Java and Bazel

This page contains resources that help you use Bazel with Java projects. It links to a tutorial, build rules, and other information specific to building Java projects with Bazel.

Contents

Working with Bazel

The following resources will help you work with Bazel on Java projects:

Migrating to Bazel

If you currently build your Java projects with Maven, follow the steps in the migration guide to start building your Maven projects with Bazel:

Best practices

In addition to general Bazel best practices, below are best practices specific to Java projects.

Directory structure

Prefer Maven’s standard directory layout (sources under src/main/java, tests under src/test/java).

BUILD files

Follow these guidelines when creating your BUILD files:

  • Use one BUILD file per package containing Java sources.

  • Every BUILD file should contain one java_library rule that looks like this:

    java_library(
    name = "directory-name",
    srcs = glob(["*.java"]),
    deps = [...],
)

  • The name of the library should be the name of the directory containing the BUILD file.

  • The sources should be a non-recursive glob of all Java files in the directory.

  • Tests should be in a matching directory under src/test and depend on this library.

Java and new rules

Note: Creating new rules is for advanced build and test scenarios. You do not need it when getting started with Bazel.

The following modules, configuration fragments, and providers will help you extend Bazel’s capabilities when building your Java projects:

Configuring the JDK

Bazel is configured to use a default OpenJDK 11 for building and testing JVM-based projects. However, you can switch to another JDK using the --java_toolchain and --javabase flags.

In short,

  • --java_toolchain: A java_toolchain target that defines the set of Java tools for building target binaries.
  • --javabase: A java_runtime target defining the Java runtime for running target JVM binaries.

The --host_java_toolchain and --host_javabase variants are meant for building and running host binaries that Bazel uses for building target binaries. These host binaries belong to --java_toolchain, which includes JavaBuilder and Turbine.

Bazel’s default flags essentially look like this:

$ bazel build \
      --host_javabase=@bazel_tools//tools/jdk:remote_jdk11 \
      --javabase=@bazel_tools//tools/jdk:remote_jdk11 \
      --host_java_toolchain=@bazel_tools//tools/jdk:toolchain_java11 \
      --java_toolchain=@bazel_tools//tools/jdk:toolchain_java11 \
      //my/java:target

@bazel_tools comes with a number of java_toolchain targets. Run the following command to list them:

$ bazel query 'kind(java_toolchain, @bazel_tools//tools/jdk:all)'

Similarly for java_runtime targets:

$ bazel query 'kind(java_runtime, @bazel_tools//tools/jdk:all)'

For example, if you’d like to use a locally installed JDK installed at /usr/lib/jvm/java-13-openjdk, use the absolute_javabase java_runtime target and the toolchain_vanilla java_toolchain target, and define ABSOLUTE_JAVABASE as the absolute path to the JDK.

bazel build \
    --define=ABSOLUTE_JAVABASE=/usr/lib/jvm/java-13-openjdk \
    --javabase=@bazel_tools//tools/jdk:absolute_javabase \
    --host_javabase=@bazel_tools//tools/jdk:absolute_javabase \
    --java_toolchain=@bazel_tools//tools/jdk:toolchain_vanilla \
    --host_java_toolchain=@bazel_tools//tools/jdk:toolchain_vanilla \
    //my/java_13:target

Optionally, you can add the flags into your project’s .bazelrc file to avoid having to specify them every time:

build --define=ABSOLUTE_JAVABASE=/usr/lib/jvm/java-13-openjdk
build --javabase=@bazel_tools//tools/jdk:absolute_javabase
build --host_javabase=@bazel_tools//tools/jdk:absolute_javabase
build --java_toolchain=@bazel_tools//tools/jdk:toolchain_vanilla
build --host_java_toolchain=@bazel_tools//tools/jdk:toolchain_vanilla

You can also write your own java_runtime and java_toolchain targets. As a tip, use bazel query --output=build @bazel_tools//tools/jdk:all to see how the built-in runtime and toolchain targets are defined.