Java NIO Selector

2025年3月21日 | 阅读 4 分钟

在 Java NIO 中，Selector 是可选择通道的复用器，它是一种可以置于非阻塞模式的特殊类型的通道。它可以检查一个或多个 NIO 通道，并确定哪些通道已准备好进行通信，即读取或写入。

Selector 的作用是什么

Selector 用于使用单个线程处理多个通道。因此，它需要更少的线程来处理通道。线程之间的切换对操作系统来说代价很高。因此，为了提高系统效率，使用了它。

让我们看看一个使用 Selector 处理 3 个通道的线程的示意图

让我们看看 Gather 原理的简单示意图

创建 Selector

我们可以通过调用 Selector.open() 方法来创建一个 Selector，如下所示

打开服务器套接字通道

让我们看看打开服务器套接字通道的示例

ServerSocketChannel serverSocket = ServerSocketChannel.open();
InetSocketAddress hostAddress = new InetSocketAddress("localhost", 8080);
serverSocket.bind(hostAddress);

使用 Selector 选择通道

在注册了一个或多个通道到 Selector 后，我们可以调用 select() 方法之一。此方法返回一个已准备好执行我们想要执行的事件（即连接、读取、写入或接受）的通道。

用于选择通道的各种 select() 方法是

int select(): select() 方法返回的整数值会告知有多少个通道已准备好进行通信。
int select(long TS): 此方法与 select() 相同，只是它会将输出阻塞最长 TS（以毫秒为单位）的时间段。
int selectNow(): 它不会阻塞输出，并立即返回任何已准备好的通道。

selectedKeys()

一旦我们调用了任何一个 select() 方法，它就会返回一个值，表明一个或多个通道已准备就绪，然后我们可以通过调用 Selector 的 selectedKeys() 方法来访问已准备好的通道，如下所示。

我们可以迭代 selected key set 来访问已准备好的通道，如下所示

Set<SelectionKey> selectedKeys = selector.selectedKeys();
Iterator<SelectionKey> keyIterator = selectedKeys.iterator();
while(keyIterator.hasNext()) {	
    SelectionKey key = keyIterator.next();
    if(key.isConnectable()) {
        // The connection was established with a remote server.
    } else if (key.isAcceptable()) {
        // The connection was accepted by a ServerSocketChannel.
    } else if (key.isWritable()) {
        //  The channel is ready for writing
    } else if (key.isReadable()) {
        // The channel is ready for reading
    }
    keyIterator.remove();
}

上面的循环迭代 selected key set 中的一个 key，以确定使用已选定的通道执行的操作。

完整的 select 循环框图如下所示

基本 Selector 示例

主程序

package com.javatpoint;

import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.nio.channels.Selector;
import java.nio.channels.SelectionKey;
import java.nio.ByteBuffer;
import java.io.IOException;
import java.util.Set;
import java.util.Iterator;
import java.net.InetSocketAddress;
public class SelectorExample {
   public static void main (String [] args)
            throws IOException {
        // Get the selector
        Selector selector = Selector.open();
        System.out.println("Selector is open for making connection: " + selector.isOpen());
        // Get the server socket channel and register using selector
        ServerSocketChannel SS = ServerSocketChannel.open();
        InetSocketAddress hostAddress = new InetSocketAddress("localhost", 8080);
        SS.bind(hostAddress);
        SS.configureBlocking(false);
        int ops = SS.validOps();
        SelectionKey selectKy = SS.register(selector, ops, null);
        for (;;) {
            System.out.println("Waiting for the select operation...");
            int noOfKeys = selector.select();
            System.out.println("The Number of selected keys are: " + noOfKeys);
            Set selectedKeys = selector.selectedKeys();
            Iterator itr = selectedKeys.iterator();
            while (itr.hasNext()) {
                SelectionKey ky = (SelectionKey) itr.next();
                if (ky.isAcceptable()) {
                    // The new client connection is accepted
                    SocketChannel client = SS.accept();
                    client.configureBlocking(false);
                    // The new connection is added to a selector
                    client.register(selector, SelectionKey.OP_READ);
                    System.out.println("The new connection is accepted from the client: " + client);
                }
                else if (ky.isReadable()) {
                    // Data is read from the client
                    SocketChannel client = (SocketChannel) ky.channel();
                    ByteBuffer buffer = ByteBuffer.allocate(256);
                    client.read(buffer);
                    String output = new String(buffer.array()).trim();
                    System.out.println("Message read from client: " + output);
                    if (output.equals("Bye Bye")) {
                        client.close();
                        System.out.println("The Client messages are complete; close the session.");
                    }
                }
                itr.remove();
            } // end of while loop
        } // end of for loop
    }
}

客户端程序

package com.javatpoint;

import java.nio.channels.SocketChannel;
import java.nio.ByteBuffer;
import java.io.IOException;
import java.net.InetSocketAddress;
public class Client {
    public static void main (String [] args)
            throws IOException, InterruptedException {
        InetSocketAddress hA = new InetSocketAddress("localhost", 8080);
        SocketChannel client = SocketChannel.open(hA);
        System.out.println("The Client is sending messages to server...");
        // Sending messages to the server	
        String [] msg = new String [] {"Time goes fast.", "What next?", "Bye Bye"};
        for (int j = 0; j < msg.length; j++) {
            byte [] message = new String(msg [j]).getBytes();
            ByteBuffer buffer = ByteBuffer.wrap(message);
            client.write(buffer);
            System.out.println(msg [j]);
            buffer.clear();
            Thread.sleep(3000);
        }
        client.close();				
    }
}

输出

主程序输出为

Selector is open for making connection: true
Waiting for the select operation...
The Number of selected keys are: 1
The new connection is accepted from the client: java.nio.channels.SocketChannel[connected 

local=/127.0.0.1:8080 remote=/127.0.0.1:51367]
Waiting for the select operation...
The Number of selected keys are: 1
Message read from client: Time goes fast.
Waiting for the select operation...
The Number of selected keys are: 1
Message read from client: What next?
Waiting for the select operation...
The Number of selected keys are: 1
Message read from client: Bye Bye
The Client messages are complete; close the session.
Waiting for the select operation...

客户端程序输出为

The Client is sending messages to server...
Time goes fast.
What next?
Bye Bye

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Java NIO Selector

Selector 的作用是什么

创建 Selector

打开服务器套接字通道

使用 Selector 选择通道

基本 Selector 示例

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创建 Selector

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基本 Selector 示例

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