NiO uses writable events to handle the situation of one-time write incompleteness

Server side

 public static void main(String[] args) throws IOException {
    
 		//  Create a server socket
        ServerSocketChannel ssc = ServerSocketChannel.open();
        //  Set asynchronous io
        ssc.configureBlocking(false);
		//  Create selector 
        Selector selector = Selector.open();
        // socket Add to selector , Focus on  accept event 
        ssc.register(selector, SelectionKey.OP_ACCEPT);
		// socket Binding port 
        ssc.bind(new InetSocketAddress(8888));

        while (true){
    
            selector.select();
            Iterator<SelectionKey> iterator = selector.selectedKeys().iterator();
            while (iterator.hasNext()){
    
                SelectionKey key = iterator.next();
                iterator.remove();
                if(key.isAcceptable()){
    
                    SocketChannel sc = ssc.accept();
                    sc.configureBlocking(false);
                    SelectionKey scKey = sc.register(selector, 0, null);
                    scKey.interestOps(SelectionKey.OP_READ);

                    StringBuilder sb = new StringBuilder();
                    for(int i=0;i<30000000;i++){
    
                        sb.append("a");
                    }

                    ByteBuffer buffer = Charset.defaultCharset().encode(sb.toString());

                    int write = sc.write(buffer);
                    System.out.println(write);
                    if(buffer.hasRemaining()){
    
                        scKey.interestOps(scKey.interestOps()+SelectionKey.OP_WRITE);

                        scKey.attach(buffer);
                    }

                }else if(key.isWritable()){
    
                    ByteBuffer buffer = (ByteBuffer) key.attachment();
                    SocketChannel sc = (SocketChannel)key.channel();
                    int write = sc.write(buffer);
                    System.out.println(write);
                    if(!buffer.hasRemaining()){
    
                        key.attach(null);
                        key.interestOps(key.interestOps()-SelectionKey.OP_WRITE);//  No longer pay attention to writable events 
                    }
                }
            }
        }
    }

client

public static void main(String[] args) throws IOException {
    
        SocketChannel sc = SocketChannel.open();

        sc.connect(new InetSocketAddress("localhost",8888));

        //  receive data 
        int count = 0;
        while (true){
    
            ByteBuffer buffer = ByteBuffer.allocate(1024 * 1024);
            count += sc.read(buffer);
            System.out.println(count);
            buffer.clear();
        }
    }