java - SelectionKey 不可写

Question

我很好奇为什么代码总是告诉我“ key 不可写”？我的代码有什么问题吗？每次套接字读到一些内容时，我都会将键设置为对 OP_WRITE 感兴趣，并测试它是否可写。但是总是说不可写。我完全是Java套接字编程的新手。

顺便说一句，我没有关闭客户端。

public final class DateServer {

    private DateServer() {
    throw new IllegalStateException("Instantiation not allowed");
    }

    public static void main(final String[] args) throws Exception {
        try (final Selector selector = Selector.open(); ServerSocketChannel serverSocket = ServerSocketChannel.open();) {
            InetSocketAddress hostAddress = new InetSocketAddress("127.0.0.1", 9999);
            serverSocket.bind(hostAddress);

            serverSocket.configureBlocking(false);
            serverSocket.register(selector, serverSocket.validOps(), null);

            while (true) {
                int numSelectedKeys = selector.select();
                if (numSelectedKeys > 0) {
                    handleSelectionKeys(selector.selectedKeys(), serverSocket);
                }
            }
        }
    }

    private static void handleSelectionKeys(Set<SelectionKey> selectionKeys, ServerSocketChannel serverSocket) throws IOException {

    Iterator<SelectionKey> selectionKeyIterator = selectionKeys.iterator();
    while (selectionKeyIterator.hasNext()) {
        SelectionKey key = selectionKeyIterator.next();

        if (key.isAcceptable()) {
            acceptClientSocket(key, serverSocket);
        } else if (key.isReadable()) {
            readRequest(key);
        }

        selectionKeyIterator.remove();
    }
    }

    private static void acceptClientSocket(SelectionKey key, ServerSocketChannel serverSocket) throws IOException {

        SocketChannel client = serverSocket.accept();
        client.configureBlocking(false);
        client.register(key.selector(), SelectionKey.OP_READ);

        System.out.println("Accepted connection from client");
    }

    private static void readRequest(SelectionKey key) throws IOException {

        SocketChannel client = (SocketChannel) key.channel();
        ByteBuffer buffer = ByteBuffer.allocate(1024);

        int bytesRead = client.read(buffer);

        if (bytesRead == -1) {
            client.close();
        } else {
            System.out.println(String.format("Request data: %s", new String(buffer.array())));
            int interestOps = 0;
            interestOps |= SelectionKey.OP_WRITE;
            key.interestOps(interestOps);

            if (key.isWritable()){
                System.out.println("key is writable");
            } else {
                System.out.println("key is not writable");
            }

            interestOps = 0;
            interestOps |= SelectionKey.OP_READ;
            key.interestOps(interestOps);
        }

    }
}

Answer 1

interestOps 仅告诉选择器下次选择什么。设置 OP_WRITE 并不会神奇地使选择键具有预测 future 的能力。您必须再次调用 select() 才能使 OP_WRITE 真正根据此代码进行设置。

但是您不需要选择器的“权限”即可写入 channel 。您只需写入，只有当写入计数为零时，您才需要担心 OP_WRITE，正如此处有关该主题的众多答案。