Goodbye JNI: Embracing Java’s Foreign Function & Memory API

Goodbye JNI, Hello FFM API The Foreign Function & Memory (FFM) API is the modern, elegant replacement for older JEPs—namely JEP 393 (Foreign-Memory Access API) and JEP 389 (Foreign Linker API)—both introduced as incubator features in JDK 16. Its mission? To make the painful days of Java Native Interface (JNI) a distant memory. JNI, while historically essential for bridging Java and native code (typically C or C++), has major shortcomings: Verbose and fragile native method signatures Difficult and unsafe memory management Tight memory limits (~2 GB) Poor performance due to complex marshalling A traditional JNI example requires external tools like javah, native compilation steps, and tedious header files. All that just to interact with a native function. Memory, Managed Differently: The Foreign Memory API Enter the Foreign Memory API, which allows off-heap memory interaction—in pure Java—with built-in safety and high performance. At its core is the MemorySegment interface. This represents a block of memory and ensures: Bounds checking Prevention of use-after-free bugs Thread confinement for consistent access patterns Lifecycle Control with Arenas To handle memory allocation lifetimes, the FFM API introduces Arena, a powerful abstraction for memory management. Four types of arenas offer flexibility: Global: Never freed, useful for constants Auto: Memory is freed by the GC Confined: Lifetime bound to a scope (try-with-resources) Shared: Thread-safe memory with explicit cleanup JEP 312 (Thread-Local Handshakes) complements shared arenas by giving fine-grained memory cleanup control across threads. Reading & Writing Memory: A Rectangle Example Suppose we want to map this native C structure into Java: struct Rectangle { int width; int height; }; With FFM in Java, you can allocate and interact with it like this: MemoryLayout RECTANGLE = MemoryLayout.structLayout( ValueLayout.JAVA_INT.withName("width"), ValueLayout.JAVA_INT.withName("height") ); static final VarHandle WIDTH_H = RECTANGLE.varHandle(PathElement.groupElement("width")); static final VarHandle HEIGHT_H = RECTANGLE.varHandle(PathElement.groupElement("height")); try (Arena arena = Arena.ofConfined()) { MemorySegment rect = arena.allocate(RECTANGLE); WIDTH_H.set(rect, 0L, 40); HEIGHT_H.set(rect, 0L, 60); int width = (int) WIDTH_H.get(rect, 0L); int height = (int) HEIGHT_H.get(rect, 0L); System.out.println("Area: " + (width * height)); // Output: Area: 2400 } This approach avoids manual offset math and gives you structured access to native memory with full type safety and runtime checks. Goodbye Boilerplate: Meet jextract To call native functions, the Foreign Function API (part of FFM) works alongside a powerful tool: jextract. It generates Java bindings from C headers automatically. Example: calling C’s abs function from stdlib.h: jextract --output classes --target-package org.cstdlib /usr/include/stdlib.h Then, in Java: import org.cstdlib.stdlib; try (Arena arena = Arena.ofConfined()) { int result = stdlib.abs(-42); System.out.println(result); // Output: 42 } No JNI glue code. No native compilation headaches. Just pure Java, talking to native code. Summary The FFM API is a breath of fresh air for developers who’ve wrestled with JNI. With safer memory management, cleaner native interop, and tools like jextract, Java now feels at home even in low-level systems programming tasks. Whether you’re allocating native structs, calling C libraries, or managing off-heap buffers, FFM brings native power into Java—without sacrificing readability or safety.

Apr 4, 2025 - 15:39

Goodbye JNI: Embracing Java’s Foreign Function & Memory API

Goodbye JNI, Hello FFM API

The Foreign Function & Memory (FFM) API is the modern, elegant replacement for older JEPs—namely JEP 393 (Foreign-Memory Access API) and JEP 389 (Foreign Linker API)—both introduced as incubator features in JDK 16. Its mission? To make the painful days of Java Native Interface (JNI) a distant memory.

JNI, while historically essential for bridging Java and native code (typically C or C++), has major shortcomings:

Verbose and fragile native method signatures
Difficult and unsafe memory management
Tight memory limits (~2 GB)
Poor performance due to complex marshalling

A traditional JNI example requires external tools like javah, native compilation steps, and tedious header files. All that just to interact with a native function.

Memory, Managed Differently: The Foreign Memory API

Enter the Foreign Memory API, which allows off-heap memory interaction—in pure Java—with built-in safety and high performance.

At its core is the MemorySegment interface. This represents a block of memory and ensures:

Bounds checking
Prevention of use-after-free bugs
Thread confinement for consistent access patterns

Lifecycle Control with Arenas

To handle memory allocation lifetimes, the FFM API introduces Arena, a powerful abstraction for memory management. Four types of arenas offer flexibility:

Global: Never freed, useful for constants
Auto: Memory is freed by the GC
Confined: Lifetime bound to a scope (try-with-resources)
Shared: Thread-safe memory with explicit cleanup

JEP 312 (Thread-Local Handshakes) complements shared arenas by giving fine-grained memory cleanup control across threads.

Reading & Writing Memory: A `Rectangle` Example

Suppose we want to map this native C structure into Java:

struct Rectangle {
    int width;
    int height;
};

With FFM in Java, you can allocate and interact with it like this:

MemoryLayout RECTANGLE = MemoryLayout.structLayout(
    ValueLayout.JAVA_INT.withName("width"),
    ValueLayout.JAVA_INT.withName("height")
);

static final VarHandle WIDTH_H = RECTANGLE.varHandle(PathElement.groupElement("width"));
static final VarHandle HEIGHT_H = RECTANGLE.varHandle(PathElement.groupElement("height"));

try (Arena arena = Arena.ofConfined()) {
    MemorySegment rect = arena.allocate(RECTANGLE);
    WIDTH_H.set(rect, 0L, 40);
    HEIGHT_H.set(rect, 0L, 60);

    int width = (int) WIDTH_H.get(rect, 0L);
    int height = (int) HEIGHT_H.get(rect, 0L);

    System.out.println("Area: " + (width * height)); // Output: Area: 2400
}

This approach avoids manual offset math and gives you structured access to native memory with full type safety and runtime checks.

Goodbye Boilerplate: Meet `jextract`

To call native functions, the Foreign Function API (part of FFM) works alongside a powerful tool: jextract. It generates Java bindings from C headers automatically.

Example: calling C’s abs function from stdlib.h:

jextract --output classes --target-package org.cstdlib /usr/include/stdlib.h

Then, in Java:

import org.cstdlib.stdlib;

try (Arena arena = Arena.ofConfined()) {
    int result = stdlib.abs(-42);
    System.out.println(result); // Output: 42
}

No JNI glue code. No native compilation headaches. Just pure Java, talking to native code.

Summary

The FFM API is a breath of fresh air for developers who’ve wrestled with JNI. With safer memory management, cleaner native interop, and tools like jextract, Java now feels at home even in low-level systems programming tasks.

Whether you’re allocating native structs, calling C libraries, or managing off-heap buffers, FFM brings native power into Java—without sacrificing readability or safety.