Java Community | Help. Code. Learn.•6d ago

How to describe nested struct layout through jdk 22 ffm APIs?

Based on the example like [1]. I am able to access C compatible function. With following code, I am also able to access my custom Rust lib.

// JVM side
 MethodHandle addNumbers = {
    Linker linker = Linker.nativeLinker();
    SymbolLookup rustlib = SymbolLookup.libraryLookup(
      "target/release/libmylib.so",
      Arena.global()
    );
    return linker.downcallHandle(
      rustlib.find("add_numbers").orElseThrow(),
      FunctionDescriptor.of(
        ValueLayout.JAVA_INT, 
        ValueLayout.JAVA_INT, 
        ValueLayout.JAVA_INT 
      )
    );
  }
  Object result = addNumbers.invokeExact(10, 20)
  System.out.println("Result:" + result)

// Rust side
#[no_mangle]
pub extern "C" fn add_numbers(x: i32, y: i32) -> i32 {
    x + y
}

// JVM side
 MethodHandle addNumbers = {
    Linker linker = Linker.nativeLinker();
    SymbolLookup rustlib = SymbolLookup.libraryLookup(
      "target/release/libmylib.so",
      Arena.global()
    );
    return linker.downcallHandle(
      rustlib.find("add_numbers").orElseThrow(),
      FunctionDescriptor.of(
        ValueLayout.JAVA_INT, 
        ValueLayout.JAVA_INT, 
        ValueLayout.JAVA_INT 
      )
    );
  }
  Object result = addNumbers.invokeExact(10, 20)
  System.out.println("Result:" + result)

// Rust side
#[no_mangle]
pub extern "C" fn add_numbers(x: i32, y: i32) -> i32 {
    x + y
}

However, I have a problem describing complicated nested struct type. For instance, I have a custom struct MyStruct, which embeds a rustc_hash.FxHashMap, which in turns embeds a hashbrown.HashMap, which again contains 2 fields - one generic type S (a DefaultHashBuilder), and a RawTable, which contains a non public struct TableInner. In this case, 1. should I construct RawTable and all corresponded data struct such as TableInner, and so on before creating my custom structLayout i.e. MyStruct? 2. if yes, how to create structLayout that represents generic type e.g. S? Many thanks.
[1]. https://belief-driven-design.com/looking-at-java-22-foreign-function-memory-api-57fde/

6 Replies

JavaBot•6d ago

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stechy1•6d ago

Hello, Recently I played a bit with the Java FFM API and I could just recommend you to try the "jextract" tool. It should generate a mapping from the provided header file to Java classes. So you don't have to write this cumbersome code by hand. https://github.com/openjdk/jextract/blob/master/doc/GUIDE.md

GitHub

jextract/doc/GUIDE.md at master · openjdk/jextract

https://openjdk.org/projects/code-tools. Contribute to openjdk/jextract development by creating an account on GitHub.

dan1st•6d ago

I'd say use jextract as suggested by stechy1 or change the native code so the function you are calling doesn't accept a hashmap. Instead, create a new native function that accepts an array of entries and call that from Java. You can also do that and use jextract If you want to use jextract, you probably need C header files for the generic type, you'd need to show how that specific generic type would be represented in C. I assume it's just whatever you want to pass directly embedded there (possibly as a pointer)

everymenOP•6d ago

Ok. I will look into 1.) jextract, and 2.) test if I can create a wrapper that accepts a simpler data type, rather than hash map. Thank you for the advice!

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How to describe nested struct layout through jdk 22 ffm APIs?

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