Foreign Function & Memory API

Learn modern native calls with the Java 21 FFM API.

Foreign Function & Memory API

The FFM API (Foreign Function & Memory API, Java 21+) is a modern alternative to JNI. It lives in the java.lang.foreign package. Linker.nativeLinker() gives a native linker, SymbolLookup finds symbols, MemorySegment models off-heap memory and MethodHandle performs the call.

It is safer than JNI: memory access is bounds-checked and the lifetime is managed with an Arena. For new code FFM API is recommended over JNI.

Memory with MemorySegment

MemorySegment represents an off-heap memory region. It is allocated with an Arena; when the arena closes the memory is freed automatically. This reduces the manual malloc/free risks of JNI.

A native function is obtained as a MethodHandle and called with invoke. Data is read and written on a MemorySegment; bounds checking catches errors early.

Exercise Prepare the linker with the FFM API

Add import java.lang.foreign.Linker;. In Main's main method create the linker with Linker linker = Linker.nativeLinker(); and print System.out.println("linker ready");.

Show example solution

import java.lang.foreign.Linker;

public class Main {
    public static void main(String[] args) {
        Linker linker = Linker.nativeLinker();
        System.out.println("linker ready");
    }
}

Test yourself

Complete the Linker creation

Linker linker = Linker.____;

Show answer

Linker linker = Linker.nativeLinker();

Which package holds the FFM API?
- java.lang.native
- java.lang.foreign
- java.foreign
- javax.jni
Show answer

java.lang.foreign — The FFM API lives in java.lang.foreign.
Match the FFM elements

Linker

Native call bridge
SymbolLookup

Symbol lookup
MemorySegment

Off-heap memory
Arena

Memory lifetime

Next lesson: Summary - JNI

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