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Java : MappedByteBuffer with Examples

Published : September 5, 2023

MappedByteBuffer (Java SE 20 & JDK 20) API Examples.
You will find code examples on most MappedByteBuffer methods.


Summary

A direct byte buffer whose content is a memory-mapped region of a file.

MappedByteBuffer (Java SE 20 & JDK 20)

Class diagram

public MappedByteBuffer createBuffer(Path file) throws IOException {
    try (final var channel = FileChannel.open(file,
            StandardOpenOption.READ, StandardOpenOption.WRITE)) {

        return channel.map(FileChannel.MapMode.READ_WRITE, 0, channel.size());
    }
}

final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

final var buffer = createBuffer(file);

{
    final var dst = new byte[buffer.capacity()];
    buffer.get(0, dst);
    System.out.println(Arrays.toString(dst)); // [10, 20, 30, 40, 50]

    final var ret = Files.readAllBytes(file);
    System.out.println(Arrays.toString(ret)); // [10, 20, 30, 40, 50]
}

buffer.put(0, (byte) -10);
buffer.put(1, (byte) -20);
buffer.put(2, (byte) -30);

{
    final var dst = new byte[buffer.capacity()];
    buffer.get(0, dst);
    System.out.println(Arrays.toString(dst)); // [-10, -20, -30, 40, 50]

    final var ret = Files.readAllBytes(file);
    System.out.println(Arrays.toString(ret)); // [-10, -20, -30, 40, 50]
}

final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file,
        StandardOpenOption.READ, StandardOpenOption.WRITE)) {

    final var buffer = channel.map(
            FileChannel.MapMode.READ_ONLY, 0, channel.size());

    {
        final var dst = new byte[buffer.capacity()];
        buffer.get(0, dst);
        System.out.println(Arrays.toString(dst)); // [10, 20, 30, 40, 50]

        final var ret = Files.readAllBytes(file);
        System.out.println(Arrays.toString(ret)); // [10, 20, 30, 40, 50]
    }

    final var src = ByteBuffer.allocate(3)
            .put((byte) -10)
            .put((byte) -20)
            .put((byte) -30)
            .clear();

    System.out.println(channel.write(src)); // 3

    {
        final var dst = new byte[buffer.capacity()];
        buffer.get(0, dst);
        System.out.println(Arrays.toString(dst)); // [-10, -20, -30, 40, 50]

        final var ret = Files.readAllBytes(file);
        System.out.println(Arrays.toString(ret)); // [-10, -20, -30, 40, 50]
    }
}

Methods

final MappedByteBuffer clear ()

Clears this buffer.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file, StandardOpenOption.READ)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

    buffer.position(2);
    buffer.limit(4);

    System.out.println(buffer); // java.nio.DirectByteBufferR[pos=2 lim=4 cap=5]

    System.out.println(buffer.get()); // 30
    System.out.println(buffer.get()); // 40
    System.out.println(buffer.hasRemaining()); // false

    System.out.println(buffer.clear()); // java.nio.DirectByteBufferR[pos=0 lim=5 cap=5]

    System.out.println(buffer.get()); // 10
    System.out.println(buffer.get()); // 20
    System.out.println(buffer.get()); // 30
    System.out.println(buffer.get()); // 40
    System.out.println(buffer.get()); // 50
    System.out.println(buffer.hasRemaining()); // false
}

abstract MappedByteBuffer compact ()

Compacts this buffer (optional operation).

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test1.data");
System.out.println(file); // R:\java-work\test1.data

final byte[] bytes = {10, 20, 30, 40, 50, 60};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file,
        StandardOpenOption.READ, StandardOpenOption.WRITE)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_WRITE, 0, channel.size());
    buffer.position(3);
    System.out.println(buffer.compact()); // java.nio.DirectByteBuffer[pos=3 lim=6 cap=6]
}

final var ret = Files.readAllBytes(file);
System.out.println(Arrays.toString(ret)); // [40, 50, 60, 40, 50, 60]

final var file = Path.of("R:", "java-work", "test2.data");
System.out.println(file); // R:\java-work\test2.data

final byte[] bytes = {10, 20, 30, 40, 50, 60};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file,
        StandardOpenOption.READ, StandardOpenOption.WRITE)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_WRITE, 0, channel.size());

    buffer.position(2);
    System.out.println(buffer.compact()); // java.nio.DirectByteBuffer[pos=4 lim=6 cap=6]
}

final var ret = Files.readAllBytes(file);
System.out.println(Arrays.toString(ret)); // [30, 40, 50, 60, 50, 60]

final var file = Path.of("R:", "java-work", "test3.data");
System.out.println(file); // R:\java-work\test3.data

final byte[] bytes = {10, 20, 30, 40, 50, 60};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file,
        StandardOpenOption.READ, StandardOpenOption.WRITE)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_WRITE, 0, channel.size());

    buffer.position(4);
    System.out.println(buffer.compact()); // java.nio.DirectByteBuffer[pos=2 lim=6 cap=6]
}

final var ret = Files.readAllBytes(file);
System.out.println(Arrays.toString(ret)); // [50, 60, 30, 40, 50, 60]

abstract MappedByteBuffer duplicate ()

Creates a new byte buffer that shares this buffer's content.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file, StandardOpenOption.READ)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

    buffer.position(1);
    buffer.limit(4);

    final var duplicated = buffer.duplicate();

    System.out.println(buffer); // java.nio.DirectByteBufferR[pos=1 lim=4 cap=5]
    System.out.println(duplicated); // java.nio.DirectByteBufferR[pos=1 lim=4 cap=5]

    System.out.println(buffer.get()); // 20
    System.out.println(buffer.get()); // 30
    System.out.println(buffer.get()); // 40

    System.out.println(duplicated.get()); // 20
    System.out.println(duplicated.get()); // 30
    System.out.println(duplicated.get()); // 40
}

final MappedByteBuffer flip ()

Flips this buffer.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file, StandardOpenOption.READ)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

    System.out.println(buffer.get()); // 10
    System.out.println(buffer.get()); // 20
    System.out.println(buffer.get()); // 30

    System.out.println(buffer); // java.nio.DirectByteBufferR[pos=3 lim=5 cap=5]
    System.out.println(buffer.flip()); // java.nio.DirectByteBufferR[pos=0 lim=3 cap=5]

    System.out.println(buffer.get()); // 10
    System.out.println(buffer.get()); // 20
    System.out.println(buffer.get()); // 30
}

final MappedByteBuffer force ()

Forces any changes made to this buffer's content to be written to the storage device containing the mapped file.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

try (final var channel = FileChannel.open(file, StandardOpenOption.READ,
        StandardOpenOption.CREATE, StandardOpenOption.WRITE)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_WRITE, 0, 3);

    {
        final var bytes = Files.readAllBytes(file);
        System.out.println(Arrays.toString(bytes)); // [0, 0, 0]
    }

    buffer.put((byte) 10);
    buffer.put((byte) 20);
    buffer.put((byte) 30);

    {
        // NOTE : In my environment, writing to the file is completed
        // before the force method is called.
        final var bytes = Files.readAllBytes(file);
        System.out.println(Arrays.toString(bytes)); // [10, 20, 30]
    }

    final var ret = buffer.force();
    System.out.println(buffer.equals(ret)); // true

    {
        final var bytes = Files.readAllBytes(file);
        System.out.println(Arrays.toString(bytes)); // [10, 20, 30]
    }
}

final MappedByteBuffer force (int index, int length)

Forces any changes made to a region of this buffer's content to be written to the storage device containing the mapped file.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

try (final var channel = FileChannel.open(file, StandardOpenOption.READ,
        StandardOpenOption.CREATE, StandardOpenOption.WRITE)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_WRITE, 0, 5);

    {
        final var bytes = Files.readAllBytes(file);
        System.out.println(Arrays.toString(bytes)); // [0, 0, 0, 0, 0]
    }

    buffer.put((byte) 10);
    buffer.put((byte) 20);
    buffer.put((byte) 30);

    {
        // NOTE : In my environment, writing to the file is completed
        // before the force method is called.
        final var bytes = Files.readAllBytes(file);
        System.out.println(Arrays.toString(bytes)); // [10, 20, 30, 0, 0]
    }

    final var pos = buffer.position();
    System.out.println(pos); // 3

    final var ret = buffer.force(0, pos);
    System.out.println(buffer.equals(ret)); // true

    {
        final var bytes = Files.readAllBytes(file);
        System.out.println(Arrays.toString(bytes)); // [10, 20, 30, 0, 0]
    }
}

final boolean isLoaded ()

Tells whether or not this buffer's content is resident in physical memory.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var os = System.getProperty("os.name");
System.out.println(os); // Windows 10

final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

final byte[] bytes = {10, 20, 30};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file, StandardOpenOption.READ)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

    System.out.println(buffer.isLoaded()); // false

    final var ret = buffer.load();
    System.out.println(buffer.equals(ret)); // true

    // The isLoaded method always returns false on Windows ...?
    System.out.println(buffer.isLoaded()); // false
}

final MappedByteBuffer limit (int newLimit)

Sets this buffer's limit.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test1.data");
System.out.println(file); // R:\java-work\test1.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file, StandardOpenOption.READ)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

    System.out.println(buffer); // java.nio.DirectByteBufferR[pos=0 lim=5 cap=5]
    System.out.println(buffer.limit()); // 5

    System.out.println(buffer.limit(3)); // java.nio.DirectByteBufferR[pos=0 lim=3 cap=5]
    System.out.println(buffer.limit()); // 3

    System.out.println(buffer.clear()); // java.nio.DirectByteBufferR[pos=0 lim=5 cap=5]
    System.out.println(buffer.limit()); // 5
}

final var file = Path.of("R:", "java-work", "test2.data");
System.out.println(file); // R:\java-work\test2.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file, StandardOpenOption.READ)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

    System.out.println(buffer.limit(3)); // java.nio.DirectByteBufferR[pos=0 lim=3 cap=5]

    System.out.println(buffer.get()); // 10
    System.out.println(buffer.get()); // 20
    System.out.println(buffer.get()); // 30

    try {
        final var ret = buffer.get();
    } catch (BufferUnderflowException e) {
        System.out.println("BufferUnderflowException!");
    }

    // Result
    // ↓
    //BufferUnderflowException!
}

final MappedByteBuffer load ()

Loads this buffer's content into physical memory.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file, StandardOpenOption.READ)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

    {
        // NOTE : In my environment, reading from the file is completed
        // before the load method is called.
        final var dst = new byte[buffer.capacity()];
        buffer.get(0, dst);
        System.out.println(Arrays.toString(dst)); // [10, 20, 30, 40, 50]
    }

    final var ret = buffer.load();
    System.out.println(buffer.equals(ret)); // true

    {
        final var dst = new byte[buffer.capacity()];
        buffer.get(0, dst);
        System.out.println(Arrays.toString(dst)); // [10, 20, 30, 40, 50]
    }
}

final MappedByteBuffer mark ()

Sets this buffer's mark at its position.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file, StandardOpenOption.READ)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

    System.out.println(buffer.get()); // 10
    System.out.println(buffer.get()); // 20

    System.out.println(buffer.position()); // 2
    System.out.println(buffer.mark()); // java.nio.DirectByteBufferR[pos=2 lim=5 cap=5]

    System.out.println(buffer.get()); // 30
    System.out.println(buffer.get()); // 40
    System.out.println(buffer.get()); // 50

    System.out.println(buffer.position()); // 5
    System.out.println(buffer.reset()); // java.nio.DirectByteBufferR[pos=2 lim=5 cap=5]

    System.out.println(buffer.get()); // 30
    System.out.println(buffer.get()); // 40
    System.out.println(buffer.get()); // 50
}

final MappedByteBuffer position (int newPosition)

Sets this buffer's position.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file, StandardOpenOption.READ)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

    System.out.println(buffer.position()); // 0

    System.out.println(buffer.get()); // 10
    System.out.println(buffer.position()); // 1

    System.out.println(buffer.get()); // 20
    System.out.println(buffer.position()); // 2

    System.out.println(buffer.get()); // 30
    System.out.println(buffer.position()); // 3

    System.out.println(buffer.get()); // 40
    System.out.println(buffer.position()); // 4

    System.out.println(buffer.get()); // 50
    System.out.println(buffer.position()); // 5

    System.out.println(buffer.position(3)); // java.nio.DirectByteBufferR[pos=3 lim=5 cap=5]

    System.out.println(buffer.get()); // 40
    System.out.println(buffer.position()); // 4

    System.out.println(buffer.get()); // 50
    System.out.println(buffer.position()); // 5
}

final MappedByteBuffer reset ()

Resets this buffer's position to the previously-marked position.

MappedByteBuffer (Java SE 20 & JDK 20)

Please see mark().

final MappedByteBuffer rewind ()

Rewinds this buffer.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file, StandardOpenOption.READ)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

    buffer.position(2);
    buffer.limit(4);

    System.out.println(buffer.position()); // 2
    System.out.println(buffer.limit()); // 4

    System.out.println(buffer.rewind()); // java.nio.DirectByteBufferR[pos=0 lim=4 cap=5]

    System.out.println(buffer.position()); // 0
    System.out.println(buffer.limit()); // 4
}

abstract MappedByteBuffer slice ()

Creates a new byte buffer whose content is a shared subsequence of this buffer's content.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file, StandardOpenOption.READ)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

    buffer.position(1);
    buffer.limit(4);

    final var sliced = buffer.slice();

    System.out.println(buffer); // java.nio.DirectByteBufferR[pos=1 lim=4 cap=5]
    System.out.println(sliced); // java.nio.DirectByteBufferR[pos=0 lim=3 cap=3]

    System.out.println(buffer.get()); // 20
    System.out.println(buffer.get()); // 30
    System.out.println(buffer.get()); // 40
    System.out.println(buffer.hasRemaining()); // false

    System.out.println(sliced.get()); // 20
    System.out.println(sliced.get()); // 30
    System.out.println(sliced.get()); // 40
    System.out.println(sliced.hasRemaining()); // false
}

abstract MappedByteBuffer slice (int index, int length)

Creates a new byte buffer whose content is a shared subsequence of this buffer's content.

MappedByteBuffer (Java SE 20 & JDK 20) 
final var file = Path.of("R:", "java-work", "test.data");
System.out.println(file); // R:\java-work\test.data

final byte[] bytes = {10, 20, 30, 40, 50};
Files.write(file, bytes);

try (final var channel = FileChannel.open(file, StandardOpenOption.READ)) {

    final var buffer = channel.map(FileChannel.MapMode.READ_ONLY, 0, channel.size());

    final var sliced = buffer.slice(2, 3);

    System.out.println(buffer); // java.nio.DirectByteBufferR[pos=0 lim=5 cap=5]
    System.out.println(sliced); // java.nio.DirectByteBufferR[pos=0 lim=3 cap=3]

    System.out.println(buffer.get()); // 10
    System.out.println(buffer.get()); // 20
    System.out.println(buffer.get()); // 30
    System.out.println(buffer.get()); // 40
    System.out.println(buffer.get()); // 50
    System.out.println(buffer.hasRemaining()); // false

    System.out.println(sliced.get()); // 30
    System.out.println(sliced.get()); // 40
    System.out.println(sliced.get()); // 50
    System.out.println(sliced.hasRemaining()); // false
}

Methods declared in ByteBuffer

alignedSlice, alignmentOffset, allocate, allocateDirect, array, arrayOffset, asCharBuffer, asDoubleBuffer, asFloatBuffer, asIntBuffer, asLongBuffer, asReadOnlyBuffer, asShortBuffer, compareTo, equals, get, get, get, get, get, get, getChar, getChar, getDouble, getDouble, getFloat, getFloat, getInt, getInt, getLong, getLong, getShort, getShort, hasArray, hashCode, isDirect, mismatch, order, order, put, put, put, put, put, put, put, put, putChar, putChar, putDouble, putDouble, putFloat, putFloat, putInt, putInt, putLong, putLong, putShort, putShort, toString, wrap, wrap

Please see the link below.

Methods declared in Buffer

capacity, hasRemaining, isReadOnly, limit, position, remaining

Please see the link below.


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