Java NIO Basics: Channels, Buffers, and Selectors

Introduction

In the previous post we discussed several I/O models; now we begin the Java NIO programming series. NIO, introduced in Java 4, provides a new API designed to overcome the limitations of traditional I/O.

Article Outline

The article is organized into four parts:

1. Basic concepts in NIO

2. Channel

3. Buffer

4. Selector

1. Basic Concepts in NIO

NIO revolves around three core concepts: Channel , Buffer , and Selector .

Channel

A Channel is analogous to a road that provides a two‑way pathway for data, unlike the one‑way streams of traditional I/O. The following code shows how traditional I/O reads a file using an InputStream:

public class Test {
    public static void main(String[] args) throws IOException {
        File file = new File("data.txt");
        InputStream inputStream = new FileInputStream(file);
        byte[] bytes = new byte[1024];
        inputStream.read(bytes);
        inputStream.close();
    }
}

In NIO, a Channel replaces the stream and can perform both read and write operations.

Buffer

A Buffer is a container (a continuous array) that holds data during I/O operations. All reads and writes in NIO must go through a Buffer; for example, data read from a Channel is placed into a Buffer before the application processes it.

Selector

The Selector is the most critical component of NIO. It polls registered Channels and, when an event occurs on a Channel, it retrieves the event for processing. This enables a single thread to manage multiple connections efficiently.

Below is an illustration of a Selector handling events (image omitted for brevity).

2. Channel Details

Common Channel types include:

FileChannel

SocketChannel

ServerSocketChannel

DatagramChannel

FileChannel reads from or writes to files; SocketChannel uses TCP for network I/O; ServerSocketChannel listens for incoming TCP connections and creates a new SocketChannel for each; DatagramChannel uses UDP.

The following example writes the string "java nio" to a file using a FileChannel:

public class Test {
    public static void main(String[] args) throws IOException {
        File file = new File("data.txt");
        FileOutputStream outputStream = new FileOutputStream(file);
        FileChannel channel = outputStream.getChannel();
        ByteBuffer buffer = ByteBuffer.allocate(1024);
        String string = "java nio";
        buffer.put(string.getBytes());
        buffer.flip(); // must flip before writing
        channel.write(buffer);
        channel.close();
        outputStream.close();
    }
}

Note: The flip() call is required before invoking channel.write() to prepare the buffer for reading.

3. Buffer Details

A Buffer is an abstract class with several concrete subclasses, such as:

ByteBuffer

IntBuffer

CharBuffer

LongBuffer

DoubleBuffer

FloatBuffer

ShortBuffer

For file I/O, any of these may be used, but for network I/O the most common is ByteBuffer. Detailed usage, including the concepts of limit, position, and capacity, will be covered in a future article.

4. Selector Details

The Selector class is the core of NIO, allowing a single thread to monitor multiple Channels for events, thereby reducing system overhead and avoiding the need for a thread per connection. Each event is represented by a SelectionKey, which together with the Selector forms the essential server‑side processing logic.

Further examples of Selector usage will be presented in subsequent posts.

Reference

Original article: http://www.cnblogs.com/dolphin0520/p/3919162.html