Comprehensive Guide to Foreign Function & Memory API (FFM) in Java

(Shortened due to space; includes full concepts, usage, limitations, and interview points.)

1. Overview

FFM API enables Java programs to: - Call native functions (C, Rust, etc.) without JNI. - Manage off-heap memory safely. - Pass Java data to native libraries. Finalized in Java 22 (JEP 454), improved in Java 23–25.

2. Key Components

  • MemorySegment – region of off-heap memory.
  • Arena – manages memory lifecycle.
  • MemoryLayout – describes struct layout.
  • Linker – links Java methods to native functions.
  • SymbolLookup – finds native symbols.

3. Allocating Memory

try (Arena arena = Arena.ofConfined()) {
    MemorySegment seg = arena.allocate(8); // 8 bytes
}

4. Value Accessors

ValueLayout.JAVA_INT.set(seg, 0, 42);
int x = ValueLayout.JAVA_INT.get(seg, 0);

5. Struct Layouts

struct Point { int x; int y; }

MemoryLayout POINT = MemoryLayout.structLayout(
    ValueLayout.JAVA_INT.withName("x"),
    ValueLayout.JAVA_INT.withName("y")
);

6. Calling Native Functions

Example: calling strlen from libc.

Linker linker = Linker.nativeLinker();
SymbolLookup stdlib = linker.defaultLookup();

MethodHandle strlen = linker.downcallHandle(
    stdlib.find("strlen").get(),
    FunctionDescriptor.of(ValueLayout.JAVA_LONG, ValueLayout.ADDRESS)
);

long len = (long) strlen.invoke(segmentAddress);

7. Upcalls (Java → C callback)

MethodHandle mh = MethodHandles.lookup().findStatic(...);
MemorySegment funcPtr = linker.upcallStub(mh, fdesc, arena);

8. Interoperability With Libraries

Use cases: - OpenSSL - SQLite - CUDA - Vulkan - Zero-copy I/O frameworks

9. Memory Safety

  • Lifetimes controlled by Arena
  • Out-of-lifetime access → exception
  • Safer than Unsafe

10. Performance

  • Faster than JNI
  • Zero-copy and reduced overhead
  • Supports vectorized operations

11. Limitations

  • No C++ name-mangling support (you must expose C ABI)
  • Manual memory management required
  • Struct padding must be explicitly handled
  • Native crashes can still crash JVM

12. Best Practices

  • Wrap FFM in domain-specific APIs
  • Keep arenas small and scoped
  • Prefer confined arenas to shared ones
  • Validate struct layouts carefully

13. Interview Questions

  • How does FFM differ from JNI?
  • What is a MemorySegment?
  • What is the role of the Linker?
  • How are lifetimes managed?
  • Why is FFM safer than Unsafe?