You need to use Java 7 and Node.js for building gerrit.

There is currently no binary distribution of Buck, so it has to be manually built and installed. Apache Ant and gcc are required. Currently only Linux and Mac OS are supported.

Clone the git and build it:

  git clone
  cd buck
  git checkout $(cat ../gerrit/.buckversion)

If you don’t have a bin/ directory in your home directory, create one:

  mkdir ~/bin

Add the ~/bin folder to the path:


Note that the buck executable needs to be available in all shell sessions, so also make sure it is appended to the path globally.

Add a symbolic link in ~/bin to the buck and buckd executables:

  ln -s `pwd`/bin/buck ~/bin/
  ln -s `pwd`/bin/buckd ~/bin/

Verify that buck is accessible:

  which buck

To enable autocompletion of buck commands, install the autocompletion script from ./scripts/buck-completion.bash in the buck project. Refer to the script’s header comments for installation instructions.


Buck requires Python version 2.7 to be installed. The Maven download toolchain requires curl to be installed.

Eclipse Integration

Generating the Eclipse Project

Create the Eclipse project:


and then follow the setup instructions.

Refreshing the Classpath

If an updated classpath is needed, the Eclipse project can be refreshed and missing dependency JARs can be downloaded:


Attaching Sources

Source JARs are downloaded by default. This allows Eclipse to show documentation or dive into the implementation of a library JAR.

To save time and bandwidth, download of source JARs can be restricted to only those that are necessary to compile Java source into JavaScript using the GWT compiler:

  tools/eclipse/ --no-src

Building on the Command Line

Gerrit Development WAR File

To build the Gerrit web application that includes GWT UI and PolyGerrit UI:

  buck build gerrit
PolyGerrit UI may require additional tools (such as npm). Please read the polygerrit-ui/ for more info.

The output executable WAR will be placed in:


To build the Gerrit web application that includes only GWT UI:

  buck build gwtgerrit

The output executable WAR will be placed in:


Headless Mode

To build Gerrit in headless mode, i.e. without the GWT Web UI:

  buck build headless

The output executable WAR will be placed in:


Extension and Plugin API JAR Files

To build the extension, plugin and GWT API JAR files:

  buck build api

Java binaries, Java sources and Java docs are generated into corresponding project directories in buck-out/gen, here as example for plugin API:


Install {extension,plugin,gwt}-api to the local maven repository:

  tools/maven/ install

Install gerrit.war to the local maven repository:

  tools/maven/ war_install


To build all core plugins:

  buck build plugins:core

The output JAR files for individual plugins will be placed in:


The JAR files will also be packaged in:


To build a specific plugin:

  buck build plugins/<name>:<name>

The output JAR file will be be placed in:


Note that when building an individual plugin, the package is not regenerated.

Additional plugins with BUCK files can be added to the build environment by cloning the source repository into the plugins subdirectory:

  git clone<name> plugins/<name>
  echo /plugins/<name> >>.git/info/exclude

Additional plugin sources will be automatically added to Eclipse the next time is run:



To build only the documentation for testing or static hosting:

  buck build docs

The generated html files will NOT come with the search box, and will be placed in:


The html files will also be bundled into in this location:


To build the executable WAR with the documentation included:

  buck build withdocs

The WAR file will be placed in:


GWT Compile Report

The GWT compiler can output a compile report (or "story of your compile"), describing the size of the JavaScript and which source classes contributed to the overall download size.

  buck build soyc

The report will be written as an HTML page to the extras directory, and can be opened and viewed in any web browser:


Only the "Split Point Report" is created, "Compiler Metrics" are not output.

Gerrit Release WAR File

To build the release of the Gerrit web application, including documentation and all core plugins:

  buck build release

The output release WAR will be placed in:


Running Unit Tests

To run all tests including acceptance tests (but not flaky tests):

  buck test --exclude flaky

To exclude flaky and slow tests:

  buck test --exclude flaky slow

To run only a specific group of acceptance tests:

  buck test --include api

The following groups of tests are currently supported:

  • acceptance

  • api

  • edit

  • flaky

  • git

  • pgm

  • rest

  • server

  • ssh

  • slow

To run a specific test group, e.g. the rest-account test group:

  buck test //gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/account:rest-account

To create test coverage report:

  buck test --code-coverage --code-coverage-format html --no-results-cache

The HTML report is created in buck-out/gen/jacoco/code-coverage/index.html.


Dependency JARs are normally downloaded automatically, but Buck can inspect its graph and download any missing JAR files. This is useful to enable subsequent builds to run without network access:


When downloading from behind a proxy (which is common in some corporate environments), it might be necessary to explicitly specify the proxy that is then used by curl:

  export http_proxy=http://<proxy_user_id>:<proxy_password>@<proxy_server>:<proxy_port>

Redirection to local mirrors of Maven Central and the Gerrit storage bucket is supported by defining specific properties in, a file that is not tracked by Git:

  echo download.GERRIT = >>
  echo download.MAVEN_CENTRAL = >>

The file may be placed in the root of the gerrit repository being built, or in ~/.gerritcodereview/. The file in the root of the gerrit repository has precedence.

Building against unpublished Maven JARs

To build against unpublished Maven JARs, like gwtorm or PrologCafe, the custom JARs must be installed in the local Maven repository (mvn clean install) and maven_jar() must be updated to point to the MAVEN_LOCAL Maven repository for that artifact:

   name = 'gwtorm',
   id = 'gwtorm:gwtorm:42',
   license = 'Apache2.0',
   repository = MAVEN_LOCAL,

Building against artifacts from custom Maven repositories

To build against custom Maven repositories, two modes of operations are supported: with rewrite in and without.

Without rewrite the URL of custom Maven repository can be directly passed to the maven_jar() function:


    name = 'gitblit',
    id = 'com.gitblit:gitblit:1.4.0',
    sha1 = '1b130dbf5578ace37507430a4a523f6594bf34fa',
    license = 'Apache2.0',
    repository = GERRIT_FORGE,

When the custom URL has to be rewritten, then the same logic as with Gerrit known Maven repository is used: Repo name must be defined that matches an entry in file:

  download.GERRIT_FORGE =

And corresponding BUCK excerpt:


    name = 'gitblit',
    id = 'com.gitblit:gitblit:1.4.0',
    sha1 = '1b130dbf5578ace37507430a4a523f6594bf34fa',
    license = 'Apache2.0',
    repository = GERRIT_FORGE,

Caching Build Results

Build results can be locally cached, saving rebuild time when switching between Git branches. Buck’s documentation covers caching in buckconfig. The trivial case using a local directory is:

  cat >.buckconfig.local <<EOF
    mode = dir
    dir = buck-cache

Cleaning The Buck Cache

The cache for the Gerrit Code Review project is located in ~/.gerritcodereview/buck-cache/locally-built-artifacts.

The Buck cache should never need to be manually deleted. If you find yourself deleting the Buck cache regularly, then it is likely that there is something wrong with your environment or your workflow.

If you really do need to clean the cache manually, then:

 rm -rf ~/.gerritcodereview/buck-cache/locally-built-artifacts

Note that the root buck-cache folder should not be deleted as it also contains the downloaded-artifacts directory, which holds the artifacts that got downloaded (not built locally).

Using Buck daemon

Buck ships with a daemon command buckd, which uses the Nailgun protocol for running Java programs from the command line without incurring the JVM startup overhead.

Using a Buck daemon can save significant amounts of time as it avoids the overhead of starting a Java virtual machine, loading the buck class files and parsing the build files for each command.

It is safe to run several buck daemons started from different project directories and they will not interfere with each other. Buck’s documentation covers daemon in buckd.

To use buckd the additional watchman program must be installed.

To disable buckd, the environment variable NO_BUCKD must be set. It’s not recommended to put it in the shell config, as it can be forgotten about it and then assumed Buck was working as it should when it should be using buckd. Prepend the variable to Buck invocation instead:

  NO_BUCKD=1 buck build gerrit

Installing watchman

Watchman is used internally by Buck to monitor directory trees and is needed for buck daemon to work properly. Because buckd is activated by default in the latest version of Buck, it searches for the watchman executable in the path and issues a warning when it is not found and kills buckd.

To prepare watchman installation on Linux:

  git clone
  cd watchman

To install it in user home directory (without root privileges):

  ./configure --prefix $HOME/watchman
  make install

To install it system wide:

  sudo make install

Put $HOME/watchman/bin/watchman in path or link to $HOME/bin/watchman.

To install watchman on OS X:

  brew install --HEAD watchman

See the original documentation for more information: Watchman installation.

Override Buck’s settings

Additional JVM args for Buck can be set in .buckjavaargs in the project root directory. For example to override Buck’s default 1GB heap size:

  cat > .buckjavaargs <<EOF
  -XX:MaxPermSize=512m -Xms8000m -Xmx16000m

Rerun unit tests

Test execution results are cached by Buck. If a test that was already run needs to be repeated, the unit test cache for that test must be removed first:

  rm -rf buck-out/bin/gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/account/.rest-account/

After clearing the cache, the test can be run again:

  buck test //gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/account:rest-account
  [-] TESTING...FINISHED 12,3s (12 PASS/0 FAIL)
  RESULTS FOR //gerrit-acceptance-tests/src/test/java/com/google/gerrit/acceptance/rest/account:rest-account
  PASS     970ms  2 Passed   0 Skipped   0 Failed
  PASS     999ms  1 Passed   0 Skipped   0 Failed
  PASS      1,2s  1 Passed   0 Skipped   0 Failed
  PASS     951ms  2 Passed   0 Skipped   0 Failed
  PASS      6,4s  2 Passed   0 Skipped   0 Failed
  PASS      1,2s  4 Passed   0 Skipped   0 Failed

An alternative approach is to use Buck’s --filters (-f) option:

  buck test -f ''
  Using buckd.
  [-] BUILDING...FINISHED 2,8s [100%] (334/701 JOBS, 110 UPDATED, 5,1% CACHE MISS)
  PASS      8,0s  2 Passed   0 Skipped   0 Failed
  PASS    <100ms  4 Passed   0 Skipped   0 Failed   //tools:util_test

When this option is used, the cache is disabled per design and doesn’t need to be explicitly deleted. Note, that this is a known issue, that python tests are always executed.

Note that when this option is used, the whole unit test cache is dropped, so repeating the

buck test

causes all tests to be executed again.

To run tests without using cached results at all, use the --no-results-cache option:

buck test --no-results-cache

Upgrading Buck

The following tests should be executed, when Buck version is upgraded:

  • buck build release

  • tools/maven/ install

  • buck test

  • buck build gerrit, change some sources in gerrit-server project, repeat buck build gerrit and verify that gerrit.war was updated

  • install and verify new gerrit site

  • upgrade and verify existing gerrit site

  • reindex existing gerrit site

  • verify that tools/eclipse/ produces sane Eclipse project

  • verify that tools/eclipse/ --src generates sources as well

  • verify that unit test execution from Eclipse works

  • verify that daemon started from Eclipse works

  • verify that GWT SDM debug session started from Eclipse works

Known issues and bugs

Buck with activated Watchman has currently a [known bug]( related to symbolic links. The symbolic links are used very often with external plugins, that are linked per symbolic link to the plugins directory. With this use case Buck is failing to rebuild the plugin artifact after it was built. All attempts to convince Buck to rebuild will fail. The only known way to recover is to weep out buck-out directory. The better workaround is to avoid using Watchman in this specific use case. Watchman can either be de-installed or disabled. See Using Buck daemon section above how to temporarily disable buckd.

Troubleshooting Buck

In some cases problems with Buck itself need to be investigated. See for example this attempt to upgrade Buck and the fix that was needed to make the update possible.

To build Gerrit with a custom version of Buck, the following steps are necessary:

  1. In the Buck git apply any necessary changes from pull requests

  2. Compile Buck with ant

  3. In the root of the Gerrit project create a .nobuckcheck file to prevent Buck from updating itself

  4. Replace the sha1 in Gerrit’s .buckversion file with the required version from the custom Buck build

  5. Build Gerrit as usual