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.
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:
Java versions
There are two relevant versions of Java that are set with configuration flags:
- the version of the source files in the repository
- the version of the Java runtime that is used to execute the code and to test it
Configuring the version of the source code in your repository
Without an additional configuration, Bazel assumes all Java source files in the
repository are written in a single Java version. To specify the version of the
sources in the repository add build --java_language_version={ver}
to
.bazelrc
file, where {ver}
is for example 11
. Bazel repository owners
should set this flag so that Bazel and its users can reference the source code's
Java version number. For more details, see
Java language version flag.
Configuring the JVM used to execute and test the code
Bazel uses one JDK for compilation and another JVM to execute and test the code.
By default Bazel compiles the code using a JDK it downloads and it executes and
tests the code with the JVM installed on the local machine. Bazel searches for
the JVM using JAVA_HOME
or path.
The resulting binaries are compatible with locally installed JVM in system libraries, which means the resulting binaries depend on what is installed on the machine.
To configure the JVM used for execution and testing use --java_runtime_version
flag. The default value is local_jdk
.
Hermetic testing and compilation
To create a hermetic compile, you can use command line flag
--java_runtime_version=remotejdk_11
. The code is compiled for, executed, and
tested on the JVM downloaded from a remote repository. For more details, see
Java runtime version flag.
Configuring compilation and execution of build tools in Java
There is a second pair of JDK and JVM used to build and execute tools, which are
used in the build process, but are not in the build results. That JDK and JVM
are controlled using --tool_java_language_version
and
--tool_java_runtime_version
. Default values are 11
and remotejdk_11
,
respectively.
Compiling using locally installed JDK
Bazel by default compiles using remote JDK, because it is overriding JDK's internals. The compilation toolchains using locally installed JDK are configured, however not used.
To compile using locally installed JDK, that is use the compilation toolchains
for local JDK, use additional flag --extra_toolchains=@local_jdk//:all
,
however, mind that this may not work on JDK of arbitrary vendors.
For more details, see configuring Java toolchains.
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 directory containing Java sources, because this improves build performance.Every
BUILD
file should contain onejava_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. This makes the label of the library shorter, that is use"//package"
instead of"//package:package"
.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.
Creating new rules for advanced Java builds
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:
- Main Java module:
java_common
- Main Java provider:
JavaInfo
- Configuration fragment:
java
Other modules:
Configuring the Java toolchains
Bazel uses two types of Java toolchains:
- execution, used to execute and test Java binaries, controlled with
--java_runtime_version
flag
- compilation, used to compile Java sources, controlled with
--java_language_version
flag
Configuring additional execution toolchains
Execution toolchain is the JVM, either local or from a repository, with some additional information about its version, operating system, and CPU architecture.
Java execution toolchains may added using local_java_repository
or
remote_java_repository
rules in the WORKSPACE
file. Adding the rule makes
the JVM available using a flag. When multiple definitions for the same operating
system and CPU architecture are given, the first one is used.
Example configuration of local JVM:
load("@bazel_tools//tools/jdk:local_java_repository.bzl", "local_java_repository")
local_java_repository(
name = "additionaljdk", # Can be used with --java_runtime_version=additionaljdk, --java_runtime_version=11 or --java_runtime_version=additionaljdk_11
version = 11, # Optional, if not set it is autodetected
java_home = "/usr/lib/jdk-15/", # Path to directory containing bin/java
)
Example configuration of remote JVM:
load("@bazel_tools//tools/jdk:remote_java_repository.bzl", "remote_java_repository")
remote_java_repository(
name = "openjdk_canary_linux_arm",
prefix = "openjdk_canary", # Can be used with --java_runtime_version=openjdk_canary_11
version = "11", # or --java_runtime_version=11
target_compatible_with = [ # Specifies constraints this JVM is compatible with
"@platforms//cpu:arm",
"@platforms//os:linux",
],
urls = ..., # Other parameters are from http_repository rule.
sha256 = ...,
strip_prefix = ...
)
Configuring additional compilation toolchains
Compilation toolchain is composed of JDK and multiple tools that Bazel uses during the compilation and that provides additional features, such as: Error Prone, strict Java dependencies, header compilation, Android desugaring, coverage instrumentation, and genclass handling for IDEs.
JavaBuilder is a Bazel-bundled tool that executes compilation, and provides the
aforementioned features. Actual compilation is executed using the internal
compiler by the JDK. The JDK used for compilation is specified by java_runtime
attribute of the toolchain.
Bazel overrides some JDK internals. In case of JDK version > 9,
java.compiler
and jdk.compiler
modules are patched using JDK's flag
--patch_module
. In case of JDK version 8, the Java compiler is patched using
-Xbootclasspath
flag.
VanillaJavaBuilder is a second implementation of JavaBuilder, which does not modify JDK's internal compiler and does not have any of the additional features. VanillaJavaBuilder is not used by any of the built-in toolchains.
In addition to JavaBuilder, Bazel uses several other tools during compilation.
The ijar
tool processes jar
files to remove everything except call
signatures. Resulting jars are called header jars. They are used to improve the
compilation incrementality by only recompiling downstream dependents when the
body of a function changes.
The singlejar
tool packs together multiple jar
files into a single one.
The genclass
tool post-processes the output of a Java compilation, and produces
a jar
containing only the class files for sources that were generated by
annotation processors.
The JacocoRunner
tool runs Jacoco over instrumented files and outputs results in
LCOV format.
The TestRunner
tool executes JUnit 4 tests in a controlled environment.
You can reconfigure the compilation by adding default_java_toolchain
macro to
a BUILD
file and registering it either by adding register_toolchains
rule to
the WORKSPACE
file or by using
--extra_toolchains
flag.
The toolchain is only used when the source_version
attribute matches the
value specified by --java_language_version
flag.
Example toolchain configuration:
load(
"@bazel_tools//tools/jdk:default_java_toolchain.bzl",
"default_java_toolchain", "DEFAULT_TOOLCHAIN_CONFIGURATION", "BASE_JDK9_JVM_OPTS", "DEFAULT_JAVACOPTS"
)
default_java_toolchain(
name = "repository_default_toolchain",
configuration = DEFAULT_TOOLCHAIN_CONFIGURATION, # One of predefined configurations
# Other parameters are from java_toolchain rule:
java_runtime = "@bazel_tools//tools/jdk:remote_jdk11", # JDK to use for compilation and toolchain's tools execution
jvm_opts = BASE_JDK9_JVM_OPTS + ["--enable_preview"], # Additional JDK options
javacopts = DEFAULT_JAVACOPTS + ["--enable_preview"], # Additional javac options
source_version = "9",
)
which can be used using --extra_toolchains=//:repository_default_toolchain_definition
or by adding register_toolchains("//:repository_default_toolchain_definition")
to the workpace.
Predefined configurations:
DEFAULT_TOOLCHAIN_CONFIGURATION
: all features, supports JDK versions >= 9VANILLA_TOOLCHAIN_CONFIGURATION
: no additional features, supports JDKs of arbitrary vendors.PREBUILT_TOOLCHAIN_CONFIGURATION
: same as default, but only use prebuilt tools (ijar
,singlejar
)NONPREBUILT_TOOLCHAIN_CONFIGURATION
: same as default, but all tools are built from sources (this may be useful on operating system with different libc)
Configuring JVM and Java compiler flags
You may configure JVM and javac flags either with flags or with
default_java_toolchain
attributes.
The relevant flags are --jvmopt
, --host_jvmopt
, --javacopt
, and
--host_javacopt
.
The relevant default_java_toolchain
attributes are javacopts
, jvm_opts
,
javabuilder_jvm_opts
, and turbine_jvm_opts
.
Package specific Java compiler flags configuration
You can configure different Java compiler flags for specific source
files using package_configuration
attribute of default_java_toolchain
.
Please refer to the example below.
load("@bazel_tools//tools/jdk:default_java_toolchain.bzl", "default_java_toolchain")
# This is a convenience macro that inherits values from Bazel's default java_toolchain
default_java_toolchain(
name = "toolchain",
package_configuration = [
":error_prone",
],
visibility = ["//visibility:public"],
)
# This associates a set of javac flags with a set of packages
java_package_configuration(
name = "error_prone",
javacopts = [
"-Xep:MissingOverride:ERROR",
],
packages = ["error_prone_packages"],
)
# This is a regular package_group, which is used to specify a set of packages to apply flags to
package_group(
name = "error_prone_packages",
packages = [
"//foo/...",
"-//foo/bar/...", # this is an exclusion
],
)
Multiple versions of Java source code in a single repository
Bazel only supports compiling a single version of Java sources in a build. build. This means that when building a Java test or an application, all dependencies are built against the same Java version.
However, separate builds may be executed using different flags.
To make the task of using different flags easier, sets of flags for a specific
version may be grouped with .bazelrc
configs":
build:java8 --java_language_version=8
build:java8 --java_runtime_version=local_jdk_8
build:java11 --java_language_version=11
build:java11 --java_runtime_version=remotejdk_11
These configs can be used with the --config
flag, for example
bazel test --config=java11 //:java11_test
.