netty源码阅读之客户端新连接之检测新连接

雨点打透心脏的1/2处 2022-05-17 12:18 162阅读 0赞

boss线程的NioEventLoop在用户代码调用bond的时候启动。

检测新连接我们从《[netty源码阅读之NioEventLoop之NioEventLoop执行-----processSelectedKey()执行][netty_NioEventLoop_NioEventLoop_-----processSelectedKey]》这一篇文章的这个方法入口：processSelectedKeysOptimized(SelectionKey\[\] selectedKeys)

看到这一步：

final Object a = k.attachment();
    
                if (a instanceof AbstractNioChannel) {
                    processSelectedKey(k, (AbstractNioChannel) a);
                } else {
                    @SuppressWarnings("unchecked")
                    NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                    processSelectedKey(k, task);
                }

之前我们的NioSocket以attachment的方式传入select key set里面，所以我们进入

processSelectedKey(k, (AbstractNioChannel) a);

这个方法里面。

private void processSelectedKeysOptimized(SelectionKey[] selectedKeys) {
            for (int i = 0;; i ++) {
                final SelectionKey k = selectedKeys[i];
                if (k == null) {
                    break;
                }
                // null out entry in the array to allow to have it GC'ed once the Channel close
                // See https://github.com/netty/netty/issues/2363
                selectedKeys[i] = null;
    
                final Object a = k.attachment();
    
                if (a instanceof AbstractNioChannel) {
                    processSelectedKey(k, (AbstractNioChannel) a);
                } else {
                    @SuppressWarnings("unchecked")
                    NioTask<SelectableChannel> task = (NioTask<SelectableChannel>) a;
                    processSelectedKey(k, task);
                }
    
                if (needsToSelectAgain) {
                    // null out entries in the array to allow to have it GC'ed once the Channel close
                    // See https://github.com/netty/netty/issues/2363
                    for (;;) {
                        i++;
                        if (selectedKeys[i] == null) {
                            break;
                        }
                        selectedKeys[i] = null;
                    }
    
                    selectAgain();
                    // Need to flip the optimized selectedKeys to get the right reference to the array
                    // and reset the index to -1 which will then set to 0 on the for loop
                    // to start over again.
                    //
                    // See https://github.com/netty/netty/issues/1523
                    selectedKeys = this.selectedKeys.flip();
                    i = -1;
                }
            }
        }
    
        private void processSelectedKey(SelectionKey k, AbstractNioChannel ch) {
            final AbstractNioChannel.NioUnsafe unsafe = ch.unsafe();
            ..
            try {
                int readyOps = k.readyOps();
                // We first need to call finishConnect() before try to trigger a read(...) or write(...) as otherwise
                // the NIO JDK channel implementation may throw a NotYetConnectedException.
                if ((readyOps & SelectionKey.OP_CONNECT) != 0) {
                    ...
                }
    
                // Process OP_WRITE first as we may be able to write some queued buffers and so free memory.
                if ((readyOps & SelectionKey.OP_WRITE) != 0) {
                    ...
                }
    
                // Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
                // to a spin loop
                if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                    unsafe.read();
                    if (!ch.isOpen()) {
                        // Connection already closed - no need to handle write.
                        return;
                    }
                }
            } catch (CancelledKeyException ignored) {
                unsafe.close(unsafe.voidPromise());
            }
        }

由于服务端的socketChannel注册的是accept事件，所以我们会进入这个：

// Also check for readOps of 0 to workaround possible JDK bug which may otherwise lead
                // to a spin loop
                if ((readyOps & (SelectionKey.OP_READ | SelectionKey.OP_ACCEPT)) != 0 || readyOps == 0) {
                    unsafe.read();
                    if (!ch.isOpen()) {
                        // Connection already closed - no need to handle write.
                        return;
                    }
                }

### NioMessageUnsafe.read() ###

服务端NioServerSocketChannel的unsafe的实现是NioMessageUnsafe（后续会有文章介绍unsafe），所以我们到达了这个方法:

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 {
                            int localRead = doReadMessages(readBuf);
                            if (localRead == 0) {
                                break;
                            }
                            if (localRead < 0) {
                                closed = true;
                                break;
                            }
    
                            allocHandle.incMessagesRead(localRead);
                        } while (allocHandle.continueReading());
                    } catch (Throwable t) {
                        exception = t;
                    }
    
                    int size = readBuf.size();
                    for (int i = 0; i < size; i ++) {
                        readPending = false;
                        pipeline.fireChannelRead(readBuf.get(i));
                    }
                    ...
    
                    if (closed) {
                        inputShutdown = true;
                        if (isOpen()) {
                            close(voidPromise());
                        }
                    }
                } finally {
                    ...
                }
            }
        }

这个方法里面有个doMessageRead()，是主要的读方法，通过while循环不停地读数据。

这个final RecvByteBufAllocator.Handle allocHandle = unsafe().recvBufAllocHandle();代码里面有个handle，是读的策略（控制连接接入的速度，每次有一个新连接就加1，默认最大只读16个连接）。在这个

allocHandle.continueReading()里面，可以知道它什么时候停下来：

@Override
            public boolean continueReading() {
                return config.isAutoRead() &&
                       attemptedBytesRead == lastBytesRead &&
                       totalMessages < maxMessagePerRead &&
                       totalBytesRead < Integer.MAX_VALUE;
            }

totalMessages<maxMessagePerRead是判断读到的连接是否大于最大连接，maxMessagePerRead默认是16。

我们继续doMessageRead()吧。

### doMessageRead() ###

@Override
        protected int doReadMessages(List<Object> buf) throws Exception {
            SocketChannel ch = javaChannel().accept();
    
            try {
                if (ch != null) {
                    buf.add(new NioSocketChannel(this, ch));
                    return 1;
                }
            } catch (Throwable t) {
                logger.warn("Failed to create a new channel from an accepted socket.", t);
    
                try {
                    ch.close();
                } catch (Throwable t2) {
                    logger.warn("Failed to close a socket.", t2);
                }
            }
    
            return 0;
        }

在这里面，SocketChannel是通过jdk实现的。然后这个buf.add(new NioSocketChannel(this, ch));新建一个客户端的NioSocketChannel，把jdk实现得SocketChannel绑定进去(把jdk底层的channel封装进去)，并且把this，也就是服务端NioServerSocketChannel也作为参数传递进去。

buf是之前传入进来的，其实他就是之前的readBuf，一个list，保存的是客户端的NioSocketChannel：

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

下一篇文章，我们分析new NioSocketChannel(this, ch)这一步做了什么事情。

[netty_NioEventLoop_NioEventLoop_-----processSelectedKey]: https://blog.csdn.net/fst438060684/article/details/81559977