Question

I have a functioning client-server apparatus which can successfully connect and send messages to each other using NIO.

Right now my only confusion is how I'm supposed to continue reading when socketChannel.read() returns zero.

I have a protocol that sends the first 4 bytes as the number of incoming bytes to expect. Even with that amount, I'm running into a potential issue.

For testing, However, there are times where I might read something like:

5 // Read 5 bytes when calling socketChannel.read()
0 // Read 0 bytes when calling socketChannel.read() immediately after

When I hit the zero, I assumed that I was done reading and need to wait for more data to come to me.

When I do this however, OP_READ doesn't seem to be triggered when I perform selectNow() again later on. I checked the key and it has it's readyops() and interestops() set to 1 (which is OP_READ), but it does not want to recognize that it's time to read again.

I found that if I continue looping to read, I might get something like:

5 // socketChannel.read()
0 // socketChannel.read()
7 // socketChannel.read() (Done since I have all my bytes)
0
0
0
...

I'm confused here because this means one of:

There is no data there, so the 0 available is legitimate, but then when the rest of the data comes in, the selector refuses to return the key with selectNow()
The data is all there, but for some reason returns 0 on reading.

Am I supposed to re-register the channel after a selectNow() returns it as an active key? (Though I didn't have to between switching from OP_CONNECT to OP_READ... so I'm guessing not). I feel like blindly circling in a loop is dangerous and will waste processing cycles.

Am I just supposed to keep polling them? That makes me confused at when OP_READ actually fires then.

Answer 1

这是由于我的错误，我没有在读取的bytebuffer上调用.clear()。这导致它返回0读取，即使数据已流入。

此示例也可能对想要查看简单客户端如何工作的人有用（尽管有非常糟糕的异常处理）。无法保证这将正常工作，并且可能会出现问题，因为它被设计为快速而肮脏的测试。

import java.io.IOException;
import java.net.InetSocketAddress;
import java.nio.ByteBuffer;
import java.nio.channels.SelectionKey;
import java.nio.channels.Selector;
import java.nio.channels.ServerSocketChannel;
import java.nio.channels.SocketChannel;

public class Test {

    public static final int PORT = 22222;

    public static void main(String[] args) throws IOException {
        Thread s = new Thread(new Server());
        Thread c = new Thread(new Client());
        s.start();
        c.start();
    }
}

class Client implements Runnable {

    public Selector selector;

    public SocketChannel sc;

    public Client() throws IOException {
        selector = Selector.open();
    }

    @Override
    public void run() {
        try {
            sc = SocketChannel.open();
            sc.socket().setTcpNoDelay(true);
            sc.configureBlocking(false);
            SelectionKey k = sc.register(selector, SelectionKey.OP_CONNECT);
            boolean firstConnect = sc.connect(new InetSocketAddress("localhost", Test.PORT));
            if (firstConnect) {
                System.out.println("Connected on first connect, de-registering OP_CONNECT");
                k.interestOps(SelectionKey.OP_READ);
            }

            while (true) {
                int keys = selector.selectNow();
                if (keys > 0) {
                    for (SelectionKey key : selector.selectedKeys()) {
                        if (key.isConnectable()) {
                            boolean finishConnectResult = sc.finishConnect();
                            key.interestOps(SelectionKey.OP_READ);
                            System.out.println("Finished connection: " + finishConnectResult);
                        }

                        if (key.isReadable()) {
                            ByteBuffer bb = ByteBuffer.allocate(2);
                            int bytesRead = 0;
                            while ((bytesRead = sc.read(bb)) > 0) {
                                bb.flip();
                                System.out.println(bytesRead + " bytes read");
                                System.out.println(bb.get() + ", " + bb.get());
                                //bb.clear(); // If this is not commented, it will not be handled properly.
                            }
                            System.out.println("Last bytes read value = " + bytesRead);
                            System.exit(0);
                        }
                    }
                }

                Thread.sleep(5);
            }
        } catch (Exception e) { 
            e.printStackTrace();
            throw new RuntimeException();
        }
    }
}

class Server implements Runnable {

    public Selector selector;

    public SocketChannel sc;

    public Server() throws IOException {
        selector = Selector.open();
        ServerSocketChannel ssc = ServerSocketChannel.open();
        ssc.configureBlocking(false);
        ssc.bind(new InetSocketAddress(Test.PORT));
        ssc.register(selector, SelectionKey.OP_ACCEPT);
    }

    @Override
    public void run() {
        boolean notSentData = true;
        try {
            while (true) {
                int keys = selector.selectNow();
                if (keys > 0) {
                    for (SelectionKey key : selector.selectedKeys()) {
                        if (key.isAcceptable()) {
                            ServerSocketChannel ssc = (ServerSocketChannel) key.channel();
                            sc = ssc.accept();
                            if (sc != null) {
                                sc.configureBlocking(false);
                                sc.socket().setTcpNoDelay(true); // Required in my application
                                sc.register(selector, SelectionKey.OP_WRITE);
                                System.out.println("Server accepted connection");
                            } else {
                                System.out.println("Got null connection");
                            }
                        }
                    }
                }

                if (sc != null && notSentData) {
                    ByteBuffer bb = ByteBuffer.allocate(4);
                    bb.put(new byte[]{ 1, 2, 3, -1});
                    bb.flip();
                    int wrote = sc.write(bb);
                    System.out.println("Wrote " + wrote + " bytes");
                    notSentData = false;
                }

                Thread.sleep(5);
            }
        } catch (Exception e) {
            e.printStackTrace();
            throw new RuntimeException();
        }
    }
}

NIO SocketChannel saying there is no data when there is (or selector is not informing me)

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