Channel

Learn the NIO channel with Channel.

The Channel concept

A Channel carries data through a buffer. Unlike streams, a channel can be bidirectional and switch to non-blocking mode. This is more efficient for high-volume IO.

Example: a WritableByteChannel takes a ByteBuffer and writes it to a target with write. With Channels.newChannel(out) an ordinary OutputStream is wrapped as a channel. So stream-based code can work with the NIO channel API. Channels are closed with close.

Channel versus stream

Streams work with single bytes or byte arrays; a channel moves data in blocks through a ByteBuffer. Streams are usually blocking; some channel types can switch to non-blocking mode.

Example: FileChannel opens a channel to a file directly; read and write take a buffer. This is faster than a stream for large files. Methods like transferTo copy data directly from channel to channel. So channels are preferred in high-performance IO.

Exercise Write bytes to a channel

Create a ByteArrayOutputStream out. Wrap the channel with WritableByteChannel ch = Channels.newChannel(out). Prepare a two-byte buffer with ByteBuffer.wrap(new byte[]{65, 66}). Write with ch.write(b), close with ch.close(). Print the result of new String(out.toByteArray()) with System.out.println.

Show example solution

import java.io.ByteArrayOutputStream;
import java.nio.ByteBuffer;
import java.nio.channels.Channels;
import java.nio.channels.WritableByteChannel;

public class ChannelDemo {
    public static void main(String[] args) throws Exception {
        ByteArrayOutputStream out = new ByteArrayOutputStream();
        WritableByteChannel ch = Channels.newChannel(out);
        ByteBuffer b = ByteBuffer.wrap(new byte[]{65, 66});
        ch.write(b);
        ch.close();
        System.out.println(new String(out.toByteArray()));
    }
}

Test yourself

Match the Channel concepts

Channel

Data channel
ByteBuffer

Carried data
write

Writes a buffer
Channels.newChannel

Stream to channel
What kind of data does a Channel work with?
- Strings
- Characters
- ByteBuffer
- Objects
Show answer

ByteBuffer — A Channel carries data through a ByteBuffer.

Complete the Channels.newChannel call

WritableByteChannel ch = Channels.____(out);

Show answer

WritableByteChannel ch = Channels.newChannel(out);

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