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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.

Practice 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");.

Starter code

Main.java
// Import Linker from the java.lang.foreign package

// Main: create the native linker and print linker ready

Sample solution

Main.java
import java.lang.foreign.Linker;

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

Expected output

linker ready

Questions and answers

Review the lesson questions together with their answers. To test yourself, solve the lesson interactively.

1 Complete the Linker creation

Question 
Linker linker = Linker.____;
Answer 
Linker linker = Linker.nativeLinker();

Explanation: Linker.nativeLinker() returns the native linker.

2 Which package holds the FFM API?

  • java.lang.native
  • java.lang.foreign Correct answer
  • java.foreign
  • javax.jni

Explanation: The FFM API lives in java.lang.foreign.

3 Match the FFM elements

Term Meaning
Linker Native call bridge
SymbolLookup Symbol lookup
MemorySegment Off-heap memory
Arena Memory lifetime

All lessons in Expert

1 JVM internals

  1. JVM architecture
  2. Class loaders
  3. Bytecode
  4. JIT compiler
  5. Runtime data areas
  6. Summary - JVM internals

2 Memory model

  1. Heap and stack
  2. happens-before
  3. volatile
  4. final field guarantees
  5. Metaspace
  6. Summary - Memory model

3 NIO

  1. Buffer
  2. Channel
  3. Selector
  4. Charset
  5. Path and Files (NIO.2)
  6. Summary - NIO

4 Reactive

  1. Flow API
  2. Publisher and Subscriber
  3. SubmissionPublisher
  4. Backpressure
  5. Project Reactor Mono
  6. Summary - Reactive

5 JNI

  1. native methods
  2. javah and headers
  3. JNIEnv
  4. C/C++ implementation
  5. Foreign Function & Memory API
  6. Summary - JNI

6 Performance and GC

  1. JMH
  2. JFR and jcmd
  3. GC tuning
  4. Heap dump
  5. JIT tuning
  6. Summary - Performance and GC

7 Bytecode (ASM)

  1. Project design
  2. ClassReader
  3. ClassVisitor
  4. Bytecode transformation
  5. Putting it together

8 Expert final

  1. Project design
  2. Reactive layer
  3. Performance layer
  4. Microservice basics
  5. Putting it together