File memory mapping

Map files larger than 2GB with deterministic cleanup using MemorySegment.

I/O Foreign Function & Memory API Performance

Code Comparison

✕ Java 8

try (FileChannel channel =
    FileChannel.open(path,
        StandardOpenOption.READ,
        StandardOpenOption.WRITE)) {
    MappedByteBuffer buffer =
        channel.map(
            FileChannel.MapMode.READ_WRITE,
            0, (int) channel.size());
    // Limited to 2GB
    // Freed by GC, no control
}

✓ Java 22+

FileChannel channel =
    FileChannel.open(path,
        StandardOpenOption.READ,
        StandardOpenOption.WRITE);
try (Arena arena = Arena.ofShared()) {
    MemorySegment segment =
        channel.map(
            FileChannel.MapMode.READ_WRITE,
            0, channel.size(), arena);
    // No size limit
    // ...
} // Deterministic cleanup

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Why the modern way wins

📏

No size limit

Map files larger than 2GB without workarounds.

🔒

Deterministic cleanup

Arena ensures memory is freed at scope exit, not GC time.

⚡

Better performance

Aligned with modern memory models and hardware.

Old Approach

MappedByteBuffer

Modern Approach

MemorySegment with Arena

Since JDK

Difficulty

Advanced

JDK Support

File memory mapping

Available

Available since JDK 22 (March 2024)

How it works

The Foreign Function & Memory API (JEP 454) introduces MemorySegment for safe and efficient memory access. Unlike MappedByteBuffer, MemorySegment supports files larger than 2GB (Integer.MAX_VALUE), provides deterministic cleanup via Arena, and offers better performance with modern hardware.

Modern HTTP client

Java 8

URL url = new URL("https://api.com/data");
HttpURLConnection con =
    (HttpURLConnection) url.openConnection();
con.setRequestMethod("GET");
BufferedReader in = new BufferedReader(
    new InputStreamReader(con.getInputStream()));
// read lines, close streams...

Java 11+

var client = HttpClient.newHttpClient();
var request = HttpRequest.newBuilder()
    .uri(URI.create("https://api.com/data"))
    .build();
var response = client.send(
    request, BodyHandlers.ofString());
String body = response.body();

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JDK 11+ →

I/O Beginner

Reading files

Java 8

StringBuilder sb = new StringBuilder();
try (BufferedReader br =
    new BufferedReader(
        new FileReader("data.txt"))) {
    String line;
    while ((line = br.readLine()) != null)
        sb.append(line).append("\n");
}
String content = sb.toString();

Java 11+

String content =
    Files.readString(Path.of("data.txt"));

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JDK 11+ →

I/O Beginner

Writing files

Java 8

try (FileWriter fw =
        new FileWriter("out.txt");
    BufferedWriter bw =
        new BufferedWriter(fw)) {
    bw.write(content);
}

Java 11+

Files.writeString(
    Path.of("out.txt"),
    content
);

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JDK 11+ →