README

mx is a command line based tool for managing the development of (primarily) Java code. It includes a mechanism for specifying the dependencies as well as making it simple to build, test, run, update, etc the code and built artifacts. mx contains support for developing code spread across multiple source repositories. mx is written in Python and is extensible.

The organizing principle of mx is a suite. A suite is both a directory and the container for the components of the suite. A suite component is either a project, library or distribution. There are various flavors of each of these. A suite may import and depend on other suites. For an execution of mx, exactly one suite is the primary suite. This is either the suite in whose directory mx is executed or the value of the global -p mx option. The set of suites reachable from the primary suite by transitive closure of the imports relation form the set that mx operates on.

Running mx

mx can be run directly (i.e., python mx/mx.py ...), but is more commonly invoked via the mx/mx bash script. Adding the mx/ directory to your PATH simplifies executing mx. The mx/mx.cmd script should be used on Windows.

The general form of the mx command line is:

mx [global options] [command] [command-specific options]

If no options or command is specified, mx prints information on the available options and commands, which will include any suite-specific options and commands. Help for a specific command is obtained via mx help <command>. Global options are expected to have wide applicability to many commands and as such precede the command to be executed.

For an example of mx usage, see README.md.

Note: There is a Bash completion script for global options and commands, located in bash_completion directory. Install it for example by sourceing this script in your ~/.bashrc file. If used, a temporary file /tmp/mx-bash-completion-<project-path-hash> is created and used for better performance.

mx-honey provides richer completions for zsh users.

Suites

The definition of a suite and its components is in a file named suite.py in the mx metadata directory of the primary suite. This is the directory named mx.<suite name> in the suite's top level directory. For example, for the compiler suite, it is mx.compiler. The format of suite.py is JSON with the following extensions:

• Python multi-line and single-quoted strings are supported
• Python hash-prefixed comments are supported

Java projects

Java source code is contained in a project. Here's an example of two Graal compiler projects:

"jdk.graal.compiler.serviceprovider" : {
  "subDir" : "src",
  "sourceDirs" : ["src"],
  "dependencies" : ["JVMCI_SERVICES"],
  "checkstyle" : "jdk.graal.compiler.graph",
  "javaCompliance" : "8",
  "workingSets" : "API,Graal",
},

"jdk.graal.compiler.serviceprovider.jdk9" : {
  "subDir" : "src",
  "sourceDirs" : ["src"],
  "dependencies" : ["jdk.graal.compiler.serviceprovider"],
  "uses" : ["jdk.graal.compiler.serviceprovider.GraalServices.JMXService"],
  "checkstyle" : "jdk.graal.compiler.graph",
  "javaCompliance" : "9+",
  "multiReleaseJarVersion" : "9",
  "workingSets" : "API,Graal",
},

The javaCompliance attribute can be a single number (e.g. 8), the lower bound of a range (e.g. 8+) or a fixed range (e.g. 9..11). This attribute specifies the following information:

• The maximum Java language level used by the project. This is the lower bound in a range. It is also used as the value for the -source and -target javac options when compiling the project.
• The JDKs providing any internal JDK API used by the project. A project that does not use any internal JDK API should specify an open range (e.g. 8+). Otherwise, a JDK matching the exact version or range is required to compile the project.

The multiReleaseJarVersion attribute is explained in the "Versioning sources for different JDK releases" section below.

Java distributions

A distribution encompasses one or more Java projects and enables the class files and related resources from projects to be packaged into a jar file. If a distribution declares itself as a module (see Java modules support), a JMOD file will also be produced when the distribution is built. The path to the jar file for a distribution is given by mx paths <distribution name>. For example:

> mx paths GRAAL
/Users/dnsimon/graal/graal/compiler/mxbuild/dists/jdk11/graal.jar

When building the jar for a distribution, mx will create the layout for the jar in a directory that is a sibling of the distribution's jar path. For example:

├── graal.jar
├── graal.jar.files
│   ├── META-INF
│   └── org

For efficiency, the files under the *.files hierarchy will be symlinks where possible. On Windows, creating symlinks is a privileged operation and so if symlinks cannot be created, files are copied instead. There are plenty of internet resources describing how to elevate your privileges on Windows to enable symlinking (e.g. here).

Exploded builds

By default, mx will produce a jar for each distribution. If a distribution defines a module, the jar is further processed to make it a multi-release modular jar and a jmod file is also created. Creating the jar and jmod files increases build time. For faster development, it's possible to leave a distribution in its exploded form, a directory with the same layout as the jar structure. To work in this mode, set MX_BUILD_EXPLODED=true. Also, ensure that exactly one JDK is specified by the union of JAVA_HOME and EXTRA_JAVA_HOMES (required since there is no equivalent of multi-release jar support for directories).

Using MX_BUILD_EXPLODED=true is roughly equivalent to building the OpenJDK with make instead of make images.

Note that MX_BUILD_EXPLODED=true should not be used when building for deployment.

Java modules support

A distribution that has a moduleInfo attribute will result in a Java module being built from the distribution. The moduleInfo attribute must specify the name of the module and can include other parts of a module descriptor.

This is best shown with examples from Truffle and Graal:

Here is an extract from the definition of the TRUFFLE_API distribution which produces the org.graavm.truffle module:

"TRUFFLE_API" : {
    "moduleInfo" : {
        "name" : "org.graalvm.truffle",
        "requires" : [
            "static java.desktop"
        ],
        "exports" : [
            "com.oracle.truffle.api.nodes to jdk.graal.compiler",
            "com.oracle.truffle.api.impl to jdk.graal.compiler, org.graalvm.locator",
            "com.oracle.truffle.api to jdk.graal.compiler, org.graalvm.locator, com.oracle.graal.graal_enterprise",
            "com.oracle.truffle.api.object to jdk.graal.compiler, com.oracle.graal.graal_enterprise",
            "com.oracle.truffle.object to jdk.graal.compiler, com.oracle.graal.graal_enterprise",
        ],
        "uses" : [
          "com.oracle.truffle.api.TruffleRuntimeAccess",
          "java.nio.file.spi.FileTypeDetector",
          "com.oracle.truffle.api.impl.TruffleLocator",
        ],
    },
    ...
    "distDependencies" : [
        # These distributions must also have `moduleInfo` attributes and the corresponding
        # modules will be added to the set of `requires` for this module.
        "sdk:GRAAL_SDK"
    ],
}

The module-info.java created by mx from the above is:

module org.graalvm.truffle {
    requires java.base;
    requires static java.desktop;
    requires java.logging;
    requires jdk.unsupported;
    requires transitive org.graalvm.sdk;
    exports com.oracle.truffle.api to com.oracle.graal.graal_enterprise, jdk.graal.compiler, org.graalvm.locator;
    exports com.oracle.truffle.api.impl to jdk.graal.compiler, org.graalvm.locator;
    exports com.oracle.truffle.api.nodes to jdk.graal.compiler;
    exports com.oracle.truffle.api.object to com.oracle.graal.graal_enterprise, jdk.graal.compiler;
    exports com.oracle.truffle.object to com.oracle.graal.graal_enterprise, jdk.graal.compiler;
    uses com.oracle.truffle.api.TruffleRuntimeAccess;
    uses com.oracle.truffle.api.impl.TruffleLocator;
    uses com.oracle.truffle.api.object.LayoutFactory;
    uses java.nio.file.spi.FileTypeDetector;
    provides com.oracle.truffle.api.object.LayoutFactory with com.oracle.truffle.object.basic.DefaultLayoutFactory;
    provides org.graalvm.polyglot.impl.AbstractPolyglotImpl with com.oracle.truffle.polyglot.PolyglotImpl;
    // conceals: com.oracle.truffle.api.debug
    // conceals: com.oracle.truffle.api.debug.impl
    // conceals: com.oracle.truffle.api.dsl
    // conceals: com.oracle.truffle.api.frame
    // conceals: com.oracle.truffle.api.instrumentation
    // conceals: com.oracle.truffle.api.interop
    // conceals: com.oracle.truffle.api.interop.impl
    // conceals: com.oracle.truffle.api.io
    // conceals: com.oracle.truffle.api.library
    // conceals: com.oracle.truffle.api.object.dsl
    // conceals: com.oracle.truffle.api.profilesLayoutFactory
    // conceals: com.oracle.truffle.api.source
    // conceals: com.oracle.truffle.api.utilities
    // conceals: com.oracle.truffle.object.basic
    // conceals: com.oracle.truffle.polyglot
    // jarpath: /Users/dnsimon/hs/graal/truffle/mxbuild/dists/jdk11/truffle-api.jar
    // dist: TRUFFLE_API
    // modulepath: org.graalvm.sdk
}

The provides clauses are automatically derived from the META-INF/services/ directory in the distribution's jar file. The generation of the provides clauses can be modified by utilizing the ignoredServiceTypes attribute. Here is an extract from the definition of the TRUFFLE_NFI distribution, which prevents adding DefaultExportProvider and EagerExportProvider implementations to provides clauses.

"TRUFFLE_NFI" : {
    "moduleInfo" : {
        "name" : "com.oracle.truffle.truffle_nfi",
        "exports" : [
            "com.oracle.truffle.nfi.api",
            "com.oracle.truffle.nfi.backend.spi",
            "com.oracle.truffle.nfi.backend.spi.types",
            "com.oracle.truffle.nfi.backend.spi.util",
        ],
        "ignoredServiceTypes" : [
            "com.oracle.truffle.api.library.DefaultExportProvider",
            "com.oracle.truffle.api.library.EagerExportProvider",
        ],
    }
    ...
}

The GRAAL distribution shows how a single exports attribute can be used to specify multiple exports clauses:

"GRAAL" : {
    "moduleInfo" : {
        "name" : "jdk.graal.compiler",
        "exports" : [
            # Qualified exports of all packages in GRAAL to modules built from
            # ENTERPRISE_GRAAL and GRAAL_MANAGEMENT distributions
            "* to com.oracle.graal.graal_enterprise,jdk.graal.compiler.management",
        ],
        ...
    },
    ...
},

This results info a module-info.java as that contains qualified exports, a small subset of which are shown below:

module jdk.graal.compiler {
    ...
    exports jdk.graal.compiler.api.directives to com.oracle.graal.graal_enterprise, jdk.graal.compiler.management;
    exports jdk.graal.compiler.api.replacements to com.oracle.graal.graal_enterprise, jdk.graal.compiler.management;
    exports jdk.graal.compiler.api.runtime to com.oracle.graal.graal_enterprise, jdk.graal.compiler.management;
    exports jdk.graal.compiler.asm to com.oracle.graal.graal_enterprise, jdk.graal.compiler.management;
    exports jdk.graal.compiler.asm.aarch64 to com.oracle.graal.graal_enterprise, jdk.graal.compiler.management;
    exports jdk.graal.compiler.asm.amd64 to com.oracle.graal.graal_enterprise, jdk.graal.compiler.management;
    ...

The jars build for a distribution are in <suite>/mxbuild/dists/jdk*/. The modular jars are in the jdk<N> directories where N >= 9. There is a modular jar built for each JDK version denoted by the javaCompliance values of the distribution's constituent projects.

Specifying required modules

If a project with a Java compliance >= 9 uses a package from a module other than java.base, it must specify these additional modules with the requires attribute. For example:

"jdk.graal.compiler.hotspot.management.jdk11" : {
    ...
    "requires" : [
        "jdk.management"
    ],
    "javaCompliance" : "11+",
    ...
},

The requires attribute is used for two purposes:

• As input to the requires attribute of the descriptor for the module encapsulating the project.
• To derive a value for the --limit-modules javac option which restricts the modules observable during compilation. This is required to support separate compilation of projects that are part of a JDK module. For example, jdk.graal.compiler.hotspot.amd64 depends on jdk.graal.compiler.hotspot and the classes of both these projects are contained in the jdk.graal.compiler module. When compiling jdk.graal.compiler.hotspot.amd64, we must compile against classes in jdk.graal.compiler.hotspot as they might be different (i.e., newer) than the classes in jdk.graal.compiler. The value of --limit-modules will omit jdk.graal.compiler in this case to achieve this hiding. In the absence of a requires attribute, only the java.base module is observable when compiling on JDK 9+.

Use of concealed packages

Concealed packages are those defined by a module but not exported by the module. If a project uses concealed packages, it must specify a requiresConcealed attribute denoting the concealed packages it accesses. For example:

"jdk.graal.compiler.lir.aarch64.jdk11" : {
    "requiresConcealed" : {
        "jdk.internal.vm.ci" : [
            "jdk.vm.ci.aarch64",
            "jdk.vm.ci.code",
        ],
    },
    "javaCompliance" : "11+",
},

This will result in --add-exports=jdk.internal.vm.ci/jdk.vm.ci.aarch64=ALL-UNNAMED and --add-exports=jdk.internal.vm.ci/jdk.vm.ci.code=ALL-UNNAMED being added to the javac command line when the jdk.graal.compiler.lir.aarch64.jdk11 project is compiled by a JDK 9+ javac.

Note that the requires and requiresConcealed attributes only apply to projects with a minimum javaCompliance value of 9 or greater. When javac from jdk 9+ is used in conjunction with -source 8 (as will be the case for projects with a minimum javaCompliance of 8 or less), all classes in the JDK are observable. However, if an 8 project would need a requires or requiresConcealed attribute were it a 9+ project, then these attributes must be applied to any module containing the project. For example, jdk.graal.compiler.serviceprovider has "javaCompliance" : "8+" and contains code that imports sun.misc.Unsafe. Since jdk.graal.compiler.serviceprovider is part of the jdk.graal.compiler module defined by the GRAAL distribution, GRAAL must include a requires attribute in its moduleInfo attribute:

"GRAAL" : {
    "moduleInfo" : {
        "name" : "jdk.graal.compiler",
        "requires" : ["jdk.unsupported"],
        ...
    }
}

Modules can be removed from the JDK. For example, JDK-8255616 removed the jdk.aot, jdk.internal.vm.compiler and jdk.internal.vm.compiler.management modules from standard JDK binaries as of JDK 16. Any requiresConcealed attributes targeting these modules must use a Java compliance qualifier so that the relevant sources can still be built on JDK 16:

"com.oracle.svm.enterprise.jdk11.test": {
    ...
    "requiresConcealed": {
        "[email protected]": [
            "jdk.graal.compiler.serviceprovider"
        ],
        ...
    }
}

As shown above, a module name in a requiresConcealed attribute can be qualified by appending @ followed by a valid Java compliance specifier. Such a module will be ignored if the JDK version used to compile the sources is not matched by the specified Java compliance. This also works for the regular requires attribute. E.g.

    "requires": [
        ...
        "[email protected]",
    ],
    ...

is needed to ensure that a given module requires module jdk.scripting.nashorn only when the specified compliance matches.

Selecting JDKs

Specifying JDKs to mx is done via the --java-home and --extra-java-homes options or via the JAVA_HOME and EXTRA_JAVA_HOMES environment variables. An option has precedence over the corresponding environment variable. Mx comes with a select_jdk.py helper that simplifies switching between different values for JAVA_HOME and EXTRA_JAVA_HOMES.

Install a JDK with fetch-jdk

The mx fetch-jdk command can download and install JDKs defined in JSON files. See mx fetch-jdk --help for more detail.

Generated artifacts

The build artifacts of mx are in directories separate from the source file directories. Output for platform dependent suite constituents is under a directory whose name reflects the current platform. For example:

<suite>/mxbuild/<project>               # Platform independent project
<suite>/mxbuild/darwin-amd64/<project>  # Platform dependent project

Partitioning build output to take the platform into account has the following advantages:

• A file system shared between different platforms (e.g. via NFS or virtualization host/guest file system sharing) keeps its platform dependent output separated.

Unless MX_OUTPUT_ROOT_INCLUDES_CONFIG=false then:

• The output for JDK dependent suite constituents is under a directory reflecting the JDK(s) specified by JAVA_HOME and EXTRA_JAVA_HOMES.
• The output for platform and JDK dependent suite constituents is under a directory reflecting both the platform and JDKs.

For example:

<suite>/mxbuild/jdk16+8/<project>               # JDK dependent project
<suite>/mxbuild/darwin-amd64/<project>          # Platform dependent project
<suite>/mxbuild/darwin-amd64-jdk16+8/<project>  # Platform and JDK dependent project

Partitioning build output to take JDK configuration into account has the following advantages:

• Avoids re-compilation after changing the value of JAVA_HOME or EXTRA_JAVA_HOMES in the case where no sources have changed since mx build was last executed with the new values.
• Avoid issues related to API changes between JDK versions. If only public JDK API was used by Java projects, this could be solved with the --release option introduced by JEP 247. However, a significant number of mx managed projects use JDK internal API in which case --release does not help.

Note that IDE configurations ignore MX_OUTPUT_ROOT_INCLUDES_CONFIG and so must be regenerated after changing the value of JAVA_HOME or EXTRA_JAVA_HOMES if you want output generated by an IDE to be visible to subsequent mx commands.

The JDK configuration dependent layout of build artifacts is best shown by an example. Consider the following directory tree containing graal and truffleruby repositories where graal defines the suites compiler, truffle and sdk and truffleruby defines a single truffleruby suite:

ws
├── graal
│   ├── compiler
│   ├── sdk
│   └── truffle
└── truffleruby

With this layout when working on macOS with $JAVA_HOME set to a JDK 8 and $EXTRA_JAVA_HOMES set to a JDK 16, after running mx build, the layout will be:

ws
├── graal
│   ├── compiler
│   │   └── mxbuild
│   │       ├── darwin-amd64
│   │       │   └── <project>
│   │       └── jdk8+16
│   │           └── <project>
│   ├── sdk
│   │   └── mxbuild
│   │       ├── darwin-amd64
│   │       │   └── <project>
│   │       └── jdk8+16
│   │           └── <project>
│   └── truffle
│       └── mxbuild
│           ├── darwin-amd64
│           │   └── <project>
│           └── jdk8+16
│               └── <project>
└── truffleruby
    └── mxbuild
        ├── darwin-amd64
        │   └── <project>
        └── jdk8+16
            └── <project>

Unit testing with Junit

The unittest command supports running Junit tests in mx suites.

The unit test harness will use any org.junit.runner.notification.RunListener objects available via java.util.ServiceLoader.load().

Executing tests on JDK 9 or later can be complicated if the tests access packages that are publicly available in JDK 8 or earlier but are not public as of JDK 9. That is, the packages are concealed by their declaring module. Such tests can be compiled simply enough by specifying their Java compliance as "1.8=". Running the tests on JDK 9 however requires that the concealed packages are exported to the test classes. To achieve this, an AddExports annotation should be applied to the test class requiring the export or to any of its super classes or super interfaces. To avoid the need for a dependency on mx, unittest harness simply looks for an annotation named AddExports that matches the following definition:

import java.lang.annotation.ElementType;
import java.lang.annotation.Retention;
import java.lang.annotation.RetentionPolicy;
import java.lang.annotation.Target;

/**
 * Specifies packages concealed in JDK modules used by a test. The mx unit test runner will ensure
 * the packages are exported to the module containing annotated test class.
 */
@Retention(RetentionPolicy.RUNTIME)
@Target(ElementType.TYPE)
public @interface AddExports {
    /**
     * The qualified name of the concealed package(s) in {@code <module>/<package>} format (e.g.,
     * "jdk.vm.ci/jdk.vm.ci.code").
     */
    String[] value() default "";
}

Coverage testing with JaCoCo

To enable code coverage testing with JaCoCo, the JaCoCo agent needs to be injected through VM command line arguments. For this, mx provides the convenience method mx_gate.get_jacoco_agent_args() which returns a list of those arguments if coverage is requested (e.g. by using mx gate --jacocout ...), otherwise None. Here is an example how it is used to enable coverage testing of the sources of the Graal compiler. Running code with the JaCoCo agent enabled outputs a jacoco.exec which can be converted into an HTML or CSV report with the mx jacocoreport command.

The packages or classes to be included in the JaCoCo report can be customized by importing mx_gate and using the helper functions:

• add_jacoco_includes (adds one or more package patterns to the list of packages to include in the report)
• add_jacoco_excludes (adds one or more package patterns to the list of packages to exclude from the report)
• add_jacoco_excluded_annotations (adds one or more annotations to the list of annotations that will cause a class to be excluded from the report)

The include patterns can include an explicit trailing .* wildcard match. The exclude patterns have an implicit trailing wildcard match. Annotation names added to the annotation exclusion list must start with an @ character.

As an example from mx_compiler.py:

mx_gate.add_jacoco_includes(['org.graalvm.*'])
mx_gate.add_jacoco_excludes(['com.oracle.truffle.*'])
mx_gate.add_jacoco_excluded_annotations(['@Snippet', '@ClassSubstitution'])

This adds classes from packages starting with org.graalvm to the report, excludes classes in packages startng with com.oracle.truffle and also excludes classes annotated with @Snippet and @ClassSubstitution.

To omit excluded classes from the JaCoCo data and report use the gate option --jacoco-omit-excluded.

Versioning sources for different JDK releases

Mx includes support for multiple versions of a Java class. The mechanism is inspired by and similar to multi-release jars. A versioned Java class has a base version and one or more versioned copies. The public signature of each copy (i.e., methods and fields accessed from outside the source file) must be identical. Note that the only API that is visible from the JAR is the one from the base version.

Versioned classes for JDK 9 or later need to be in a project with a javaCompliance greater than or equal to 9 and a multiReleaseJarVersion attribute whose value is also greater or equal to 9. The versioned project must have the base project as a dependency.

Versioned classes for JDK 8 or earlier need to be in a project with a javaCompliance less than or equal to 8 and an overlayTarget attribute denoting the base project.

Profiling with proftool

Mx includes proftool, a utility for capturing and examining profiles of Java programs. Further details are here.

URL rewriting

Mx includes support for the primary suite to be able to override the source URLs of imported suites. The suite level urlrewrites attribute allows regular expression URL rewriting, and, optionally, digest rewriting. For example:

  "urlrewrites" : [
    {
      "https://git.acme.com/(.*).git" : {
        "replacement" : r”https://my.company.com/foo-git-cache/\1.git",
        "digest" : "sha1:da39a3ee5e6b4b0d3255bfef95601890afd80709",
      }
    },
    {
      "https://hg.acme.com/(.*)" : {
        "replacement" : r”https://my.company.com/foo-hg-cache/\1",
      }
    }
  ],

The rules are applied in definition order. Only rewrite rules from the primary suite are used meaning a suite may have to replicate the rewrite rules of its suite dependencies. This allows the primary suite to retain full control over where its dependencies are sourced from.

Rewrite rules can also be specified by the MX_URLREWRITES environment variable. The value of this variable must either be a JSON object describing a single rewrite rule, a JSON array describing a list of rewrite rules or a file containing one of these JSON values. Rewrites rules specified by MX_URLREWRITES are applied after rules specified by the primary suite.

IDE configuration generation

Mx supports generating IDE configurations using the mx ideinit command. There are also specific commands that generate configurations for Eclipse (mx eclipseinit), Netbeans (mx netbeansinit) or IntelliJ (mx intellijinit) individually. Please see here for details.

Environment variable processing

Suites might require various environment variables to be defined for the suite to work and mx provides env files to cache this information for a suite. Each suite can have an env file in suite/mx.suite/env and a default env file can be provided for the user in ~/.mx/env. Env files are loaded in the following order and override any value provided by the shell environment.

1. ~/.mx/env is loaded first.

2. The primary suite's env file is loaded before loading of the suites begins.

3. The env files of any subsuites are loaded in a depth first fashion such that subsuite env files are loaded before their dependents.

4. The primary suite's env file is reloaded so that it overrides any definitions provided by subsuites.

The -v option to mx will show the loading of env files during suite parsing.

Multiple suites per repository

Sometimes it might be convenient to group multiple suites inside a single repository. In particular, this helps ensure that all these suites are synchronized and tested together.

• A suite inside a 'big repo' must be in a directory that has the same name as the suite
• If you depend on a suite that is inside a 'big repo', you have to set subdir to True in the suite import.
• If you depend on a suite that is in the same 'big repo' as the current suite, you should not specify urls in the suite import.
• In order to sclone something that is inside a 'big repo' you have to use the --subdir argument for sclone which tells in which directory the suite that you want to clone is
• In order to dynamically import a suite that is inside a 'big repo' you have to use --dynamicimport bigrepo/suite (e.g., --dynamicimport graal-enterprise/substratrevm)

Note that a suite in a "big repo" should not have a dependency to a suite in a different repository that in turn has a transitive dependency to the same "big repo". In other words, there should be no back-and-forth to the same repo.

Preview features

Java projects may use language or runtime features which are considered preview features in certain Java versions, in which case preview features must be enabled for compilation (--enable-preview). This is specified using the javaPreviewNeeded attribute, which is a version specification in the same format as javaCompliance, for example: "javaPreviewNeeded": "19..20" If the compiling JDK matches that version or version range, preview features are enabled for compilation. Given that javac and the JVM must be on the same JDK version for preview features (see here for details), compiling a project with preview features will force the javac -source and -target options to N where N is the minimum of:

• the version of the JDK being used for compilation (i.e. JAVA_HOME) and
• the lowest version where --enable-preview is not needed.

The following table of examples should make this clearer:

JDK	javaPreviewNeeded	-target / -source	--enable-preview
19	19+	19	Yes
20	19+	20	Yes
20	19	20	No
21	19	20	No
22	20	21	No
22	19..20	21	No

System dependent configuration

A project can specify system dependent configuration options depending on which operating system and architecture are in use. The following example shows how the bar property can be set to A on Windows and B on all other operating systems.

"project" : {
  "foo" : "A",
  "os" : {
    "windows" : {
      "bar" : "A"
    },
    "<others>" : {
      "bar" : "B"
    }
  }
}

Commonly supported operating system names are darwin, linux and windows. The <others> value can be used as a wildcard to match any other operating system. A warning is emitted if no operating system is matched.

The arch property can be used to alter the configuration depending on which system architecture is used. Common examples of examples of system architectures are amd64 and aarch64. The following example shows how the bar property can be set to A on amd64 and to B on all other platforms.

"project" : {
  "foo" : "A",
  "arch" : {
    "amd64" : {
      "bar" : "A"
    },
    "<others>" : {
      "bar" : "B"
    }
  }
}

Configuration options that should depend on both the operating system and the architecture value can be specified using the os_arch property as follows. The following configuration example sets the property bar to A on amd64 linux systems, and to B for all other systems.

"project" : {
  "foo" : "A",
  "os_arch" : {
    "linux" : {
      "amd64" : {
        "bar" : "A"
      },
      "<others>" : {
        "bar" : "B"
      }
    },
    "<others>" : {
      "<others>" : {
        "bar" : "B"
      }
    }
  }
}

It is only possible to specify one of either the os, arch or os_arch options for any project.