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Working with external dependencies

Bazel can depend on targets from other projects. Dependencies from these other projects are called external dependencies.

The WORKSPACE file in the workspace directory tells Bazel how to get other projects' sources. These other projects can contain one or more BUILD files with their own targets. BUILD files within the main project can depend on these external targets by using their name from the WORKSPACE file.

For example, suppose there are two projects on a system:

/
  home/
    user/
      project1/
        WORKSPACE
        BUILD
        srcs/
          ...
      project2/
        WORKSPACE
        BUILD
        my-libs/

If project1 wanted to depend on a target, :foo, defined in /home/user/project2/BUILD, it could specify that a repository named project2 could be found at /home/user/project2. Then targets in /home/user/project1/BUILD could depend on @project2//:foo.

The WORKSPACE file allows users to depend on targets from other parts of the filesystem or downloaded from the internet. Users can also write custom repository rules to get more complex behavior.

This WORKSPACE file uses the same syntax as BUILD files, but allows a different set of rules. The full list of built-in rules are in the Build Encyclopedia's Workspace Rules.

Supported types of external dependencies

A few basic types of external dependencies can be used:

Depending on other Bazel projects

If you want to use targets from a second Bazel project, you can use local_repository, git_repository or http_archive to symlink it from the local filesystem, reference a git repository or download it (respectively).

For example, suppose you are working on a project, my-project/, and you want to depend on targets from your coworker's project, coworkers-project/. Both projects use Bazel, so you can add your coworker's project as an external dependency and then use any targets your coworker has defined from your own BUILD files. You would add the following to my_project/WORKSPACE:

local_repository(
    name = "coworkers-project",
    path = "/path/to/coworkers-project",
)

If your coworker has a target //foo:bar, your project can refer to it as @coworkers-project//foo:bar.

Depending on non-Bazel projects

Rules prefixed with new_ (e.g., new_local_repository, new_git_repository and new_http_archive ) allow you to create targets from projects that do not use Bazel.

For example, suppose you are working on a project, my-project/, and you want to depend on your coworker's project, coworkers-project/. Your coworker's project uses make to build, but you'd like to depend on one of the .so files it generates. To do so, add the following to my_project/WORKSPACE:

new_local_repository(
    name = "coworkers-project",
    path = "/path/to/coworkers-project",
    build_file = "coworker.BUILD",
)

build_file specifies a BUILD file to overlay on the existing project, for example:

java_library(
    name = "some-lib",
    srcs = glob(["**"]),
    visibility = ["//visibility:public"],
)

You can then depend on @coworkers-project//:some-lib from your project's BUILD files.

Depending on external packages

Maven repositories

Use the rule maven_jar (and optionally the rule maven_server) to download a jar from a Maven repository and make it available as a Java dependency.

Fetching dependencies

By default, external dependencies are fetched as needed during bazel build. If you would like to disable this behavior or prefetch dependencies, use bazel fetch.

Using Proxies

Bazel will pick up proxy addresses from the HTTPS_PROXY and HTTP_PROXY environment variables and use these to download HTTP/HTTPS files (if specified).

Transitive dependencies

Bazel only reads dependencies listed in your WORKSPACE file. If your project (A) depends on another project (B) which list a dependency on a third project (C) in its WORKSPACE file, you'll have to add both B and C to your project's WORKSPACE file. This requirement can balloon the WORKSPACE file size, but hopefully limits the chances of having one library include C at version 1.0 and another include C at 2.0.

Large WORKSPACE files can be generated using the tool generate_workspace. For details, see Generate external dependencies from Maven projects.

Caching of external dependencies

Bazel caches external dependencies and re-downloads or updates them when the WORKSPACE file changes.

Layout

External dependencies are all downloaded and symlinked under a directory named external. You can see this directory by running:

ls $(bazel info output_base)/external

Note that running bazel clean will not actually delete the external directory. To remove all external artifacts, use bazel clean --expunge.