Netty 之 NioSocketChannel 源碼分析

NioSocketChannel

public class NioSocketChannel extends AbstractNioByteChannel implements io.netty.channel.socket.SocketChannel {

    private static final SelectorProvider DEFAULT_SELECTOR_PROVIDER = SelectorProvider.provider();

    private static SocketChannel newSocket(SelectorProvider provider) {
        try {
            return provider.openSocketChannel();
        } catch (IOException e) {
            throw new ChannelException("Failed to open a socket.", e);
        }
    }

    public NioSocketChannel() {
        this(DEFAULT_SELECTOR_PROVIDER);
    }

    public NioSocketChannel(SelectorProvider provider) {
        this(newSocket(provider));
    }

    public NioSocketChannel(SocketChannel socket) {
        this(null, socket);
    }

    public NioSocketChannel(Channel parent, SocketChannel socket) {
        super(parent, socket);
        config = new NioSocketChannelConfig(this, socket.socket());
    }

從上面代碼中可以看出 NioSocketChannel 封裝了 Nio 中的 SocketChannel。
SocketChannel 是通過 SelectorProvider.provider().openSocketChannel() 創建的。

    @Override
    public ServerSocketChannel parent() {
        return (ServerSocketChannel) super.parent();
    }

NioSocketChannel.parent() 是 NioServerSocketChannel。
如果是 NioServerSocketChannel.parent() 爲 null。

    @Override
    protected SocketChannel javaChannel() {
        return (SocketChannel) super.javaChannel();
    }

javaChannel() 獲取的是 Nio 的 SocketChannel。

    @Override
    public boolean isActive() {
        SocketChannel ch = javaChannel();
        return ch.isOpen() && ch.isConnected();
    }

查看 SocketChannel 是否是open，並且是已經連接狀態。

    @Override
    public boolean isOutputShutdown() {
        return javaChannel().socket().isOutputShutdown() || !isActive();
    }

查看 SocketChannel 寫是否關閉。
調用 SocketChannel.shutdownOutput() 可用關閉寫操作。

    @Override
    public boolean isInputShutdown() {
        return javaChannel().socket().isInputShutdown() || !isActive();
    }

查看 SocketChannel 讀是否關閉。
調用 SocketChannel.shutdownInput() 可用關閉讀操作。

    @Override
    public boolean isShutdown() {
        Socket socket = javaChannel().socket();
        return socket.isInputShutdown() && socket.isOutputShutdown() || !isActive();
    }
    @Override
    public boolean isActive() {
        SocketChannel ch = javaChannel();
        return ch.isOpen() && ch.isConnected();
    }

判斷SocketChannel 是否是Shutdown 狀態：如果 SocketChannel 不支持讀寫 或者不是打開和連接狀態

    @Override
    public InetSocketAddress localAddress() {
        return (InetSocketAddress) super.localAddress();
    }

    @Override
    public InetSocketAddress remoteAddress() {
        return (InetSocketAddress) super.remoteAddress();
    }

獲取 Socket 的本地地址和遠端地址

    @Override
    protected void doBind(SocketAddress localAddress) throws Exception {
        doBind0(localAddress);
    }

    private void doBind0(SocketAddress localAddress) throws Exception {
        if (PlatformDependent.javaVersion() >= 7) {
            SocketUtils.bind(javaChannel(), localAddress);
        } else {
            SocketUtils.bind(javaChannel().socket(), localAddress);
        }
    }

調用 SocketChannel 進行綁定 IP 地址。

    @Override
    protected boolean doConnect(SocketAddress remoteAddress, SocketAddress localAddress) throws Exception {
        if (localAddress != null) {
            doBind0(localAddress);
        }

        boolean success = false;
        try {
            boolean connected = SocketUtils.connect(javaChannel(), remoteAddress);
            if (!connected) {
                selectionKey().interestOps(SelectionKey.OP_CONNECT);
            }
            success = true;
            return connected;
        } finally {
            if (!success) {
                doClose();
            }
        }
    }

客戶端向服務端發起連接操作。
1、首先綁定 本地IP
2、然後發起連接，返回連接的結果true、false。
3、如果未連接成功，則向 Selector 上註冊 OP_CONNECT 事件。

連接結果有 3 種可能
1、連接成功，返回true
2、連接未成功，返回false，表示已經發送連接請求還未收到連接成功的狀態。
3、連接網絡異常，在 finally 中關閉該連接。

    @Override
    protected void doDisconnect() throws Exception {
        doClose();
    }

    @Override
    protected void doClose() throws Exception {
        super.doClose();
        javaChannel().close();
    }

關閉 SocketChannel 連接。

@Override
    protected int doReadBytes(ByteBuf byteBuf) throws Exception {
        final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
        allocHandle.attemptedBytesRead(byteBuf.writableBytes());
        return byteBuf.writeBytes(javaChannel(), allocHandle.attemptedBytesRead());
    }

從 SocketChannel 中讀取數據到 byteBuf 中

    @Override
    protected int doWriteBytes(ByteBuf buf) throws Exception {
        final int expectedWrittenBytes = buf.readableBytes();
        return buf.readBytes(javaChannel(), expectedWrittenBytes);
    }

buf 數據寫入到 SocketChannel 中

    @Override
    protected long doWriteFileRegion(FileRegion region) throws Exception {
        final long position = region.transferred();
        return region.transferTo(javaChannel(), position);
    }

NioSocketChannel 除了支持寫 ByteBuf ，還支持寫 FileRegion類型。
FileRegion 是使用 FileChannel.transferTo() 直接寫 Socket的，使用的零拷貝。

@Override
    protected void doWrite(ChannelOutboundBuffer in) throws Exception {
        SocketChannel ch = javaChannel();
        int writeSpinCount = config().getWriteSpinCount();
        do {
            // 1、寫緩存待寫入的字節數爲0，則取消寫事件
            if (in.isEmpty()) {
                // All written so clear OP_WRITE
                clearOpWrite();
                // Directly return here so incompleteWrite(...) is not called.
                return;
            }

            // Ensure the pending writes are made of ByteBufs only.
            int maxBytesPerGatheringWrite = ((NioSocketChannelConfig) config).getMaxBytesPerGatheringWrite();
            ByteBuffer[] nioBuffers = in.nioBuffers(1024, maxBytesPerGatheringWrite);
            // 2、獲取 ChannelOutboundBuffer 中 ByteBuf 的數量
            int nioBufferCnt = in.nioBufferCount();

            // Always us nioBuffers() to workaround data-corruption.
            // See https://github.com/netty/netty/issues/2761
            switch (nioBufferCnt) {
                case 0:
                    // 3、如果nioBuffers個數爲0，可能是其他內容的數據，比如FileRegion，則調用父類AbstractNioByteChannel的doWriter方法
                    writeSpinCount -= doWrite0(in);
                    break;
                case 1: {
                    // 4、如果只有一個ByteBuffer，則不需要使用nio的 gathering write。
                    // Zero length buffers are not added to nioBuffers by ChannelOutboundBuffer, so there is no need
                    // to check if the total size of all the buffers is non-zero.
                    ByteBuffer buffer = nioBuffers[0];
                    int attemptedBytes = buffer.remaining();
                    final int localWrittenBytes = ch.write(buffer);
                    if (localWrittenBytes <= 0) {
                        incompleteWrite(true);
                        return;
                    }
                    adjustMaxBytesPerGatheringWrite(attemptedBytes, localWrittenBytes, maxBytesPerGatheringWrite);
                    in.removeBytes(localWrittenBytes);
                    --writeSpinCount;
                    break;
                }
                default: {
                    // 5、如果有多個 ByteBuf 則使用 Nio 的 gather 聚合寫入
                    // to check if the total size of all the buffers is non-zero.
                    // We limit the max amount to int above so cast is safe
                    long attemptedBytes = in.nioBufferSize();
                    final long localWrittenBytes = ch.write(nioBuffers, 0, nioBufferCnt);
                    if (localWrittenBytes <= 0) {
                        incompleteWrite(true);
                        return;
                    }
                    // Casting to int is safe because we limit the total amount of data in the nioBuffers to int above.
                    adjustMaxBytesPerGatheringWrite((int) attemptedBytes, (int) localWrittenBytes,
                            maxBytesPerGatheringWrite);
                    in.removeBytes(localWrittenBytes);
                    --writeSpinCount;
                    break;
                }
            }
        } while (writeSpinCount > 0);

        incompleteWrite(writeSpinCount < 0);
    }

1、判斷 ChannelOutboundBuffer 中的鏈表數據是否爲空，如果爲空，則取消註冊在 Selector 上的寫事件。
2、判斷 ChannelOutboundBuffer 中待寫的 ByteBuf 數量。
3、ByteBuf 個數爲0，可能待寫入的類型不是 ByteBuf，而是 FileRegion 類型的。則調用 AbstractNioByteChannel.doWrite0() 嘗試寫入 FileRegion 類型的數據。FileRegion 類型的數據以後文章在分析。
4、如果 ByteBuf 個數爲1，則不使用 Nio 的 gather 聚合 Buffer 寫入，而是直接寫入就可以。
5、如果 ByteBuf 個數大於1，則使用 Nio 的gather 聚合 Buffer 寫入，一次就可以多去多個 Buffer 的數據。

NIO 知識點 Gather 和 Scatter

聚集（gather）寫入Channel是指在寫操作時將多個buffer的數據寫入同一個Channel，因此，Channel 將多個Buffer中的數據“聚集（gather）”後發送到Channel。

分散（scatter）從Channel中讀取是指在讀操作時將讀取的數據寫入多個buffer中。因此，Channel將從Channel中讀取的數據“分散（scatter）”到多個Buffer中。

NioSocketChannelConfig

private final class NioSocketChannelConfig extends DefaultSocketChannelConfig {
        private volatile int maxBytesPerGatheringWrite = Integer.MAX_VALUE;
        private NioSocketChannelConfig(NioSocketChannel channel, Socket javaSocket) {
            super(channel, javaSocket);
            calculateMaxBytesPerGatheringWrite();
        }

        @Override
        protected void autoReadCleared() {
            clearReadPending();
        }

        @Override
        public NioSocketChannelConfig setSendBufferSize(int sendBufferSize) {
            super.setSendBufferSize(sendBufferSize);
            calculateMaxBytesPerGatheringWrite();
            return this;
        }

        @Override
        public <T> boolean setOption(ChannelOption<T> option, T value) {
            if (PlatformDependent.javaVersion() >= 7 && option instanceof NioChannelOption) {
                return NioChannelOption.setOption(jdkChannel(), (NioChannelOption<T>) option, value);
            }
            return super.setOption(option, value);
        }

        @Override
        public <T> T getOption(ChannelOption<T> option) {
            if (PlatformDependent.javaVersion() >= 7 && option instanceof NioChannelOption) {
                return NioChannelOption.getOption(jdkChannel(), (NioChannelOption<T>) option);
            }
            return super.getOption(option);
        }

        @SuppressWarnings("unchecked")
        @Override
        public Map<ChannelOption<?>, Object> getOptions() {
            if (PlatformDependent.javaVersion() >= 7) {
                return getOptions(super.getOptions(), NioChannelOption.getOptions(jdkChannel()));
            }
            return super.getOptions();
        }

        void setMaxBytesPerGatheringWrite(int maxBytesPerGatheringWrite) {
            this.maxBytesPerGatheringWrite = maxBytesPerGatheringWrite;
        }

        int getMaxBytesPerGatheringWrite() {
            return maxBytesPerGatheringWrite;
        }

        private void calculateMaxBytesPerGatheringWrite() {
            // Multiply by 2 to give some extra space in case the OS can process write data faster than we can provide.
            int newSendBufferSize = getSendBufferSize() << 1;
            if (newSendBufferSize > 0) {
                setMaxBytesPerGatheringWrite(getSendBufferSize() << 1);
            }
        }

        private SocketChannel jdkChannel() {
            return ((NioSocketChannel) channel).javaChannel();
        }
    }

NioSocketChannelConfig 是 NioSocketChannel 的配置類。
1、設置 Socket 參數
2、設置gather 最大寫入數據

設置 Socket 參數

首先調用 DefaultSocketChannelConfig.setOption() 進行設置

    @Override
    public <T> boolean setOption(ChannelOption<T> option, T value) {
        validate(option, value);

        if (option == SO_RCVBUF) {
            setReceiveBufferSize((Integer) value);
        } else if (option == SO_SNDBUF) {
            setSendBufferSize((Integer) value);
        } else if (option == TCP_NODELAY) {
            setTcpNoDelay((Boolean) value);
        } else if (option == SO_KEEPALIVE) {
            setKeepAlive((Boolean) value);
        } else if (option == SO_REUSEADDR) {
            setReuseAddress((Boolean) value);
        } else if (option == SO_LINGER) {
            setSoLinger((Integer) value);
        } else if (option == IP_TOS) {
            setTrafficClass((Integer) value);
        } else if (option == ALLOW_HALF_CLOSURE) {
            setAllowHalfClosure((Boolean) value);
        } else {
            return super.setOption(option, value);
        }

        return true;
    }

這些參數都是 socket 參數，具體詳見 socket 參數說明。

DefaultChannelConfig.setOption()

    @Override
    @SuppressWarnings("deprecation")
    public <T> boolean setOption(ChannelOption<T> option, T value) {
        validate(option, value);

        if (option == CONNECT_TIMEOUT_MILLIS) {
            setConnectTimeoutMillis((Integer) value);
        } else if (option == MAX_MESSAGES_PER_READ) {
            setMaxMessagesPerRead((Integer) value);
        } else if (option == WRITE_SPIN_COUNT) {
            setWriteSpinCount((Integer) value);
        } else if (option == ALLOCATOR) {
            setAllocator((ByteBufAllocator) value);
        } else if (option == RCVBUF_ALLOCATOR) {
            setRecvByteBufAllocator((RecvByteBufAllocator) value);
        } else if (option == AUTO_READ) {
            setAutoRead((Boolean) value);
        } else if (option == AUTO_CLOSE) {
            setAutoClose((Boolean) value);
        } else if (option == WRITE_BUFFER_HIGH_WATER_MARK) {
            setWriteBufferHighWaterMark((Integer) value);
        } else if (option == WRITE_BUFFER_LOW_WATER_MARK) {
            setWriteBufferLowWaterMark((Integer) value);
        } else if (option == WRITE_BUFFER_WATER_MARK) {
            setWriteBufferWaterMark((WriteBufferWaterMark) value);
        } else if (option == MESSAGE_SIZE_ESTIMATOR) {
            setMessageSizeEstimator((MessageSizeEstimator) value);
        } else if (option == SINGLE_EVENTEXECUTOR_PER_GROUP) {
            setPinEventExecutorPerGroup((Boolean) value);
        } else {
            return false;
        }

        return true;
    }