Exception in thread "main" java.nio.channels.NotYetConnectedException

 import java.nio.channels.AsynchronousServerSocketChannel;
 import java.nio.channels.AsynchronousSocketChannel;
 import java.net.InetSocketAddress;
 import java.util.concurrent.Future;
 import java.nio.ByteBuffer;

 public class SimpleAIOServer{
     static final int PORT = 30000;
     public static void main(String[] args) throws Exception{
         try(
             //创建AsynchronousServerSocketChannel对象
             AsynchronousServerSocketChannel serverChannel = AsynchronousServerSocketChannel.open()
         ){
             //指定在指定地址、端口监听
             serverChannel.bind(new InetSocketAddress(PORT));
             while(true){
                 //采用循环接受来自客户端的连接
                 Future<AsynchronousSocketChannel> future = serverChannel.accept();
                 //获取连接完成后返回的AsynchronousSocketChannel
                 AsynchronousSocketChannel socketChannel = future.get();
                 //执行输出
                 socketChannel.write(ByteBuffer.wrap("欢迎你来到AIO的世界！".getBytes("UTF-8"))).get();
             }
         }
     }
 }

 import java.nio.channels.AsynchronousSocketChannel;
 import java.nio.charset.Charset;
 import java.net.InetSocketAddress;
 import java.util.concurrent.Future;
 import java.nio.ByteBuffer;

 public class SimpleAIOClient{
     static final int PORT = 30000;
     public static void main(String[] args) throws Exception{
         //用于读取数据的ByteBuffer
         ByteBuffer buff = ByteBuffer.allocate(1024);
         Charset utf = Charset.forName("utf-8");
         try(
             //创建AsynchronousSocketChannel对象
             AsynchronousSocketChannel clientChannel = AsynchronousSocketChannel.open()
         ){
             //连接远程服务器
             clientChannel.connect(new InetSocketAddress("127.0.0.1", PORT));
             buff.clear();
             //从clientChannel中读取数据
             clientChannel.read(buff).get();
             buff.flip();
             //将buff中的内容转换为字符串
             String content = utf.decode(buff).toString();
             System.out.println("服务器信息：" + content);
         }
     }
 }

运行上述代码会出现Exception in thread "main" java.nio.channels.NotYetConnectedException并提示在at SimpleAIOClient.main(SimpleAIOClient.java:21)报错。

NotYetConnectedException是尚未连接的错误，代码出错原因：

客户端和服务端没有建立连接就执行了Socket通信，代码上的错误位置是：

SimpleAIOClient.java中第18行：

//连接远程服务器
clientChannel.connect(new InetSocketAddress("127.0.0.1", PORT));

因为这一行没有检查异步IO操作是否完成，只有异步IO操作完成客户端和服务端的连接才能建立。

而异步IO操作是否完成的标志是clientChannel有一个Future返回值，得到它才能确保异步IO操作执行完成：

//连接远程服务器
clientChannel.connect(new InetSocketAddress("127.0.0.1", PORT)).get();//得到Future返回值，否则连接不会建立。