Java : FileChannel with Examples

FileChannel (Java SE 22 & JDK 22) in Java with Examples.
You will find code samples for most of the FileChannel methods.

Annotation :

Most of the code examples in this article are run on Windows.
Some of the code examples are run on Linux for POSIX.

Summary

A channel for reading, writing, mapping, and manipulating a file.

Class diagram

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

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

    final var src = ByteBuffer.allocate(5)
            .put((byte) 10)
            .put((byte) 20)
            .put((byte) 30)
            .put((byte) 40)
            .put((byte) 50)
            .clear();

    final var ret = channel.write(src);
    System.out.println(ret); // 5
}

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

    final var dst = ByteBuffer.allocate(5);
    final var ret = channel.read(dst);
    System.out.println(ret); // 5

    if (dst.hasArray()) {
        // [10, 20, 30, 40, 50]
        System.out.println(Arrays.toString(dst.array()));
    }
}

Constructors

FileChannel ()

Initializes a new instance of this class.

protected. I think it's rare to create a subclass of this class. Therefore, the code example is omitted.

Methods

abstract void force (boolean metaData)

Forces any updates to this channel's file to be written to the storage device that contains it.

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

System.out.println(Files.notExists(path)); // true

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

    final byte[] array = {10, 20, 30, 40, 50};
    final var src = ByteBuffer.wrap(array);

    // --- PowerShell ---
    //PS R:\java-work> ls | Format-Table -Property Length, Name
    //
    //Length Name
    //------ ----
    //     0 test.data

    System.out.println(channel.write(src)); // 5
    System.out.println(channel.size()); // 5

    // --- PowerShell ---
    //PS R:\java-work> ls | Format-Table -Property Length, Name
    //
    //Length Name
    //------ ----
    //     0 test.data

    channel.force(true);

    // --- PowerShell ---
    //PS R:\java-work> ls | Format-Table -Property Length, Name
    //
    //Length Name
    //------ ----
    //     5 test.data
}

final FileLock lock ()

Acquires an exclusive lock on this channel's file.

Please see also : FileLock

public class Child {
    public static void main(String[] args) throws IOException, InterruptedException {
        if (args.length != 1) {
            throw new IllegalArgumentException();
        }

        final var name = "child " + args[0];

        final var file = Path.of("lock.txt");
        try (final var channel = FileChannel.open(
                file, StandardOpenOption.CREATE, StandardOpenOption.WRITE)) {

            System.out.printf("  %s : lock start ...%n", name);
            try (final var lock = channel.lock()) {

                TimeUnit.MILLISECONDS.sleep(100);

                System.out.printf("  %s :   *** lock OK! ***%n", name);
                System.out.printf("  %s :   sleep 5 seconds ...%n", name);

                TimeUnit.SECONDS.sleep(5);
                System.out.printf("  %s :   sleep end%n", name);
            }

            System.out.printf("  %s : lock end%n", name);
        }
    }
}

public class Main {
    public static void main(String[] args) throws IOException, InterruptedException {
        System.out.println("main : start");

        final var p1 = new ProcessBuilder("java", "Child", "1").inheritIO().start();
        TimeUnit.MILLISECONDS.sleep(10);
        final var p2 = new ProcessBuilder("java", "Child", "2").inheritIO().start();

        p1.waitFor();
        p2.waitFor();

        System.out.println("main : end");
    }
}

// Result
// ↓
//> java Main
//main : start
//  child 1 : lock start ...
//  child 2 : lock start ...
//  child 1 :   *** lock OK! ***
//  child 1 :   sleep 5 seconds ...
//  child 1 :   sleep end
//  child 1 : lock end
//  child 2 :   *** lock OK! ***
//  child 2 :   sleep 5 seconds ...
//  child 2 :   sleep end
//  child 2 : lock end
//main : end

abstract FileLock lock (long position, long size, boolean shared)

Acquires a lock on the given region of this channel's file.

An example for position and size parameters.

public class Child {
    public static void main(String[] args) throws IOException, InterruptedException {
        if (args.length != 3) {
            throw new IllegalArgumentException();
        }

        final var name = "child " + args[0];
        final var position = Long.parseLong(args[1]);
        final var size = Long.parseLong(args[2]);

        final var file = Path.of("lock.txt");
        try (final var channel = FileChannel.open(
                file, StandardOpenOption.CREATE, StandardOpenOption.WRITE)) {

            System.out.printf("  %s : lock start (position=%d, size=%d) ...%n",
                    name, position, size);
            try (final var lock = channel.lock(position, size, false)) {

                TimeUnit.MILLISECONDS.sleep(100);

                System.out.printf("  %s :   *** lock OK! ***%n", name);
                System.out.printf("  %s :   sleep 5 seconds ...%n", name);

                TimeUnit.SECONDS.sleep(5);
                System.out.printf("  %s :   sleep end%n", name);
            }

            System.out.printf("  %s : lock end%n", name);
        }
    }
}

public class Main {
    public static void main(String[] args) throws IOException, InterruptedException {
        if (args.length != 2) {
            throw new IllegalArgumentException();
        }

        System.out.println("main : start");

        final var p1 = new ProcessBuilder("java", "Child", "1", "0", "10").inheritIO().start();
        TimeUnit.MILLISECONDS.sleep(10);
        final var p2 = new ProcessBuilder("java", "Child", "2", args[0], args[1]).inheritIO().start();

        p1.waitFor();
        p2.waitFor();

        System.out.println("main : end");
    }
}

// Result
// ↓
//> java Main 0 10
//main : start
//  child 1 : lock start (position=0, size=10) ...
//  child 2 : lock start (position=0, size=10) ...
//  child 1 :   *** lock OK! ***
//  child 1 :   sleep 5 seconds ...
//  child 1 :   sleep end
//  child 1 : lock end
//  child 2 :   *** lock OK! ***
//  child 2 :   sleep 5 seconds ...
//  child 2 :   sleep end
//  child 2 : lock end
//main : end
//
//> java Main 11 5
//main : start
//  child 1 : lock start (position=0, size=10) ...
//  child 2 : lock start (position=11, size=5) ...
//  child 1 :   *** lock OK! ***
//  child 1 :   sleep 5 seconds ...
//  child 2 :   *** lock OK! ***
//  child 2 :   sleep 5 seconds ...
//  child 1 :   sleep end
//  child 1 : lock end
//  child 2 :   sleep end
//  child 2 : lock end
//main : end

An example for the shared parameter.

public class Child {
    public static void main(String[] args) throws IOException, InterruptedException {
        if (args.length != 2) {
            throw new IllegalArgumentException();
        }

        final var name = "child " + args[0];
        final var shared = "shared".equals(args[1]);

        final var file = Path.of("lock.txt");
        try (final var channel = FileChannel.open(file, StandardOpenOption.CREATE,
                StandardOpenOption.WRITE, StandardOpenOption.READ)) {

            System.out.printf("  %s : lock start (shared=%b) ...%n", name, shared);
            try (final var lock = channel.lock(0, Long.MAX_VALUE, shared)) {

                TimeUnit.MILLISECONDS.sleep(100);

                System.out.printf("  %s :   *** lock OK! ***%n", name);
                System.out.printf("  %s :   sleep 5 seconds ...%n", name);

                TimeUnit.SECONDS.sleep(5);
                System.out.printf("  %s :   sleep end%n", name);
            }

            System.out.printf("  %s : lock end%n", name);
        }
    }
}

public class Main {
    public static void main(String[] args) throws IOException, InterruptedException {
        if (args.length != 2) {
            throw new IllegalArgumentException();
        }

        System.out.println("main : start");

        final var p1 = new ProcessBuilder("java", "Child", "1", args[0]).inheritIO().start();
        TimeUnit.MILLISECONDS.sleep(10);
        final var p2 = new ProcessBuilder("java", "Child", "2", args[1]).inheritIO().start();

        p1.waitFor();
        p2.waitFor();

        System.out.println("main : end");
    }
}

// Result
// ↓
//> java Main shared shared
//main : start
//  child 1 : lock start (shared=true) ...
//  child 2 : lock start (shared=true) ...
//  child 1 :   *** lock OK! ***
//  child 1 :   sleep 5 seconds ...
//  child 2 :   *** lock OK! ***
//  child 2 :   sleep 5 seconds ...
//  child 1 :   sleep end
//  child 1 : lock end
//  child 2 :   sleep end
//  child 2 : lock end
//main : end
//
//> java Main shared exclusive
//main : start
//  child 1 : lock start (shared=true) ...
//  child 2 : lock start (shared=false) ...
//  child 1 :   *** lock OK! ***
//  child 1 :   sleep 5 seconds ...
//  child 1 :   sleep end
//  child 1 : lock end
//  child 2 :   *** lock OK! ***
//  child 2 :   sleep 5 seconds ...
//  child 2 :   sleep end
//  child 2 : lock end
//main : end
//
//> java Main exclusive shared
//main : start
//  child 1 : lock start (shared=false) ...
//  child 2 : lock start (shared=true) ...
//  child 1 :   *** lock OK! ***
//  child 1 :   sleep 5 seconds ...
//  child 1 :   sleep end
//  child 1 : lock end
//  child 2 :   *** lock OK! ***
//  child 2 :   sleep 5 seconds ...
//  child 2 :   sleep end
//  child 2 : lock end
//main : end
//
//> java Main exclusive exclusive
//main : start
//  child 1 : lock start (shared=false) ...
//  child 2 : lock start (shared=false) ...
//  child 1 :   *** lock OK! ***
//  child 1 :   sleep 5 seconds ...
//  child 1 :   sleep end
//  child 1 : lock end
//  child 2 :   *** lock OK! ***
//  child 2 :   sleep 5 seconds ...
//  child 2 :   sleep end
//  child 2 : lock end
//main : end

abstract MappedByteBuffer map (FileChannel.MapMode mode, long position, long size)

Maps a region of this channel's file directly into memory.

Please see also : MappedByteBuffer

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

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

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

    final var buffer = channel.map(
            FileChannel.MapMode.READ_WRITE, 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(path);
        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(path);
        System.out.println(Arrays.toString(ret)); // [-10, -20, -30, 40, 50]
    }
}

MemorySegment map (FileChannel.MapMode mode, long offset, long size, Arena arena)

Maps a region of this channel's file into a new mapped memory segment, with the given offset, size and arena.

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

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

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

    final var segment = channel.map(
            FileChannel.MapMode.READ_WRITE, 0, channel.size(), Arena.ofAuto());

    {

        final var dst = segment.toArray(ValueLayout.JAVA_BYTE);
        System.out.println(Arrays.toString(dst)); // [10, 20, 30, 40, 50]

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

    segment.set(ValueLayout.JAVA_BYTE, 0, (byte) -10);
    segment.set(ValueLayout.JAVA_BYTE, 1, (byte) -20);
    segment.set(ValueLayout.JAVA_BYTE, 2, (byte) -30);

    {
        final var dst = segment.toArray(ValueLayout.JAVA_BYTE);
        System.out.println(Arrays.toString(dst)); // [-10, -20, -30, 40, 50]

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

static FileChannel open (Path path, OpenOption... options)

Opens or creates a file, returning a file channel to access the file.

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

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

    final var src = ByteBuffer.allocate(5)
            .put((byte) 10)
            .put((byte) 20)
            .put((byte) 30)
            .put((byte) 40)
            .put((byte) 50)
            .clear();

    final var ret = channel.write(src);
    System.out.println(ret); // 5
}

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

    final var dst = ByteBuffer.allocate(5);
    final var ret = channel.read(dst);
    System.out.println(ret); // 5

    if (dst.hasArray()) {
        // [10, 20, 30, 40, 50]
        System.out.println(Arrays.toString(dst.array()));
    }
}

static FileChannel open (Path path, Set<? extends OpenOption> options, FileAttribute<?>... attrs)

Opens or creates a file, returning a file channel to access the file.

Please see also open(Path path, OpenOption... options) for parameters other than the attrs parameter.

The following code runs on Linux.

public class Main {
    public static void main(String[] args) throws IOException {
        System.out.println(System.getProperty("os.name")); // Linux

        {
            final var file = Path.of("sample1.txt");
            try (final var _ = FileChannel.open(file,
                    Set.of(StandardOpenOption.CREATE, StandardOpenOption.WRITE))) {
            }

            // --- Linux Terminal ---
            //$ ls -l sample1.txt
            //-rw-rw-r-- 1 xxxx xxxx 0 ... sample1.txt
        }

        {
            final var permissions = PosixFilePermissions.fromString("rw-r-----");
            final var attrs = PosixFilePermissions.asFileAttribute(permissions);

            final var file = Path.of("sample2.txt");
            try (final var _ = FileChannel.open(file,
                    Set.of(StandardOpenOption.CREATE, StandardOpenOption.WRITE),
                    attrs)) {
            }

            // --- Linux Terminal ---
            //$ ls -l sample2.txt
            //-rw-r----- 1 xxxx xxxx 0 ... sample2.txt
        }
    }
}

abstract long position ()

Returns this channel's file position.

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

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

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

    final var ret1 = channel.position(2);
    System.out.println(channel.equals(ret1)); // true

    {
        System.out.println(channel.position()); // 2

        final var buffer = ByteBuffer.allocate(3);
        final var ret = channel.read(buffer);
        System.out.println(ret); // 3

        System.out.println(channel.position()); // 5

        if (buffer.hasArray()) {
            // [30, 40, 50]
            System.out.println(Arrays.toString(buffer.array()));
        }
    }

    final var ret2 = channel.position(0);
    System.out.println(channel.equals(ret2)); // true

    {
        System.out.println(channel.position()); // 0

        final var buffer = ByteBuffer.allocate(5);
        final var ret = channel.read(buffer);
        System.out.println(ret); // 5

        System.out.println(channel.position()); // 5

        if (buffer.hasArray()) {
            // [10, 20, 30, 40, 50]
            System.out.println(Arrays.toString(buffer.array()));
        }
    }
}

abstract FileChannel position (long newPosition)

Sets this channel's file position.

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

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

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

    final var ret1 = channel.position(2);
    System.out.println(channel.equals(ret1)); // true

    {
        System.out.println(channel.position()); // 2

        final var buffer = ByteBuffer.allocate(3);
        final var ret = channel.read(buffer);
        System.out.println(ret); // 3

        System.out.println(channel.position()); // 5

        if (buffer.hasArray()) {
            // [30, 40, 50]
            System.out.println(Arrays.toString(buffer.array()));
        }
    }

    final var ret2 = channel.position(0);
    System.out.println(channel.equals(ret2)); // true

    {
        System.out.println(channel.position()); // 0

        final var buffer = ByteBuffer.allocate(5);
        final var ret = channel.read(buffer);
        System.out.println(ret); // 5

        System.out.println(channel.position()); // 5

        if (buffer.hasArray()) {
            // [10, 20, 30, 40, 50]
            System.out.println(Arrays.toString(buffer.array()));
        }
    }
}

abstract int read (ByteBuffer dst)

Reads a sequence of bytes from this channel into the given buffer.

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

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

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

    final var array = new byte[5];
    final var dst = ByteBuffer.wrap(array);

    System.out.println(dst); // java.nio.HeapByteBuffer[pos=0 lim=5 cap=5]
    System.out.println(Arrays.toString(array)); // [0, 0, 0, 0, 0]

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

    final var ret = channel.read(dst);
    System.out.println(ret); // 3

    System.out.println(channel.position()); // 3

    System.out.println(dst); // java.nio.HeapByteBuffer[pos=3 lim=5 cap=5]
    System.out.println(Arrays.toString(array)); // [10, 20, 30, 0, 0]
}

final long read (ByteBuffer[] dsts)

Reads a sequence of bytes from this channel into the given buffers.

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

final byte[] bytes = {10, 20, 30, 40, 50, 60, 70, 80, 90};
Files.write(path, bytes);

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

    final ByteBuffer[] dsts = {
            ByteBuffer.allocate(2),
            ByteBuffer.allocate(3),
            ByteBuffer.allocate(4),
    };

    final var ret = channel.read(dsts);
    System.out.println(ret); // 9

    System.out.println("-- dsts --");
    for (final var dst : dsts) {
        if (dst.hasArray()) {
            System.out.println(Arrays.toString(dst.array()));
        }
    }

    // Result
    // ↓
    //-- dsts --
    //[10, 20]
    //[30, 40, 50]
    //[60, 70, 80, 90]
}

abstract long read (ByteBuffer[] dsts, int offset, int length)

Reads a sequence of bytes from this channel into a subsequence of the given buffers.

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

final byte[] bytes = {10, 20, 30, 40, 50, 60, 70, 80, 90};
Files.write(path, bytes);

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

    final ByteBuffer[] dsts = {
            ByteBuffer.allocate(1),
            ByteBuffer.allocate(2),
            ByteBuffer.allocate(3),
            ByteBuffer.allocate(4),
            ByteBuffer.allocate(5),
    };

    final var ret = channel.read(dsts, 1, 3);
    System.out.println(ret); // 9

    System.out.println("-- dsts --");
    for (final var dst : dsts) {
        if (dst.hasArray()) {
            System.out.println(Arrays.toString(dst.array()));
        }
    }

    // Result
    // ↓
    //-- dsts --
    //[0]
    //[10, 20]
    //[30, 40, 50]
    //[60, 70, 80, 90]
    //[0, 0, 0, 0, 0]
}

abstract int read (ByteBuffer dst, long position)

Reads a sequence of bytes from this channel into the given buffer, starting at the given file position.

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

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

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

    final var array = new byte[5];
    final var dst = ByteBuffer.wrap(array);

    System.out.println(dst); // java.nio.HeapByteBuffer[pos=0 lim=5 cap=5]
    System.out.println(Arrays.toString(array)); // [0, 0, 0, 0, 0]

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

    final var ret = channel.read(dst, 2);
    System.out.println(ret); // 3

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

    System.out.println(dst); // java.nio.HeapByteBuffer[pos=3 lim=5 cap=5]
    System.out.println(Arrays.toString(array)); // [30, 40, 50, 0, 0]
}

abstract long size ()

Returns the current size of this channel's file.

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

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

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

    final var size = channel.size();
    System.out.println(size); // 5

    final var dst = ByteBuffer.allocate(Math.toIntExact(size));
    System.out.println(channel.read(dst)); // 5

    if (dst.hasArray()) {
        // [10, 20, 30, 40, 50]
        System.out.println(Arrays.toString(dst.array()));
    }
}

abstract long transferFrom (ReadableByteChannel src, long position, long count)

Transfers bytes into this channel's file from the given readable byte channel.

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

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

Files.write(srcPath, new byte[]{10, 20, 30});

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

    System.out.println(channel.size()); // 0

    try (final var src = FileChannel.open(srcPath, StandardOpenOption.READ)) {

        final var ret = channel.transferFrom(src, 0, src.size());
        System.out.println(ret); // 3
    }
}

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

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

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

Files.write(srcPath, new byte[]{10, 20, 30});
Files.write(dstPath, new byte[]{-1, -2, -3, -4, -5, -6, -7});

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

    System.out.println(channel.size()); // 7

    try (final var src = FileChannel.open(srcPath, StandardOpenOption.READ)) {

        final var ret = channel.transferFrom(src, 2, 3);
        System.out.println(ret); // 3
    }
}

final var bytes = Files.readAllBytes(dstPath);
System.out.println(Arrays.toString(bytes)); // [-1, -2, 10, 20, 30, -6, -7]

abstract long transferTo (long position, long count, WritableByteChannel target)

Transfers bytes from this channel's file to the given writable byte channel.

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

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

Files.write(srcPath, new byte[]{10, 20, 30});

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

    System.out.println(channel.size()); // 3

    try (final var target = FileChannel.open(dstPath,
            StandardOpenOption.CREATE, StandardOpenOption.WRITE)) {

        final var ret = channel.transferTo(0, channel.size(), target);
        System.out.println(ret); // 3
    }
}

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

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

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

Files.write(srcFile, new byte[]{10, 20, 30, 40, 50, 60, 70});

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

    System.out.println(channel.size()); // 7

    try (final var target = FileChannel.open(dstFile,
            StandardOpenOption.CREATE, StandardOpenOption.WRITE)) {

        final var ret = channel.transferTo(2, 3, target);
        System.out.println(ret); // 3
    }
}

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

abstract FileChannel truncate (long size)

Truncates this channel's file to the given size.

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

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

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

    final var ret = channel.truncate(3);
    System.out.println(channel.equals(ret)); // true

    final var size = channel.size();
    System.out.println(size); // 3
}

// [10, 20, 30]
System.out.println(Arrays.toString(Files.readAllBytes(path)));

final FileLock tryLock ()

Attempts to acquire an exclusive lock on this channel's file.

Please see also : FileLock

public class Child {
    public static void main(String[] args) throws IOException, InterruptedException {
        if (args.length != 1) {
            throw new IllegalArgumentException();
        }

        final var name = "child " + args[0];

        final var file = Path.of("lock.txt");
        try (final var channel = FileChannel.open(
                file, StandardOpenOption.CREATE, StandardOpenOption.WRITE)) {

            System.out.printf("  %s : lock start ...%n", name);

            while (true) {
                try (final var lock = channel.tryLock()) {

                    TimeUnit.MILLISECONDS.sleep(100);

                    if (lock != null) {
                        System.out.printf("  %s :   *** lock OK! ***%n", name);
                        System.out.printf("  %s :   sleep 3 seconds ...%n", name);
                        TimeUnit.SECONDS.sleep(3);
                        break;
                    } else {
                        System.out.printf("  %s :   ### lock NG! ###%n", name);
                        System.out.printf("  %s :   sleep 1 second ...%n", name);
                        TimeUnit.SECONDS.sleep(1);
                    }
                }
            }

            System.out.printf("  %s : lock end%n", name);
        }
    }
}

public class Main {
    public static void main(String[] args) throws IOException, InterruptedException {
        System.out.println("main : start");

        final var p1 = new ProcessBuilder("java", "Child", "1").inheritIO().start();
        TimeUnit.MILLISECONDS.sleep(10);
        final var p2 = new ProcessBuilder("java", "Child", "2").inheritIO().start();

        p1.waitFor();
        p2.waitFor();

        System.out.println("main : end");
    }
}

// Result
// ↓
//> java Main
//main : start
//  child 1 : lock start ...
//  child 2 : lock start ...
//  child 1 :   *** lock OK! ***
//  child 1 :   sleep 3 seconds ...
//  child 2 :   ### lock NG! ###
//  child 2 :   sleep 1 second ...
//  child 2 :   ### lock NG! ###
//  child 2 :   sleep 1 second ...
//  child 2 :   ### lock NG! ###
//  child 2 :   sleep 1 second ...
//  child 1 : lock end
//  child 2 :   *** lock OK! ***
//  child 2 :   sleep 3 seconds ...
//  child 2 : lock end
//main : end

abstract FileLock tryLock (long position, long size, boolean shared)

Attempts to acquire a lock on the given region of this channel's file.

Please see :

tryLock() for the basic behavior of this method.
lock(long position, long size, boolean shared) for position, size and shared parameters.

abstract int write (ByteBuffer src)

Writes a sequence of bytes to this channel from the given buffer.

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

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

    final byte[] array = {10, 20, 30, 40, 50};
    final var src = ByteBuffer.wrap(array);

    System.out.println(src); // java.nio.HeapByteBuffer[pos=0 lim=5 cap=5]
    System.out.println(channel.position()); // 0

    final var ret = channel.write(src);
    System.out.println(ret); // 5

    System.out.println(src); // java.nio.HeapByteBuffer[pos=5 lim=5 cap=5]
    System.out.println(channel.position()); // 5
}

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

final long write (ByteBuffer[] srcs)

Writes a sequence of bytes to this channel from the given buffers.

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

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

    final ByteBuffer[] srcs = {
            ByteBuffer.wrap(new byte[]{10, 20}),
            ByteBuffer.wrap(new byte[]{30, 40, 50}),
            ByteBuffer.wrap(new byte[]{60, 70, 80, 90}),
    };

    final var ret = channel.write(srcs);
    System.out.println(ret); // 9
}

final var bytes = Files.readAllBytes(path);

// [10, 20, 30, 40, 50, 60, 70, 80, 90]
System.out.println(Arrays.toString(bytes));

abstract long write (ByteBuffer[] srcs, int offset, int length)

Writes a sequence of bytes to this channel from a subsequence of the given buffers.

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

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

    final ByteBuffer[] srcs = {
            ByteBuffer.wrap(new byte[]{10}),
            ByteBuffer.wrap(new byte[]{20, 30}),
            ByteBuffer.wrap(new byte[]{40, 50, 60}),
            ByteBuffer.wrap(new byte[]{70, 80, 90, 100}),
    };

    final var ret = channel.write(srcs, 1, 2);
    System.out.println(ret); // 5
}

final var bytes = Files.readAllBytes(path);

// [20, 30, 40, 50, 60]
System.out.println(Arrays.toString(bytes));

abstract int write (ByteBuffer src, long position)

Writes a sequence of bytes to this channel from the given buffer, starting at the given file position.

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

Files.write(path, new byte[]{-1, -2, -3, -4, -5, -6, -7});

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

    final byte[] array = {10, 20, 30};
    final var src = ByteBuffer.wrap(array);

    System.out.println(src); // java.nio.HeapByteBuffer[pos=0 lim=3 cap=3]
    System.out.println(channel.position()); // 0

    final var ret = channel.write(src, 2);
    System.out.println(ret); // 3

    System.out.println(src); // java.nio.HeapByteBuffer[pos=3 lim=3 cap=3]
    System.out.println(channel.position()); // 0
}

final var bytes = Files.readAllBytes(path);
System.out.println(Arrays.toString(bytes)); // [-1, -2, 10, 20, 30, -6, -7]

Methods declared in AbstractInterruptibleChannel

begin, close, end, implCloseChannel, isOpen

Please see the link below.

AbstractInterruptibleChannel with Examples

Methods declared in Channel

isOpen