To use Bazel, first clone the Github repo and build Bazel (follow the instructions in the README to install prerequisites):
$ git clone https://github.com/google/bazel.git $ cd bazel $ ./compile.sh
./compile.sh creates the bazel executable in output/bazel.
Note: Bazel may support a binary installation at a later time.
Bazel is oriented around a workspace – a directory which contains the tools and libraries it needs and projects it builds. A workspace is any directory that contains a WORKSPACE file (marking the directory as a workspace root) and contains its toolchain, third-party libraries, project subdirectories, etc. The workspace is also the directory where build outputs can be easily accessed. Bazel creates bazel-out, bazel-bin and others in the workspace root, which are symbolic links to the real output directories.
One workspace can be shared among multiple projects, if desired. To get started, we'll focus on a simple example with one project.
Suppose that you have an existing project in a directory, say, ~/gitroot/my-project/. Create an empty file at ~/gitroot/my-project/WORKSPACE to show Bazel where your project's root is.
To make sure everything is set up correctly in your build root, build one of the examples from the examples/ directory.
$ cd ~/gitroot/my-project $ bazel build examples/java-native/src/main/java/com/example/myproject:hello-world Extracting Bazel installation... ........... INFO: Found 1 target... Target //examples/java-native/src/main/java/com/example/myproject:hello-world up-to-date: bazel-bin/examples/java-native/src/main/java/com/example/myproject/hello-world.jar bazel-bin/examples/java-native/src/main/java/com/example/myproject/hello-world INFO: Elapsed time: 3.040s, Critical Path: 1.14s $ bazel-bin/examples/java-native/src/main/java/com/example/myproject/hello-world Hello world
Bazel puts binaries it has built under bazel-bin/. Note that you can always look at the build command's output to find output file paths.
Now you can create your own BUILD file and start adding build rules. This example assumes that my-project/ is a Java project. See the build encyclopedia for advice on adding build rules for other languages.
Note that when we ran “bazel build” above, the third argument started with a filesystem path (“examples/java”), followed by a colon. When you run bazel build examples/java-native/src/main/java/com/example/myproject:hello-world, Bazel will look for a special file named BUILD in the examples/java-native/src/main/java/com/example/myproject/ subdirectory. This BUILD file defines rules about how Bazel should build things in this subdirectory.
Thus, to add build rules to my-project, create a file named BUILD in the my-project/ directory. Add the following lines to this BUILD file:
# ~/gitroot/base_workspace/my-project/BUILD java_binary( name = "my-runner", srcs = glob(["**/*.java"]), main_class = "com.example.ProjectRunner", )
BUILD files are Python-like scripts. BUILD files cannot contain arbitrary Python, but each build rule looks like a Python function call and you can use “#” to start a single-line comment.
java_binary is the type of thing this rule will build. name is how you'll refer to the rule when you run “bazel build” (in the “examples/java:hello-world” build above the name was “hello-world”). srcs lists the Java source files Bazel should compile into a Java binary. glob(["**/*.java"]) is a handy shorthand for “recursively include every file that ends with .java” (see the user manual for more information about globbing). Replace com.example.ProjectRunner with the class that contains the main method.
If you have no actual Java project you're using, you can use the following commands to make a fake project for this example:
$ # If you're not already there, move to your build root directory. $ cd ~/gitroot/base_workspace $ mkdir -p my-project/java/com/example $ cat > my-project/java/com/example/ProjectRunner.java <<EOF package com.example; public class ProjectRunner { public static void main(String args[]) { Greeting.sayHi(); } } EOF $ cat > my-project/java/com/example/Greeting.java <<EOF package com.example; public class Greeting { public static void sayHi() { System.out.println("Hi!"); } } EOF
Now build your project:
$ bazel build my-project:my-runner INFO: Found 1 target... Target //my-project:my-runner up-to-date: bazel-bin/my-project/my-runner.jar bazel-bin/my-project/my-runner INFO: Elapsed time: 1.021s, Critical Path: 0.83s $ bazel-bin/my-project/my-runner Hi!
Congratulations, you've created your first Bazel BUILD file!
Creating one rule to build your entire project may be sufficient for small projects, but as projects get larger it‘s important to break up the build into self-contained libraries that can be assembled into a final product. This way the entire world doesn’t need to be rebuilt on small changes and Bazel can parallelize more of the build steps.
To break up a project, create separate rules for each subcomponent and then make them depend on each other. For the example above, add the following rules to the my-project/BUILD file:
java_binary( name = "my-other-runner", srcs = ["java/com/example/ProjectRunner.java"], main_class = "com.example.ProjectRunner", deps = [":greeter"], ) java_library( name = "greeter", srcs = ["java/com/example/Greeting.java"], )
Now you can build and run my-project:my-other-runner:
$ bazel run my-project:my-other-runner INFO: Found 1 target... Target //my-project:my-other-runner up-to-date: bazel-bin/my-project/my-other-runner.jar bazel-bin/my-project/my-other-runner INFO: Elapsed time: 2.454s, Critical Path: 1.58s INFO: Running command line: bazel-bin/my-project/my-other-runner Hi!
If you edit ProjectRunner.java and rebuild my-other-runner, only ProjectRunner.java needs to be rebuilt (greeter is unchanged).
For larger projects, you will often be dealing with several directories. You can refer to targets defined in other BUILD files using the syntax //package-name:target-name. For example, suppose my-project/java/com/example/ has a cmdline/ subdirectory with the following file:
$ mkdir my-project/java/com/example/cmdline $ cat > my-project/java/com/example/cmdline/Runner.java <<EOF package com.example.cmdline; import com.example.Greeting; public class Runner { public static void main(String args[]) { Greeting.sayHi(); } } EOF
We could add a BUILD file at my-project/java/com/example/cmdline/BUILD that contained the following rule:
# ~/gitroot/base_workspace/my-project/java/com/example/cmdline/BUILD java_binary( name = "runner", srcs = ["Runner.java"], main_class = "com.example.cmdline.Runner", deps = ["//my-project:greeter"] )
However, by default, build rules are private. This means that they can only be referred to by rules in the same BUILD file. This prevents libraries that are implementation details from leaking into public APIs, but it also means that you must explicitly allow runner to depend on my-project:greeter. As is, if we build runner we'll get a permissions error:
$ bazel build my-project/java/com/example/cmdline:runner ERROR: /usr/local/google/home/kchodorow/gitroot/base_workspace/my-project/java/com/example/cmdline/BUILD:2:1: Target '//my-project:greeter' is not visible from target '//my-project/java/com/example/cmdline:runner'. Check the visibility declaration of the former target if you think the dependency is legitimate. ERROR: Analysis of target '//my-project/java/com/example/cmdline:runner' failed; build aborted. INFO: Elapsed time: 0.091s
You can make a rule visibile to rules in other BUILD files by adding a visibility = level attribute. Change the greeter rule in my-project/BUILD to be visible to our new rule:
java_library( name = "greeter", srcs = ["java/com/example/Greeting.java"], visibility = ["//my-project/java/com/example/cmdline:__pkg__"], )
This makes //my-project:greeter visible to any rule in the //my-project/java/com/example/cmdline package. Now we can build and run the binary:
$ bazel run my-project/java/com/example/cmdline:runner INFO: Found 1 target... Target //my-project/java/com/example/cmdline:runner up-to-date: bazel-bin/my-project/java/com/example/cmdline/runner.jar bazel-bin/my-project/java/com/example/cmdline/runner INFO: Elapsed time: 1.576s, Critical Path: 0.81s INFO: Running command line: bazel-bin/my-project/java/com/example/cmdline/runner Hi!
See the build encyclopedia for more visibility options.
If you look at the contents of bazel-bin/my-project/java/com/example/cmdline/runner.jar, you can see that it only contains Runner.class, not its dependencies (Greeting.class):
$ jar tf bazel-bin/my-project/java/com/example/cmdline/runner.jar META-INF/ META-INF/MANIFEST.MF com/ com/example/ com/example/cmdline/ com/example/cmdline/Runner.class
To deploy a runner binary, we need a self-contained jar. To build this, build runner_deploy.jar (or, more generally, _<target-name>deploy.jar):
$ bazel build my-project/java/com/example/cmdline:runner_deploy.jar INFO: Found 1 target... Target //my-project/java/com/example/cmdline:runner_deploy.jar up-to-date: bazel-bin/my-project/java/com/example/cmdline/runner_deploy.jar INFO: Elapsed time: 1.700s, Critical Path: 0.23s
runner_deploy.jar will contain all of its dependencies.
You can now create your own targets and compose them. See the build encyclopedia and Bazel user manual for more information. Let us know if you have any questions!