Java Academy · Lesson

Layouts and Structs

Describe native memory layouts.

Layouts and Structs is a free Java Academy lesson on CoddyKit — lesson 4 of 4. You can read the complete lesson below for free — then practise it hands-on in the browser with a built-in code editor and a 24/7 AI tutor. It is part of the Java Academy learning path, one of 4 lessons in the course, and your progress syncs across the web and the CoddyKit app.

Describing Native Memory

Native data is rarely a single value. C uses structs, arrays, and unions. FFM models these shapes with memory layouts, letting you read and write structured native memory safely.

The Layout Family

The layout types form a small hierarchy:

ValueLayout: a primitive (int, long, double, address)
SequenceLayout: a fixed-size array of a layout
StructLayout: fields laid out sequentially
UnionLayout: overlapping fields

A Value Layout

The simplest layouts are the built-in value layouts. They carry size, alignment, and byte order.

import java.lang.foreign.ValueLayout;

public class Main {
    public static void main(String[] args) {
        System.out.println("int size: " + ValueLayout.JAVA_INT.byteSize());
        System.out.println("double size: " + ValueLayout.JAVA_DOUBLE.byteSize());
    }
}

Defining a Struct

Model a C struct with MemoryLayout.structLayout(...), naming each field with withName. Consider C's struct Point { int x; int y; }.

import java.lang.foreign.*;

public class Main {
    public static void main(String[] args) {
        StructLayout point = MemoryLayout.structLayout(
            ValueLayout.JAVA_INT.withName("x"),
            ValueLayout.JAVA_INT.withName("y"));
        System.out.println("Point size: " + point.byteSize());
    }
}

Var Handles for Fields

To read or write a named field, derive a VarHandle using a path expression. groupElement("x") selects the field by name.

import java.lang.foreign.*;
import java.lang.invoke.VarHandle;

public class Main {
    public static void main(String[] args) {
        StructLayout point = MemoryLayout.structLayout(
            ValueLayout.JAVA_INT.withName("x"),
            ValueLayout.JAVA_INT.withName("y"));
        VarHandle xHandle = point.varHandle(
            MemoryLayout.PathElement.groupElement("x"));
        System.out.println("Built VarHandle for x: " + xHandle);
    }
}

Reading and Writing a Struct

Allocate a segment sized to the layout, then use the var handles to set and get fields. This mirrors filling a C struct.

import java.lang.foreign.*;
import java.lang.invoke.VarHandle;

public class Main {
    public static void main(String[] args) {
        StructLayout point = MemoryLayout.structLayout(
            ValueLayout.JAVA_INT.withName("x"),
            ValueLayout.JAVA_INT.withName("y"));
        VarHandle xh = point.varHandle(MemoryLayout.PathElement.groupElement("x"));
        VarHandle yh = point.varHandle(MemoryLayout.PathElement.groupElement("y"));
        try (Arena arena = Arena.ofConfined()) {
            MemorySegment p = arena.allocate(point);
            xh.set(p, 0L, 3);
            yh.set(p, 0L, 4);
            System.out.println("Point = (" + xh.get(p, 0L) + ", " + yh.get(p, 0L) + ")");
        }
    }
}

Padding and Alignment

C compilers insert padding so fields are aligned. FFM does not add it automatically, so you must include MemoryLayout.paddingLayout(n) to match the C layout exactly.

Getting alignment wrong corrupts every field after the gap.

Explicit Padding Example

For struct { char c; int n; }, the int must align to 4 bytes, so 3 padding bytes follow the char.

import java.lang.foreign.*;

public class Main {
    public static void main(String[] args) {
        StructLayout s = MemoryLayout.structLayout(
            ValueLayout.JAVA_BYTE.withName("c"),
            MemoryLayout.paddingLayout(3),
            ValueLayout.JAVA_INT.withName("n"));
        System.out.println("Aligned struct size: " + s.byteSize());
    }
}

Sequence Layouts

A SequenceLayout describes an array. MemoryLayout.sequenceLayout(count, element) models, say, an array of 10 doubles.

import java.lang.foreign.*;

public class Main {
    public static void main(String[] args) {
        SequenceLayout arr = MemoryLayout.sequenceLayout(10, ValueLayout.JAVA_DOUBLE);
        System.out.println("Array bytes: " + arr.byteSize());
    }
}

Nested Layouts

Layouts compose. A struct can contain a sequence, and a sequence can contain structs. Path expressions like sequenceElement() and groupElement(name) navigate deep into nested data.

This handles real-world C data structures of arbitrary complexity.

Offsets and Slicing

You can query a field's byteOffset from its path and use segment.asSlice(offset, size) to obtain a sub-segment, useful for passing one field of a struct to a native function.

Quick Check

Recall a common pitfall with structs.

Recap

You learned to describe native data:

Layout family: ValueLayout, SequenceLayout, StructLayout, UnionLayout
Name fields with withName; access via VarHandle and path elements
Add explicit paddingLayout for correct alignment
Compose nested layouts and slice with asSlice

That completes the Foreign Function and Memory API course.

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Describe native memory layouts. You practise Java Academy with hands-on code you run directly in the browser, and a 24/7 AI tutor answers your questions as you work through the lesson.

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