[JAVA] I made a plugin to execute jextract with Gradle task

I made a gradle plugin as the title says. We are currently applying for the gradle repository. Click here for source code

• jnr-ffi Plugin

jnr-ffi Plugin

This plugin makes it easier to use the features under development (to be implemented in Java 13) in Project Panama.

In particular, it aims to make JEP 191: Foreign Function Interface more convenient.

JNR related articles are summarized in this Qiita, so please do not hesitate to contact us.

• Links to articles related to jnr-ffi

Operating environment

Version 13 or higher for both the system JDK and project JDK?

The operation-guaranteed JDK is Project Panama Early-Access Builds only.

If the JDK version is 12 or less, a warning message will be issued, and if jextract does not exist in the system JDK, an error message will be issued.

jextract

What is jextract?

jextract is a tool that generates the corresponding Java interface from .h files such as C language.

It is attached to early-access version JDK and is attached to Service Loader. It can also be called programmatically by using api / java / util / ServiceLoader.html).

jextract task

This plugin defines a task called jextract.

The jextract task uses jextract to generate a jar file (corresponding interface) from the .h files in the specified folder.

(It also searches for nested files and associates the directory with the package to generate a jar)

Options for the jextract task can be specified in build.gradle as follows:

build.gradle

jextract{
    // .h The path of the directory containing the file
    //The initial value is"src/main/resources/"
    sourceRoot = "head/"
    //The path of the directory to put the generated jar
    //The initial value is"libs/"
    outPath = "jar"
    //The root name of the package to which the generated interface belongs
    // <packageRoot>.<Directory name>.<Directory name>Is named like
    //The initial value is""
    packageRoot = "pkg"
    //Whether the package name includes the sourceRoot directory
    //If set to false, it is directly under SourceRoot..The h file package becomes packageRoot
    //Initial value is false
    includeRoot = false
}

Production process

Call jextract

Call jextract using ServiceLoader. By the way, if you try to call com.sun.tools.jextract.Main $ JextractToolProvider directly, you will get a compile error. (Is it because of resources / Message.properties?)

Now, let's write the Java code.

Test.java

public class Test{
    public static void main(String[] args){
        //Get the implementation list of ToolProvider with ServiceLoader.
        //Those in the JDK bin, such as jshell and jextract, will be returned.
        //The tool name of jextract is"jextract"So use the one that matches.
        ServiceLoader<ToolProvider> providers = load(ToolProvider.class);
        ToolProvider provider = null;
        for (ToolProvider tool : providers) {
            if (tool.name().equals("jextract")) provider = tool;
        }
        if(provider == null) System.out.println("error:jextract does not exist.");
        //Pass the command line arguments when calling jextract to the last argument array.
        //Separate elements instead of separating them with spaces.
        provider.run(System.out, System.err, String[]{"src/main/resources/test.h"});
    }
}

test.h

int hoge(void);

If you create test.h in src / main / resources / and execute Test.main, a file called test.h.jar will be created directly under the project.

• [I used jnr-ffi (made it easier to use)](https://qiita.com/Kakky/items/a54ccc68365707765a5a#interface%E7%94%9F%E6%88%90%E3%81%AE% E8% 87% AA% E5% 8B% 95% E5% 8C% 96)

In addition, the options used this time are excerpted.

For example

jextract -t pkg -o out.jar test.h

When you execute the command, or

Test.java

public class Test{
    public static void main(String[] args){
        ServiceLoader<ToolProvider> providers = load(ToolProvider.class);
        ToolProvider provider = null;
        for (ToolProvider tool : providers) {
            if (tool.name().equals("jextract")) provider = tool;
        }
        provider.run(System.out, System.err, String[]{"-o", "out.jar", "-t", "pkg", "test.h"});
    }
}

When I ran the program

test.h

int hoge(void);

If there is a file called out.jar, a file called out.jar will be generated.

out.jar
|-META-INF
| \jextract.properties
\pkg
  \test.class

The internal structure of out.jar is as follows, and you can see that a package directory called pkg is generated.

If you decompile Test.class using jad, you will see that it looks like this:

Test.jad

// Decompiled by Jad v1.5.8g. Copyright 2001 Pavel Kouznetsov.
// Jad home page: http://www.kpdus.com/jad.html
// Decompiler options: packimports(3) 

package pkg;


public interface test
{

    public abstract int hoge();
}

The .h file is firmly transpiled to the interface.

Also, when I made it a plugin, ServiceLoader didn't find jextract, so after changing the system JDK to 13, I changed the call a bit.

public class Test{
    public static void main(String[] args){
        //Use the system class loader
        ServiceLoader<ToolProvider> providers = load(ToolProvider.class, ClassLoader.getSystemClassLoader());
        ToolProvider provider = null;
        for (ToolProvider tool : providers) {
            if (tool.name().equals("jextract")) provider = tool;
        }
        provider.run(System.out, System.err, String[]{"-o", "out.jar", "-t", "pkg", "test.h"});
    }
}

Make a plugin

I referred to this article.

• Note on how to make a Gradle plugin

Be careful

• When adding the jar to the plugin in the test project, I also added the dependent libraries to the classpath.
• I also set the JDK for test projects and systems to 13.
• Failed if group is not specified when raising the plugin to gradle.