Java深入学习16：NIO详解2-Selector

Java深入学习16：NIO详解2-Selector

一、概念介绍

1- Selector（选择器）介绍

　　选择器（Selector） 是 SelectableChannle 对象的多路复用器，Selector 可以同时监控多个 SelectableChannel 的 IO 状况，也就是说，利用 Selector 可使一个单独的线程管理多个 Channel。Selector 是非阻塞 IO 的核心。

　　使用Selector的好处在于： 使用更少的线程来就可以来处理通道了， 相比使用多个线程，避免了线程上下文切换带来的开销。

 2- 阻塞与非阻塞

　　传统的 IO 流都是阻塞式的。也就是说，当一个线程调用 read() 或 write() 时，该线程被阻塞，直到有一些数据被读取或写入，该线程在此期间不能执行其他任务。因此，在完成网络通信进行 IO 操作时，由于线程会阻塞，所以服务器端必须为每个客户端都提供一个独立的线程进行处理，当服务器端需要处理大量客户端时，性能急剧下降。

　　Java NIO 是非阻塞模式的。当线程从某通道进行读写数据时，若没有数据可用时，该线程可以进行其他任务。线程通常将非阻塞 IO 的空闲时间用于在其他通道上执行 IO 操作，所以单独的线程可以管理多个输入和输出通道。因此，NIO 可以让服务器端使用一个或有限几个线程来同

时处理连接到服务器端的所有客户端。

二、代码示例(对方法做了简单的解释)

 1- Socket方案1：客户端传数据，客户端接收

package nio.blockingnio;

import org.junit.Test;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.SocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;

/*
 * 一、使用 NIO 完成网络通信的三个核心：
 *
 * 1. 通道（Channel）：负责连接
 *
 *        java.nio.channels.Channel 接口：
 *             |--SelectableChannel
 *                 |--SocketChannel
 *                 |--ServerSocketChannel
 *                 |--DatagramChannel
 *
 *                 |--Pipe.SinkChannel
 *                 |--Pipe.SourceChannel
 *
 * 2. 缓冲区（Buffer）：负责数据的存取
 *
 * 3. 选择器（Selector）：是 SelectableChannel 的多路复用器。用于监控 SelectableChannel 的 IO 状况
 *
 */
public class BlockingNIOTest {
    @Test
    public void client() throws IOException {

        //1-获取Socket通道和文件通道
        // SocketChannel.open(): Opens a socket channel and connects it to a remote address.
        //InetSocketAddress: This class implements an IP Socket Address (IP address + port number)
        SocketChannel socketChannel = SocketChannel.open(new InetSocketAddress("127.0.0.1", 8001));
        FileChannel inChannel = FileChannel.open(Paths.get("1.jpg"), StandardOpenOption.READ);
        //2-分配指定大小的缓冲区
        ByteBuffer buf = ByteBuffer.allocate(10240);
        //3-读取本地文件，并发送到服务器
        while(inChannel.read(buf) != -1){
            buf.flip();
            socketChannel.write(buf);
            buf.clear();
        }
        //4-关闭通道
        inChannel.close();
        socketChannel.close();

    }
    @Test
    public void server() throws IOException {
        //1-获取ServerSocke通道并绑定链接
        // ServerSocketChannel.open(): Opens a server-socket channel.
        // ServerSocketChannel.bind(): Binds the channel's socket to a local address and configures the socket to listen for connections.
        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open().bind(new InetSocketAddress(8001));
        //2-获取本地文件写入通道
        FileChannel outChannel = FileChannel.open(Paths.get("5.jpg"),StandardOpenOption.READ,StandardOpenOption.WRITE,StandardOpenOption.CREATE);

        //3-获取客户端连接通道
        //ServerSocketChannel.accept(): Accepts a connection made to this channel's socket.
        SocketChannel socketChannel = serverSocketChannel.accept();

        //4-分配指定大小的缓冲区
        ByteBuffer buf = ByteBuffer.allocate(10240);
        //5-接收客户端的数据，并保存到本地
        while(socketChannel.read(buf) != -1){
            buf.flip();
            outChannel.write(buf);
            buf.clear();
        }
        //6-关闭通道
        socketChannel.close();
        outChannel.close();
        serverSocketChannel.close();


    }
}

 2- Socket方案2：客户端传数据，客户端接收数据后，并返回数据给客户端(使用socketChannel.shutdownOutput()关闭客户端数据传输)

import org.junit.Test;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.net.ServerSocket;
import java.nio.ByteBuffer;
import java.nio.channels.FileChannel;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.nio.file.Paths;
import java.nio.file.StandardOpenOption;

public class BlockingNIOTest2 {

    @Test
    public void client() throws IOException {

        //1- 获取SocketChannel通道，并写入数据，发送到服务器
        SocketChannel socketChannel = SocketChannel.open(new InetSocketAddress("127.0.0.1", 8001));
        FileChannel inChannel = FileChannel.open(Paths.get("1.jpg"), StandardOpenOption.READ);
        ByteBuffer buffer = ByteBuffer.allocate(10240);
        while(inChannel.read(buffer) != -1){
            buffer.flip();
            socketChannel.write(buffer);
            buffer.clear();
        }

        //2-关闭SocketChannel写数据的链接，但是不关闭通道本身。
        //SocketChannel.shutdownOutput(): Shutdown the connection for writing without closing the channel.
        socketChannel.shutdownOutput();

        //3-接收服务端的反馈
        int len = 0;
        while((len = socketChannel.read(buffer)) != -1){
            buffer.flip();
            System.out.println("client output: " + new String(buffer.array(), 0, len));
            buffer.clear();
        }
        //4-关闭通道
        inChannel.close();;
        socketChannel.close();
    }


    @Test
    public void server() throws IOException {

        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open().bind(new InetSocketAddress(8001));

        FileChannel outChannel = FileChannel.open(Paths.get("6.jpg"), StandardOpenOption.CREATE, StandardOpenOption.WRITE, StandardOpenOption.READ);

        ByteBuffer buffer = ByteBuffer.allocate(10240);

        SocketChannel socketChannel = serverSocketChannel.accept();

        while(socketChannel.read(buffer) != -1){
            buffer.flip();
            outChannel.write(buffer);
            buffer.clear();
        }


        //传输数据
        buffer.put("helleo tyj".getBytes());
        buffer.flip();
        socketChannel.write(buffer);
        buffer.clear();


        socketChannel.close();;
        outChannel.close();
        serverSocketChannel.close();

    }
}

3-Socket方案3：客户端传数据，客户端接收；使用选择器Selector

import org.junit.Test;

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.Buffer;
import java.nio.ByteBuffer;
import java.nio.channels.*;
import java.time.LocalDateTime;
import java.util.Iterator;
import java.util.Scanner;

public class NonBlockingNIOTest {

    @Test
    public  void client() throws IOException {

        SocketChannel socketChannel = SocketChannel.open(new InetSocketAddress("127.0.0.1", 8001));

        socketChannel.configureBlocking(false);

        ByteBuffer byteBuffer = ByteBuffer.allocate(1024);

        Scanner scanner = new Scanner(System.in);


        String str = "";
        while(scanner.hasNext()){
            str = scanner.next();
            byteBuffer.put((LocalDateTime.now().toString()+"  client: 
" + str).getBytes());
            byteBuffer.flip();
            socketChannel.write(byteBuffer);
            byteBuffer.clear();
        }

        scanner.close();
    }

    @Test
    public  void server() throws IOException {

        //1-获取通道
        ServerSocketChannel serverSocketChannel = ServerSocketChannel.open();

        //2-切换成非阻塞模式
        //ServerSocketChannel.configureBlocking(): Adjusts this channel's blocking mode.
        serverSocketChannel.configureBlocking(false);

        //3-绑定连接
        serverSocketChannel.bind(new InetSocketAddress(8001));

        //4-获取选择器
        //Selector.open(): Opens a selector.
        Selector selector = Selector.open();

        //5-将通道注册到选择器上，并指定"监听接收事件"
        //ServerSocketChannel.register(): Registers this channel with the given selector, returning a selection key.
        serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);

        //6-轮询的方式获取选择器上已经“准备就绪”的事件
        //selector.select(): Selects a set of keys whose corresponding channels are ready for I/O  operations.
        while(selector.select() > 0){

            //7-获取当前选择器上所有注册的“选择键(已经就绪的监听事件)”
            //selector.selectedKeys(): Returns this selector's selected-key set.
            Iterator<SelectionKey> keyIterator = selector.selectedKeys().iterator();
            while(keyIterator.hasNext()){

                //8-获取"准备就绪"的事件
                SelectionKey selectionKey = keyIterator.next();

                //9-判断具体是什么事件就绪
                // selectionKey.isAcceptable(): Tests whether this key's channel is ready to accept a new socket connection.
                if(selectionKey.isAcceptable()){

                    //10-如果是“接收就绪”，获取客户端连接
                    SocketChannel socketChannel = serverSocketChannel.accept();

                    //11-切换成非阻塞模式
                    socketChannel.configureBlocking(false);

                    //12-将该通道注册到选择器上
                    socketChannel.register(selector,SelectionKey.OP_READ);

                }else if(selectionKey.isReadable()){

                    //13-获取当前选择器上“读就绪”状态的通道
                    //selectionKey.channel(): Returns the channel for which this key was created.  This method will continue to return the channel even after the key is cancelled.
                    SocketChannel socketChannel = (SocketChannel)selectionKey.channel();

                    //14-获取通道，读取数据
                    ByteBuffer byteBuffer = ByteBuffer.allocate(1024);

                    int len = 0;
                    while((len = socketChannel.read(byteBuffer)) != -1){
                        byteBuffer.flip();
                        System.out.println(new String(byteBuffer.array(),0,len));
                        byteBuffer.clear();
                    }
                }
                //15-取消选择建SelectionKey
                keyIterator.remove();
            }
        }
    }
}

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