MNI: from a Java method to a datapack function

MNI connects MCFPP with Java. During a build, the compiler invokes Java methods to obtain values or add Minecraft commands. Minecraft later executes the generated datapack.

This page completes a small project: create it, implement Java code, declare a source function, build it, and test it. The example makes answer() return 42.

1. Create the project

Prepare JDK 21. Choose MCFPP MNI in IDEA, or run:

mcfpp init hello-mni --mni
cd hello-mni

The CLI creates these main files:

hello-mni/
├─ mcfpp.toml
├─ src/main/mcfpp/main.mcfpp
├─ src/main/java/example/mni/Example.java
└─ src/test/java/example/mni/ExampleTest.java

main.mcfpp contains the caller. Example.java contains the Java implementation. ExampleTest.java checks the Java calculation. During a build, the CLI prepares Gradle and dependencies. The first preparation needs network access.

2. Read the Java implementation

src/main/java/example/mni/Example.java:

package example.mni;

import top.mcfpp.annotations.MNIFunction;
import top.mcfpp.core.lang.MCInt;
import top.mcfpp.util.ValueWrapper;

public final class Example {
    public static int answerValue() {
        return 42;
    }

    @MNIFunction(returnType = "int")
    public static void answer(ValueWrapper<MCInt> result) {
        result.set(new MCInt(answerValue(), "answer"));
    }
}

The two methods have these roles:

Method or type	Role in this example
answerValue()	Calculate the ordinary Java integer 42
@MNIFunction	Mark a Java entry that MCFPP can bind
MCInt	Represent the result as a compiler integer
ValueWrapper<MCInt> result	Deliver the integer result to the MCFPP caller

The entry uses public static void. Java void means that this method supplies its result through result.set(...). For MCFPP callers, answer() still returns int.

3. Declare and call the source function

src/main/mcfpp/main.mcfpp:

func answer(): int = example.mni.Example.answer

func main() {
    print(answer())
}

The left side declares the MCFPP interface: answer takes zero arguments and returns int. The right side identifies the Java package example.mni, class Example, and method answer.

This declaration connects the two sides. You can then call answer() like an ordinary function.

4. Build and run in the game

mcfpp build

The CLI compiles Java files. The compiler invokes the Java entry at the answer() call and generates a datapack. The output directory is build/hello-mni/.

Copy that directory into the world's datapacks/ directory. Run these commands in the game:

reload
function hello-mni:main

The chat displays 42. Java participates during compilation. Minecraft executes the resulting datapack commands.

5. Test the Java calculation

src/test/java/example/mni/ExampleTest.java:

package example.mni;

import org.junit.jupiter.api.Test;
import static org.junit.jupiter.api.Assertions.assertEquals;

class ExampleTest {
    @Test
    void answerIs42() {
        assertEquals(42, Example.answerValue());
    }
}

mcfpp test

This test checks the Java calculation in answerValue(). mcfpp build checks Java compilation, source binding, and datapack generation together. Keeping calculations in ordinary Java methods makes them straightforward to unit test.

Continue reading

• Parameters and results: accept MCFPP data and process runtime integers.
• Generate commands from Java: produce operations that execute in the game.
• Gradle and test configuration: add Java dependencies and test settings.
• MNI macros: generate expressions, loops, and user callbacks for macro libraries.