《Java 网络编程基础》附录 A Socket 与 NIO API 速查

InetAddress loopback = InetAddress.getByName("127.0.0.1");
System.out.println(loopback.getHostAddress());     // 127.0.0.1
System.out.println(loopback.isLoopbackAddress());  // true

InetAddress bound = new InetSocketAddress(9090).getAddress();
System.out.println(bound.isAnyLocalAddress());     // true，端口构造器用的是通配地址

try (Socket socket = new Socket()) {
    socket.connect(new InetSocketAddress("127.0.0.1", 9090), 1000);
    socket.setSoTimeout(2000);
    socket.setTcpNoDelay(true);

    OutputStream out = socket.getOutputStream();
    out.write("GET /healthz HTTP/1.0\r\n\r\n".getBytes(StandardCharsets.US_ASCII));
    out.flush();

    InputStream in = socket.getInputStream();
    byte[] buf = new byte[1024];
    int n = in.read(buf);          // 超时抛 SocketTimeoutException
    System.out.println("read=" + n);
}

try (ServerSocket server = new ServerSocket()) {
    server.setReuseAddress(true);
    server.bind(new InetSocketAddress("127.0.0.1", 9090), 128);
    server.setSoTimeout(5000);
    while (true) {
        try (Socket c = server.accept()) {
            c.getInputStream().read();
        } catch (SocketTimeoutException e) {
            // 五秒没有新连接，回到循环继续等
        }
    }
}

try (SocketChannel ch = SocketChannel.open()) {
    ch.configureBlocking(false);
    boolean done = ch.connect(new InetSocketAddress("127.0.0.1", 9090));
    while (!done) {
        done = ch.finishConnect();   // 真实事件循环里等 OP_CONNECT，这里只是示意
    }
    ch.setOption(StandardSocketOptions.TCP_NODELAY, true);

    ByteBuffer out = ByteBuffer.wrap("ping".getBytes(StandardCharsets.UTF_8));
    while (out.hasRemaining()) {
        ch.write(out);
    }
}

try (ServerSocketChannel ssc = ServerSocketChannel.open()) {
    ssc.configureBlocking(false);
    ssc.bind(new InetSocketAddress("127.0.0.1", 9090), 128);
    try (Selector selector = Selector.open()) {
        ssc.register(selector, SelectionKey.OP_ACCEPT);
        // 之后在 select 循环里 accept
    }
}

try (Selector selector = Selector.open()) {
    ssc.register(selector, SelectionKey.OP_ACCEPT);   // ssc 来自上一节的监听 channel
    while (true) {
        selector.select(1000);
        Iterator<SelectionKey> it = selector.selectedKeys().iterator();
        while (it.hasNext()) {
            SelectionKey key = it.next();
            it.remove();                       // 先移除，再处理
            if (!key.isValid()) {
                continue;
            }
            if (key.isAcceptable()) {
                SocketChannel ch = ((ServerSocketChannel) key.channel()).accept();
                ch.configureBlocking(false);
                ch.register(selector, SelectionKey.OP_READ, ByteBuffer.allocate(1024));
            } else if (key.isReadable()) {
                // 读处理，见上一节
            }
        }
    }
}

java -cp . Prov

sun.nio.ch.KQueueSelectorProvider

static String state(ByteBuffer b) {
    return "pos=" + b.position() + " lim=" + b.limit()
         + " cap=" + b.capacity() + " rem=" + b.remaining();
}

ByteBuffer b = ByteBuffer.allocate(8);
System.out.println(state(b));   // allocate
b.putInt(0x01020304);
System.out.println(state(b));   // putInt
b.flip();
System.out.println(state(b));   // flip
System.out.println("0x" + Integer.toHexString(b.getInt()));
b.clear();
System.out.println(state(b));   // clear

allocate  pos=0 lim=8 cap=8 rem=8
putInt    pos=4 lim=8 cap=8 rem=4
flip      pos=0 lim=4 cap=8 rem=4
0x1020304
clear     pos=0 lim=8 cap=8 rem=8

long pos = 0;
long size = in.size();
while (pos < size) {
    long moved = in.transferTo(pos, size - pos, out);
    if (moved == 0) {
        break;                 // 目标通道暂时写不动，交给上层重试
    }
    pos += moved;
}