Java : MappedByteBuffer with Examples
MappedByteBuffer (Java SE 22 & JDK 22) in Java with Examples.
You will find code samples for most of the MappedByteBuffer methods.
Summary
A direct byte buffer whose content is a memory-mapped region of a file.
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.
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).
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.
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.
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.
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.
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.
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.
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 {
var _ = buffer.get();
} catch (BufferUnderflowException e) {
System.out.println("BufferUnderflowException!");
}
// Result
// ↓
//BufferUnderflowException!
}
final MappedByteBuffer load ()
Loads this buffer's content into physical memory.
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.
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.
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
// java.nio.DirectByteBufferR[pos=3 lim=5 cap=5]
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
}
final MappedByteBuffer reset ()
Resets this buffer's position to the previously-marked position.
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 rewind ()
Rewinds this buffer.
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.
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.
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.
Related posts
- API Examples