Example for getting known folder ids using Java Foreign Function & Memory API

JavaDir.java

import java.lang.foreign.AddressLayout;
import java.lang.foreign.Arena;
import java.lang.foreign.FunctionDescriptor;
import java.lang.foreign.GroupLayout;
import java.lang.foreign.Linker;
import java.lang.foreign.MemoryLayout;
import java.lang.foreign.MemorySegment;
import java.lang.foreign.SymbolLookup;
import java.lang.foreign.ValueLayout;
import java.lang.invoke.MethodHandle;

import static java.lang.foreign.ValueLayout.JAVA_BYTE;
import static java.lang.foreign.ValueLayout.JAVA_CHAR;

public class JavaDir {
	private static final String FOLDERID_LocalAppData = "{F1B32785-6FBA-4FCF-9D55-7B8E7F157091}";
	private static final SymbolLookup SYMBOL_LOOKUP = SymbolLookup.loaderLookup().or(Linker.nativeLinker().defaultLookup());
	public static final ValueLayout.OfBoolean C_BOOL = ValueLayout.JAVA_BOOLEAN;
	public static final ValueLayout.OfByte C_CHAR = ValueLayout.JAVA_BYTE;
	public static final ValueLayout.OfShort C_SHORT = ValueLayout.JAVA_SHORT;
	public static final ValueLayout.OfInt C_INT = ValueLayout.JAVA_INT;
	public static final ValueLayout.OfLong C_LONG_LONG = ValueLayout.JAVA_LONG;
	public static final ValueLayout.OfFloat C_FLOAT = ValueLayout.JAVA_FLOAT;
	public static final ValueLayout.OfDouble C_DOUBLE = ValueLayout.JAVA_DOUBLE;
	public static final AddressLayout C_POINTER = ValueLayout.ADDRESS
			.withTargetLayout(MemoryLayout.sequenceLayout(java.lang.Long.MAX_VALUE, JAVA_BYTE));
	public static final ValueLayout.OfInt C_LONG = ValueLayout.JAVA_INT;
	public static final ValueLayout.OfDouble C_LONG_DOUBLE = ValueLayout.JAVA_DOUBLE;

	public static void main(String[] args) {
		System.loadLibrary("ole32");
		System.loadLibrary("shell32");

		try (var arena = Arena.ofConfined()) {
			MemorySegment guid = arena.allocate(_GUID_LAYOUT);
			if (CLSIDFromString(createSegmentFromString(FOLDERID_LocalAppData, arena), guid) != 0) {
				System.exit(-1);
			}
			MemorySegment path = arena.allocate(C_POINTER);
			SHGetKnownFolderPath(guid, 0, MemorySegment.NULL, path);
			System.out.println(createStringFromSegment(path.get(C_POINTER, 0)));
		}
	}

	/**
	 * Creates a memory segment as a copy of a Java string.
	 * <p>
	 * The memory segment contains a copy of the string (null-terminated, UTF-16/wide characters).
	 * </p>
	 *
	 * @param str   the string to copy
	 * @param arena the arena for the memory segment
	 * @return the resulting memory segment
	 */
	public static MemorySegment createSegmentFromString(String str, Arena arena) {
		// allocate segment (including space for terminating null)
		var segment = arena.allocate(JAVA_CHAR, str.length() + 1L);
		// copy characters
		segment.copyFrom(MemorySegment.ofArray(str.toCharArray()));
		return segment;
	}

	/**
	 * Creates a copy of the string in the memory segment.
	 * <p>
	 * The string must be a null-terminated UTF-16 (wide character) string.
	 * </p>
	 *
	 * @param segment the memory segment
	 * @return copied string
	 */
	public static String createStringFromSegment(MemorySegment segment) {
		var len = 0;
		while (segment.get(JAVA_CHAR, len) != 0) {
			len += 2;
		}

		return new String(segment.asSlice(0, len).toArray(JAVA_CHAR));
	}

	static MemorySegment findOrThrow(String symbol) {
		return SYMBOL_LOOKUP.find(symbol)
				.orElseThrow(() -> new UnsatisfiedLinkError("unresolved symbol: " + symbol));
	}

	private static final GroupLayout _GUID_LAYOUT = MemoryLayout.structLayout(
			C_LONG.withName("Data1"),
			C_SHORT.withName("Data2"),
			C_SHORT.withName("Data3"),
			MemoryLayout.sequenceLayout(8, C_CHAR).withName("Data4")
	).withName("_GUID");

	/**
	 * {@snippet lang=c :
	 * extern HRESULT CLSIDFromString(LPCOLESTR lpsz, LPCLSID pclsid)
	 * }
	 */
	public static int CLSIDFromString(MemorySegment lpsz, MemorySegment pclsid) {
		var mh$ = CLSIDFromString.HANDLE;
		try {
			return (int)mh$.invokeExact(lpsz, pclsid);
		} catch (Throwable ex$) {
			throw new AssertionError("should not reach here", ex$);
		}
	}

	private static class CLSIDFromString {
		public static final FunctionDescriptor DESC = FunctionDescriptor.of(
				C_LONG,
				C_POINTER,
				C_POINTER
		);

		public static final MethodHandle HANDLE = Linker.nativeLinker().downcallHandle(
				findOrThrow("CLSIDFromString"), DESC);
	}

	/**
	 * {@snippet lang=c :
	 * extern HRESULT SHGetKnownFolderPath(const KNOWNFOLDERID *const rfid, DWORD dwFlags, HANDLE hToken, PWSTR *ppszPath)
	 * }
	 */
	public static int SHGetKnownFolderPath(MemorySegment rfid, int dwFlags, MemorySegment hToken, MemorySegment ppszPath) {
		var mh$ = SHGetKnownFolderPath.HANDLE;
		try {
			return (int)mh$.invokeExact(rfid, dwFlags, hToken, ppszPath);
		} catch (Throwable ex$) {
			throw new AssertionError("should not reach here", ex$);
		}
	}

	private static class SHGetKnownFolderPath {
		public static final FunctionDescriptor DESC = FunctionDescriptor.of(
				C_LONG,
				C_POINTER,
				C_LONG,
				C_POINTER,
				C_POINTER
		);

		public static final MethodHandle HANDLE = Linker.nativeLinker().downcallHandle(
				findOrThrow("SHGetKnownFolderPath"), DESC);
	}

}

run.ps1

$jdk = "C:\Program Files\Java\jdk-22"
New-Alias -Name javac -Value "$jdk\bin\javac" -Force
New-Alias -Name java -Value "$jdk\bin\java" -Force
javac --enable-preview --source=22 -d . *.java
java --source=22 --enable-preview --enable-native-access=ALL-UNNAMED JavaDir.java