java

Run doomkit from pure Java (Project Panama / FFM)

"Pure Java" means no JNI and no hand-written C glue. Java 22's Foreign Function & Memory API (java.lang.foreign) can both call native functions (downcalls) and expose Java methods as C function pointers (upcalls) — which is exactly what doomkit's callback design needs.

Concepts in Doom.java

FFM piece	Used for
SymbolLookup.libraryLookup(...)	open libdoomgeneric
Linker.downcallHandle(...)	call dg_create, dg_tick, dg_resx, ...
Linker.upcallStub(...)	turn onInit, onDrawFrame, ... into C function pointers
Arena / MemorySegment	the callback struct, the argv array, and reading the framebuffer

Requirements

• JDK 22 or newer (FFM is stable there; on 21 it was preview and a couple of method names differ).

Build & run

1. Build libdoomgeneric (see bindings/README.md).
2. Single-file source launch (no compile step needed):

   java --enable-native-access=ALL-UNNAMED \
        -Djava.library.path=/path/to/lib \
        Doom.java -iwad /path/to/doom1.wad

   It writes frame.ppm at frame 100. --enable-native-access silences the restricted-method warning; -Djava.library.path is where it looks for the native library.

Notes

• The Arena (ARENA) is shared and never closed on purpose: it owns the upcall stubs, so it must outlive every engine callback.
• dg_screen_buffer() returns a zero-length segment; .reinterpret(w*h*4) gives it a real size so you can read pixels (0x00RRGGBB).
• Real input: return 1 from onGetKey and write the DOOM key code / pressed flag into the two MemorySegment out-parameters with set(JAVA_INT/JAVA_BYTE...).