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:
-
Modules:
-
Configuration fragments:
-
Providers:
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
: Ajava_toolchain
target that defines the set of Java tools for building target binaries.--javabase
: Ajava_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.