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。