Gazelle is a build file generator for Bazel projects. It can create new BUILD.bazel files for a project that follows language conventions, and it can update existing build files to include new sources, dependencies, and options. Gazelle natively supports Go and protobuf, and it may be extended to support new languages and custom rule sets.

Gazelle may be run by Bazel using the gazelle rule, or it may be installed and run as a command line tool. Gazelle can also generate build files for external repositories as part of the go_repository rule.

Gazelle is under active development. Its interface and the rules it generates may change. Gazelle is not an official Google product.

Mailing list: bazel-go-discuss

Slack: #go on Bazel Slack, #bazel on Go Slack

rules_go and Gazelle are getting community maintainers! If you are a regular user of either project and are interested in helping out with development, code reviews, and issue triage, please drop by our Slack channels (linked above) and say hello!

See also:

• Architecture of Gazelle
• Repository rules
  • go_repository
• Extending Gazelle
• Avoiding conflicts with proto rules

Gazelle can generate Bazel BUILD files for many languages:

• Go

  Go supported is included here in bazel-gazelle, see below.

• Haskell

  Tweag's rules_haskell has two extensions: gazelle_cabal, for generating rules from Cabal files and gazelle_haskell_modules for even more fine-grained build definitions.

• Java

  bazel-contrib's rules_jvm extensions include a gazelle extension for generating java_library, java_binary, java_test, and java_test_suite rules.

• JavaScript / TypeScript

  Aspect provides JavaScript and TypeScript Support in aspect-cli (also usable separately).

  BenchSci's rules_nodejs_gazelle supports generating ts_project, js_library, jest_test, and web_asset rules, and is able to support module bundlers like Webpack and Next.js

• Kotlin

  Aspect Build provides some Kotlin Support in the repo of their aspect-cli (also usable separately). Still under development, please check the README for currently available features.

• Protocol Buffers

  Support for the proto_library rule, as well as go_proto_library is in this repository, see below. Other language-specific proto rules are not supported here. stackb/rules_proto is a good resource for these rules.

• Python

  rules_python has an extension for generating py_library, py_binary, and py_test rules.

• R

  rules_r has an extension for generating rules for R package builds and tests.

• Rust

  gazelle_rust is an extension for generating rules_rust targets.

• Starlark

  bazel-skylib has an extension for generating bzl_library rules. See bazel_skylib//gazelle/bzl.

• Swift

  rules_swift_package_manager has an extension for generating swift_library, swift_binary, and swift_test rules. It also includes facilities for resolving, downloading and building external Swift packages for a Bazel workspace.

If you know of an extension which could be linked here, please open a PR!

More languages can be added by Extending Gazelle. Chat with us in the #gazelle channel on Bazel Slack if you'd like to discuss your design.

If you've written your own extension, please consider open-sourcing it for use by the rest of the community. Note that such extensions belong in a language-specific repository, not in bazel-gazelle. See discussion in #1030.

To use Gazelle in a new project, add the bazel_gazelle repository and its dependencies to your WORKSPACE file and call gazelle_dependencies. It should look like this:

load("@bazel_tools//tools/build_defs/repo:http.bzl", "http_archive")

http_archive(
    name = "io_bazel_rules_go",
    sha256 = "80a98277ad1311dacd837f9b16db62887702e9f1d1c4c9f796d0121a46c8e184",
    urls = [
        "https://mirror.bazel.build/github.com/bazelbuild/rules_go/releases/download/v0.46.0/rules_go-v0.46.0.zip",
        "https://github.com/bazelbuild/rules_go/releases/download/v0.46.0/rules_go-v0.46.0.zip",
    ],
)

http_archive(
    name = "bazel_gazelle",
    integrity = "sha256-MpOL2hbmcABjA1R5Bj2dJMYO2o15/Uc5Vj9Q0zHLMgk=",
    urls = [
        "https://mirror.bazel.build/github.com/bazelbuild/bazel-gazelle/releases/download/v0.35.0/bazel-gazelle-v0.35.0.tar.gz",
        "https://github.com/bazelbuild/bazel-gazelle/releases/download/v0.35.0/bazel-gazelle-v0.35.0.tar.gz",
    ],
)


load("@io_bazel_rules_go//go:deps.bzl", "go_register_toolchains", "go_rules_dependencies")
load("@bazel_gazelle//:deps.bzl", "gazelle_dependencies", "go_repository")

############################################################
# Define your own dependencies here using go_repository.
# Else, dependencies declared by rules_go/gazelle will be used.
# The first declaration of an external repository "wins".
############################################################

go_rules_dependencies()

go_register_toolchains(version = "1.20.5")

gazelle_dependencies()

gazelle_dependencies supports optional argument go_env (dict-mapping) to set project specific go environment variables. If you are using a WORKSPACE.bazel file, you will need to specify that using:

gazelle_dependencies(go_repository_default_config = "//:WORKSPACE.bazel")

Add the code below to the BUILD or BUILD.bazel file in the root directory of your repository.

Important: For Go projects, replace the string after prefix with the portion of your import path that corresponds to your repository.

load("@bazel_gazelle//:def.bzl", "gazelle")

# gazelle:prefix github.com/example/project
gazelle(name = "gazelle")

After adding this code, you can run Gazelle with Bazel.

$ bazel run //:gazelle

This will generate new BUILD.bazel files for your project. You can run the same command in the future to update existing BUILD.bazel files to include new source files or options.

You can write other gazelle rules to run alternate commands like update-repos.

gazelle(
    name = "gazelle-update-repos",
    args = [
        "-from_file=go.mod",
        "-to_macro=deps.bzl%go_dependencies",
        "-prune",
    ],
    command = "update-repos",
)

You can also pass additional arguments to Gazelle after a -- argument.

$ bazel run //:gazelle -- update-repos -from_file=go.mod -to_macro=deps.bzl%go_dependencies

After running update-repos, you might want to run bazel run //:gazelle again, as the update-repos command can affect the output of a normal run of Gazelle.

If you have a Go toolchain installed, you can install Gazelle with the command below:

go install github.com/bazelbuild/bazel-gazelle/cmd/gazelle@latest

Make sure to re-run this command to upgrade Gazelle whenever you upgrade rules_go in your repository.

To generate BUILD.bazel files in a new project, run the command below, replacing the prefix with the portion of your import path that corresponds to your repository.

gazelle -go_prefix github.com/example/project

Most of Gazelle's command-line arguments can be expressed as special comments in build files. See Directives below. You may want to copy this line into your root build files to avoid having to type -go_prefix every time.

# gazelle:prefix github.com/example/project

Gazelle is compatible with supported releases of Go, per the Go Release Policy. The Go Team officially supports the current and previous minor releases. Older releases are not supported and don't receive bug fixes or security updates.

Gazelle may use language and library features from the oldest supported release.

Gazelle generates build files that use features in newer versions of rules_go. Newer versions of Gazelle may generate build files that work with older versions of rules_go, but check the table below to ensure you're using a compatible version.

gazelle <command> [flags...] [package-dirs...]

The first argument to Gazelle may be one of the commands below. If no command is specified, update is assumed. The remaining arguments are specific to each command and are documented below.

update

Scans sources files, then generates and updates build files.

fix

Same as the update command, but it also fixes deprecated usage of rules.

update-repos

Adds and updates repository rules in the WORKSPACE file.

Gazelle may be run via a rule. See Running Gazelle with Bazel for setup instructions. This rule builds Gazelle and generates a wrapper script that executes Gazelle with baked-in set of arguments. You can run this script with bazel run, or you can copy it into your workspace and run it directly.

The following attributes are available on the gazelle rule.

The update command is the most common way of running Gazelle. Gazelle scans sources in directories throughout the repository, then creates and updates build files.

The fix command does everything update does, but it also fixes deprecated usage of rules, analogous to go fix. For example, cgo_library will be consolidated with go_library. This command may delete or rename rules, so it's not on by default. See Fix command transformations for details.

Both commands accept a list of directories to process as positional arguments. If no directories are specified, Gazelle will process the current directory. Subdirectories will be processed recursively.

The following flags are accepted:

The update-repos command updates repository rules. It can write the rules to either the WORKSPACE (by default) or a .bzl file macro function. It can be used to add new repository rules or update existing rules to the specified version. It can also import repository rules from a go.mod or a go.work file.

WARNING: This command is mainly used for managing external Go dependencies in Bazel's WORKSPACE mode. For managing external Go dependencies in Bazel's BzlMod mode, please check: https://github.com/bazelbuild/rules_go/blob/master/docs/go/core/bzlmod.md#external-dependencies

# Add or update a repository to latest version by import path
$ gazelle update-repos example.com/new/repo

# Add or update a repository to specified version/commit by import path
$ gazelle update-repos example.com/new/repo@v1.3.1

# Import repositories from go.mod
$ gazelle update-repos -from_file=go.mod

# Import repositories from go.work
$ gazelle update-repos -from_file=go.work

# Import repositories from go.mod and update macro
$ gazelle update-repos -from_file=go.mod -to_macro=repositories.bzl%go_repositories

# Import repositories from go.work and update macro
$ gazelle update-repos -from_file=go.work -to_macro=repositories.bzl%go_repositories

The following flags are accepted:

Gazelle can be configured with directives, which are written as top-level comments in build files. Most options that can be set on the command line can also be set using directives. Some options can only be set with directives.

Directive comments have the form # gazelle:key value. For example:

load("@io_bazel_rules_go//go:def.bzl", "go_library")

# gazelle:prefix github.com/example/project
# gazelle:build_file_name BUILD,BUILD.bazel

go_library(
    name = "go_default_library",
    srcs = ["example.go"],
    importpath = "github.com/example/project",
    visibility = ["//visibility:public"],
)

Directives apply in the directory where they are set and in subdirectories. This means, for example, if you set # gazelle:prefix in the build file in your project's root directory, it affects your whole project. If you set it in a subdirectory, it only affects rules in that subtree.

The following directives are recognized:

# gazelle:resolve go example.com/foo //foo:go_def
# gazelle:resolve proto go foo/foo.proto //foo:fo

# gazelle:resolve_regexp go example.com/.* //foo:
# gazelle:resolve_regexp proto go foo/.*\.proto /

# gazelle:default_visibility //foo:__subpackages_

Gazelle also reads directives from the WORKSPACE file. They may be used to discover custom repository names and known prefixes. The fix and update commands use these directives for dependency resolution. update-repos uses them to learn about repository rules defined in alternate locations.

In addition to directives, Gazelle supports # keep comments that protect parts of build files from being modified. # keep may be written before a rule, before an attribute, or after a string within a list.

# keep comments might take one of 2 forms; the # keep literal or a description prefixed by # keep:.

Suppose you have a library that includes a generated .go file. Gazelle won't know what imports to resolve, so you may need to add dependencies manually with # keep comments.

load("@io_bazel_rules_go//go:def.bzl", "go_library")
load("@com_github_example_gen//:gen.bzl", "gen_go_file")

gen_go_file(
    name = "magic",
    srcs = ["magic.go.in"],
    outs = ["magic.go"],
)

go_library(
    name = "go_default_library",
    srcs = ["magic.go"],
    visibility = ["//visibility:public"],
    deps = [
        "@com_github_example_gen//:go_default_library",  # keep
        "@com_github_example_gen//a/b/c:go_default_library",  # keep: this is also important
    ],
)

One of Gazelle's most important jobs is resolving library import strings (like import "golang.org/x/sys/unix") to Bazel labels (like @org_golang_x_sys//unix:go_default_library). Gazelle follows the rules below to resolve dependencies:

1. If the import to be resolved is part of a standard library, no explicit dependency is written. For example, in Go, you don't need to declare that you depend on "fmt".
2. If a # gazelle:resolve directive matches the import to be resolved, the label at the end of the directive will be used.
3. If proto rule generation is enabled, special rules will be used when importing certain libraries. These rules may be disabled by adding # gazelle:proto disable_global to a build file (this will affect subdirectories, too) or by passing -proto disable_global on the command line.
   1. Imports of Well Known Types are mapped to rules in @io_bazel_rules_go//proto/wkt.
   2. Imports of Google APIs are mapped to @go_googleapis.
   3. Imports of github.com/golang/protobuf/ptypes, descriptor, and jsonpb are mapped to special rules in @com_github_golang_protobuf. See Avoiding conflicts with proto rules.
4. If the import to be resolved is in the library index, the import will be resolved to that library. If -index=true, Gazelle builds an index of library rules in the current repository before starting dependency resolution, and this is how most dependencies are resolved.
   1. For Go, the match is based on the importpath attribute.
   2. For proto, the match is based on the srcs attribute.
5. If -index=false and a package is imported that has the current go_prefix as a prefix, Gazelle generates a label following a convention. For example, if the build file in //src set the prefix with # gazelle:prefix example.com/repo/foo, and you import the library "example.com/repo/foo/bar, the dependency will be "//src/foo/bar:go_default_library".
6. Otherwise, Gazelle will use the current external mode to resolve the dependency.
   1. In external mode (the default), Gazelle will transform the import string into an external repository label. For example, "golang.org/x/sys/unix" would be resolved to "@org_golang_x_sys//unix:go_default_library". Gazelle does not confirm whether the external repository is actually declared in WORKSPACE, but if there is a go_repository in WORKSPACE with a matching importpath, Gazelle will use its name. Gazelle does not index rules in external repositories, so it's possible the resolved dependency does not exist.
   2. In static mode, Gazelle has the same behavior as external mode, except that it will not call out to the network for resolution when no matching import is found within WORKSPACE. Instead, it will skip the unknown import. This is the default mode for go_repository rules.
   3. In vendored mode, Gazelle will transform the import string into a label in the vendor directory. For example, "golang.org/x/sys/unix" would be resolved to "//vendor/golang.org/x/sys/unix:go_default_library". This mode is usually not necessary, since vendored libraries will be indexed and resolved using rule 4.

Gazelle will generate and update build files when invoked with either gazelle update or gazelle fix (update is the default). Both commands perform several transformations to fix deprecated usage of the Go rules. update performs a safe set of tranformations, while fix performs some additional transformations that may delete or rename rules.

The following transformations are performed:

Migrate library to embed (fix and update): Gazelle replaces library attributes with embed attributes.

Migrate gRPC compilers (fix and update): Gazelle converts go_grpc_library rules to go_proto_library rules with compilers = ["@io_bazel_rules_go//proto:go_grpc"].

Flatten srcs (fix and update): Gazelle converts srcs attributes that use OS and architecture-specific select expressions to flat lists. rules_go filters these sources anyway.

Squash cgo libraries (fix only): Gazelle will remove cgo_library rules named cgo_default_library and merge their attributes with a go_library rule in the same package named go_default_library. If no such go_library rule exists, a new one will be created. Other cgo_library rules will not be removed.

Squash external tests (fix only): Gazelle will squash go_test rules named go_default_xtest into go_default_test. Earlier versions of rules_go required internal and external tests to be built separately, but this is no longer needed.

Remove legacy protos (fix only): Gazelle will remove usage of go_proto_library rules loaded from @io_bazel_rules_go//proto:go_proto_library.bzl and filegroup rules named go_default_library_protos. Newly generated proto rules will take their place. Since filegroup isn't needed anymore and go_proto_library has different attributes and was always written by hand, Gazelle will not attempt to merge anything from these rules with the newly generated rules.

This transformation is only applied in the default proto mode. Since Gazelle will run in legacy proto mode if go_proto_library.bzl is loaded, this transformation is not usually applied. You can set the proto mode explicitly using the directive # gazelle:proto default.

Update loads of gazelle rule (fix and update): Gazelle will remove loads of gazelle from @io_bazel_rules_go//go:def.bzl. It will automatically add a load from @bazel_gazelle//:def.bzl if gazelle is not loaded from another location.