rust-maven-plugin

Build Rust Cargo crates within a Java Maven Project.

$ mvn clean package
...
[INFO] --- rust-maven-plugin:1.1.1:build (str-reverse) @ rust-maven-jni-example ---
[INFO] Working directory: /home/adam/questdb/repos/rust-maven-plugin/rust-maven-jni-example/src/main/rust/str-reverse
[INFO] Environment variables:
[INFO]   REVERSED_STR_PREFIX='Great Scott, A reversed string!'
[INFO] Running: cargo build --target-dir /home/adam/questdb/repos/rust-maven-plugin/rust-maven-jni-example/target/rust-maven-plugin/str-reverse --release
[INFO]    Compiling proc-macro2 v1.0.49
[INFO]    Compiling quote v1.0.23
[INFO]    Compiling unicode-ident v1.0.6
[INFO]    Compiling syn v1.0.107
...

Plugin Features

• The plugin delegates the build to cargo and supports most of cargo build's features.
• The primary use case is to simplify the build process of Rust JNI libs inside a Java Maven project.
• Additionally, the plugin can also compile binaries.
• The plugin can copy complied binaries to a custom location, so they can be bundled inside of .jar files.
  • We use the same naming scheme as JNA to support loading binaries from JARs, without having JNA as a dependency.
• Support for invoking cargo test during mvn test.
• Points to https://www.rust-lang.org/tools/install if cargo isn't found.

Optional supporting loader library

For your convenience, we've also made jar-jni available: An optional Java library to load JNI dynamic libraries from JARs.

Both the plugin and the library support a common directory naming convention to organize and find compiled artifacts for a multitude of platforms.

Complete Examples

When integrating Rust, we recommend using JNI due to better performance and features, but JNA is also supported.

Rust JNI Example

See the rust-maven-jni-example directory for a working example using Java JNI via the Rust jni crate.

Thi example also uses the jar-jni library to load the Rust binaries from the compiled JAR file.

Rust JNA Example

See the rust-maven-jna-example directory for calling Rust from Java via JNA.

Note that neither rust-maven-plugin nor jar-jni depend on JNA: Instead both the plugin and the library support a common directory naming convention to organize and find compiled artifacts for a multitude of platforms.

Basic Plugin Configuration

Edit your pom.xml to add the plugin:

<project ...>
    ...

    <!-- Note: Don't add rust-maven-plugin to <dependencies>. -->

    <build>
        <plugins>
            <plugin>
                <groupId>org.questdb</groupId>
                <artifactId>rust-maven-plugin</artifactId>
                <version>1.1.1</version>
                <executions>
                    <execution>
                        <id>rust-build-id</id>
                        <goals>
                            <goal>build</goal>
                        </goals>
                        <configuration>
                            <path>src/main/rust/your-rust-crate</path>
                            <copyTo>${project.build.directory}/classes/io/questdb/jni/example/rust/libs</copyTo>
                            <copyWithPlatformDir>true</copyWithPlatformDir>
                        </configuration>
                    </execution>
                </executions>
            </plugin>
            ...
        </plugins>
    </build>

</project>

Here, <path>..</path> is the path to the Rust crate to build, and it is relative to the pom.xml file itself. The plugin will invoke cargo build on the crate.

The <id> is an arbitrary string that you can use to identify the execution. It does not need to match the crate name.

If you need to build multiple crates, you can add multiple executions.

Testing

The plugin can also invoke cargo test during mvn test.

To enable running tests:

• Duplicate the <execution> block above.
• Change it's <id> to a new name.
• Change the <goal> to test.

<execution>
    <id>str-reverse-test</id>
    <goals>
        <goal>test</goal>
    </goals>
    <configuration>
        <path>src/main/rust/str-reverse</path>
    </configuration>
</execution>

Customizing the build and tests steps

The settings below go in the <configuration> section of the <execution> block.

Custom path to the cargo command

If cargo isn't in your PATH, you can specify the path to the cargo command with the <cargoPath> configuration option.

You can also specify this on the command line via mvn ... -DcargoPath=....

Verbosity

The plugin can be configured to forward various verbosity flags to cargo by setting <verbosity>-v</verbosity> (or other value) in the <configuration> block.

Accepted values are:

Value	Description
<verbosity></verbosity> (or no tag)	Default - no additional flags passed to cargo
<verbosity>-q</verbosity>	Quiet
<verbosity>-v</verbosity>	Verbose
<verbosity>-vv</verbosity>	Very verbose

Release builds

The plugin can be configured to build in release mode by setting <release>true</release> in the <configuration> block.

Building --release will cut down binary size considerably and should be taken into consideration when shipping binaries in .jar files.

Specifying Crate Features

The equivalent of cargo build --features feat1,feat2,feat3 is

<features>
    <feature>feat1</feature>
    <feature>feat2</feature>
    <feature>feat3</feature>
</features>

You can also specify --all-features via <all-features>true</all-features> and --no-default-features via <no-default-features>true</no-default-features>.

Note that you can drive features with maven profiles by introducing variables.

<!-- in the plugin configuration -->
<features>
    <feature>$rustFeature1</feature>
    <feature>$rustFeature2</feature>
</features>

<!-- in the profiles section -->
<profile>
    <id>feat-ssl</id>
    <properties>
        <rustFeature1>ssl</rustFeature1>
        <rustFeature1>use-rustls</rustFeature1>
    </properties>
</profile>

Then building with mvn package -P feat-ssl will call cargo build --features ssl,use-rustls.

Additional cargo arguments

Additional arguments to can go in the <extra-args> configuration section.

<extra-args>
    <extra-arg>--verbose</extra-arg>
    <extra-arg>--color=always</extra-arg>
</extra-args>

Skipping an execution

If you need to skip an execution, you can set <skip>true</skip> in the <configuration> block of the <execution> of the plugin.

<skip>true</skip>  <!-- or `false`, the default value. -->

This is probably most useful when driven from a property.

<skip>${skipRustBuild}</skip>

The property can be defaulted in the <properties> section of the pom.xml file.

<properties>
    <skipRustBuild>false</skipRustBuild>
</properties>

And then overridden on the command line with -DskipRustBuild=true.

$ mvn package -DskipRustBuild=true

Overriding Environment Variables

The plugin can be configured to override environment variables during the build. This might be useful for setting RUSTFLAGS.

In the <configuration> section, add:

<environmentVariables>
    <RUSTFLAGS>-C target-cpu=native</RUSTFLAGS>
</environmentVariables>

Cleaning the Rust build

Regular mvn clean will also clean the Rust build without additional config. This is because the plugin builds crates inside Maven's target build directory, via cargo build --target-dir ....

De-duplicating build directories when invoking cargo build without Maven

If you (or your IDE) end up invoking cargo build on your Rust crate without the plugin, you'll notice this creates a duplicate target dir, inside the crate's directory, that will not be cleaned at the next mvn clean.

To avoid this duplicate target directory problem, consider adding .cargo/config.toml files configured to match the --target-dir argument passed by this plugin.

See .cargo/config.toml from the str-reverse crate in the example.

Bundling binaries in the .jar file

The <copyTo> configuration (as shown in the example) allows copying the binaries any path. The example however choses to copy them to ${project.build.directory}/classes/.... Anything placed there gets bundled into the JAR file. The classes directory sits within the target directory, outside the source tree.

Binaries in the source tree

If you prefer to keep your binaries in the source tree, you can instead configure the plugin to copy binaries to the resources directory instead:

<copyTo>src/main/resources/io/questdb/jni/example/rust/libs</copyTo>
<copyWithPlatformDir>true</copyWithPlatformDir>

In such case, you may opt to move the rust-maven-plugin inside a Maven Profile and only build the Rust code when you need to.

<project ...>
    ...
    <profiles>
        <profile>
            <id>rust</id>
            <build>
                <plugins>
                    <plugin>
                        <groupId>org.questdb</groupId>
                        <artifactId>rust-maven-plugin</artifactId>
                        <version>1.1.1</version>
...

You can then enable the profile in Maven via mvn clean package -P rust ....

Supporting Multiple Platforms

During the binary copy step, the <copyWithPlatformDir>true</copyWithPlatformDir> config setting (used in the examples above) will further nest the binaries in a directory named after the platform.

target
    classes
        io/questdb/jni/example/rust/libs/
            linux-x86-64/libstr_reverse.so
            darwin-aarch64/libstr_reverse.dylib
            win32-x86-64/str_reverse.dll

If you only intend to target one single platform (e.g. linux-amd64), then you don't need <copyWithPlatformDir>true</copyWithPlatformDir> and the plugin will not create a nested directory.

Whilst we don't depend on the JNA library, the plugin's naming convention matches that of JNA's Platform.RESOURCE_PREFIX.

Loading binaries from the .jar with jar-jni

The jar-jni library is configured as so:

<!-- pom.xml -->
<project ...>
    ...
    <dependencies>
      <dependency>
          <groupId>org.questdb</groupId>
          <artifactId>jar-jni</artifactId>
          <version>1.1.1</version>
      </dependency>
    </dependencies>
    ...
</project>

It helps with bundling JNI native code in .jar files by establishing a directory naming convention for organising binaries for different operating systems and architectures.

Assuming you've compiled with <copyWithPlatformDir>true</copyWithPlatformDir>, load the binary from the .jar file with:

JarJniLoader.loadLib(
    Main.class,

    // A platform-specific path is automatically suffixed to path below.
    "/io/questdb/jni/example/rust/libs",

    // The "lib" prefix and ".so|.dynlib|.dll" suffix are added automatically as needed.
    "str_reverse");

If instead you compiled with <copyWithPlatformDir>false</copyWithPlatformDir>, then:

JarJniLoader.loadLib(
    Main.class,
    "/io/questdb/jni/example/rust/libs",
    "str_reverse",
    null);

IntelliJ Integration

If you're using IntelliJ, you'll notice that the rust-maven-plugin is not invoked when you run Build -> Build Project or Build -> Rebuild Project.

This is because IntelliJ uses its own build system, and does not invoke Maven.

To work around this, you can add a Maven Run Configuration that invokes an Ant task before each build. The Ant task then invokes the Maven build step.

Whilst it does sound a little scary to also involve Ant, the actual XML is (for once) very simple.

<your-proj-root>/rust-intellij.xml:

<project name="str-reverse-intellij-integration" default="str-reverse-intellij-build" basedir=".">
    <description>
        IntelliJ integration to trigger maven steps to build the Rust code via the rust-maven-plugin.
    </description>
    <target name="str-reverse-intellij-build">
        <exec executable="mvn">
            <arg value="org.questdb:rust-maven-plugin:build@str-reverse"/>
        </exec>
    </target>
</project>

In the XML above, str-reverse is the <id> of the <execution> block in the rust-maven-plugin configuration in the pom.xml file.

You can add multiple executions here, for example to also build release binaries.

Check the file into your source control.

You can then go to the Ant window in IntelliJ (View -> Tool Windows -> Ant) and select the configured ant target, right-click, Execute On -> Before Build.

This will generate a new .idea/ant.xml file that will be picked up by IntelliJ and will trigger the Ant task before each build which you probably want to add to source control.

This file will look something like this:

<?xml version="1.0" encoding="UTF-8"?>
<project version="4">
  <component name="AntConfiguration">
    <buildFile url="file://$PROJECT_DIR$/rust-intellij.xml">
      <executeOn event="beforeCompilation" target="str-reverse-intellij-build" />
    </buildFile>
  </component>
</project>

Here is the IntelliJ reference for this feature: https://www.jetbrains.com/help/idea/configuring-triggers-for-ant-build-target.html#db2565bc

Contributing & Support

• Test cases, features, docs, tutorials, etc are always welcome.
• Raise an issue if you find bugs.
• We've got a list of open issues.
• Raise a pull request if you need a new feature.

If you want to talk to us, we're on Slack.

Also read the Developer's Notes with instructions on building and running from source.