Setting Up Java

Creating a MethodHandle for Rust Functions

Set up a MethodHandle: This will act as a function name wrapper in Java, allowing you to call Rust functions as if they were Java methods.

public class ClassName {
    static MethodHandle yourMethodName;// Creates MethodHandle
    // ...
}

Create a static block: This block initializes the native linker and loads the Rust library.

Step-by-Step: Inside the static Block

1. Initialize the Native Linker

static{
    Linker linker = Linker.nativeLinker(); // Initializes the native linker
    // ...
}  

The linker is used to bind Java code to native functions from the Rust library.

2. Load the Rust Shared Library

SymbolLookup lib = SymbolLookup.libraryLookup("/* path/to/your/FileName.dylib */", Arena.global()); // Loads the Rust library

Replace "/* path/to/your/FileName.dylib */" with the actual path to the shared library (e.g., librust_lib.dylib on macOS). The Arena.global() ensures global memory scope for linked symbols.

3. Link the Functions

 yourMethodName = linker.downcallHandle(
    lib.find("/* function_name */").orElseThrow(), // Replace with the Rust function name
    FunctionDescriptor.of(
        ValueLayout.JAVA_INT,                     // Match Java's return type to Rust's return type
        ValueLayout.JAVA_INT,                     // Match Java's first parameter type to Rust's first parameter type
        ValueLayout.JAVA_INT                      // Match Java's second parameter type to Rust's second parameter type
    )
);

• Replace /* function_name */ with the name of your Rust function (e.g., add_numbers).
• Use FunctionDescriptor to map Java types to Rust types.
• Refer to the Value Layout appendix for a full table of Java and Rust type mappings.

Full Java Binding for Adding Two Numbers

import java.lang.foreign.*;
import java.lang.invoke.MethodHandle;

public class RustBindings {
    static MethodHandle addNumbers; // Wrapper for the Rust function

    static {
        // Initialize the linker
        Linker linker = Linker.nativeLinker();

        // Load the Rust library
        SymbolLookup lib = SymbolLookup.libraryLookup("/* path/to/your/FileName.dylib */", Arena.global());

        // Link the Rust function
        addNumbers = linker.downcallHandle(
            lib.find("add_numbers").orElseThrow(), // Replace with the function name from Rust
            FunctionDescriptor.of(
                ValueLayout.JAVA_INT,              // Rust's return type: i32
                ValueLayout.JAVA_INT,              // Rust's first parameter: i32
                ValueLayout.JAVA_INT               // Rust's second parameter: i32
            )
        );
    }
}

Main

public class Main {
    public static void main(String[] args) throws Throwable {
        // Call the Rust function
        int result = (int) RustBindings.addNumbers.invokeExact(10, 20);
        System.out.println("Result: " + result); // Output should be 30
    }
}

Make sure to add throws Throwable to main function.

.invokeExact() is a powerful and strict method for calling functions using a MethodHandle. It ensures precise type matching, which is especially important for interoperability with external libraries (like Rust) where argument types are fixed. Always ensure that the types match exactly to avoid runtime exceptions.