Netty-NioServerSocketChannel与NioSocketChannel

NioServerSocketChannel

NioServerSocketChannel是netty服务端的channel。在ServerbootStrap的bind方法中,通过反射,实例化对象NioServerSocketChannel。

NioServerSocketChannel对象实例化的过程中。

  1. AbstractChannel中实例化channel的id,unsafe,channelpipeline。
  2. AbstractNioChannel保存ServerSocketChannel,并设置感兴趣的事件SelectionKey.OP_ACCEPT,设置ServerSocketChannel#configureBlocking为false。
  3. NioServerSocketChannel中对NioServerSocketChannelConfig对象进行构建,爆保存到类变量config中。NioServerSocketChannelConfig中包含了当前对象和通过ServerSocketChannel获取到的serverSocket对象。

AbstractNioMessageChannel这个父类中没有对属性赋值。它主要是对AbstractChannel中的抽象方法newUnsafe的实现。Unsafe的落地实现主要是处理NioEventLoop--》run--》processSelectedKeys--》processSelectedKey--》unsafe.read(),其中unsafe是当前channel父类abstractChannel中的unsafe。也就是说,当NioEventLoop中处理IO事件的时候,读取数据的时候,落地实现在AbstractNioMessageChannel的内部类NioMessageUnsafe

NioMessageUnsafe中的doReadMessages方法,将数据读取到readBuf中。doReadMessages也是一个抽象方法,落地实现是NioServerSocketChannel。主要是做的事情是通过ServerSocketChannel.accept方法获取到SocketChannel,将这个SocketChannel封装成NioSocketChannel,添加到readBuf中

NioMessageUnsafe中的fireChannelRead方法,将readBuf集合中的数据遍历调用channelPipeline的fireChannelRead方法。readBuf集合中存储的是NioSocketChannel。之前的学习我们知道,NioServerSocketChannel中的channelPipeline的链表结构为:headContext-->ServerBootstrapAcceptor-->tailContext

ServerBootstrapAcceptor中的channelRead方法,将客户端的请求socketChannel绑定到childGroup中的一个NioEventLoop上。

下面是AbstractNioMessageChannel 的原代码。

public abstract class AbstractNioMessageChannel extends AbstractNioChannel {
    boolean inputShutdown;

    /**
     * @see AbstractNioChannel#AbstractNioChannel(Channel, SelectableChannel, int)
     */
    protected AbstractNioMessageChannel(Channel parent, SelectableChannel ch, int readInterestOp) {
        super(parent, ch, readInterestOp);
    }

    @Override
    protected AbstractNioUnsafe newUnsafe() {
       // 看这里,看这里,看这里。是对AbstractChannel的newUnsafe抽象方法的实现。
        return new NioMessageUnsafe();
    }

    @Override
    protected void doBeginRead() throws Exception {
        if (inputShutdown) {
            return;
        }
        super.doBeginRead();
    }

    protected boolean continueReading(RecvByteBufAllocator.Handle allocHandle) {
        return allocHandle.continueReading();
    }

 // 看这里,看这里,看这里。内部类实现Unsafe
    private final class NioMessageUnsafe extends AbstractNioUnsafe {

        private final List<Object> readBuf = new ArrayList<Object>();

        @Override
        public void read() {
            assert eventLoop().inEventLoop();
            final ChannelConfig config = config();
            final ChannelPipeline pipeline = pipeline();
            final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();
            allocHandle.reset(config);

            boolean closed = false;
            Throwable exception = null;
            try {
                try {
                    do {
                        // 看这里,看这里,看这里。doReadMessages也是一个抽象方法,落地实现是NioServerSocketChannel。主要是做的事情是通过ServerSocketChannel.accept方法获取到SocketChannel,将这个SocketChannel封装成NioSocketChannel,添加到readBuf中。
                        int localRead = doReadMessages(readBuf);
                        if (localRead == 0) {
                            break;
                        }
                        if (localRead < 0) {
                            closed = true;
                            break;
                        }

                        allocHandle.incMessagesRead(localRead);
                    } while (continueReading(allocHandle));
                } catch (Throwable t) {
                    exception = t;
                }

                int size = readBuf.size();
                for (int i = 0; i < size; i ++) {
                    readPending = false;
                    // 看这里,看这里,看这里。调用NioServerSocketChannel的channelPipeline进行事件发布。readBuf里面存的是NioSocketChannel。
                    pipeline.fireChannelRead(readBuf.get(i));
                }
                readBuf.clear();
                allocHandle.readComplete();
                pipeline.fireChannelReadComplete();

                if (exception != null) {
                    closed = closeOnReadError(exception);

                    pipeline.fireExceptionCaught(exception);
                }

                if (closed) {
                    inputShutdown = true;
                    if (isOpen()) {
                        close(voidPromise());
                    }
                }
            } finally {
                // Check if there is a readPending which was not processed yet.
                // This could be for two reasons:
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
                //
                // See https://github.com/netty/netty/issues/2254
                if (!readPending && !config.isAutoRead()) {
                    removeReadOp();
                }
            }
        }
    }

    @Override
    protected void doWrite(ChannelOutboundBuffer in) throws Exception {
        final SelectionKey key = selectionKey();
        final int interestOps = key.interestOps();

        int maxMessagesPerWrite = maxMessagesPerWrite();
        while (maxMessagesPerWrite > 0) {
            Object msg = in.current();
            if (msg == null) {
                break;
            }
            try {
                boolean done = false;
                for (int i = config().getWriteSpinCount() - 1; i >= 0; i--) {
                    if (doWriteMessage(msg, in)) {
                        done = true;
                        break;
                    }
                }

                if (done) {
                    maxMessagesPerWrite--;
                    in.remove();
                } else {
                    break;
                }
            } catch (Exception e) {
                if (continueOnWriteError()) {
                    maxMessagesPerWrite--;
                    in.remove(e);
                } else {
                    throw e;
                }
            }
        }
        if (in.isEmpty()) {
            // Wrote all messages.
            if ((interestOps & SelectionKey.OP_WRITE) != 0) {
                key.interestOps(interestOps & ~SelectionKey.OP_WRITE);
            }
        } else {
            // Did not write all messages.
            if ((interestOps & SelectionKey.OP_WRITE) == 0) {
                key.interestOps(interestOps | SelectionKey.OP_WRITE);
            }
        }
    }

    /**
     * Returns {@code true} if we should continue the write loop on a write error.
     */
    protected boolean continueOnWriteError() {
        return false;
    }

    protected boolean closeOnReadError(Throwable cause) {
        if (!isActive()) {
            // If the channel is not active anymore for whatever reason we should not try to continue reading.
            return true;
        }
        if (cause instanceof PortUnreachableException) {
            return false;
        }
        if (cause instanceof IOException) {
            // ServerChannel should not be closed even on IOException because it can often continue
            // accepting incoming connections. (e.g. too many open files)
            return !(this instanceof ServerChannel);
        }
        return true;
    }

    /**
     * Read messages into the given array and return the amount which was read.
     */
    protected abstract int doReadMessages(List<Object> buf) throws Exception;

    /**
     * Write a message to the underlying {@link java.nio.channels.Channel}.
     *
     * @return {@code true} if and only if the message has been written
     */
    protected abstract boolean doWriteMessage(Object msg, ChannelOutboundBuffer in) throws Exception;
}

NioSocketChannel

NioSocketChannel是netty封装的客户端请求的channel。

与上面的NioServerSocketChannel相比。继承类中将AbstractNioMessageChannel 替换为了AbstractNioByteChannel。从名称上看,是对字节数据的处理。下面就单独研究一下AbstractNioByteChannel。

AbstractNioByteChannel。同样的,在NioSocketChannel实例化的时候,这个类没有属性赋值。它主要也是实现AbstractChannel中的newUnsafe抽象方法。

newUnsafe的实现也是AbstractNioByteChannel的内部类NioByteUnsafe 。NioByteUnsafe 读取数据是调用抽象方法doReadBytes。doReadBytes的落地实现在NioSocketChannel中。

以下是AbstractNioByteChannel 的原码。

public abstract class AbstractNioByteChannel extends AbstractNioChannel {
    private static final ChannelMetadata METADATA = new ChannelMetadata(false, 16);
    private static final String EXPECTED_TYPES =
            " (expected: " + StringUtil.simpleClassName(ByteBuf.class) + ", " +
            StringUtil.simpleClassName(FileRegion.class) + ')';

    private final Runnable flushTask = new Runnable() {
        @Override
        public void run() {
            // Calling flush0 directly to ensure we not try to flush messages that were added via write(...) in the
            // meantime.
            ((AbstractNioUnsafe) unsafe()).flush0();
        }
    };
    private boolean inputClosedSeenErrorOnRead;

    /**
     * Create a new instance
     *
     * @param parent            the parent {@link Channel} by which this instance was created. May be {@code null}
     * @param ch                the underlying {@link SelectableChannel} on which it operates
     */
    protected AbstractNioByteChannel(Channel parent, SelectableChannel ch) {
        super(parent, ch, SelectionKey.OP_READ);
    }

    /**
     * Shutdown the input side of the channel.
     */
    protected abstract ChannelFuture shutdownInput();

    protected boolean isInputShutdown0() {
        return false;
    }

    @Override
    protected AbstractNioUnsafe newUnsafe() {
       // 看这里,看这里,看这里。实现是内部类NioByteUnsafe
        return new NioByteUnsafe();
    }

    @Override
    public ChannelMetadata metadata() {
        return METADATA;
    }

    final boolean shouldBreakReadReady(ChannelConfig config) {
        return isInputShutdown0() && (inputClosedSeenErrorOnRead || !isAllowHalfClosure(config));
    }

    private static boolean isAllowHalfClosure(ChannelConfig config) {
        return config instanceof SocketChannelConfig &&
                ((SocketChannelConfig) config).isAllowHalfClosure();
    }

 // 看这里,看这里,看这里,实现newUnsafe的内部类
    protected class NioByteUnsafe extends AbstractNioUnsafe {

        private void closeOnRead(ChannelPipeline pipeline) {
            if (!isInputShutdown0()) {
                if (isAllowHalfClosure(config())) {
                    shutdownInput();
                    pipeline.fireUserEventTriggered(ChannelInputShutdownEvent.INSTANCE);
                } else {
                    close(voidPromise());
                }
            } else if (!inputClosedSeenErrorOnRead) {
                inputClosedSeenErrorOnRead = true;
                pipeline.fireUserEventTriggered(ChannelInputShutdownReadComplete.INSTANCE);
            }
        }

        private void handleReadException(ChannelPipeline pipeline, ByteBuf byteBuf, Throwable cause, boolean close,
                RecvByteBufAllocator.Handle allocHandle) {
            if (byteBuf != null) {
                if (byteBuf.isReadable()) {
                    readPending = false;
                    pipeline.fireChannelRead(byteBuf);
                } else {
                    byteBuf.release();
                }
            }
            allocHandle.readComplete();
            pipeline.fireChannelReadComplete();
            pipeline.fireExceptionCaught(cause);

            // If oom will close the read event, release connection.
            // See https://github.com/netty/netty/issues/10434
            if (close || cause instanceof OutOfMemoryError || cause instanceof IOException) {
                closeOnRead(pipeline);
            }
        }

       // 看这里,看这里,看这里,我们重点看看read()方法。
        @Override
        public final void read() {
           // 获取NioSocketChannelConfig对象
            final ChannelConfig config = config();
            if (shouldBreakReadReady(config)) {
                clearReadPending();
                return;
            }
            // 获取NioSocketChannel的channelPipeline
            final ChannelPipeline pipeline = pipeline();
            final ByteBufAllocator allocator = config.getAllocator();
            final RecvByteBufAllocator.Handle allocHandle = recvBufAllocHandle();
            allocHandle.reset(config);

            ByteBuf byteBuf = null;
            boolean close = false;
            try {
                do {
                    byteBuf = allocHandle.allocate(allocator);
                    // 看这里,看这里,看这里,将数据读取到byteBuf中。doReadBytes
                    allocHandle.lastBytesRead(doReadBytes(byteBuf));
                    if (allocHandle.lastBytesRead() <= 0) {
                        // nothing was read. release the buffer.
                        byteBuf.release();
                        byteBuf = null;
                        close = allocHandle.lastBytesRead() < 0;
                        if (close) {
                            // There is nothing left to read as we received an EOF.
                            readPending = false;
                        }
                        break;
                    }

                    allocHandle.incMessagesRead(1);
                    readPending = false;
                    // 看这里,看这里,看这里,将读取到的数据在pipeline中进行事件传播。
                    pipeline.fireChannelRead(byteBuf);
                    byteBuf = null;
                } while (allocHandle.continueReading());

                allocHandle.readComplete();
                pipeline.fireChannelReadComplete();

                if (close) {
                    closeOnRead(pipeline);
                }
            } catch (Throwable t) {
                handleReadException(pipeline, byteBuf, t, close, allocHandle);
            } finally {
                // Check if there is a readPending which was not processed yet.
                // This could be for two reasons:
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelRead(...) method
                // * The user called Channel.read() or ChannelHandlerContext.read() in channelReadComplete(...) method
                //
                // See https://github.com/netty/netty/issues/2254
                if (!readPending && !config.isAutoRead()) {
                    removeReadOp();
                }
            }
        }
    }

    /**
     * Write objects to the OS.
     * @param in the collection which contains objects to write.
     * @return The value that should be decremented from the write quantum which starts at
     * {@link ChannelConfig#getWriteSpinCount()}. The typical use cases are as follows:
     * <ul>
     *     <li>0 - if no write was attempted. This is appropriate if an empty {@link ByteBuf} (or other empty content)
     *     is encountered</li>
     *     <li>1 - if a single call to write data was made to the OS</li>
     *     <li>{@link ChannelUtils#WRITE_STATUS_SNDBUF_FULL} - if an attempt to write data was made to the OS, but no
     *     data was accepted</li>
     * </ul>
     * @throws Exception if an I/O exception occurs during write.
     */
    protected final int doWrite0(ChannelOutboundBuffer in) throws Exception {
        Object msg = in.current();
        if (msg == null) {
            // Directly return here so incompleteWrite(...) is not called.
            return 0;
        }
        return doWriteInternal(in, in.current());
    }

    private int doWriteInternal(ChannelOutboundBuffer in, Object msg) throws Exception {
        if (msg instanceof ByteBuf) {
            ByteBuf buf = (ByteBuf) msg;
            if (!buf.isReadable()) {
                in.remove();
                return 0;
            }

            final int localFlushedAmount = doWriteBytes(buf);
            if (localFlushedAmount > 0) {
                in.progress(localFlushedAmount);
                if (!buf.isReadable()) {
                    in.remove();
                }
                return 1;
            }
        } else if (msg instanceof FileRegion) {
            FileRegion region = (FileRegion) msg;
            if (region.transferred() >= region.count()) {
                in.remove();
                return 0;
            }

            long localFlushedAmount = doWriteFileRegion(region);
            if (localFlushedAmount > 0) {
                in.progress(localFlushedAmount);
                if (region.transferred() >= region.count()) {
                    in.remove();
                }
                return 1;
            }
        } else {
            // Should not reach here.
            throw new Error();
        }
        return WRITE_STATUS_SNDBUF_FULL;
    }

    @Override
    protected void doWrite(ChannelOutboundBuffer in) throws Exception {
        int writeSpinCount = config().getWriteSpinCount();
        do {
            Object msg = in.current();
            if (msg == null) {
                // Wrote all messages.
                clearOpWrite();
                // Directly return here so incompleteWrite(...) is not called.
                return;
            }
            writeSpinCount -= doWriteInternal(in, msg);
        } while (writeSpinCount > 0);

        incompleteWrite(writeSpinCount < 0);
    }

    @Override
    protected final Object filterOutboundMessage(Object msg) {
        if (msg instanceof ByteBuf) {
            ByteBuf buf = (ByteBuf) msg;
            if (buf.isDirect()) {
                return msg;
            }

            return newDirectBuffer(buf);
        }

        if (msg instanceof FileRegion) {
            return msg;
        }

        throw new UnsupportedOperationException(
                "unsupported message type: " + StringUtil.simpleClassName(msg) + EXPECTED_TYPES);
    }

    protected final void incompleteWrite(boolean setOpWrite) {
        // Did not write completely.
        if (setOpWrite) {
            setOpWrite();
        } else {
            // It is possible that we have set the write OP, woken up by NIO because the socket is writable, and then
            // use our write quantum. In this case we no longer want to set the write OP because the socket is still
            // writable (as far as we know). We will find out next time we attempt to write if the socket is writable
            // and set the write OP if necessary.
            clearOpWrite();

            // Schedule flush again later so other tasks can be picked up in the meantime
            eventLoop().execute(flushTask);
        }
    }

    /**
     * Write a {@link FileRegion}
     *
     * @param region        the {@link FileRegion} from which the bytes should be written
     * @return amount       the amount of written bytes
     */
    protected abstract long doWriteFileRegion(FileRegion region) throws Exception;

    /**
     * Read bytes into the given {@link ByteBuf} and return the amount.
     */
    protected abstract int doReadBytes(ByteBuf buf) throws Exception;

    /**
     * Write bytes form the given {@link ByteBuf} to the underlying {@link java.nio.channels.Channel}.
     * @param buf           the {@link ByteBuf} from which the bytes should be written
     * @return amount       the amount of written bytes
     */
    protected abstract int doWriteBytes(ByteBuf buf) throws Exception;

    protected final void setOpWrite() {
        final SelectionKey key = selectionKey();
        // Check first if the key is still valid as it may be canceled as part of the deregistration
        // from the EventLoop
        // See https://github.com/netty/netty/issues/2104
        if (!key.isValid()) {
            return;
        }
        final int interestOps = key.interestOps();
        if ((interestOps & SelectionKey.OP_WRITE) == 0) {
            key.interestOps(interestOps | SelectionKey.OP_WRITE);
        }
    }

    protected final void clearOpWrite() {
        final SelectionKey key = selectionKey();
        // Check first if the key is still valid as it may be canceled as part of the deregistration
        // from the EventLoop
        // See https://github.com/netty/netty/issues/2104
        if (!key.isValid()) {
            return;
        }
        final int interestOps = key.interestOps();
        if ((interestOps & SelectionKey.OP_WRITE) != 0) {
            key.interestOps(interestOps & ~SelectionKey.OP_WRITE);
        }
    }
}
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