Selector

Learn multi-channel management with Selector.

Multiplexing with Selector

A Selector lets a single thread watch many channels. Channels register with the selector through a SelectionKey; the select call waits for channels that are ready. This is known as multiplexing.

Example: a server can watch hundreds of client channels with a single Selector. Each channel is set to non-blocking mode and registered. When select returns, the keys of ready channels are processed. This is far more efficient than spawning a thread per client.

Non-blocking channels

To use a Selector the channel must be in non-blocking mode. channel.configureBlocking(false) does this. Then the channel is registered to a selector with register, along with which events to watch.

Example: channel.register(sel, SelectionKey.OP_READ) waits for the channel to be ready to read. When select returns, the channel is taken from the ready key and data is processed. A blocking channel cannot be registered, so setting the mode is mandatory.

Exercise Open a Selector and print its state

Open a selector with Selector.open() and assign it to sel. Print the result of sel.isOpen() with System.out.println. Then close it with sel.close().

Show example solution

import java.nio.channels.Selector;

public class SelectorDemo {
    public static void main(String[] args) throws Exception {
        Selector sel = Selector.open();
        System.out.println(sel.isOpen());
        sel.close();
    }
}

Test yourself

Complete the Selector.open call

Selector sel = Selector.____();

Show answer

Selector sel = Selector.open();

Match the Selector concepts

Selector.open

Opens a selector
select

Waits for an event
isOpen

Asks for state
close

Closes the selector
What does a Selector provide?
- File deletion
- A single instance
- String encoding
- Many channels with one thread
Show answer

Many channels with one thread — A Selector watches many channels with one thread.

Next lesson: Charset

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