/meta/config` branch.  If a plugin
needs configuration on project level (e.g. to enable its functionality
only for certain projects), this configuration should be stored in a
`plugin` subsection in the project's `project.config` file.

This approach of storing the plugin configuration is only suitable for
plugins that have a simple configuration that only consists of
key-value pairs. With this approach it is not possible to have
subsections in the plugin configuration. Plugins that require a complex
configuration need to store their configuration in their own
configuration file where they can make use of subsections. On the other
hand storing the plugin configuration in a 'plugin' subsection in the
`project.config` file has the advantage that project owners have all
configuration parameters in one file, instead of having one
configuration file per plugin.

To avoid conflicts with other plugins, it is recommended that plugins
only use the `plugin` subsection with their own name. For example the
`helloworld` plugin should store its configuration in the
`plugin.helloworld` subsection:

----
  [plugin "helloworld"]
    enabled = true
----

Via the `com.google.gerrit.server.config.PluginConfigFactory` class a
plugin can easily access its project specific configuration and there
is no need for a plugin to parse the `project.config` file on its own:

[source,java]
----
@Inject
private com.google.gerrit.server.config.PluginConfigFactory cfg;

[...]

boolean enabled = cfg.getFromProjectConfig(project, "helloworld")
                     .getBoolean("enabled", false);
----

It is also possible to get missing configuration parameters inherited
from the parent projects:

[source,java]
----
@Inject
private com.google.gerrit.server.config.PluginConfigFactory cfg;

[...]

boolean enabled = cfg.getFromProjectConfigWithInheritance(project, "helloworld")
                     .getBoolean("enabled", false);
----

Project owners can edit the project configuration by fetching the
`refs/meta/config` branch, editing the `project.config` file and
pushing the commit back.

[[capabilities]]
Plugin Owned Capabilities
-------------------------

Plugins may provide their own capabilities and restrict usage of SSH
commands to the users who are granted those capabilities.

Plugins define the capabilities by overriding the `CapabilityDefinition`
abstract class:

[source,java]
----
public class PrintHelloCapability extends CapabilityDefinition {
  @Override
  public String getDescription() {
    return "Print Hello";
  }
}
----

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

[source,java]
----
@Export("printHello")
public class PrintHelloCapability extends CapabilityDefinition {
  [...]
}
----

Otherwise the capability must be bound in a plugin module:

[source,java]
----
public class HelloWorldModule extends AbstractModule {
  @Override
  protected void configure() {
    bind(CapabilityDefinition.class)
      .annotatedWith(Exports.named("printHello"))
      .to(PrintHelloCapability.class);
  }
}
----

With a plugin-owned capability defined in this way, it is possible to restrict
usage of an SSH command or `UiAction` to members of the group that were granted
this capability in the usual way, using the `RequiresCapability` annotation:

[source,java]
----
@RequiresCapability("printHello")
@CommandMetaData(name="print", description="Print greeting in different languages")
public final class PrintHelloWorldCommand extends SshCommand {
  [...]
}
----

Or with `UiAction`:

[source,java]
----
@RequiresCapability("printHello")
public class SayHelloAction extends UiAction<RevisionResource>
  implements RestModifyView<RevisionResource, SayHelloAction.Input> {
  [...]
}
----

Capability scope was introduced to differentiate between plugin-owned
capabilities and core capabilities. Per default the scope of the
`@RequiresCapability` annotation is `CapabilityScope.CONTEXT`, that means:

* when `@RequiresCapability` is used within a plugin the scope of the
capability is assumed to be that plugin.

* If `@RequiresCapability` is used within the core Gerrit Code Review server
(and thus is outside of a plugin) the scope is the core server and will use
the `GlobalCapability` known to Gerrit Code Review server.

If a plugin needs to use a core capability name (e.g. "administrateServer")
this can be specified by setting `scope = CapabilityScope.CORE`:

[source,java]
----
@RequiresCapability(value = "administrateServer", scope =
    CapabilityScope.CORE)
  [...]
----

[[ui_extension]]
UI Extension
------------

Plugins can contribute their own UI actions on core Gerrit pages.
This is useful for workflow customization or exposing plugin functionality
through the UI in addition to SSH commands and the REST API.

For instance a plugin to integrate Jira with Gerrit changes may contribute its
own "File bug" button to allow filing a bug from the change page or plugins to
integrate continuous integration systems may contribute a "Schedule" button to
allow a CI build to be scheduled manually from the patch set panel.

Two different places on core Gerrit pages are currently supported:

* Change screen
* Project info screen

Plugins contribute UI actions by implementing the `UiAction` interface:

[source,java]
----
@RequiresCapability("printHello")
class HelloWorldAction implements UiAction<RevisionResource>,
    RestModifyView<RevisionResource, HelloWorldAction.Input> {
  static class Input {
    boolean french;
    String message;
  }

  private Provider<CurrentUser> user;

  @Inject
  HelloWorldAction(Provider<CurrentUser> user) {
    this.user = user;
  }

  @Override
  public String apply(RevisionResource rev, Input input) {
    final String greeting = input.french
        ? "Bonjour"
        : "Hello";
    return String.format("%s %s from change %s, patch set %d!",
        greeting,
        Strings.isNullOrEmpty(input.message)
            ? Objects.firstNonNull(user.get().getUserName(), "world")
            : input.message,
        rev.getChange().getId().toString(),
        rev.getPatchSet().getPatchSetId());
  }

  @Override
  public Description getDescription(
      RevisionResource resource) {
    return new Description()
        .setLabel("Say hello")
        .setTitle("Say hello in different languages");
  }
}
----

Sometimes plugins may want to be able to change the state of a patch set or
change in the `UiAction.apply()` method and reflect these changes on the core
UI. For example a buildbot plugin which exposes a 'Schedule' button on the
patch set panel may want to disable that button after the build was scheduled
and update the tooltip of that button. But because of Gerrit's caching
strategy the following must be taken into consideration.

The browser is allowed to cache the `UiAction` information until something on
the change is modified. More accurately the change row needs to be modified in
the database to have a more recent `lastUpdatedOn` or a new `rowVersion`, or
the +refs/meta/config+ of the project or any parents needs to change to a new
SHA-1. The ETag SHA-1 computation code can be found in the
`ChangeResource.getETag()` method.

The easiest way to accomplish this is to update `lastUpdatedOn` of the change:

[source,java]
----
@Override
public Object apply(RevisionResource rcrs, Input in) {
  // schedule a build
  [...]
  // update change
  ReviewDb db = dbProvider.get();
  db.changes().beginTransaction(change.getId());
  try {
    change = db.changes().atomicUpdate(
      change.getId(),
      new AtomicUpdate<Change>() {
        @Override
        public Change update(Change change) {
          ChangeUtil.updated(change);
          return change;
        }
      });
    db.commit();
  } finally {
    db.rollback();
  }
  [...]
}
----

`UiAction` must be bound in a plugin module:

[source,java]
----
public class Module extends AbstractModule {
  @Override
  protected void configure() {
    install(new RestApiModule() {
      @Override
      protected void configure() {
        post(REVISION_KIND, "say-hello")
            .to(HelloWorldAction.class);
      }
    });
  }
}
----

The module above must be declared in pom.xml for Maven driven plugins:

[source,xml]
----
<manifestEntries>
  <Gerrit-Module>com.googlesource.gerrit.plugins.cookbook.Module</Gerrit-Module>
</manifestEntries>
----

or in the BUCK configuration file for Buck driven plugins:

[source,python]
----
manifest_entries = [
  'Gerrit-Module: com.googlesource.gerrit.plugins.cookbook.Module',
]
----

In some use cases more user input must be gathered, for that `UiAction` can be
combined with the JavaScript API. This would display a small popup near the
activation button to gather additional input from the user. The JS file is
typically put in the `static` folder within the plugin's directory:

[source,javascript]
----
Gerrit.install(function(self) {
  function onSayHello(c) {
    var f = c.textfield();
    var t = c.checkbox();
    var b = c.button('Say hello', {onclick: function(){
      c.call(
        {message: f.value, french: t.checked},
        function(r) {
          c.hide();
          window.alert(r);
          c.refresh();
        });
    }});
    c.popup(c.div(
      c.prependLabel('Greeting message', f),
      c.br(),
      c.label(t, 'french'),
      c.br(),
      b));
    f.focus();
  }
  self.onAction('revision', 'say-hello', onSayHello);
});
----

The JS module must be exposed as a `WebUiPlugin` and bound as
an HTTP Module:

[source,java]
----
public class HttpModule extends HttpPluginModule {
  @Override
  protected void configureServlets() {
    DynamicSet.bind(binder(), WebUiPlugin.class)
        .toInstance(new JavaScriptPlugin("hello.js"));
  }
}
----

The HTTP module above must be declared in pom.xml for Maven driven plugins:

[source,xml]
----
<manifestEntries>
  <Gerrit-HttpModule>com.googlesource.gerrit.plugins.cookbook.HttpModule</Gerrit-HttpModule>
</manifestEntries>
----

or in the BUCK configuration file for Buck driven plugins

[source,python]
----
manifest_entries = [
  'Gerrit-HttpModule: com.googlesource.gerrit.plugins.cookbook.HttpModule',
]
----

If `UiAction` is annotated with the `@RequiresCapability` annotation, then the
capability check is done during the `UiAction` gathering, so the plugin author
doesn't have to set `UiAction.Description.setVisible()` explicitly in this
case.

The following prerequisities must be met, to satisfy the capability check:

* user is authenticated
* user is a member of the Administrators group, or
* user is a member of a group which has the required capability

The `apply` method is called when the button is clicked. If `UiAction` is
combined with JavaScript API (its own JavaScript function is provided),
then a popup dialog is normally opened to gather additional user input.
A new button is placed on the popup dialog to actually send the request.

Every `UiAction` exposes a REST API endpoint. The endpoint from the example above
can be accessed from any REST client, i. e.:

====
  curl -X POST -H "Content-Type: application/json" \
    -d '{message: "François", french: true}' \
    --digest --user joe:secret \
    http://host:port/a/changes/1/revisions/1/cookbook~say-hello
  "Bonjour François from change 1, patch set 1!"
====

A special case is to bind an endpoint without a view name.  This is
particularly useful for DELETE requests:

[source,java]
----
public class Module extends AbstractModule {
  @Override
  protected void configure() {
    install(new RestApiModule() {
      @Override
      protected void configure() {
        delete(PROJECT_KIND)
            .to(DeleteProject.class);
      }
    });
  }
}
----

For a `UiAction` bound this way, a JS API function can be provided.

Currently only one restriction exists: per plugin only one `UiAction`
can be bound per resource without view name. To define a JS function
for the `UiAction`, "/" must be used as the name:

[source,javascript]
----
Gerrit.install(function(self) {
  function onDeleteProject(c) {
    [...]
  }
  self.onAction('project', '/', onDeleteProject);
});
----

[[top-menu-extensions]]
Top Menu Extensions
-------------------

Plugins can contribute items to Gerrit's top menu.

A single top menu extension can have multiple elements and will be put as
the last element in Gerrit's top menu.

Plugins define the top menu entries by implementing `TopMenu` interface:

[source,java]
----
public class MyTopMenuExtension implements TopMenu {

  @Override
  public List<MenuEntry> getEntries() {
    return Lists.newArrayList(
               new MenuEntry("Top Menu Entry", Lists.newArrayList(
                      new MenuItem("Gerrit", "http://gerrit.googlecode.com/"))));
  }
}
----

Plugins can also add additional menu items to Gerrit's top menu entries
by defining a `MenuEntry` that has the same name as a Gerrit top menu
entry:

[source,java]
----
public class MyTopMenuExtension implements TopMenu {

  @Override
  public List<MenuEntry> getEntries() {
    return Lists.newArrayList(
               new MenuEntry(GerritTopMenu.PROJECTS, Lists.newArrayList(
                      new MenuItem("Browse Repositories", "https://gerrit.googlesource.com/"))));
  }
}
----

If no Guice modules are declared in the manifest, the top menu extension may use
auto-registration by providing an `@Listen` annotation:

[source,java]
----
@Listen
public class MyTopMenuExtension implements TopMenu {
  [...]
}
----

Otherwise the top menu extension must be bound in the plugin module used
for the Gerrit system injector (Gerrit-Module entry in MANIFEST.MF):

[source,java]
----
package com.googlesource.gerrit.plugins.helloworld;

public class HelloWorldModule extends AbstractModule {
  @Override
  protected void configure() {
    DynamicSet.bind(binder(), TopMenu.class).to(MyTopMenuExtension.class);
  }
}
----

[source,manifest]
----
Gerrit-ApiType: plugin
Gerrit-Module: com.googlesource.gerrit.plugins.helloworld.HelloWorldModule
----

[[http]]
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:

[source,java]
----
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");
  }
}
----

The auto registration only works for standard servlet mappings like
`/foo` or `/foo/*`. Regex style bindings must use a Guice ServletModule
to register the HTTP servlets and declare it explicitly in the manifest
with the `Gerrit-HttpModule` attribute:

[source,java]
----
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
----

[[data-directory]]
Data Directory
--------------

Plugins can request a data directory with a `@PluginData` File
dependency. A data directory will be created automatically by the
server in `$site_path/data/$plugin_name` and passed to the plugin.

Plugins can use this to store any data they want.

[source,java]
----
@Inject
MyType(@PluginData java.io.File myDir) {
  new FileInputStream(new File(myDir, "my.config"));
}
----

[[download-commands]]
Download Commands
-----------------

Gerrit offers commands for downloading changes using different
download schemes (e.g. for downloading via different network
protocols). Plugins can contribute download schemes and download
commands by implementing
`com.google.gerrit.extensions.config.DownloadScheme` and
`com.google.gerrit.extensions.config.DownloadCommand`.

The download schemes and download commands which are used most often
are provided by the Gerrit core plugin `download-commands`.

[[documentation]]
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 the `static/` directory in the JAR will be
available as `/plugins/helloworld/static/resource`. This prefix is
configurable by setting the `Gerrit-HttpStaticPrefix` attribute.

Documentation files under the `Documentation/` directory in the JAR
will be available as `/plugins/helloworld/Documentation/resource`. This
prefix is configurable by setting the `Gerrit-HttpDocumentationPrefix`
attribute.

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`.

[[macros]]
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.

[[auto-index]]
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 is named `about.md` or `about.html`, its
content will be inserted in an 'About' section at the top of the
auto-generated index page.  If both `about.md` and `about.html`
exist, only the first discovered file will be used.

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

If a discovered file name beings with `servlet-` it will be clustered
into a 'Servlets' section of the generated index page.

If a discovered file name beings with `rest-api-` it will be clustered
into a 'REST APIs' 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]]
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.

Disabled plugins can be re-enabled using the
link:cmd-plugin-enable.html[plugin enable] command.

SEE ALSO
--------

* link:js-api.html[JavaScript API]
* link:dev-rest-api.html[REST API Developers' Notes]

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