java NIO Selector 概念和例子

java NIO Selector 可以監控多個Channel 實例，確定哪個Channel需要準備讀或者寫。單個線程管理多個Channel。

為什麼使用Selector，為什麼Selector要用單線程？

因為切換線程和創建都需要消耗系統的資源，下面塗是Selector管理Channel的圖：

下面我們看一個例子：

public class NIOClient {
 SocketChannel channel;
 public void initClient(String host,int port) throws IOException{
 InetSocketAddress servAdds = new InetSocketAddress(host,port); //地址
 this.channel = SocketChannel.open(servAdds); //SocketChannel.open() 通過InetSocketAddress打開一個通道
 }
 public void sendAndRecv(String words) throws IOException{
 byte[] msg = new String(words).getBytes();
 ByteBuffer buffer = ByteBuffer.wrap(msg);
 System.out.println("client 發送數據:"+words);
 channel.write(buffer); //通過buffer寫數據到channel
 buffer.clear();
 channel.read(buffer); //讀數據
 System.out.println("client 接收到數據："+new String(buffer.array()).trim());
 channel.close(); //channel用完關閉
 }
 public static void main(String[] args) throws IOException{
 NIOClient client = new NIOClient();
 client.initClient("localhost",9999);
 client.sendAndRecv("showm me the money");
 }
}

package xin.clips.nio.selector;
import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;
import java.util.Iterator;
public class NIOServer {
 private Selector selector;
 private ServerSocketChannel serverSocketChannel;
 public void initServer(int port) throws IOException {
 serverSocketChannel = ServerSocketChannel.open(); //通過靜態open()方法打開ServerSocketChannel
 serverSocketChannel.configureBlocking(false); //設置為非阻塞模式
 serverSocketChannel.socket().bind(new InetSocketAddress("localhost", port)); //服務器監聽 

 this.selector = Selector.open();
 serverSocketChannel.register(selector, SelectionKey.OP_ACCEPT);
 }
 public void listen() throws IOException {
 System.out.println("服務器啟動成功! ");
 while (true) { //select 會出現空循環，浪費CPU資源
 selector.select(); // 進行阻塞blocking
 Iterator<selectionkey> its = selector.selectedKeys().iterator();
 while (its.hasNext()) {
 SelectionKey key = its.next();
 if (key.isAcceptable()) {
 SocketChannel channel = serverSocketChannel.accept(); //打開通道
 channel.configureBlocking(false);//設置非阻塞模式
 channel.register(selector, SelectionKey.OP_READ);
 } else if (key.isReadable()) {
 //準備讀取
 recvAndReply(key);
 }
 its.remove(); //移除
 }
 }
 }
 public void recvAndReply(SelectionKey key) throws IOException {
 SocketChannel channel = (SocketChannel) key.channel();
 ByteBuffer buffer = ByteBuffer.allocate(256);
 int i = channel.read(buffer);
 if (i != -1) {
 //能獲取到數據
 String msg = new String(buffer.array()).trim();
 System.out.println("nio 服務器收到信息:" + msg);
 System.out.println("nio 服務器返回:" + msg);
 channel.write(ByteBuffer.wrap(msg.getBytes())); //返回數據
 } else {
 //沒有數據
 channel.close();
 }
 }
 public static void main(String[] args) throws IOException {
 NIOServer server = new NIOServer();
 server.initServer(9999);
 server.listen();
 }
}
/<selectionkey> 

NIO server 程序輸出結果：

nio 服務器收到信息:showm me the money
nio 服務器返回:showm me the money
nio 服務器收到信息:showm me the money
nio 服務器返回:showm me the money

NIO Client 程序輸出結果：

client 發送數據:showm me the money
client 接收到數據：showm me the money
Disconnected from the target VM, address: '127.0.0.1:64825', transport: 'socket'
Process finished with exit code 0

程序分析：

1、Selector創建需要通過Selector.open() 方法，通過Selector工廠類開啟

2、在Channel中註冊Selector，通過channel.register(select,SelectionKey.OP_READ);方法

3、Channel需要使用Selector必須使用非阻塞的Channel，FileChannel是阻塞模式的，所以它不能配合使用Selector

4、register()方法表示"興趣集"，表示Selector需要監聽的Channel的10:25:35，比如Connect，Accept，Read，Write，這4種事件對應SelectionKey中的常量：

SelectionKey.OP_CONNECT

SelectionKey.OP_ACCEPT

SelectionKey.OP_READ

SelectionKey.OP_WRITE

5、select.select()方法調用後，服務器端程序進入阻塞狀態，等到監聽事件觸發後，select()方法結果，通過迭代select.selectedKeys()進行匹配處理定義的情況。

代碼地址：https://github.com/yuanzongyu/tutorials/tree/master/core-java-io/src/main/java/xin/clips/nio