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Introduction to Bazel: Build a Java Project

In this tutorial, you'll learn the basics of building Java applications with Bazel. You will set up your workspace and build a simple Java project that illustrates key Bazel concepts, such as targets and BUILD files.

Estimated completion time: 30 minutes.

What you'll learn

In this tutorial you'll learn how to:

  • Build a target
  • Visualize the project's dependencies
  • Split the project into multiple targets and packages
  • Control target visibility across packages
  • Reference targets through labels
  • Deploy a target

Contents

Before you begin

Install Bazel

To prepare for the tutorial, first Install Bazel if you don't have it installed already.

Get the sample project

Retrieve the sample project from Bazel's GitHub repository:

git clone https://github.com/bazelbuild/examples/

The sample project for this tutorial is in the examples/java-tutorial directory and is structured as follows:

java-tutorial
│
├── BUILD
├── src
│   └── main
│       └── java
│           └── com
│               └── example
│                   ├── cmdline
│                   │   ├── BUILD
│                   │   └── Runner.java
│                   ├── Greeting.java
│                   └── ProjectRunner.java
└── WORKSPACE

Build with Bazel

Set up the workspace

Before you can build a project, you need to set up its workspace. A workspace is a directory that holds your project's source files and Bazel's build outputs. It also contains files that Bazel recognizes as special:

  • The WORKSPACE file, which identifies the directory and its contents as a Bazel workspace and lives at the root of the project's directory structure,

  • One or more BUILD files, which tell Bazel how to build different parts of the project. (A directory within the workspace that contains a BUILD file is a package. You will learn about packages later in this tutorial.)

To designate a directory as a Bazel workspace, create an empty file named WORKSPACE in that directory.

When Bazel builds the project, all inputs and dependencies must be in the same workspace. Files residing in different workspaces are independent of one another unless linked, which is beyond the scope of this tutorial.

Understand the BUILD file

A BUILD file contains several different types of instructions for Bazel. The most important type is the build rule, which tells Bazel how to build the desired outputs, such as executable binaries or libraries. Each instance of a build rule in the BUILD file is called a target and points to a specific set of source files and dependencies. A target can also point to other targets.

Take a look at the java-tutorial/BUILD file:

java_binary(
    name = "ProjectRunner",
    srcs = glob(["src/main/java/com/example/*.java"]),
)

In our example, the ProjectRunner target instantiates Bazel's built-in java_binary rule. The rule tells Bazel to build a .jar file and a wrapper shell script (both named after the target).

The attributes in the target explicitly state its dependencies and options. While the name attribute is mandatory, many are optional. For example, in the ProjectRunner rule target, name is the name of the target, srcs specifies the source files that Bazel uses to build the target, and main_class specifies the class that contains the main method. (You may have noticed that our example uses glob to pass a set of source files to Bazel instead of listing them one by one.)

Build the project

Let's build your sample project. Change into the java-tutorial directory and run the following command:

bazel build //:ProjectRunner

Notice the target label - the // part is the location of our BUILD file relative to the root of the workspace (in this case, the root itself), and ProjectRunner is what we named that target in the BUILD file. (You will learn about target labels in more detail at the end of this tutorial.)

Bazel produces output similar to the following:

   INFO: Found 1 target...
   Target //:ProjectRunner up-to-date:
      bazel-bin/ProjectRunner.jar
      bazel-bin/ProjectRunner
   INFO: Elapsed time: 1.021s, Critical Path: 0.83s

Congratulations, you just built your first Bazel target! Bazel places build outputs in the bazel-bin directory at the root of the workspace. Browse through its contents to get an idea for Bazel's output structure.

Now test your freshly built binary:

bazel-bin/ProjectRunner

Review the dependency graph

Bazel requires build dependencies to be explicitly declared in BUILD files. Bazel uses those statements to create the project's dependency graph, which enables accurate incremental builds.

Let's visualize our sample project's dependencies. First, generate a text representation of the dependency graph (run the command at the workspace root):

bazel query  --nohost_deps --noimplicit_deps 'deps(//:ProjectRunner)' \
  --output graph

The above command tells Bazel to look for all dependencies for the target //:ProjectRunner (excluding host and implicit dependencies) and format the output as a graph.

Then, paste the text into GraphViz.

As you can see, the project has a single target that build two source files with no additional dependencies:

Dependency graph of the target 'ProjectRunner'

Now that you have set up your workspace, built your project, and examined its dependencies, let's add some complexity.

Refine your Bazel build

While a single target is sufficient for small projects, you may want to split larger projects into multiple targets and packages to allow for fast incremental builds (that is, only rebuild what's changed) and to speed up your builds by building multiple parts of a project at once.

Specify multiple build targets

Let's split our sample project build into two targets. Replace the contents of the java-tutorial/BUILD file with the following:

java_binary(
    name = "ProjectRunner",
    srcs = ["src/main/java/com/example/ProjectRunner.java"],
    main_class = "com.example.ProjectRunner",
    deps = [":greeter"],
)

java_library(
    name = "greeter",
    srcs = ["src/main/java/com/example/Greeting.java"],
)

With this configuration, Bazel first builds the greeter library, then the ProjectRunner library. The deps attribute in java_binary tells Bazel that the greeter library is required to build the ProjectRunner binary.

Let's build this new version of our project. Run the following command:

bazel build //:ProjectRunner

Bazel produces output similar to the following:

INFO: Found 1 target...
Target //:ProjectRunner up-to-date:
  bazel-bin/ProjectRunner.jar
  bazel-bin/ProjectRunner
INFO: Elapsed time: 2.454s, Critical Path: 1.58s

Now test your freshly built binary:

bazel-bin/ProjectRunner

If you now modify ProjectRunner.java and rebuild the project, Bazel only recompiles that file.

Looking at the dependency graph, you can see that ProjectRunner depends on the same inputs as it did before, but the structure of the build is different:

Dependency graph of the target 'ProjectRunner' after adding a dependency

You've now built the project with two targets. The ProjectRunner target builds two source files and depends on one other target (:greeter), which builds one additional source file.

Use multiple packages

Let’s now split the project into multiple packages. If you take a look at the src/main/java/com/example/cmdline directory, you can see that it also contains a BUILD file, plus some source files. Therefore, to Bazel, the workspace now contains two packages, //src/main/java/com/example/cmdline and // (since there is a BUILD file at the root of the workspace).

Take a look at the src/main/java/com/example/cmdline/BUILD file:

java_binary(
    name = "runner",
    srcs = ["Runner.java"],
    main_class = "com.example.cmdline.Runner",
    deps = ["//:greeter"]
)

The runner target depends on the greeter target in the // package (hence the target label //:greeter) - Bazel knows this through the deps attribute. Take a look at the dependency graph:

Dependency graph of the target 'runner'

However, for the build to succeed, you must explicitly give the runner target in //src/main/java/com/example/cmdline/BUILD visibility to targets in //BUILD using the visibility attribute. This is because by default targets are only visible to other targets in the same BUILD file. (Bazel uses target visibility to prevent issues such as libraries containing implementation details leaking into public APIs.)

To do this, add the visibility attribute to the greeter target in java-tutorial/BUILD as shown below:

java_library(
    name = "greeter",
    srcs = ["src/main/java/com/example/Greeting.java"],
    visibility = ["//src/main/java/com/example/cmdline:__pkg__"],
    )

Let's now build the new package. Run the following command at the root of the workspace:

bazel build //src/main/java/com/example/cmdline:runner

Bazel produces output similar to the following:

INFO: Found 1 target...
Target //src/main/java/com/example/cmdline:runner up-to-date:
  bazel-bin/src/main/java/com/example/cmdline/runner.jar
  bazel-bin/src/main/java/com/example/cmdline/runner
  INFO: Elapsed time: 1.576s, Critical Path: 0.81s

Now test your freshly built binary:

./bazel-bin/src/main/java/com/example/cmdline/runner

You've now modified the project to build as two packages, each containing one target, and understand the dependencies between them.

Use labels to reference targets

In BUILD files and at the command line, Bazel uses target labels to reference targets - for example, //:ProjectRunner or //src/main/java/com/example/cmdline:runner. Their syntax is as follows:

//path/to/package:target-name

If the target is a rule target, then path/to/package is the path to the directory containing the BUILD file, and target-name is what you named the target in the BUILD file (the name attribute). If the target is a file target, then path/to/package is the path to the root of the package, and target-name is the name of the target file, including its full path.

When referencing targets within the same package, you can skip the package path and just use //:target-name. When referencing targets within the same BUILD file, you can even skip the // workspace root identifier and just use :target-name.

For example, for targets in the java-tutorial/BUILD file, you did not have to specify a package path, since the workspace root is itself a package (//), and your two target labels were simply //:ProjectRunner and //:greeter.

However, for targets in the //src/main/java/com/example/cmdline/BUILD file you had to specify the full package path of //src/main/java/com/example/cmdline and your target label was //src/main/java/com/example/cmdline:runner.

Package a Java target for deployment

Let’s now package a Java target for deployment by building the binary with all of its runtime dependencies. This lets you run the binary outside of your development environment.

As you remember, the java_binary build rule produces a .jar and a wrapper shell script. Take a look at the contents of runner.jar using this command:

jar tf bazel-bin/src/main/java/com/example/cmdline/runner.jar

The contents are:

META-INF/
META-INF/MANIFEST.MF
com/
com/example/
com/example/cmdline/
com/example/cmdline/Runner.class

As you can see, runner.jar contains Runner.class, but not its dependency, Greeting.class. The runner script that Bazel generates adds greeter.jar to the classpath, so if you leave it like this, it will run locally, but it won't run standalone on another machine. Fortunately, the java_binary rule allows you to build a self-contained, deployable binary. To build it, add the _deploy.jar suffix to the file name when building runner.jar (_deploy.jar):

bazel build //src/main/java/com/example/cmdline:runner_deploy.jar

Bazel produces output similar to the following:

INFO: Found 1 target...
Target //src/main/java/com/example/cmdline:runner_deploy.jar up-to-date:
  bazel-bin/src/main/java/com/example/cmdline/runner_deploy.jar
INFO: Elapsed time: 1.700s, Critical Path: 0.23s

You have just built runner_deploy.jar, which you can run standalone away from your development environment since it contains the required runtime dependencies.

Further reading

Happy building!