Java and Bazel

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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 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. 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:

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 the local_java_repository or remote_java_repository repo rules in a module extension. 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("@rules_java//toolchains: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("@rules_java//toolchains: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 MODULE.bazel 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(
  "@rules_java//toolchains: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 = "@rules_java//toolchains: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 >= 9
  • VANILLA_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("@rules_java//toolchains: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.