从未选择用于多个 OP 代码的 Java SocketChannel register()

Question

我用 Java 构建了一个基于选择器的系统，可以接受多个客户端。它有一个在 OP_ACCEPT 下注册的 ServerSocketChannel，它接受传入的连接并再次向选择器注册生成的 SocketChannel。这是一点:

ServerSocketChannel insock = ServerSocketChannel.open();
    insock.configureBlocking(false);
    insock.socket().bind(new InetSocketAddress(6789));

    Selector sel = Selector.open();
    SelectionKey joinchannel = insock.register(sel, SelectionKey.OP_ACCEPT);

    System.out.println("Ready to accept incoming connections.");

    while (true) {
        int ready = sel.selectNow();
        if (ready == 0)
            continue;
        Set<SelectionKey> selectedKeys = sel.selectedKeys();
        Iterator<SelectionKey> keyIterator = selectedKeys.iterator();
        while (keyIterator.hasNext()) {
            SelectionKey key = keyIterator.next();
            if(key.isAcceptable()){
                SocketChannel newConnection = insock.accept();
                System.out.println("New client "+newConnection+" connected.");
                newConnection.configureBlocking(false);
                newConnection.register(sel, SelectionKey.OP_READ).attach(new DGPlayer());
            }

如果我为 OP_READ 注册新的 SocketChannel，这工作正常。 isReadable() 检查成功，读取数据。这是一点:

else if(key.isReadable()){
                ByteBuffer buf = ByteBuffer.allocate(1024);
                int trans = ((SocketChannel)key.channel()).read(buf); buf.flip();
                byte[] ba = new byte[buf.remaining()]; buf.get(ba);
                String msg = new String(ba, 0, ba.length, Charset.forName("UTF-8"));

                if(trans > 0){
                    DGPlayer client = (DGPlayer) key.attachment();
                    System.out.println(client.name+": "+msg.trim());
                }
            }
                //              else if(key.isWritable()){
//                  ByteBuffer buf = ByteBuffer.allocate(48);
//                  buf.clear();
//                  buf.put(((String)key.attachment()).getBytes());
//                  buf.flip();
//                  
//                  SocketChannel target = ((SocketChannel)key.channel());
//                  while(buf.hasRemaining()) {
//                      target.write(buf);
//                  }
//                  
//                  key.attach(null);
//              }

但是，如果我为 OP_READ|OP_WRITE 注册 SocketChannel，则什么也没有发生。据我所知，选择器从未发现 channel 是 readable() 或 writable()。唯一的变化是注册 channel 以获得 WRITE 权限。

知道为什么会这样吗？

编辑 - 为了完成，这里有一些客户端代码:

while (true) {
        int readyChannels = sel.select();
        if (readyChannels == 0)
            continue;

        Set<SelectionKey> selectedKeys = sel.selectedKeys();
        Iterator<SelectionKey> keyIterator = selectedKeys.iterator();

        while (keyIterator.hasNext()) {
            SelectionKey key = keyIterator.next();

            if (key.isReadable()) {
                ByteBuffer buf = ByteBuffer.allocate(1024);
                int trans = ((SocketChannel)key.channel()).read(buf); buf.flip();
                byte[] ba = new byte[buf.remaining()]; buf.get(ba);
                String msg = new String(ba, 0, ba.length, Charset.forName("UTF-8"));

                if(msg.trim().length() != 0)
                    System.out.println("Response from server: "+msg.trim());
            }
            else if(key.isWritable() && messages.size() > 0){
                String message = messages.remove();
                ByteBuffer buf = ByteBuffer.allocate(48);
                buf.clear();
                buf.put(message.getBytes(Charset.forName("UTF-8")));

                buf.flip();

                int written = outsock.write(buf);
                while(buf.hasRemaining()){
                    outsock.write(buf);
                }

                System.out.println("Wrote '"+message+"' to server");
            }
            keyIterator.remove();
        }
    }

(客户端为 OP_READ|OP_WRITE 注册服务器)

Answer 1

一般来说，如果您没有任何要主动写入的内容，那么为 OP_WRITE 注册一个 channel 是个坏主意。大多数 channel 大部分时间都可以写入，因此每次调用 Selector.select 都会无阻塞地返回并消耗资源。最好在SelectionKey.interestOps中加上writing flag当您准备好向 channel 写入内容时，并在完成后删除标志。