How to represent C++ class in java to be used via FFI?

Question

I have following sample code in C++

class SomeClass: public ISomeClass
{
private:
    int myMember;

public:
    SomeClass(int value) : myMember(value) {}
    __declspec(dllexport)
    int GetCount() override
    {
        return myMember;
    }
};

extern "C" __declspec(dllexport)
int doSomething(ISomeClass** someCl)
{
    *someCl = new SomeClass(600);
    return (*someCl)->GetCount() + 2;
}

And I'm successfully using this in Java with FFI with the following code:

try(Arena arena = Arena.ofConfined()) {
        Linker linker = Linker.nativeLinker();
        SymbolLookup my = SymbolLookup.libraryLookup("C:\\Path\\to\\my.dll", arena);
        MethodHandle doSomething = linker.downcallHandle(
                my.find("doSomething").orElseThrow(),
                FunctionDescriptor.of(ValueLayout.JAVA_INT, ValueLayout.ADDRESS)
        );
        MemorySegment someAddress = arena.allocate(ValueLayout.ADDRESS);
        int res = (int)doSomething.invoke(someAddress);
        System.out.println("Result is " + res);
    }
    catch (Throwable t)
    {
        System.out.println("Error is " + t.getMessage());
    }

And it correctly outputs 602.

My question is - how can I represent C++'s SomeClass in Java, so that I can pass that representation from Java to C++ instead of MemorySegment someAddress on the line doSomething.invoke(someAddress) and that I can later use that representation to call ThatRepresentation->GetCount() method directly on that class inside Java?

Answer 1

What would be the easiest way to create some C wrapper around this dll as a bridge between java and C++?

Typically, a good way to represent a C++ class in Java is as an opaque pointer, where all interactions with the object happen on the native side.

If you have a C++ library you would like to access, check to see if it already has a C interface that you can link against directly using FFM. If not, you would have to define a C interface yourself to link against. This is an art, more than something that can be done mechanically, unfortunately.

For the example class you've given, I might write something like this:

Create a header file for the C interface (CHeader.h):

#ifdef __cplusplus
extern "C" {
#endif

typedef struct SomeClass SomeClass;

__declspec(dllexport)
SomeClass* new_SomeClass(int);

__declspec(dllexport)
void delete_SomeClass(SomeClass*);

__declspec(dllexport)
int SomeClass_GetCount(SomeClass*);

#ifdef __cplusplus
}
#endif

Then create a C++ implementation (Impl.cpp):

#include "CHeader.h"
#include "CppHeader.hpp"

extern "C" {

__declspec(dllexport)
SomeClass* new_SomeClass(int value) {
    return new SomeClass(value);
}

__declspec(dllexport)
void delete_SomeClass(SomeClass* inst) {
    delete inst;
}

__declspec(dllexport)
int SomeClass_GetCount(SomeClass* inst) {
    return inst->GetCount();
}

}

(where CppHeader.hpp contains the definition of SomeClass).

After compiling (cl /LD Impl.cpp), I can then use the C interface through FFM:

System.loadLibrary("Impl");
Linker linker = Linker.nativeLinker();
SymbolLookup lookup = SymbolLookup.loaderLookup();
MethodHandle newSomeClass = linker.downcallHandle(
    lookup.find("new_SomeClass").orElseThrow(),
    FunctionDescriptor.of(ADDRESS, JAVA_INT));
MethodHandle deleteSomeClass = linker.downcallHandle(
    lookup.find("delete_SomeClass").orElseThrow(),
    FunctionDescriptor.ofVoid(ADDRESS));
MethodHandle SomeClassGetCount = linker.downcallHandle(
    lookup.find("SomeClass_GetCount").orElseThrow(),
    FunctionDescriptor.of(JAVA_INT, ADDRESS));

MemorySegment inst = (MemorySegment) newSomeClass.invoke(600);
System.out.println((int) SomeClassGetCount.invoke(inst));
deleteSomeClass.invoke(inst);

I think you can see the pattern here: for every C++ member function you would like to access, define an extern "C" wrapper that takes a pointer to the class, and then calls the function you want to call.

---

If your C++ class is a standard layout type (or if you can reliably derive the layout from a C++ class), you may also access its fields more directly, by creating a GroupLayout that represents the layout of the class, and then deriving var handles to access its fields.