当客户端连接关闭时,服务器select()不会阻塞,然后一直分发读就绪操作,且读到的字节长度都是0,这是什么情况。

服务器代码:

try {
    ServerSocketChannel serverChannel = ServerSocketChannel.open();
    serverChannel.bind(new InetSocketAddress(666));
    serverChannel.configureBlocking(false);
    Selector selector = Selector.open();
    serverChannel.register(selector, SelectionKey.OP_ACCEPT);
    while (true) {
        int count = selector.select(); //阻塞
        if (count > 0) {
            Set<SelectionKey> keys = selector.selectedKeys();
            Iterator<SelectionKey> iterator = keys.iterator();
            while (iterator.hasNext()) {
                SelectionKey key = iterator.next();
                iterator.remove();
                if (key.isAcceptable()) {
                    System.out.println("client connect");
                    ServerSocketChannel serverSocketChannel = (ServerSocketChannel) key.channel();
                    SocketChannel sc = serverSocketChannel.accept();
                    sc.configureBlocking(false);
                    sc.register(selector, SelectionKey.OP_READ);
                }
                if (key.isReadable()) {
                    SocketChannel socketChannel = (SocketChannel) key.channel();
                    ByteBuffer buffer = ByteBuffer.allocate(512);
                    socketChannel.read(buffer);
                    buffer.flip();
                    System.out.println("on read size:" + buffer.remaining());
                }
            }
        }
    }
} catch (IOException e) {
    e.printStackTrace();
}

客户端代码:

public class NIOClientTest{
    public static void main(String[] args) throws UnknownHostException, IOException{
        try {
            Socket socket = new Socket("127.0.0.1",666);
            try(OutputStreamWriter output = new OutputStreamWriter(socket.getOutputStream());){
                output.write(1);
                output.flush();
            }catch (Exception e) {
                e.printStackTrace();
            }
            socket.close();
        } catch (Exception e1) {
            e1.printStackTrace();
        }
    }
}

解决:



当客户端主动切断连接的时候,服务端 Socket 的读事件(FD_READ)仍然起作用,也就是说,服务端 Socket 的状态仍然是有东西可读,当然此时读出来的字节肯定是 0。

socketChannel.read(buffer) 是有返回值的,这种情况下返回值是 -1,所以如果 read 方法返回的是 -1,就可以关闭和这个客户端的连接了。

SocketChannel.read 的返回值:
1fe9c9e4-d605-4faf-a61e-4cba3e7d9e68-image.png

这种情况也有可能会抛出 IOException,需要捕获异常并判断。



nio的客户端如果关闭了,服务端还是会收到该channel的读事件,但是数目为0,而且会读到-1,其实-1在网络io中就是socket关闭的含义,在文件时末尾的含义,所以为了避免客户端关闭服务端一直收到读事件,必须检测上一次的读是不是-1,如果是-1,就关闭这个channel。

ByteBuffer buffer = ByteBuffer.allocate(100);
SocketChannel sc = (SocketChannel) key.channel();
StringBuffer buf = new StringBuffer();
int c = 0;
while ((c = sc.read(buffer)) > 0) {
    buf.append(new String(buffer.array()));
}
if (c == -1) {
    System.out.println("断开");
    sc.close();
}
String msg = buf.toString();
System.out.println(msg);