java - 在 JAVA nio 选择器中，我应该什么时候注册 'write operation' ？

Question

我正在学习 Java nio 选择器。在我的理解中，我认为使用选择器的步骤是首先注册我感兴趣的操作，然后我可以检查就绪集，最后我可以执行与我感兴趣的操作相对应的操作。我不知道为什么在这段代码中，写入过程可以发生在 if (key.isReadable()){...} 而不是 if (key.isWritable){ ...} 为什么没有注册写操作？

Iterator keys = sel.selectedKeys().iterator();
while (keys.hasNext()) {
    SelectionKey key = (SelectionKey)keys.next();

    if (!key.isValid()) 
        continue;

    if (key.isAcceptable()) {
        // increase the counter
        connection++;

        // remove accept request
        keys.remove();

        // ACCEPT: get the server channel
        ServerSocketChannel ssc = 
                (ServerSocketChannel) key.channel();

        // init a socket for a client
        SocketChannel nsc = ssc.accept();
        nsc.configureBlocking(false);

        // register the socket for READ
        nsc.register(sel, SelectionKey.OP_READ);
    } 
}


while (count < COUNT_MAX + NUM_CHILD - 1) {
    sel.select();

    // Get all pending events and iterate
    Iterator keys = sel.selectedKeys().iterator();
    while ( keys.hasNext() ) {
        SelectionKey key = (SelectionKey)keys.next();
        keys.remove();

        if (!key.isValid()) 
            continue;

        if (key.isReadable()) {
            // READ: get the channel
            SocketChannel nsc = (SocketChannel) key.channel();

            // clear buffer for reading
            readBuffer.clear();
            int nBytes = nsc.read(readBuffer);
            if (nBytes == -1) {// Check if the client closed the socket
                key.channel().close();
                key.cancel();
                continue;
            }

            // Read a message 
            DataInputStream ist = new DataInputStream (
                    new ByteArrayInputStream(readBuffer.array()));
            String msg = ist.readUTF();
            System.out.print(msg + "\n");

            // Clear the write buffer 
            writeBuffer.clear();

            // Write the counter value on the buffer
            count++;
            if (count < COUNT_MAX)
                writeBuffer.putInt(count);
            else
                writeBuffer.putInt(-1);
            // flip the buffer and write on the channel
            writeBuffer.flip();
            // Reply to a client
            nsc.write(writeBuffer);
        } 
    } // while (key)

Answer 1

您不需要注册对 OP_WRITE 的兴趣，因为通常 channel 已准备好写入。然而一个WritableChannel ，如果处于非阻塞模式，可能无法成功写入给定 ByteBuffer 的所有内容。在其 java 文档中查看此处:

Some types of channels, depending upon their state, may write only some of the bytes or possibly none at all. A socket channel in non-blocking mode, for example, cannot write any more bytes than are free in the socket's output buffer.

在这种情况下，您需要在选择器上注册对OP_WRITE 的兴趣，以便在 channel 再次准备好写入时收到通知，这样您就可以完成写入您的ByteBuffer.

在这里查看相关的SO question .