development description for prql-java module

Implementation

We implement Rust bindings to Java with JNI.

First, define a native method -- public static native String toSql(String query, String target, boolean format, boolean signature) for PrqlCompiler, toJson is same.

And then implement it in Rust with this crate.

Build

For ease of use to users, we need pre-build dynamic libs for different platforms. This process is combined into the build of Java module.

We use Maven to build the Java library. To add the Rust cross compilation into the Maven build process, we add the following XML segment to the pom.xml:

<plugin>
    <artifactId>exec-maven-plugin</artifactId>
    <groupId>org.codehaus.mojo</groupId>
    <version>1.6.0</version>
    <executions>
        <execution>
            <id>Build for release</id>
            <phase>generate-resources</phase>
            <goals>
                <goal>exec</goal>
            </goals>
            <configuration>
                <executable>../cross.sh</executable>
                <arguments>
                    <argument>${project.basedir}/../</argument>
                </arguments>
            </configuration>
        </execution>
    </executions>
</plugin>

When we build, it will execute the cross.sh script to get all the Rust cdylibs. This process is time consuming.

As to cross compilation toolchains, we use cross.

Publish (for maintainer)

To publish the Java library to Maven public repo, project maintainer need first register a project in the Maven Nexus repository, by the doc: https://central.sonatype.org/publish/publish-guide/.

And then, we can release our artifact in the release workflow. The action we used is action-maven-publish. Project maintainer has to configure some personal information, those used in the first step, by the action's doc, such as nexus_username, nexus_password, gpg_private_key, gpg_passphrase.